Model Limitations and Uncertainties

1. Spokane Valley – Rathdrum Prairie Aquifer Study Model Limitations and Uncertainties Presented by Paul Hsieh U.S. Geological Survey

2. Outline • Intended use of model • Assessment of model fit • Non-uniqueness & alternative models • Recommendations for future work

3. Intended use of model • The primary purpose of the model is to serve as a tool for: • analyzing SVRP aquifer inflows and outflows, • simulating the effects of future changes in ground-water withdrawals, • evaluating aquifer management strategies. • The scale of the model and the level of detail are intended for analysis of aquifer-wide water-supply issues.

4. Assessment of Model Fit In general, the simulated water levels and flows (streamflow gains and losses) are in good agreement with the measured water levels and flows throughout most of the aquifer. However, discrepancies between measured and simulated quantities provide indications of model limitations.

5. 90 92 94 96 98 00 02 04 In well 251, the simulated rise in water level during 1996 and 1997 is not as large as the measured rise. The simulated fluctuations during 2004 and 2005 are somewhat larger in magnitude than the measured fluctuations. 251

6. 90 92 94 96 98 00 02 04 In well 143, the character of the simulated fluctuations during 2004–05 does not match the character of the measured fluctuations. This suggests that the temporal distribution of recharge to southern Rathdrum Prairie might not be accurately represented. 143

7. 90 92 94 96 98 00 02 04 In well 92, the discrepancy between simulated and measured water levels during early fall indicates that simulated interchange between aquifer and river might be inaccurate during this period. 92

8. Harvard Road 8/20/03 554 cfs Looking west

9. Water level (ft) 90 92 94 96 98 00 02 04 The relatively poor fits to measured water levels in wells 115 and 99 indicate that the lower unit might not be represented accurately by the model. 99 115

10. Non-uniqueness Non-uniqueness occurs if alternative models can fit the measured data nearly as well as the calibrated model. Non-uniqueness is illustrated by two alternative models (A and D) for which parameters values such as horizontal hydraulic conductivity are varied from those of the calibrated model.

11. Calibrated Model Horizontal hydraulic conductivity (ft/d)

12. Subregions for flow analysis

13. Calibrated Model 10-year Average Simulated Flows (cfs) 67 286 823 274 377 293 138 264 623 1,280 Note: Not all flow components are shown.

14. Alternative Model A 500 ft/d

15. Alternative Model A Change in Flow from Calibrated Model (cfs) +40 +40 -54 +13 +1 0 -109 -46 -7 -53

16. Alternative Model D 45 cfs

17. Alternative Model D +45 Change in Flow from Calibrated Model (cfs) +29 +74 +32 -9 0 0 -33 +32 +2 +32

18. Recommendations for Future Work

19. Monitoring water levels & aquifer tests • in and near Spirit and Hoodoo Valleys • in vicinity of Coeur d’Alene Lake • in upper and lower units in Hillyard Trough and Little Spokane River Arm • Improve estimation of recharge from precipitation on Rathdrum Prairie • weather station and monitoring of soil moisture • use of Unsaturated Zone Flow package of MODFLOW-2005 to simulate infiltration of recharge to water table.

20. Monitor change in wetted perimeter of Spokane River bed during early autumn, when river stage begins to rise after summer low flow conditions.