1. Idaho Water Resources Research Institute
Idaho Water Issues Background and Science Focus: Snake River Plain Aquifer
Dr. Gary S. Johnson
Dr. Donna M. Cosgrove
Mr. Bryce A. Contor
Dr. John Tracy
Idaho Water Resources Research Institute
University of Idaho

2. Topics for today Hydrology Basics Conjunctive Management Challenges
Snake Plain Example
Aquifer Management Opportunities

3. The aquifer is a large, leaky tank
recharge varies dramatically
Aquifer
Springs
Output is
more constant
Characteristics:
Flow
Recharge and Discharge
Water Budget
Storage

4. Aquifer Flow

5. Aquifer Flow

6. Aquifer Recharge Water entering the aquifer is called Recharge
Common sources of recharge:
Infiltrating precipitation
River or lake-bed seepage
Subsurface inflows from nearby aquifers
Seepage from human activities
Storm drains
Irrigation

7. Aquifer Discharge Water leaving the aquifer is called Discharge
Common sources of discharge:
Spring discharge
Seepage to rivers or lakes
Wetlands
Subsurface outflows to adjacent aquifers
Ground-water pumping
Note that rivers can be a source of recharge or discharge

8. Water Budget
Since we cannot create or destroy water, a basic law exists
Inflow = Outflow +/- Change in Storage
It is like a bank account, if we spend more money than we make, our account balance goes down
Aquifer

9. Aquifer Storage
Aquifer storage is the volume of water stored in the aquifer
We can detect increases in aquifer storage by rising aquifer water levels
Changes in storage impact connected surface water
Long term decreases in storage indicate aquifer mining

10. Well Water Levels
Seasonal Variation
Long-term Change

11. River/Aquifer Interconnection
A gaining river or lake is said to be ‘hydraulically connected’
Springs and seeps discharge to rivers and lakes
The gain is not at a constant rate
As aquifer elevation rises, the gain increases
As aquifer elevation declines, the gain decreases
If aquifer elevation is well below river elevation, river is a ‘losing’ reach

15. Conceptual Aquifer System
Recharge
Pumping
Discharge to
River
BASIC TRUTH: A gallon taken out of the aquifer represents a gallon that never makes it to the river
ISSUE: When, where depletion effects are felt

16. Conjunctive Management
Conjunctive Management is the simultaneous management of the surface-water resource and the interconnected ground-water resource
Not all aquifers are interconnected with rivers or lakes
The Snake Plain Aquifer is intimately interconnected with the river

17. Why is Conjunctive Management So Difficult?
Water Law originally intended to manage surface water
Effects are immediate, visible and down-stream only
With ground-water, the effects exist but are more difficult to identify
Effects are spatially distributed
Effects are spread out over time

18. COMPLICATION
NO. 1

19. COMPLICATION
NO. 2:
TIME ATTENUATED
IMPACTS

20. How do we sort this out?
Ground-water model can predict the timing and location of impacts
Identify arrival time of impacts
Identify arrival locations
Ground-water model is a numerical representation of a physical system
Example: Snake Plain Aquifer Model

21. Some Snake Plain Background

22. Major Sources of Recharge and
Precipitation
Tributary
Underflow
Evapotranspiration
Surface
Irrigation
Spring
Discharge
Major Sources
of Recharge and 1

25. Snake Plain Aquifer Water Budget

27. Water Budget on Snake Plain
Approximately 6.5 million acre-feet (8,000 cfs) of water recharges the aquifer annually
Discharge to Thousand Springs reach is 5,000 to 6,000 cfs
Discharge to American Falls reach is 2,500 to 3,000 cfs
We are actually water-rich

28. History of Irrigation on Snake Plain
Surface water irrigation started around 1890s
Water table rose 10s to 100s of feet
New springs emerged
Earliest claims on spring flow in Thousand Springs started around 1920s
Ground-water pumping introduced in 1950s
Rural electrification
Deep pump technology

29. Water levels on Plain peaked in 1950s Primarily declining since then
Impacts of ground-water pumping
Conversion from flood irrigation to more efficient sprinkler systems
Enlargement of irrigated areas
Drought
The aquifer seems to be very responsive to drought

32. Changes in diversions/time

33. Changes in Ground Water Irrigated Acres Over Time
1 million acres
~ 2 MAF/yr or 2,700 cfs

34. Model is Translation of Physical System to Numerical Model Representation
Hydrologic
Properties
Aquifer
Properties
Boundaries
Recharge/
Discharge
Starting
Conditions
Model
Ending
Water
Levels
Spring Discharge
River Gains and Losses

35. Snake Plain Model Enhancement
Completed over past four years
Appropriation from state, contribution from Idaho Power, in-kind from USGS
Collaborative process
Multi-agency
Consultants for water users
Model calibrated to 22 years of data, with 15,000 measured data points

36. Steady State Water Table

37. Comparison of Modeled to Measured Aquifer Water Levels

38. Future Snake Plain Challenges
Continued changes in irrigation practices
Canal lining
Continued conversion to sprinkler irrigation
Land use changing
Subdivisions taking over agricultural land
Industry moving in
Dairies moving in
Societal desires changing
Species protection
Potential for impacts of climate changes

39. Is this just an Eastern Idaho Problem?
Snake Plain may be ahead of other basins in facing these challenges
How we sort out the problems in the Snake Plain will have far-reaching impacts throughout the state
Bear River Drainage
Spokane Valley/Rathdrum Prairie Aquifer
Treasure Valley Aquifer

40. Toolbox for Aquifer Management
Managed Recharge
Aquifer storage of excess spring run-off in high water years
Conservation Reserve Program
Taking acres out of production
Ground-water Banking
Conversion of some ground-water irrigation back to surface irrigation

41. Toolbox for Aquifer Management (cont’d)
Partial or full curtailment of use during low water years
Buy-out of some threatened water rights

42. Summary
Spring discharge and aquifer water levels are changing in response to:
Man-induced effects: pumping and recharge
Weather variability
Demands exceed supplies
Some technical tools available to minimize conflict
State in the position of balancing priorities
Protection of
Senior Rights
Full Economic
Use

43. More information? Gary S. Johnson Donna M. Cosgrove Bryce A. Contor
Idaho Water Resources Research Institute
University of Idaho
Donna M. Cosgrove
Bryce A. Contor