ASDSO Survey Hydrologic Model Calibration/Uncertainty ... · response parameters and uncertainty....

ASDSO Survey – Hydrologic Model Calibration/Uncertainty

August 2016

Information request from Montana:

Montana Dam Safety has assembled a group of experts to evaluate our methodology for determining

the inflow design flood for spillway analysis. This “Extreme Storm Working Group” is focusing on basin

response parameters and uncertainty. They have asked us to conduct a survey of other states to better

understand the national state of practice. We realize that there are other recent surveys dealing with

inflow design flood determination. The purpose of this survey is to query states on how they deal with

model calibration and uncertainty. We would appreciate of few moments of your time in responding to

the following questions:

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)?

A. If so, do your regulatory rules require your “guidelines” to be followed? A link to your guidelines would be appreciated.

#2 Montana is looking closely at how hydrologic models are pseudo-calibrated / verified. We rarely have sufficient gage data and need to improvise. We would be interested in hearing your thoughts. Specifically:

A. Do you use regional (e.g. USGS Peak Flow) regression equations as a calibration tool? Or do you

prefer that local regression equations be developed and utilized? B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? C. Other comments or advice?

#3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation?

A. If yes, how do you calibrate/verify these models? #4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)? #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept? #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification. Any other comments, suggestions or advice? (M. Lemieux)

State Responses:

Alaska - (C. Cobb)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? Guidelines reference HEC-HMS or TR-55.


No, but see Chapter 6 of the Guidelines for Cooperation with the Alaska Dam Safety Program. A 2016 update is in progress.


A. Do you use regional (e.g. USGS Peak Flow) regression equations as a calibration tool? Or do you prefer that local regression equations be developed and utilized?

Regional regression equations may be used for low hazard dams or for checking other methods, but not as calibration tool. NOAA Atlas 14 is preferred for AEP values.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? No specific requirements.

C. Other comments or advice? #3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation? No

A. If yes, how do you calibrate/verify these models? #4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)? When USGS regression equations are used, they may have to pick the 84th percentile extreme value rather than the median. #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept? No specific requirements. #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification. Engineer’s judgement.

http://dnr.alaska.gov/mlw/water/dams/AK_Dam_Safety_Guidelines062005.pdf

California – (W. Daniel Meyershon)

Before addressing your questions, it may be beneficial explaining the philosophy on hydrologic dam safety evaluations implemented at the California Department of Water Resources, Division of Safety of Dams (DSOD) on Jurisdictional-size dams. Recognizing that hydrology is a science of extremes and there is significant inherent uncertainty, the Division has adopted a long-standing tradition of performing hydrologic evaluations independently of those of dam owners or their consultants. This independence helps in addressing the effects epistemic uncertainties and increases confidence in hydrologic estimates, especially when different methods yield similar results. Because of this approach to our hydrologic evaluation, we do not require dam owners to use specific hydrologic methods in their evaluation provided these methods are accepted by the engineering community and demonstrated to provide reasonable predictive capabilities. Our only requirement is related to the size of the evaluation (or design) storm. The magnitude of the design storm is based on an internal evaluation of the downstream damage potential as well as dam height and reservoir size. From this information the Division assesses the return period of the storm and its associated standard deviation. The minimum return period for the evaluation storm is 1,000 years and the maximum storm is capped at the PMP. In cases where the design storm is smaller than the PMP but has a return period of more than 1,000 years, the Division provides the required return period to the owner for their hydrologic evaluation. #1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? Yes


The California Department of Water Resources, Division of Safety of Dams (DSOD) has developed over the years procedures to evaluate the magnitude of design storms and develop pertinent basin parameters for hydrologic routing. As stated above, these procedures are used only internally by our staff. Dam owners and their consultants are not required to use the same procedures used by DSOD. In fact, the Division expects that dam owners or their consultants select the most appropriate hydrologic model and develop the dam watershed model independently of our own evaluation. #2 Montana is looking closely at how hydrologic models are pseudo-calibrated / verified. We rarely have sufficient gage data and need to improvise. We would be interested in hearing your thoughts. Specifically:


prefer that local regression equations be developed and utilized? Normally, we do not use regression equations developed from stream gage information as a basis for hydrologic studies. We rely more heavily on rainfall data. Stream gage flow data, if available and of good quality, are only used for calibrating basin parameters used in

hydrologic routing models. The reason for relying on rainfall data is simply because rainfall measurements are much more reliable and subject to fewer systematic and random errors compared to stream gages. In addition, the network of rainfall stations is much more extensive than that of stream flow. We face the same dilemma in that there are only a few gaged watersheds and only a few of them have captured sufficiently large storms with good enough resolution and data quality to be used for model calibration. Therefore, our hydrologic strategy is to establish rainfall scenarios and then route them through hydrologic models to estimate inflows to reservoirs. Our internal procedure to evaluate the magnitude of the design storm for dams with storms smaller than PMP is based on an assessment of the return period. We use the Pearson Type III distribution with regional skew factors for Northern and Southern California. Depth-duration curves are then estimated using the local Mean Annual Precipitation and rainfall data from rain gages located within or nearby the subject basin. For small dams where the return period is 1,000 years, we compare the depth-duration curve from our internal procedure with the depth-duration curve from NOAA Atlas 14 for a 1,000-year return period. Regarding the watershed model, we normally used the Clark unit hydrograph for each basin comprising the dam’s watershed. The characteristics of each basin are evaluated individually (time of concentration, storage coefficient, precipitation loss model parameters, etc.) With the assistance of the USACE, Hydrologic Engineering Center (HEC), we have developed procedures to evaluate basin parameters for the development of the unit hydrograph and precipitation losses. These procedures are semi-empirical in nature and were based on flow measurements associated with low-return period events (around 10 years). Unfortunately, data variability was not incorporated in the formulae used for our hydrologic procedures and, as a result, we cannot quantify uncertainty/variability in our hydrologic evaluations.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? We do not require a particular size storm to be used for calibration of a hydrologic model as needed data to perform such calibration are normally not available. Nevertheless, if good quality rain and flow data are available we normally use the largest storm available to develop pertinent basin parameters for hydrologic routing. C. Other comments or advice?

