Presentation on theme: "Capture1 The Interaction between Groundwater Pumping, Surface Water and Evapotranspiration: The Concept of Capture Tom Maddock."— Presentation transcript:
Capture1 The Interaction between Groundwater Pumping, Surface Water and Evapotranspiration: The Concept of Capture Tom Maddock
Capture2 Talk Outline Groundwater/Surface Water Interactions What is Capture? Is Capture of Critical Interest? How do you calculate Capture? Capture calculation example What are the capture model basic parts? Modeling mischief
4 If the water level in the aquifer is above the above the stream stage elevation, the stream is a gaining stream ( I ). If the water level in the in the aquifer is below the stream stage elevation ( D w <3W ), the stream is a losing stream ( II ). For these two systems, Q RIV =C RIV (H RIV h A ) That is, the flow is proportional to the head difference between the stream ( H RIV ) and the aquifer ( h A ). DW is an indicator of the difference between aquifer water level and stream stage elevation.
Capture5 As the water level in in the aquifer drops, the seepage becomes less dependent upon the head in the aquifer ( III ). Ultimately the hydraulic connection between the bottom of the stream bed and the water table will break ( IV ). The interval below the stream bed is unsaturated, but the stream bed is assumed to remain saturated. DW is an indicator of the difference between aquifer water level and stream stage elevation.
7 CONCEPT OF CAPTURE Under natural conditions…previous to the development of wells, aquifers are in a state of approximate dynamic equilibrium. PRE-DEVELOPMENT R D Average recharge R = Average discharge D
Capture8 CONCEPT OF CAPTURE Pre-development Recharge and Discharge Recharge: Losing stream (LS) Underflow in (UI) Mountain front recharge (MFR Discharge: Gaining stream (GS) Underflow out (UO) Evapotranspiration (ET)
Capture9 CONCEPT OF CAPTURE Discharge by wells is thus a new discharge superimposed upon a previously stable system, and it must be balanced by an increase in recharge of the aquifer, or a decrease in the old natural discharge, or by a loss of storage in the aquifer, or by a combination of these. DEVELOPMENT R+ ΔR D-ΔD Q Stress Q is introduced The system may respond in three different ways: increase in rechargeR R+ ΔR decrease in dischargeD D ΔD change in aquifer storage ΔS
Capture10 CONCEPT OF CAPTURE There is a new equilibrium: remembering gives the term ΔR+ΔD is called capture.
Capture11 Reduced water table CONCEPT OF CAPTURE Stream Evapotranspiration
Capture19 Capture Is Calculated with Models There will be a surface water model and a groundwater model. There will be a historical model and a base case model. The models will consist of control variables, state variables and parameters. There is no capture data values to compare or calibrate with calculated values.
Capture20 MODEL CHARACTERISTICS Surface water model is usually an accounting model that matches stress periods of the groundwater model. Groundwater model is distributed parameter (Two or Three Dimensional). MODFLOW is an example. Interaction between surface and ground- waters if governed by Darcys law
Capture21 HISTORICAL MODEL Attempts to match historical processes Can be calibrated with temporal and spatial data Used to demonstrate the viability, accuracy and robustness of the model Does not calculate capture.
Capture22 BASE MODEL Based on little or no data May be fictional or artificial in nature May be the result of a negotiation process or imposed by the court Should be composed of the same physical based parameters as the historical model Examples: Classical (Steady State), Seasonal (Steady Oscillatory), Complex (Constrained Process)
Capture23 Classical STEADY STATE The natural recharge and discharge are equal for all time periods (R=D) Time steps are annual There is no loss of groundwater storage
Capture24 Seasonal STEADY OSCILLATORY Like the steady state but recharge and discharge can vary from season-to-season but these variations are the same each year ( R i D i ). There may be a storage loss or a storage gain each season but the total season storage loss plus the total seasonal storage gain is zero and.
Capture25 Complex CONSTRAINED PROCESS Recharge and discharge may vary from time step to time step but are the same for both the base and historical models. Some process such as pumping or diversions is constrained and is different from the historical model.
Capture26 Surface Water Model Subtracting the historical streamflows from the base streamflows provides an estimate of surface water capture by groundwater pumping. Groundwater Model A capture is the increase to a previous [base-case] recharge and/or the decrease to a previous [base- case] discharge due to groundwater withdrawal from wells
Capture38 STATE VARIABLES Is a variable that describes the state of the system (e.g. water levels, stream discharge, precipitation) Water managers have no direct control over state variables
Capture39 CONTROL VARIABLES Is a variable that describes something that can be controlled (e.g. Well pumping, streamflow diversions)
Capture40 PARAMETERS Variable specified by the modeler and are determine by the calibration process Physically or scientifically based parameters – Actual measurements Calibration Factors – No measurements (or bounds) Calibration of the models physically based parameters provides a measure of the natural error of the model. Calibration factors mask the natural error of the model and may improperly influence the Base Model