Measuring Water Resources Unit 2.1: Student Exercise – High Plains AquiferJon Harvey (Fort Lewis College) and Becca Walker (Mt. San Antonio College)

Now that you are well-versed in the water cycle and the myriad reservoirs and transport pathways it comprises, it is time to look at some real data from real locations with real users and real implications when water supplies decrease.

IntroductionLet’s start by thinking about GROUNDWATER. Groundwater is stored in aquifers, which can be as small as the ribbon of sand and gravel beneath a creek to vast rock formations hundreds of feet thick and hundreds of miles wide. Water users around the world rely on aquifers to provide reliable, clean, and abundant water. In particular, large areas of the central and western US use groundwater to irrigate millions of acres of crops. These aquifers face increasing demand - can the supply keep up with it? (read more here: https://www.nature.com/articles/nclimate2425 )We can keep track of how much water is in an aquifer, its water storage, with a relatively simple equation:

Schange = P - ET - R – WChange in water storage = Precipitation - EvapoTranspiration - Loss to Rivers – WithdrawalsAlthough the equation is relatively simple, it only works if we have a decent handle on the terms on the right side of the equation:

Precipitation - We can measure this with rain gauges and Snotel stations, but it is rather spatially variable, and difficult to accurately captureEvapotranspiration - We can model this using algorithms based on land cover, vegetation type, soil moisture, and weather/climatic conditions experienced at the site, however, there is substantial uncertainty in model predictions, limited by sparse data inputs.Rivers - The loss/gain of groundwater to/from rivers can be estimated by comparing river discharge measurements in the downstream direction. However, this may not be possible on ungaged streams.

Withdrawals - Hard to keep track of unless users are voluntarily reporting their withdrawalsWith so many unknowns in the equation, changes in the total volume of water in the aquifer can be difficult to predict. That is where some of the methods you learned about in Unit 1 can fill in the story. These methods attempt to more directly measure the LEFT HAND side of the equation: the water storage itself. These methods include:

-Depth to water table in water wells-GRACE satellite observations-Vertical GPS measurements

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 1

Unit 2.1: Student Exercise – High Plains Aquifer

REVIEW

Measuring changes in water storage with depth to water table in wellsThe traditional way to track changes in water storage is by directly measuring just how

deep the water table is beneath the surface. By measuring changes in depth to groundwater, we can get an idea of whether that reservoir is gaining or losing water. One challenge of this data source is that water table elevations are very sensitive to local withdrawals and recharge, and aquifer complexity may mean that a well in one location could behave very differently than one only a few miles away. In order to track regional water storage changes, one must include data from several wells across a region.Measuring changes in water storage with GRACE satellite

Recall from Unit 1 that the GRACE satellite is equipped to measure subtle changes in the strength of gravity across Earth’s surface (spatial patterns) and over time (temporal patterns). Although many factors influence how hard the tug of gravity is across the surface, one of the strongest factors exhibiting temporal changes is how much water is stored in the aquifers beneath our feet. Measurements from GRACE help scientists estimate the change in water storage in aquifers, including the HPA.Measuring changes in water storage with vertical GPS

Recall from Unit 1 that as aquifers gain water they tend to expand, and as they are drained they tend to contract. We can see that shrink/swell behavior as vertical shifts in the Earth’s surface. In some aquifers, including the HPA, as water storage increases, the ground surface should move slightly upward. Conversely, we would expect the surface to sink slightly as aquifer water storage decreases. Scientists track this vertical motion using permanent GPS stations. Similar to GPS stations in a smartphone or in a navigation device, GPS stations are always recalculating their position on Earth, including latitude, longitude, and elevation. Similar to the groundwater data, GPS data is station-based. To study changes in groundwater storage across broad areas, several different GPS stations must be included in the analysis.

Together, these tools help scientists and water managers keep account of water resources and how they are changing through time. Long-term studies of water resources in major aquifers in the US show that many are losing large volumes of water (Figure 1). Among the most depleted is the High Plains Aquifer.

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 2

Figure 1. Cumulative net groundwater depletion in major aquifer systems or groups in the United States, 1900-2008. (From Konikow, 2011, Geophysical Research Letters) TAHOE = 151 km^3.