Recently, we have been exploring new methodologies that incorporate nonlinearities in basin response by regarding the time of concentration as dependent on the magnitude of the rainfall intensity (Meyersohn, 2016). Our methodology can be used for basin calibration, especially if used with relatively large storms, as fewer calibrating factors would be needed. Meyersohn, W. D. (2016). "Runoff Prediction for Dam Safety Evaluations Based on Variable Time of Concentration." J. Hydrol. Eng. , 10.1061/(ASCE)HE.1943-5584.0001406 , 04016031.


For large watersheds, we normally divide it into smaller, more manageable basins. Ideally, we try to keep the maximum size of each basin to about 30 to 40 square miles, which is the upper limit of applicability of our basin empirical models. However, in dividing a watershed into basins we look at the local morphology, elevation range, presence of upstream reservoirs, variation in Mean Annual Precipitation, and rain gage variability. Area reduction factors area applied to these large watersheds to estimate the magnitude of the depth-duration curves for the different basins.

A. If yes, how do you calibrate/verify these models? The best way to calibrate a hydrologic model with actual measurements. #4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)? As stated above, our philosophy is to perform hydrologic evaluations independently of those of dam owners. While this practice does not address the inherent variability in hydrology, at least it serves a confirmation of applicability of hydrologic models, especially if results are not different from each other. When differences arise, technical discussion would normally identify which component of either model is responsible for the difference. We agree with the State of Montana that inherent uncertainty and variability is an issue that deserves some consideration. While uncertainty in rainfall is accounted to some extent by the use of actual rainfall data and extreme event distributions, we normally do not quantify the magnitude of the uncertainty other than estimating the return period for the design storm. While we do not incorporate direct parameter variability in our evaluations, we often perform sensitivity analyses to identify parameters that have the most influence in predicted flow but we do not attempt to formally quantify variability. #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept? Our precipitation loss model is prescriptive in nature rather than predictive. Considering the large storms used for dam safety evaluations, we do not consider existing loss models (Green-Ampt, CN method, etc.) fully applicable. These methods attempt to predict the amount of water lost at single locations with defined properties and not at large areas with almost random soil properties. In addition, these methods have been developed essentially for much smaller storms. Instead of predicting the losses and attempting to evaluate the influence of antecedent storms, we select a target net precipitation loss, say 60 or 80 percent. We have developed guidelines to estimate an appropriate net loss target. We prefer the exponential loss model for our evaluations because it is amenable for a prescribed approach. Our implementation of the exponential loss model has been calibrated for Northern and Southern California and only a few parameters are varied to obtain the desired targeted loss.

Dam owners and their consultant are free to select any appropriate loss model for their evaluation. We have noticed that the CN method appears to be quite popular among consultants. #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification. We assume that a series of antecedent storms would occur before the design storm such that the soil

is saturated. This eliminates uncertainty regarding the need to account for initial abstraction and

antecedent soil moisture.

Colorado – (K. Bauer)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? YES


We have Hydrologic Basin Response Parameter Estimation guidelines

http://water.state.co.us/DWRIPub/Documents/HyBasinRsp200805.pdf #2 Montana is looking closely at how hydrologic models are pseudo-calibrated / verified. We rarely have sufficient gage data and need to improvise. We would be interested in hearing your thoughts. Specifically:


prefer that local regression equations be developed and utilized? We typically use regression equations to evaluate the peak flow yield of the basin (cfs/sq-mi) as a validation check of the peak runoff calculation for the dam. PMP events typically have a peak basin yield one order of magnitude higher than the 100-/500-year regression equations.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? C. Other comments or advice?

#3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation? Our guidelines state that basins should be sub-divided based on significant changes in soil types or vegetative cover.

A. If yes, how do you calibrate/verify these models? We may do a sensitivity analysis to see how much it impacts. #4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)? We usually do best estimates to get the most likely value then perform sensitivity analysis to see what parameters most influence runoff and spillway sizing. If one parameter appears to control, we may recommend adjusting that parameter to yield more conservative results, or allow the engineer to adjust the other way if it can be justified with data and/or calculations.

http://water.state.co.us/DWRIPub/Documents/HyBasinRsp200805.pdf

#5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept? Recommendations are in our guidelines. We prefer Greene & Ampt or Initial and Constant loss methods. There are none that we across the board do not accept. We always check by comparing to one of our preferred methods if an engineer selects another methodology. #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification. It depends on the characteristics of the basin. If extreme storms are likely to occur early in the season

when the snowpack has just melted out, then saturated AMC may be appropriate. Likewise in an

agricultural basin where the storm may occur during irrigation season. There are recommended tables

in our guidelines, engineers can vary from these with justification.

