Streamflow Generation: An Introduction · 2 The Question: Why does stream- runoff behave the way it...

Quantitative Elements of Physical Hydrology

Streamflow Generation:An Introduction

Contact information:Jack HermanceEnvironmental Geophysics/HydrologyDepartment of Geological SciencesBrown UniversityProvidence, RI 02912-1846Tel: 401-863-3830e-mail: John_Hermance@Brown.Edu

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The Question: Why does stream-runoff behave the way it does?

The Thesis: Historically, streamflow shows a strong correlation with precipitation. Knowing the nature of this relationship, we can predict stream

discharge from known or predicted precipitation.

Compare Stream Discharge (Q) to Precipitation (P).

Changes in the streamflow of the Pawcatuck River for aspecific precipitation event.

Example of a single "event".( We want to explore “cause & effect” in a watershed system.)

A classic problem in the "response" of physical systems.

The transfer function of a system allows one to predict the expected output process due to a specified input process.

Changes in the streamflow of the Pawcatuck River for a precipitation event.

A precipitation eventon October 9 ...

Changes in the streamflow of the Pawcatuck River for a precipitation event.

… causes a streamflow peakon October 11 .

Quantitative Elements of Physical HydrologyQuantitative Elements of Physical Hydrology

Why do these systems behave theway they do?

What are the fundamental relationships between precipitation and streamflow

in a watershed?

Quantitative Elements of Physical HydrologyQuantitative Elements of Physical Hydrology

A Review of the Water Cycle Elements.

Skip to Dynamic interaction . . .

Here we look at the composite effect of lateral flow to a stream.

A schematic view of water pathways.

We want to look in detail ata single 2D “slice” through

this system.

(When discussing the following paradigms, it is important to keep

in mind that whatever water enters the stream from its side is

carried away by flow.)

Review of the connectivity of the watershed elements.

An Inventory of Watershed Elements

Infiltration and direct recharge Infiltration and direct recharge

ThroughflowThroughflow

InterflowInterflow

Seepage (springs) Seepage (springs)

Return FlowReturn Flow

Overland Flow(Horton & Saturated)

Groundwater Flow(in saturated zone)Groundwater Flow(in saturated zone)

Baseflow:Groundwater flow

through the saturatedzone to streams

Baseflow:Groundwater flow

through the saturatedzone to streams

WatertableMoundingWatertableMounding

In some cases there can be substantial “bank storage”.In some cases there can be substantial “bank storage”.

The Principal Watershed Elements

The Dynamic Interaction ofWatershed Elements:

Transverse Contributions to Streamflow

We next look at the time and space sequencing of precipitation, surface runoff, groundwater flow

and streamflow generation.

(Note: No precipitation for anextended period.Groundwater table in"equilibrium" with streamlevel.)

(Note development of “bank storage”.)

This is only one of many scenarios for the time-space interaction of precipitation and runoff.

A more typical “initial condition” for astormflow event follows.

The Thesis: Historically, streamflow shows a strong correlation with precipitation. Knowing the nature of this relationship, we can predict stream

discharge from known or predicted precipitation.

Compare Stream Discharge (Q) to Precipitation (P).

Longitudinal Stream Development

Next we look at the composite effect of longitudinal flow to a stream along its axis.

A schematic view of water pathways.

We want consider how these discrete “slices”provide an aggregate

streamflow.

Surface runoff and baseflowact in parallel to feed

streamflow.

This lateral contribution is comprised of local Horton and saturated overland flow, return flow from seepage, and baseflow (groundwater).

Use this symbol to denote thesum total of all surface runoff

and baseflow deliveredby this slice.

Other “slices” or 2D elementsalso contribute to

streamflow.

Which integrates to the composite

aggregate stream discharge, Q

1 flow unit

6 flow units

Plan view illustrating the longitudinal integration of lateral inputs to streamflow.

The thickness of the arrow is used to denote the local magnitude of stream discharge.The basic idea is that streamflow is generated by the integrated contributions of surface runoff and baseflow along its length.

Summary:The Paradigm: Longitudinal stream development is the composite effect of lateral flow to a stream along its axis. We saw an example of lateral input from a discrete 2D element. We assumed that each 2D “slice” provides a discrete contribution to the total streamflow. The superposition of transverse surface and subsurface flow from these discrete slices is what we term the “longitudinal integration of lateral inputs” to generating and sustaining streamflow.

Streamflow discharge (Q) progressively increases downstream due to lateral inflow from surface runoff, interflow and groundwater base flow.

A Caveat: Longitudinal stream attenuation

Note: Streams can lose water along their channel, or reach, as well as gain water.

End of Presentation(Streamflow Generation: An Introduction)

Quantitative Elements of Physical Hydrology

Streamflow Generation: An Introduction · 2 The Question: Why does stream- runoff behave the way it...

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