Unit 2.1: Student Exercise – High Plains Aquifer

In this unit, you will explore one of the largest and most important aquifers in the United States - the High Plains Aquifer (HPA). Underlying portions of eight states in the central US, the HPA is a shallowly-buried apron of gravel and sand that that was shed off the growing Rocky Mountains (Figure 2). Its water is used to irrigate millions of acres of grain in America’s “Bread Basket” – In fact, nearly a third of the United States’ irrigated crops rely on the HPA. That grain goes on to be processed into food for humans, food for livestock, and ethanol production. In addition, over 80 percent of the residents within the HPA rely on it for their domestic drinking water. Clearly, the HPA is of utmost importance not only to the residents of the High Plains but for the whole US economy!

Users of HPA water primarily access it by pumping from groundwater wells. Unfortunately, in much of the HPA pumping has exceeded natural recharge of the aquifer, so water levels are dropping (Figures 1, 4).

Read more about the HPA here:

https://ne.water.usgs.gov/ogw/hpwlms/physsett.html and here: https://ne.water.usgs.gov/ogw/hpwlms/hydsett.html TAKE IMPORTANT STUFF AND PUT IN PACKET? OR INCLUDE AS SUPPLEMENTAL READING? RIGHT NOW NOT ASKING ANYTHING FROM 2 nd LINK

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 3

Figure 2. Map showing location of High Plains Aquifer in gray. Locations of groundwater stations (black) and GPS station (red) used in the assignment are included.

Unit 2.1: Student Exercise – High Plains Aquifer

1.) Describe the spatial pattern of precipitation within the HPA (where are higher values? lower values?). What is the range of annual precipitation values?

2.) When in U.S. History did the amount of irrigated acreage increase most dramatically?

3.) How many acres were irrigated by the HPA as of 2002?

Read more about challenges facing the HPA here: https://www.geosociety.org/gsatoday/archive/27/6/pdf/GSATG318GW.1.pdf and here: https://ne.water.usgs.gov/ogw/hpwlms/files/HPAq_WLC_pd_2013_SIR_2014_5218_pubs_brief.pdf

4.) What area(s) of the HPA are struggling most with dropping water tables? How does that compare to your answer to #1 (what is the spatial pattern of rainfall in the HPA)?

5.) By how much has the water table dropped (1950 to 2013) in the most depleted parts of the HPA?

6.) What was the loss during that time (pre-development to 2013) measured in acre-feet?

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 4

Unit 2.1: Student Exercise – High Plains Aquifer

7.) The state of Kansas is ~52 million acres. If we could put the amount of water in your previous answer onto the state of Kansas, how deep would that water be (if evenly distributed)?

8.) List three ways that HPA states are trying to curb losses in HPA water.

Unit 2.1 – Measuring Water Resources in the High Plains Aquifer (HPA)In the spreadsheet provided by your instructor, you will find historic hydrologic data showing how water storage in the High Plains Aquifer has changed as seen from the multiple methods of measuring groundwater storage described above (page 2). Open up the spreadsheet file and take a look!

You should see several tabs at the base of the document - each takes you to a different spreadsheet in this Excel Workbook.

This first tab, GW-1, shows depth-to-groundwater data from a well in southwest Nebraska. This well has data going back to 1984. You’ll see a chart with a blue line running across it. This plot is showing the depth of the water table through time.

1. What values are being plotted on the Y-axis?

2. What values are being plotted on the X-axis?

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 5

Figure 3. Screenshot showing tabs at base of workbook.

Unit 2.1: Student Exercise – High Plains Aquifer

3. Without any specialized technology, how one could measure DTWT in a simple backyard water well? Include a sketch in your answer. PROVIDE A SKETCH OF SIMPLE BACKYARD WATER WELL

Depth-to-water-table sensors like this one are one component of the United States Geological Survey’s (USGS) Water Data program.

Here is a map showing the USGS’ observation network for the HPA:https://groundwaterwatch.usgs.gov/net/OGWNetwork.asp?ncd=hpn

Try these two links to see where else we have similar water data in the US:https://groundwaterwatch.usgs.gov/default.asphttps://water.usgs.gov/ogw/ PROBABLY REMOVE THIS

4. Describe the coverage that this groundwater observation network has (e.g. dense? Regularly-spaced? Sparse? Random?). How is the coverage in your state?CUT?

Now let’s get back into the data in sheet GW-1. This station in central Nebraska (See Figure 2 for location) exhibits some interesting behavior that we can use to learn about how the annual water cycle works in this location. Examine the X-axis to determine where a given calendar year starts and ends. Now look at the curve showing the position of the water table over time. Note that if you ‘mouse-over’ the curve you should see a pop-up with the date and value.