Idaho – (J. Falk)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? NO



A. Do you use regional (e.g. USGS Peak Flow) regression equations as a calibration tool?

YES – Stream Stats is an excellent tool for Q100 Or do you prefer that local regression equations be developed and utilized? Of course; but why reinvent the wheel? If a local regression equation has been developed for the particular watershed, we will use the local model.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? Q100 C. Other comments or advice?

It is very important to consider the size of the watershed when selecting or requiring the design engineer to select any particular model; for example, what areal extent does the regional storm predominate the discussion (peak-cfs vs. volume-acft)?

#3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation? NO, but again would point out that spillway capacity at dams occupying watersheds greater than XX (?)-square miles are not simply evaluated based on Peak Q. For larger-sized watersheds, recent trend now is to view through the window of RISK Analysis to determine whether or not a pure “one-death” to “PMF capacity” correlation is appropriate for sizing the spillway.

A. If yes, how do you calibrate/verify these models?

#4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)? Well, to be honest, we really don’t. For High hazard dams, we require the spillway to pass Q500 up to Full PMF (depending on the reservoir capacity). Let’s please don’t forget that all parts of this topic incorporates a great deal of uncertainty.....but, now that we have become a nation so aware of ourselves, I suppose a great deal of money will be spent trying to prove how much we really know (not) about the earth’s climate, storm potential, or other natural occurring extremes.

#5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept? Loss of one human life; let’s see....would that be 1 x 101 ?? #6 How do you deal with initial abstraction and antecedent moisture conditions? Please define exactly what you mean by the term “antecedent moisture”. Most locations in the state of Idaho consist of high desert and even higher mountains largely removed from coastal influences. Our encounters with antecedent moisture typically occur in the form of snow (not wet ground per se). In the past, we have required hydrologic analysis to include Q100-SNOW, onto which is precipitated the design threshold rainfall. Note if you handle this differently during model calibration/verification. Any other comments, suggestions or advice? Fortunately, I live in a geographic area that has a great forecasting view of approaching storms with

large moisture-generating potential. The big unknown for us each year is how fast accumulated snow

in any particular watershed will melt; of course, largely dependent on how much snow remains at any

given time during any given season. For watersheds greater than ±10 mi2, variables of SNOW and the

ELEVATION of the watershed is the big issue; not rainfall events.

Kentucky – (R. Gruzesky)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? Yes. 401 Kentucky Administrative Regulation 4:030 “Design criteria for dams and associated structures Engineering Memorandum No. 5 (EM5) incorporated by regulation, 401 KAR 4:030. A. If so, do your regulatory rules require your “guidelines” to be followed? A link to your guidelines would be appreciated. Yes. 401 Kentucky Administrative Regulation 4:030 Section 2 outlines procedures in “Design of Small Dams” shall be the minimum criteria. Also, Engineering Memorandum No. 5 (EM5) incorporated by regulation, 401 KAR 4:030 states that procedures for hydrologic design as contained in USDA Soil Conservation Service National Engineering Handbook will be accepted. Link to guidelines: http://water.ky.gov/damsafety/Pages/default.aspx



Kentucky does not use regional regression equations nor develop localized regression equation to calibrate hydrologic models.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? Kentucky does not require calibration for any type of storm frequency. Kentucky lies within the domain of National Weather Services (NWS) Hydrometeorological Report No. 51 (HMR 51). HMR 51 along with Technical Paper 40 and Technical Paper 49 are the basis for the established rainfall frequency values and are adequate for most engineering design purposes. The rainfall values used in NWS publications are for ten (10) square mile areas in Kentucky.

C. Other comments or advice? Kentucky’s minimum hydrologic criteria are set forth in Engineering Memorandum No. 5 (EM5). EM5 has a set of equations with established rainfall frequency values for the development of each hydrograph for hazard class. The rainfall frequency values are obtained from several sources developed by NWS and the sources are listed in Engineering Memorandum No. 5. The rainfall values are tabulated in a convenient form for each county in a publication called ‘Rainfall Frequency Values For Kentucky’ (Engineering Memorandum No. 2). #3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation? Kentucky has not developed guidance for model sub basin delineation.

A. If yes, how do you calibrate/verify these models? #4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)? We do agree that there is a degree of uncertainty inherent in the study of hydrology. Best estimate parameters vary greatly in the engineering community and with a wide range of solutions. Kentucky by

http://water.ky.gov/damsafety/Pages/default.aspx

regulation uses the rainfall frequency values obtained from the aforementioned listed publication sources developed by NWS. Originally, Kentucky’s rainfall values were adequate but today lack updated rainfall information necessary to incorporate more site-specific consideration that can provide improved estimates. #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept? Kentucky uses the loss methods in SCS National Engineering Handbook (NEH). SCS NEH uses any one of four loss methods: 1) Runoff curve number using rainfall data and watershed’s characteristics, 2) Runoff volume maps, 3) Regionalization and transposition of volume-duration-probability analyses, 4) Stream-flow data with provision of sufficient documentation on the method and results. These methods and other hydrologic criteria only establish the lowest limit of design considered acceptable and does not eliminate the need for sound engineering judgment. #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification. Kentucky deals with direct runoff. Initial abstraction (Ia) consist of interception, evaporation, and storage that must be exhausted before direct runoff may begin. All runoff volumes for design purposes are based on Antecedent Moisture Condition II or greater.

