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

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

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

Aquifer Flow

Aquifer Flow

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

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

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

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

Well Water Levels Seasonal Variation Long-term Change

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

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

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

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

COMPLICATION NO. 1

COMPLICATION NO. 2: TIME ATTENUATED IMPACTS

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

Some Snake Plain Background

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

Snake Plain Aquifer Water Budget

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

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

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

Changes in diversions/time

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

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

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

Steady State Water Table

Comparison of Modeled to Measured Aquifer Water Levels

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

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

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

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

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

More information? Gary S. Johnson Donna M. Cosgrove Bryce A. Contor Idaho Water Resources Research Institute University of Idaho 208-282-7985 johnson@if.uidaho.edu Donna M. Cosgrove 208-282-7914 cosgrove@if.uidaho.edu Bryce A. Contor 208-282-7846 contor@if.uidaho.edu