1. Snake River Plain Model Upgrade Base Case Scenario Donna M. Cosgrove Idaho Water Resources Research Institute University of Idaho

2. Fellow Contributors Bryce Contor Bryce Contor Gary Johnson Gary Johnson Allan Wylie Allan Wylie Nathan Rinehart Nathan Rinehart Technical Direction and Oversight Technical Direction and Oversight –IDWR –Eastern Snake Hydrologic Modeling Committee

3. Overview Executive Summary Executive Summary Snake Plain Model Re-cap Snake Plain Model Re-cap Definition of Base Case Scenario Definition of Base Case Scenario How Base Case Scenario was calculated How Base Case Scenario was calculated Results of Base Case Scenario Results of Base Case Scenario Effects of drought Effects of drought

4. Executive Summary Snake Plain water use mostly in balance over long period Snake Plain water use mostly in balance over long period High degree of natural variability in water supply High degree of natural variability in water supply Will continue to experience weather- induced declines Will continue to experience weather- induced declines

5. Snake Plain Model Re-cap

6. Model Grid 1 mile x 1 mile

7. 11 Interconnected River and Drain Reaches

8. Representative Barraclough had suggested that we compare model outputs with historical, measured data Representative Barraclough had suggested that we compare model outputs with historical, measured data

9. Model’s Ability to Match Historical Measured Data

10. Matching of Measured Data To test model, we compared model output with values measured over a 17-year period To test model, we compared model output with values measured over a 17-year period –10,000 measured aquifer water levels over the 17-year period –725 measured river reach gains in six reaches –1500 measured spring discharges in nine springs

11. Aquifer Water Levels Historical Measured Aquifer Water Levels Model Predicted Values

12. Aquifer Water Levels Historical Measured Aquifer Water Levels Model Predicted Values

13. Measured Springs Blue Lakes Blue Lakes Historical Measured Spring Discharges Model Predicted Values

14. Scenarios Scenarios are model runs intended to answer questions such as Scenarios are model runs intended to answer questions such as –What will happen if water supply changes? –What will happen if we recharge? Evaluate impacts to river gains and spring discharges due to some specified change in practice Evaluate impacts to river gains and spring discharges due to some specified change in practice Base Case Scenario is first evaluation using enhanced Snake Plain Model Base Case Scenario is first evaluation using enhanced Snake Plain Model

15. Base Case Scenario designed to answer following question: “If current land and water use practices continue and if the 22-year period from 1980 to 2002 represents future water supply conditions, what will be the effect on spring discharge and Snake River gains and losses?”

16. Base Case Scenario Objectives Evaluate degree to which aquifer inflow and outflow have been out of balance during 1980-2002 period Evaluate degree to which aquifer inflow and outflow have been out of balance during 1980-2002 period Estimate how representative 1980-2002 period is of average conditions Estimate how representative 1980-2002 period is of average conditions Predict expected changes in spring discharges and river flows, with no change in water use Predict expected changes in spring discharges and river flows, with no change in water use

17. Caveat The Base Case Scenario is just one of many scenarios to be evaluated The Base Case Scenario is just one of many scenarios to be evaluated Together, these scenarios give us a broad picture of water use and impacts on the eastern Snake River Plain Together, these scenarios give us a broad picture of water use and impacts on the eastern Snake River Plain Any one scenario is only a piece of this larger picture Any one scenario is only a piece of this larger picture

18. Major Assumptions Past 22-years is representative of average water use and supply Past 22-years is representative of average water use and supply Future water use will not significantly change Future water use will not significantly change Water budget is accurate accounting of past 22 years Water budget is accurate accounting of past 22 years

19. Approach Two major aspects to approach Two major aspects to approach –Water budget analysis The balance between the water supply and water use causes changes in the aquifer The balance between the water supply and water use causes changes in the aquifer Changes are represented in the water budget which drives modeling results Changes are represented in the water budget which drives modeling results –Running of aquifer model Recharge and discharge terms of the water budget are applied to the model Recharge and discharge terms of the water budget are applied to the model Model predicts spring flows and river gains Model predicts spring flows and river gains

20. Water Budget Approach

21. Evaluate degree to which inflows and outflows were balanced during 1980-2002 Evaluate degree to which inflows and outflows were balanced during 1980-2002 Determine how representative 1980-2002 is of the long term Determine how representative 1980-2002 is of the long term

22. Imbalance in aquifer recharge and discharge Average aquifer inflow balanced against average aquifer outflow Average aquifer inflow balanced against average aquifer outflow Imbalance on average of 180,000 acre- ft/year Imbalance on average of 180,000 acre- ft/year –More outflow than inflow –Includes two years of drought