Maryland: (B. Harrington)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit

hydrograph parameters, etc.)?

We calibrate most all our Computer Models (HEC1, HMS, TR20, HydroCad) to USGS Gage data. We have a statewide GIS Porgram that instantly computes watershed area, Cn’s, Tc’s, Soils, landuse, impervious area, 2yr through 500yr discharges. The website is http://www.gishydro.eng.umd.edu/ through the University of Maryland. We use the “Maryland Hydrology Panel Report, Third Edition September 2010 for guidance and calibration. The report can be download at this site: http://www.gishydro.eng.umd.edu/panel.htm


See above links. #2 Montana is looking closely at how hydrologic models are pseudo-calibrated / verified. We rarely have sufficient gage data and need to improvise. We would be interested in hearing your thoughts. Specifically:

A. Do you use regional (e.g. USGS Peak Flow) regression equations as a calibration tool? Or do you prefer that local regression equations be developed and utilized? See above.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? 100-yr and 500-yr

C. Other comments or advice?



We are lucky in Maryland with the GisHydro Model . It can calculate all hydrologic parameters from almost any size watershed from 400 acres to 10,000 square miles

#4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)?

We only request this for very large dams with inadequate spillways not passing full PMF. A detailed PFMA would apply a probable failure frequency for overtopping and soil piping. I agree that the frequency is a guess with PMF being somewhere between 10,000 and 1 million year

http://www.gishydro.eng.umd.edu/

http://www.gishydro.eng.umd.edu/panel.htm

storm. The frequency applied to soil piping failure is just as hard or harder to apply a failure frequency. Justification may show that the structure is more likely to fail by a non-overtopping event and full PMF passage is not justified. Also an incremental failure analysis may show no increased flood dangers with increasing the spillway size.

#5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept?

In general we use SCS curve number loss method. Other methods can be used if calibration to a particular extreme event is computed.

#6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification.

Standard initial abstraction = 0.2 S and AMC 2 (SCS) is assumed unless a particular storm is calibrated. If calibrated, hydrologic variables can be changed to achieve the best fit to gage data or high water marks.

Minnesota – (J. Boyle)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? NO




prefer that local regression equations be developed and utilized? ONLY AS A LAST RESORT IF THERE IS NO BETTER AVAILABLE INFORMATION. WE HAVE PRETTY GOOD GAGE DATA AND LAKE LEVEL DATA.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? 100 YEAR IS RECOMMENDED. C. Other comments or advice?

#3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation? WE HAVE LOTS OF LAKE TO CALIBRATE TO.

A. If yes, how do you calibrate/verify these models? #4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)?

WE HAVE VERY FEW HIGH HAZARD DAM REQUIRING PASSAGE OF AN EXTREME STORM. OFTEN, THE INCREMENTAL RISK ANALYSIS DRIVES THE INFLOW DESIGN FLOOD EVENT.


NO REQUIREMENTS, GENERALLY USE INITIAL AND CONSTANT. #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this

differently during model calibration/verification.

Mississippi – (D. Myers)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? No


N/A #2 Montana is looking closely at how hydrologic models are pseudo-calibrated / verified. We rarely have sufficient gage data and need to improvise. We would be interested in hearing your thoughts. Specifically:

A. Do you use regional (e.g. USGS Peak Flow) regression equations as a calibration tool? Or do you prefer that local regression equations be developed and utilized? We use regional regression equations.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? 100-YR

C. Other comments or advice? No


We divide into sub basins based on the % of the drainage basin represented by different tributaries and the potential for the Tc of that sub basin to be significantly different than if it were represented as part of a larger watershed based on professional judgement. A. If yes, how do you calibrate/verify these models? We don’t.


We don’t address the uncertainty. #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept?

We generally use the SCS Curve Number Method. #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification.

We use the SCS method, so Ia = 0.2S and an AMC of 2.

Nebraska – (T. Gokie)


Yes, we utilize NRCS TR-60 and other associated NRCS technical references for our design guidelines. (http://dnr.nebraska.gov/dam/resources)

A. If so, do your regulatory rules require your “guidelines” to be followed? A link to your

guidelines would be appreciated.

No. #2 Montana is looking closely at how hydrologic models are pseudo-calibrated / verified. We rarely have sufficient gage data and need to improvise. We would be interested in hearing your thoughts. Specifically:

A. Do you use regional (e.g. USGS Peak Flow) regression equations as a calibration tool? Or do you prefer that local regression equations be developed and utilized? We generally find all regression equations in Nebraska to be unreliable. We use stream gage data if calibration is necessary.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? We generally do not require calibration/verification unless the drainage area for the dam is over 50 square miles. If calibration is required, we discuss the calibration method with the design engineer and come to an agreement on the calibration method that will be used.

C. Other comments or advice? #3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation?

A. If yes, how do you calibrate/verify these models? We follow NRCS guidance for diverse watersheds (i.e. large drainage basins should be divided into hydrologically homogeneous sub-basins and the size of these sub-basins should not exceed 20 square miles.) Stream gage data may be used to help calibrate the model.


