Overview of modeling methods to estimate stream depletions due to pumping

Transcription

1 Overview of modeling methods to estimate stream depletions due to pumping Gordon McCurry, PhD, PG Senior Hydrogeologist, Geomega, Inc. Colorado Aquifer Management Conference November 28, 2012

2 Outline Background on Aquifer Response to Pumping Sources of water to a well Pumping Effects Lagged Depletions Analytical Models Simplifying Assumptions Models in Use Numerical Models Modeling Approaches URFs and Response Functions

3 Aquifer Response To Pumping Pumping initially takes water from aquifer storage Next stage is capturing water that would discharge to a stream Later stage is inducing flow from the stream

4 Effect of Pumping on Streamflow Pumping reduces streamflow, but short-term changes in flow still exist

5 Effect of Well Proximity on Depletions Wells closer to a stream have more immediate effects and are more likely to induce stream flow

6 Effect of Well Proximity on Depletions Wells closer to a stream affect a smaller reach than wells farther away

7 Effect of Multiple Wells Effects of multiple pumping wells are additive Incremental effects of individual wells can result in large stream depletions

8 Effect of Streambed Sediment

9 Lagged Effect on Depletions Depletions occur after pumping ends, until the cone of depression is replenished. Lagged effects can last months, with peak depletions in winter months

10 Analytical Modeling Approaches Require simplified aquifer settings and assumptions Are quick to implement, easy to explain Q s = Q w erfc(a 2 S/4Tt) Q s = stream depletion Q w = well pumping rate a = distance from well to stream S = aquifer storage coefficient T = aquifer transmissivity t = duration of pumping Long history of use in Colorado; well accepted

11 Key Analytical Stream Depletion Models Glover and Balmer, 1954 Hantush, 1965 (streambed sediment layer) Jenkins, 1968 (SDF concept) Q s = 0.28* Q w ( = time to depletion) Colorado DWR, 1978 (SDF model) CSU IDS, 2004 (AWAS model) USGS, 2008 (STRMDEPL08 model)

12 Example SDF Map (AK River Valley)

13 Analytical Model Assumptions Alluvial aquifer settings are greatly simplified (homogenous, uniform properties) Do not allow for meandering rivers, tributaries, complex aquifer boundaries Can lead to over-prediction of stream depletions Real Aquifer Modeled Aquifer

14 Recent Analytical Approaches Include partially penetrating streams, streambed layers (Hunt, Butler, Fox, Chen) and leaky aquifer confining layers, lateral boundary effects (Zlotnik, Miller, Hunt)

15 Numerical Modeling Approaches Includes heterogeneous aquifers and multiple stream tributaries Aquifer settings represented in numerical grids Computationally intensive; large input & output files Yields more accurate results for complex settings

16 Numerical Modeling Approaches -2 Depletion effects estimated by: Running the model without the pumping well and recording river flows over time Running the model with a pumping well at a unit rate for a unit duration (1cfs for 1 month, etc) and recording river flows over time Determine pumping-based depletions from the difference in the model results ( = Unit Response Functions, URFs) Tabulating depletions by river reach

17 Pumping Response Functions Developed using analytical or numerical models Describes the amount of pumping that comes from one or more nearby streams

18 Response Function Examples Developed for San Luis Valley, Colorado, for the RGDSS 93 zones represent unit response to pumping Zones oriented parallel to rivers, with increasing width away from rivers One well placed in each zone, pumped at 1 cfs for 20 years

19 Response Function Examples Platte River Basin, NE 28% depletion after 40 years of pumping

20 Response Function Examples Loup River, NE Map is percent of pumping from stream depletions after 50 years of pumping of 1 cfs in a given cell Developed using numerical model with unit response placed in each of 22,000 model cells

21 Response Function Examples San Pedro River, AZ Depletions after 10 years of pumping Developed using numerical model (Modflow), with unit response input per model cell (~ 10,000 cells)

22 Summary and Conclusions Pumping depletes stream flow by capturing discharge and inducing flow from the steam Analytical methods are commonly used due to precedence and ease of use; can over-predict effects Numerical methods include complex aquifer settings & tributary streams; yield more accurate results More accurate methods are appropriate as water use efficiency becomes more critical Unit response functions developed from numerical methods may be the most effective approach

23 Thank You