23. On average, inflows are less than outflows by 180,000 acre-feet per year

24. Another viewpoint is to look at aquifer storage Another viewpoint is to look at aquifer storage High degree of variability in water supply High degree of variability in water supply –Some years, water into storage –Some years, water out of storage Cumulative graph crosses zero several times Cumulative graph crosses zero several times –Overall, indicates that we are generally balanced in our water use

26. Important to note, however, that we never really recovered from the last drought Important to note, however, that we never really recovered from the last drought –Even despite high water year of 1997

27. Fullest extent of our recovery From last dry period

28. How representative is the 1980- 2002 period? Two comparisons between aquifer recharge and discharge and measured data Two comparisons between aquifer recharge and discharge and measured data –Precipitation at Aberdeen –Flow past Heise Visually, can see that aquifer recharge and discharge reasonably (not perfectly) follow these two indicators Visually, can see that aquifer recharge and discharge reasonably (not perfectly) follow these two indicators Can compare these two indicators to their long- term averages Can compare these two indicators to their long- term averages

31. Comparison with Long-term Precipitation at Aberdeen Precipitation at Aberdeen –1914-present average was 8.63 inches –1980-2002 average was 9.14 inches –1980-2002 period about 6% above normal Flow past Heise Flow past Heise –1910-present average was 5.04 MAF/yr –1980-2002 average was 5.23 MAF/yr –1980-2002 period about 4% above normal Conclude that 1980-2002 was a bit wetter than normal Conclude that 1980-2002 was a bit wetter than normal

32. Modeling Approach We are evaluating the long-term, expected impacts to spring discharges and river gains We are evaluating the long-term, expected impacts to spring discharges and river gains Three simulations Three simulations –Run 22-year cycle of recharge and discharge over and over Use ending water levels of one 22-year cycle as starting for next Use ending water levels of one 22-year cycle as starting for next –Average recharge and discharge terms for 22-year period Run as transient data set Run as transient data set –Average run as steady state data set

33. Modeling Approach (cont’d) Repeat of 22-year cycle will give us predicted highs and lows over long term Repeat of 22-year cycle will give us predicted highs and lows over long term –Assumes future will look like past –Obviously, cannot predict water supply for future Average recharge/discharge will give us smoothed average impacts to springs and rivers Average recharge/discharge will give us smoothed average impacts to springs and rivers Steady state will give us impacts after infinitely long time Steady state will give us impacts after infinitely long time

34. Figure 7. Simulation Results for the Malad Reach. 1050 1100 1150 1200 1250 1300 1350 5/1/19805/1/20025/1/20245/1/20465/1/2068 Spring Discharge (cfs) 22 yr Calibration PeriodRepeated 22 yr RechargeAverage of 22 yr Recharge Base Case Scenario Steady State

36. What these graphs mean Blue line (22 years repeated) shows the highs and lows around the average Blue line (22 years repeated) shows the highs and lows around the average –Reflects the natural variability of the water supply –Provides some estimate of the worst it can get assuming water use stays the same Pink (average) line shows that we reach equilibrium pretty quickly Pink (average) line shows that we reach equilibrium pretty quickly –Spring discharges and river gains already showing full effects of our water use –No surprises coming down the aquifer (except for weather) Black dot shows final impact value Black dot shows final impact value

37. What effect is drought having? Although water use is balanced on average, we will continue to see declines in water levels due to drought

38. 21 Year Declines in Aquifer Water Levels 0-5 ft. 5-10 ft.

39. The very next year: 0-5 ft declines

40. 0-5 ft 5-10 ft 10-15 ft Over the whole 22-year Period:

41. Half of the ground water declines between 1980 and 2002 occurred in the last year Half of the ground water declines between 1980 and 2002 occurred in the last year The aquifer is very responsive to drought The aquifer is very responsive to drought We can expect further declines since 2002 We can expect further declines since 2002

42. Senator Burtenshaw had suggested looking at Big Springs, which would be unaffected by ground water pumping Senator Burtenshaw had suggested looking at Big Springs, which would be unaffected by ground water pumping

44. Summary Snake Plain water use mostly in balance over 22-year period Snake Plain water use mostly in balance over 22-year period The Snake Plain aquifer was close to an equilibrium as of 2002 The Snake Plain aquifer was close to an equilibrium as of 2002 This assumes water use and supply stay the same This assumes water use and supply stay the same High degree of natural variability in water supply High degree of natural variability in water supply Will continue to experience weather- induced declines Will continue to experience weather- induced declines