We generally don’t use regression equations due to their unreliability in Nebraska. We use the PMP for design of high hazard dams and 70% PMP for the evaluation of existing high hazard dams in Nebraska.


We generally require SCS curve number method. #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification.

We follow the SCS method. NRCS TR-60 generally requires antecedent runoff condition II. The antecedent runoff condition must be taken into consideration during calibration.

http://dnr.nebraska.gov/dam/resources

Nevada – (L. Opperman)

Currently the Nevada Dam Safety Program (NDSP) doesn’t regularly utilized modeling tools in house and

thus lacks much experience in many of the specific questions you are asking. Basin response parameters

in numeric modeling tools, their calibration, variability in spatial distributions, etc.. are all considerations

we ask design engineers to discuss in their design reports for our review process. Sometimes we ask for

justifications or further details and could even ask a consultant to assist in our review if needed.

#1 NV has no specific guidelines for basin response parameters.

#2 NV also lacks significant gage data and we have seen both regional and local regression equations

developed used in design reports, but still there is a bunch of uncertainty. I think the most recent

projects understand the variability in design assumptions and take a conservative approach to overbuild

the spillway to be more comfortable and increase factors of safety.

#3 currently, this office lacks guidance on large basins/watershed modeling details.

#4 Many of our newer high hazard facilities are designed with uncertainty in mind and do include

sensitivity analysis to address this, but NDSP lacks guidance on procedures or minimum

standards. Another consideration, is have conduct an Incremental Damage Analysis to determine

consequences from inundation if inflows are designed to low. http://www.leg.state.nv.us/nac/NAC-

535.html 3. The State Engineer will approve plans that use an inflow design flood which is less than

those set forth in subsection 2 if the applicant provides an incremental damage analysis that

demonstrates, to the satisfaction of the State Engineer, that a lesser event is appropriate

#5 I do not recall off hand what loss methods have been submitted.

#6 No experience in initial abstraction and antecedent moisture verification.

http://cp.mcafee.com/d/5fHCN0q3wUSyMeKCMqerII6XCQkrzHCzAQsInjdEET78ThK_sSyzstsQsCzByX9J56UV56X9EVd79JmF53U0F3lqqXYOT00jq6GQRTVBK00CZoupjQs_R-p7f9LZuVtdBdXDAn7SuEyCJtdmWb7axVZicHs3jq9JcTsTsS02fYzR9Xf-wpVKCgSS0Mg4yXXb_0USvlprUGX056dDYuwpUzkOrjohKUr1oQAq80S3Jjh02_5C_FcQgeNGGq81KI9X3jh0nd40MT4PVoQglwq82pp2djWApmd42Jkffd45JfAgzIVlxzsbv6uOxcIq82VEwmf_quq88-Iumd410Iq87IhvdETvdTCDOqFrD

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New Jersey – (D. Shaffer)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? - No




prefer that local regression equations be developed and utilized? NJ’s design standard is based on theoretical rainfall distribution (NRCS/SCS type III rainfall for the 100 year or HMR 51/52 for the PMP or some fraction thereof). We permit USGS streamflow data to be used as a calibration tool only when reliable rainfall data (amount and distribution) can be correlated to the stream data (this is rarely possible). In our region, peak streamflow frequencies rarely correlate well with the rainfall frequencies (i.e. 100 year peak stream flow is typically much less than the 100 year rainfall runoff computed using standard methodologies such as TR-55).

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? We have not established a storm frequency for calibration.


#3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation? No. NJ drainage basins are relatively small.

A. If yes, how do you calibrate/verify these models? #4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)? NJ does not require a sensitivity analysis unless computed/assumed model parameters produce an unexpected result (such as an unexpectedly low runoff value). #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept? SCS Curve Number is by far the most common loss method used in NJ. . #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification. Initial abstraction is typically computed using the SCS CN method: I=.2[(1000/CN)-10]. Antecedent moisture condition (AMC) is assumed to be type II (average moisture). If calibrating a model to a specific rainfall event, it may be appropriate to adjust AMC.

New Mexico – (C. Thompson)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? Yes.

A. If so, do your regulatory rules require your “guidelines” to be followed? Although not stated explicitly in the NM Rules and Regulations Governing Dam Design, Construction and Dam Safety (hereafter referred to as “Rules and Regs”), Bureau staff engineers generate review comments to consulting engineers based on a hydrologic guidelines (White Paper) document developed internally by the NMOSE Dam Safety Bureau in 2008. The majority of the consulting engineers submitting Hydrologic Reports to the NMOSE Dam Safety Bureau are aware either through direct discussion with Bureau staff or via the Bureau’s website that hydrologic guidance is available for dams in New Mexico. Hence, although not stated directly in the Rules and Regs, the Bureau requires that the guidelines be followed, but will consider variations to hydrologic modeling so long as appropriate justification by way of supporting documentation and/or computations are provided. A link to your guidelines would be appreciated. The link is as follows: http://www.ose.state.nm.us/DS/documents/HydrologicAnalysisForDams.pdf



