Streamflow Generation: An Introduction

Transcription

1 Quantitative Elements of Physical Hydrology Streamflow Generation: An Introduction Contact information: Jack Hermance Environmental Geophysics/Hydrology Department of Geological Sciences Brown University Providence, RI Tel: January 28, 2007 (Information for the Presenter) Conventions for Prompting Slide Changes. This is the toggle symbol to prompt presenter to return to previous slide for quick comparisons with the current slide. The following traffic light signals appear in lower right corner of the slide to prompt presenter accordingly. Move forward quickly, probably the next slide has the essential information (e.g. labels). Move forward after a slight pause to assess whether content deserves brief comment. Move forward after a longer pause, perhaps a few brief comments are required. This is a pausing point at which motivating comments can be made, or the contents thoroughly discussed. End of section. 1

2 The Question: Why does streamrunoff behave the way it does? Compare Stream Discharge (Q) to Precipitation (P). 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. 2

3 Example of a single "event". ( We want to explore cause & effect in a watershed system.) Changes in the streamflow of the Pawcatuck River for a specific precipitation event. 3

4 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. A precipitation event on October 9... Changes in the streamflow of the Pawcatuck River for a precipitation event. 4

5 causes a streamflow peak on October 11. Changes in the streamflow of the Pawcatuck River for a precipitation event. Quantitative Elements of Physical Hydrology Why do these systems behave the way they do? January 28,

6 What are the fundamental relationships between precipitation and streamflow in a watershed? Quantitative Elements of Physical Hydrology A Review of the Water Cycle Elements. Skip to Dynamic interaction... January 28,

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8 Here we look at the composite effect of lateral flow to a stream. 8

9 A schematic view of water pathways. We want to look in detail at a single 2D slice through this system. 9

10 (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.) An Inventory of Watershed Elements Review of the connectivity of the watershed elements. 10

11 An Inventory of Watershed Elements An Inventory of Watershed Elements Infiltration and direct recharge 11

12 An Inventory of Watershed Elements Throughflow An Inventory of Watershed Elements Interflow 12

13 An Inventory of Watershed Elements Seepage (springs) An Inventory of Watershed Elements Return Flow 13

14 An Inventory of Watershed Elements Overland Flow (Horton & Saturated) An Inventory of Watershed Elements Groundwater Flow (in saturated zone) 14

15 An Inventory of Watershed Elements Baseflow: Groundwater flow through the saturated zone to streams An Inventory of Watershed Elements Watertable Mounding 15

16 An Inventory of Watershed Elements In some cases there can be substantial bank storage. The Principal Watershed Elements 16

17 The Dynamic Interaction of Watershed Elements: Transverse Contributions to Streamflow January 28,

18 We next look at the time and space sequencing of precipitation, surface runoff, groundwater flow and streamflow generation. (Note: No precipitation for an extended period. Groundwater table in "equilibrium" with stream level.) 18

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21 (Note development of bank storage.) 21

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26 View sequence again 26

27 This is only one of many scenarios for the timespace interaction of precipitation and runoff. A more typical initial condition for a stormflow event follows. 27

28 (Discuss) Compare Stream Discharge (Q) to Precipitation (P). 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. 28

29 Longitudinal Stream Development January 28, 2007 Next we look at the composite effect of longitudinal flow to a stream along its axis. 29

30 A schematic view of water pathways. 30

31 We want consider how these discrete slices provide an aggregate streamflow. Surface runoff and baseflow act in parallel to feed streamflow. 31

32 This lateral contribution is comprised of local Horton and saturated overland flow, return flow from seepage, and baseflow (groundwater). Use this symbol to denote the sum total of all surface runoff and baseflow delivered by this slice. 32

33 Other slices or 2D elements also contribute to streamflow. Which integrates to the composite aggregate stream discharge, Q 33

34 Plan view illustrating the longitudinal integration of lateral inputs to streamflow. 1 flow unit 6 flow units 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. January 28,

35 A Caveat: Longitudinal stream attenuation Note: Streams can lose water along their channel, or reach, as well as gain water. Quantitative Elements of Physical Hydrology End of Presentation (Streamflow Generation: An Introduction) January 28,