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 6

Unit 2.1: Student Exercise – High Plains Aquifer

5. Can you make out any annual and/or seasonal cycles in the water table elevation? If so, describe them and speculate on what might be controlling them (hint – look at climate graph on sheet GW-1)?

6. Your background reading described the challenges posed by declining groundwater levels in the HPA. How does water storage look from the perspective of this well? Make note of any years when water storage is increasing or decreasing based on the curve in the DTWT chart. Remember that when the DTWT increases, it means the water table is falling, and vice-versa.years of significant INCREASE in water storage:years of significant DECREASE in water storage:

One common way to keep track of wet and dry years is with the USA Drought Monitor. They publish a weekly drought assessment map that shows how drought conditions vary across the country. An example map from December 2018 is shown below. Notice that darker shades of orange/red indicate more extreme drought conditions:

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 7

Unit 2.1: Student Exercise – High Plains Aquifer

Now look at Figure 6, which shows the % of the HPA in a “drought” state at a given time:

7. Based on figure 6, list any periods that look particularly dry in the HPA:

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 8

Figure 6. Chart showing percent of HPA area that is in a state of drought between 2000 and 2019 (use Word to zoom in if the axes are illegible)

Figure 5. Map showing areas that are in a state of drought as of Dec 25, 2018

Unit 2.1: Student Exercise – High Plains Aquifer

8. Now look back to your chart for GW-1. How did the groundwater table respond in those years of more significant drought?

9. Based on your answers to the above questions based on well GW-1, what can you say about the long-term health of the HPA at this location?

OK - Now let’s take a look at another nearby groundwater monitoring well. Data from this well is displayed on tab GW-2. This well is in NW Kansas, still within the HPA (See Figure 2 for location).

10. Can you discern any short-term (annual and/or seasonal) cycles in groundwater levels? If so, how do they compare to what you saw in GW-1 in terms of timing within the year?

11. Describe any long-term (multi-year) trends in groundwater levels. How does that compare to what you saw in GW-1?

What explains the differences between GW-1 and GW-2 if they are all in the same general region? Often such big differences are the result of the wells tapping water stored in different aquifers that may receive water recharge from different sources and over different timelines.

12. Look at the websites for the two stations (GW1: Click Here and GW2: Click Here), and compare the descriptions of the wells, how deep they were drilled, and what ‘local aquifer’ they were completed in.

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 9

Unit 2.1: Student Exercise – High Plains Aquifer

Well GW-1 GW-2Depth    Local Aquifer    

13. *Think Deeper Question* Describe the difference in reliability of water derived from GW-1 and GW-2, and explain those differences using what you know about the water cycle and the history/status of the HPA. Take a paragraph or two to explain your answer, referring to specific features in the plots that led you to your conclusion.

Now we can say something about the status of the HPA in the locations sampled by wells GW-1 and GW-2. Although it is great to have hard data from those locations, sometimes we might want to know about groundwater supplies in places where wells are few and far between. That’s where other datasets can come in handy.

Let’s look at how the groundwater data compares with data derived from the GRACE satellite. Remember, the one that measures small changes in gravity due to redistribution of mass

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 10

Unit 2.1: Student Exercise – High Plains Aquifer

around on Earth’s surface? You may recall that one of the exciting applications of GRACE is to use it to estimate changes in water storage over broad areas.

For the sheets GW-1 and GW-2, you can ‘activate’ GRACE data on each plot by:-Right-clicking somewhere in the plot area-Select ‘Select Data’-Check the box for GRACE.-You should now see an orange line on the plot that shows GRACE data as water storage anomaly (difference from normal) in units of Water Equivalent Height (WEH). Values of WEH shows on the alternate Y-axis.NEED ALTERNATE INSTRUCTIONS FOR A MAC

14. Check back to your notes from Unit 1 - what is Water Equivalent Height as measured by GRACE? What does it mean for water storage when the measured WEH increases or decreases?

15. How well does the GRACE curve match the DTWT data for GW-1 and GW-2? What about the particular wet and dry years you identified in question 6? Do they show up as you might expect them to in GRACE data?