USGS regional regression equations as well as local regression equations if available are considered acceptable on a case-by-case basis. As expected, the preference would be to utilize local regression equations if they have been properly vetted. Given the push for the preparation of flood inundation mapping for Emergency Action Plans for dams, the vast majority of the dams currently being evaluated by Bureau staff have either a hazard potential rating of significant or high and therefore, depending on the geographic location of the drainage basin, the inflow design flood is based on HMR 55A or HMR 49. To meet the requirements of Section 19.25.12.18.F of the NM Rules and Regs, breach analyses for the Sunny-day failure, the flood discharge corresponding to the governing PMP event failure, and any other (third) event deemed appropriate by the dam owner must be determined. For the “third” event, many owners, as advised by their respective engineers, include analyses for frequency storm events to determine which event (e.g.: 100-yr 24-hr, 500-yr 24-hr, etc.) correspond to 100 percent of spillway capacity for a particular dam. A breach analysis is then performed for a “top-of-dam” failure using the frequency event that corresponds to the spillway capacity for the dam. Hence, indirectly, it can be stated that regression equations are used as a calibration tool even though the majority of the analyses are centered around development of PMP events, and as stated above, the preference would be to use local regression equations if avaialble.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? There is nothing specific in the NM Rules and Regs or the hydrologic guidance document concerning calibration/verification. However, either of the frequency events listed above, or perhaps others, could be acceptable on a case-by-case basis.

C. Other comments or advice? The guidance document available for download at the link provided in response to Comment 1.A can be referred to for more detailed information.

#3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation? Possibly, if this is being asked in the context of suggested limitations to sub-basin/basin size; however, the guidance is limited.

A. If yes, how do you calibrate/verify these models? Please refer to the guidance document available for download from the link provided in response to Comment 1.A.


We do not currently look at a risk based approach, but rather, utilize a deterministic approach. #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept?

Please refer Item 12 of the guidance document available for download from the link provided in response to Comment 1.A for more specific information pertaining to loss methods.


Please refer to the guidance document available for download from the link provided in response to Comment 1.A for more specific information pertaining to initial abstraction and antecedent moisture conditions.

Any other comments, suggestions or advice?

Nothing specific, as stated above and reiterated herein, please refer to the guidance document

available for download from the link provided in response to Comment 1.A for more specific

information concerning the state of the practice in New Mexico for hydrologic report submittals.

North Carolina – (S. Cook)

We don’t do any calibration of models on the dam safety side in North Carolina. However, our NC Risk

Map / Floodplain mapping program does a lot of that. They now have models built that can accurately

predict flooding in real time. I didn’t realize until a few weeks ago that when the flooding happened in

South Carolina last year, they were testing their new system out to see how accurate it was based on

the flooding we were seeing in North Carolina. You may find it helpful to call John Dorman or Tom

Langan at the NC Floodplain mapping group and see how they are calibrating their models. They may

give you some ideas.

Ohio – (D. Barnhouse)


We do not have guidelines at this point. However, the rules in the Ohio Administrative Code state that the inflow must be determined from actual streamflow and frequency records or from synthetic criteria based on publications prepared by USCOE, USGS, NOAA, and others deemed acceptable. Mostly, we try to ensure that accepted procedures are followed correctly and are appropriate to the watershed.


guidelines would be appreciated.


When “calibrating” a model, we do use the USGS regression equations for the 100 year flood. Based on comparisons of frequency floods to some record flood information and then extrapolating, we have drawn a loose conclusion that a 100 year flood is approximately 12% of the PMF. It is understood that there has not been a correlation between flood frequency and the PMF; the USGS peak flows are used to make sure the model produces results in the right “ballpark”. A. Do you use regional (e.g. USGS Peak Flow) regression equations as a calibration tool? Or do you

prefer that local regression equations be developed and utilized? B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? C. Other comments or advice?


Very difficult question. In the past, we have accepted results of uncalibrated models as long as it appeared to be conservative (subdividing into small basins, conservative rainfall distributions, conservative unit hydrographs, etc.). We have just ensured that the published procedures were followed. Now we do a comparison to USGS regression equation results as mentioned in question 2 to try to make sure it’s too far off. To refine an analysis, our advice would be to try to find a gage in the watershed or a gage for a similar watershed that could be used to either calibrate an existing model or to calibrate parameters.

We recently had an inquiry from a dam owner who wanted to re-evaluate the flood capacity of their dam which has a very large drainage area. There is only one stream gage in the basin. This basin size is out of the range of available synthetic unit hydrographs for a single basin. We had 2 recommendations: 1. Subdivide the basin into sub-basins that fit the criteria for an established unit hydrograph method, 2. Look into developing parameters from gage information to develop unit hydrographs for another method such as the Snyder UH Method that may be more appropriate for a basin this size. We are working under the assumption that the unit hydrograph method is valid for floods as large as a PMF and that calibrated parameters will produce a more calibrated model. It’s very unusual to find gage information where you need it.



We have not discussed this specifically. #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept?

The most common loss method that we see is the SCS Curve Number. To my knowledge, we have not had a method presented that we did not accept if there is some basis for its use.


We usually accept normal (type 2) conditions but have also had discussions that wet (type 3)

conditions may be more appropriate for PMF type events.

Oklahoma – (Z. Hollandsworth)


No specific parameters are required or restricted from use. It is up to the modeler to provide justification for the selected parameters and assumptions.


guidelines would be appreciated. It is not stated in our State rules; however, all plans must be approved by dam safety staff who will only recommend a plan’s approval by our Board if the guidelines are met.


A. Do you use regional (e.g. USGS Peak Flow) regression equations as a calibration tool? Or do you prefer that local regression equations be developed and utilized? We have used USGS Peak Flow regression equations that were developed for our state.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? For high hazard dams we have requested that the model be verified using both the 100-year and 500-year storms.



Not specifically. The guidelines state that if a basin is highly irregular that the modeler should delineate the basin into separate sub-basins to account for that variability.

A. If yes, how do you calibrate/verify these models? We rely on good engineering judgment.


We do not use a risked based spillway standard. Uncertainty is often counteracted with increased freeboard.


Curve number is most often used. We have no restrictions on acceptable loss methods, as long as the modeler provides justification for the use of their chosen method.


We use no initial abstraction. Any other comments, suggestions or advice?

Rarely do we see a model of a reservoir with a drainage area greater than 10 square miles.

Models of high hazard dams are also less common. In the case of large or high hazard dams we

would ask for some sensitivity analysis, but we have not explicitly stated so in our guidance

documents. For high hazard dams we usually work with the engineers to reduce the risk of

flooding during normal high flow scenarios, such as providing some attenuation for the 100-year

storm, if possible. Many times the uncertainty of the analysis is taken into account by adding

additional freeboard.

Oregon – (K. Mills)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? We have a joint publication with the USGS for return flows where basin information is already included. It provides information up to the 0.1 percent AEP, but I do not believe it is reliable above the 1 percent AEP. It includes basin cover type and very general infiltration parameters. It is also not appropriate for basins smaller than those that are typically gaged. It is also not appropriate for wildfires

A. If so, do your regulatory rules require your “guidelines” to be followed? A link to your guidelines would be appreciated. We do not require a set method, but rather require 2 different methods of the engineer of records choice (within limits of reason).



prefer that local regression equations be developed and utilized? I am not sure how one would develop a local regression if there is no stream gage in the basin. I require a comparison of flows calculated from precipitation intensities and also, at least for larger basins, from the regression equations. As with Montana, Oregon is one of only 5 states without NOAA Atlas 14, which provides current precipitation recurrence by duration. Other missing states WA-ID-WY) We have been working with the Corps of Engineers to try to get these updated, but have resistance from our Department of Transportation (Departments of Transportation usually lead these efforts).

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? I do not believe we have any idea what the 500 year (0.2 percent) AEP is, since we have a much shorter record, and often lose gages in the extreme events. And these may be dropped from the record, and without some reconstruction. With the short period of record, I believe any analysis for the over 1 percent AEP flood must also include precipitation, not just stream gage information.

C. Other comments or advice? Do not rely on one methodology. Especially in the West, with our

more limited flow and precipitation data.

#3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation? No. I am more concerned with the short periods of record, and the potential loss of the most significant flood or flood producing events.

A. If yes, how do you calibrate/verify these models? #4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1

standard deviation above a regression equation result)? How do you do a sensitivity analysis unless you have a good handle on extreme events, and how these events are affected by cover. For our two states, (MT, OR) wildfire can be a big factor, and we do not have good records of extreme precipitation and changes in peak flows after fires. WA has some interesting findings, and in Oregon I only have anecdotal information. #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept? This is out of my area of expertise. #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification. Depends on the hazard. If its high, I want snow, plus saturated soils, without canopy (if there is any canopy) storage. Any other comments, suggestions or advice? This issue is of highest importance for us in Oregon. I

often have low confidence that IDF’s are realistic. But I do not have anything good to offer design

engineers, either. I would interested in working on a committee to address this, and also see that our

states have an accepted precipitation atlas.

South Dakota – (T. Schaal)

#1 Does your state have guidelines for determining basin response parameters (infiltration, unit hydrograph parameters, etc.)? No




prefer that local regression equations be developed and utilized? This has really not been an issues with us, I’m sure we would be comfortable with USGS equations.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? C. Other comments or advice?

#3 We are struggling with how best to pseudo-calibrate / verify models that have large drainage basins with diverse land cover. Do you have any guidance for model sub basin delineation? No

A. If yes, how do you calibrate/verify these models? #4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)? This has not been an issue for us. #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept? This has not been an issue for us. #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this differently during model calibration/verification. This has not been an issue for us.

Tennessee – (L. Bentley)


Tennessee doesn’t have guidelines, per se. Internally, we usually use the NRCS NEH 4 methodolgy.



A. Do you use regional (e.g. USGS Peak Flow) regression equations as a calibration tool? Or do you prefer that local regression equations be developed and utilized? No.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? We don’t often verify or calibrate. C. Other comments or advice?


No.

A. If yes, how do you calibrate/verify these models? #4 With a risk based spillway standard, the uncertainty inherent in hydrology can be important. Our current line of thinking is that the modeler should strive for a best estimate, with an awareness of the uncertainty. This “awareness” comes from doing a sensitivity analysis. How do you deal with the large uncertainty associated with determining extreme storm flows in your state (for example, calibrating 1 standard deviation above a regression equation result)? #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept?

The NRCS curve number method is usually used. I don’t know of any loss methods we specifically would not approve.


We use an AMC of 2 (normal conditions) and the initial abstraction is determined from the curve number as determined by the HEC-1 or HEC-HMS.