OK. Now let’s zoom out and look at long-term water storage of the HPA as a whole. Go to the tab ‘WetDry’. Here is a plot showing an average of GRACE WEH data for the entire HPA. This plot should, theoretically, tell us about long-term changes in water storage in the aquifer as a whole.

16. Describe the typical annual cycle in terms of the timing of increasing vs decreasing water storage.

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 11

Unit 2.1: Student Exercise – High Plains Aquifer

You are also presented with two ‘Drought Monitor’ maps - these are produced every week by the USDA to help the public keep track of developing and prolonged drought problems. These dates represent particularly wet (May 2010) and dry (Nov 2012) times in the HPA for our consideration.

17. Describe the drought status of the HPA region at those two dates.

18. Find the two dates (May 2010 and Nov 2012) as colored dots on the GRACE plot. What Water-Equivalent Height values correspond to those dates? Keep in mind that negative values imply water deficit.

So, here we have two dates with starkly different water storage values as measured by GRACE. We can tell that water storage decreased between May 2010 and Oct 2012. That’s good, but to put it into context, let’s figure out the actual volume of water that was lost from the HPA region during that time. In order to do that we will have to multiply the difference in WEH (depth) for those dates by the area of the HPA (~453,000 km2).

19. Calculate the volume of water

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 12

Unit 2.1: Student Exercise – High Plains Aquifer

lost between May 2010 and Oct 2012 in cubic kilometers (km3). Be careful, you must first convert the WEH to km by dividing by 100,000, the number of cm in one km.

20. Let’s compare that number to some other volumes of water that may help with understanding just how much water was lost during that period.

a. According to the EPA, the average American family uses ~300 gallons of water per day. In a year that totals to 0.000000414 km3. How many average American families could be served by the water lost from the HPA during the period in question?

b. A typical Olympic swimming pool has a volume of ~0.0000025 km3. How many Olympic swimming pools were lost from the HPA during the period in question?

c. The Mississippi River releases, on average, around 1.45 km3 of water per day into the Gulf of Mexico. How many days of Mississippi River flow was lost from the HPA during the period in question?

d. The largest reservoir in the USA, Lake Mead, has a capacity of ~35.7 km3. How many Lake Meads were lost during period in question?

OK - Let’s look at the HPA from one final perspective - that of VERTICAL GPS. This system helps scientists track movements of the ground surface over time. Vertical GPS can be used to track water resources by detecting tiny vertical shifts in the ground surface as a result of changing water storage. This works because the weight of groundwater tends causes the ground to sink ever so slightly, and when that weight is removed the ground springs back up. Athough that relationship holds true in many deep aquifers, it should be noted that in some aquifers, like in the Central Valley of California, we get the opposite response due to aquifers that expand and contract with water addition and withdrawal, respectively.

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 13

Figure 7. Diagram showing that volume can be calculated by multiplying area by depth.

Unit 2.1: Student Exercise – High Plains Aquifer

Take a look at the map of GPS stations within UNAVCO’s Plate Boundary Observatory at this page: https://www.unavco.org/instrumentation/networks/status/pbo

21. Explore the map of GPS stations. Where do we have the best coverage? Worst? How is the coverage in the HPA?

Now let’s look at the data from station P039 - which is situated in far-northeastern New Mexico (Figure 2). These data can be found on the tab ‘GPS’ in the Excel file.

22. What does it mean for the position of the GPS station when the Y-axis value goes up?

23. What does it mean for water storage when the Y-axis value goes up?

24. Can you make out any long-term trends? Explain how water storage in the area is changing according to the vertical GPS data.

Right-click the plot and add the GRACE data as you have in the other plots.

25. Do the GRACE data support your hypothesis in the previous question? If not, explain.

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 14

Unit 2.1: Student Exercise – High Plains Aquifer

Summing it up:26. Characterize the difference in the water security of shallow aquifers and deeper bedrock

aquifers. Which is likely a more resilient supply for human use? Explain your answer, mentioning environmental and human factors that may influence the water available for use in those two reservoirs.

27. In this unit you looked at three methods for keeping tabs on underground water storage.a. Which method(s) is/are more appropriate for keeping track of local groundwater

supplies?

b. Which method(s) is/are more appropriate for keeping track of long-term trends in regional aquifers like the HPA?

28. How could farmers who rely on the HPA for irrigation prepare for predicted increases in water scarcity in the future? (there are many possible answers for this one)

Questions or comments please contact education AT unavco.org. Version Dec 30, 2018. Page 15