Washington – (M. Walther)


We do not have specific guidelines for basin response parameters. These would be covered by our general requirement for submittals to be prepared by a Professional Engineer and to conform to accepted engineering practice and our Dam Safety Guidelines.


guidelines would be appreciated. We have a general requirement for submittals to conform to accepted engineering practice and our Dam Safety Guidelines, but no specific guidelines for basin response parameters.



prefer that local regression equations be developed and utilized? Yes, we use the USGS regression equations as a calibration tool for our own calculations. However, we do not have a requirement or expectation that submittals to us by other engineers would use the USGS equations to calibrate their hydrology model. We do not have a requirement or expectation that local regression equations would be developed or utilized.

B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? We calibrate to both the 100-year and 500-year USGS flows. We have three dam safety design storm scenarios (short, intermediate and long durations). In our calibration efforts, we try to adjust the basin parameters to get the peak flows from all three storm scenarios to fall within the range of mean flow to mean plus one standard deviation. As noted above, this is primarily for our own calculations. We do not have a requirement or expectation that other engineers would use the USGS equations to calibrate their hydrology model.


Two comments. The first is that our observation has been that the un-calibrated watershed parameters tend to calculate higher runoff peak flows than the USGS equations would predict. If the spillway design is based on a hydrology model that estimates flows on the high side, the design is still approvable by Dam Safety. The second comment is that our calibration efforts tend to focus on the unit hydrograph lag time rather than the hydrologic losses (such as the CN value or soil infiltration rate). Unless there is a specific physical justification, I am reluctant to simply lose water (increase the hydrology losses) in order to reduce the peak flows from the hydrology model to get better agreement with the USGS equations. Losing water without physical justification seems to violate the principle of Conservation of Mass. My preference is to stretch out the unit hydro-

graph lag time until the model peak flows and the USGS flows are in reasonable agreement. There may not be a physical justification for stretching the lag time, but at least this approach still accounts for all the water.


No specific guidance on this topic in our regulations or published guidelines, just use good engineering practice. As a general principle, we try to select sub-basins that have a relatively modest range of variation for the loss parameter within the sub-basin to avoid the average parameter for the sub-basin from being dominated by either extremely high or extremely low values. Also, we try to select sub-basins that will have somewhat modest variation in the range of times of concentration and unit hydrograph lag times between the various sub-basins to avoid using a very small calculation interval based on the smallest UH lag time that results in a very high number of unit hydrograph ordinates for sub-basins with much longer UH lag times.


We try to calibrate each sub-basin to the USGS equations, then attempt to calibrate the model for the overall watershed to the USGS equations. At the 100-year and 500-year recurrence intervals, if the peak flows for all three dam safety storms fall within the range of mean to mean plus one standard deviation, for each sub-basin and for the overall watershed, we call it good.


Similar answer as for question 2.B. We calibrate to both the 100-year and 500-year USGS flows, and we try to adjust the basin parameters to get the peak flows from all three storm scenarios to fall within the range of mean flow to mean plus one standard deviation. We might try to calibrate closer to the mean or closer to the mean plus standard deviation depending on soil types and range of soil infiltration rates from the Web Soil Survey. High soil infiltration rates would be expected to yield lower runoff, so we might calibrate that water-shed closer to the USGS mean flows; conversely, lower infiltration rates would be expected to yield higher runoff, so we might calibrate that basin closer to the mean plus one standard deviation.


Use of the NRCS curve number (CN) method is very common for submittals to Dam Safety. Our own calculations often use a constant infiltration rate or the Green-Ampt equation. Except as discussed as follows, any of these loss methods is acceptable. The CN method is acceptable with one caveat. For the dam safety Short duration storm, the CN value must be artificially increased to estimate a runoff volume consistent with the soil

infiltration rate from the Web Soil Survey. Our guidance on this topic and a spreadsheet to perform the calculations are available on the Washington State Dam Safety web site. The CN values from published tables are acceptable for the Long and Intermediate storms, but our experience has been that the table CN values under-estimate the runoff from the brief but intense Short storm. The Short storm CN value is calculated based on a mass-balance analysis of the storm hyetograph comparing the rainfall intensity to the soil infiltration rate.


We estimate typical antecedent moisture conditions, AMC-II. We might estimate a higher AMC or lower infiltration rate if the initial comparison to USGS equations has model runoff flows that are too low, although we don’t see this very often, usually the model flows are higher than the USGS equations. Antecedent moisture and initial abstractions are handled the same during the calibration runs and during the actual hydrologic analysis model runs.

Any other comments, suggestions or advice?

For watersheds in higher elevations or other areas where snowmelt is likely to be a factor in the high flows used in the USGS analyses to develop their regression equations, we consider snowpack and possible snowmelt in our calculations, both for the calibration runs and for the actual hydrology analysis model runs.

Wyoming – (N. Graves)


Draft guidelines, nothing approved, yet.




prefer that local regression equations be developed and utilized? No B. What frequency storm do you require for calibration/verification (i.e. 100 year, 500 year etc.)? C. Other comments or advice?


A. If yes, how do you calibrate/verify these models? No


Leave it up to the design engineer. #5 What loss method do you generally see used (or require), and are there any loss methods that you will not accept?

None required. #6 How do you deal with initial abstraction and antecedent moisture conditions? Note if you handle this

differently during model calibration/verification.

Leave it up to the design engineer.

ASDSO Survey Hydrologic Model Calibration/Uncertainty ... · response parameters and uncertainty....

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