Understanding Ground Water Modeling Gary Johnson Donna Cosgrove Idaho Water Resources Research Institute University of Idaho Idaho Falls

Idaho Water Resources Research Institute Goal: link up water research and education needs with Idaho university researchers and funding National program – an institute in each state Housed at U of I Objective: Unbiased research and evaluation

Outline Modeling temperature and altitude Comparison with ground water model Ground water model description Model calibration Using the model (scenarios) What a model will and will not do

Modeling Temperature and Altitude

The model analogy Altitude Mean Temp MODEL: Temp= Altitude Model fits a relationship To measurements or observations

Model can yield a prediction or estimate an effect SCENARIO: What is the effect of climbing 1000 feet higher on the mountain ? Altitude degrees Temperature drops by 3.2 degrees MODEL RESULTS:

Concern: Will it really cool by exactly 3.2 degrees as you climb another 1000 feet ? Probably not. But it is your best estimate

Ground Water Flow Model Input/Output Comparison Altitude/Temp Model Model Input: Altitude Model Output: Temperature Ground Water Flow Model Model Input: Recharge and discharge at all locations for time frame of interest Model Output: Aquifer water levels Spring discharge & river gains and losses

Ground Water Flow Model Construction Comparison Altitude/Temp Model Fitting the model (line) to measurements of altitude and temperature Ground Water Flow Model Fitting the model: model calibration to water level measurements and river gains and losses

Ground Water Flow Model Scenario Comparison Altitude/Temp Model Scenario: What is the effect of climbing another 1000 feet in altitude ? Ground Water Flow Model Scenario: What is the effect on spring discharge of pumping 100 AF at a given location ?

Ground Water Flow Model Comparison Altitude/Temp Model Concern: Will temperature fall exactly 3.2 degrees as you climb 1000 feet ? No – but best estimate Ground Water Flow Model Concern: Will spring discharge decline by exactly the modeled amount in response to pumping? No – but best estimate

Ground Water Model Description

Model Ins and Outs Aquifer Recharge Precipitation Canals S.W. Irrig. Streams Trib. Valleys Aquifer Discharge Pumping Aquifer Head Exchange With Surface Water Computer Model

Inside the Black Box Thousands of equations Aquifer boundary representations Aquifer property estimates Computer Model

Greatest Challenges Estimating Inputs (Recharge and Discharge) Estimating Aquifer Properties –Ability to transmit water (transmissivity) –Ability to store water (storativity) –River/Spring interconnection with the aquifer

Addressing Challenges: 1) Estimating Recharge and Discharge Approach: Lots of data collection coupled with reasonable assumptions Precipitation Soil characteristics Irrigated areas Evapotranspiration Diversions and return flows Pumping rates Managed recharge Variation with location (each model cell) Variation with time (every 6 months from )

Example: Evapotranspiration

Addressing Challenges: 2) Estimating Aquifer Properties “Model Calibration” Similar to fitting line through elevation/temperature data Altitude Mean Temp Good Model Bad Model

Model Calibration MODEL Estimates of aquifer recharge and discharge Simulated water levels and spring discharges Measured water levels and spring discharges Comparison Adjust estimates of aquifer properties Automated

Snake Plain Model Calibration Matching change in water level in a well

Snake Plain Model Calibration Matching Measured Discharge At Springs

Why not perfect match ? Model is an approximation of reality –e.g. assume everything is uniform throughout entire aquifer thickness (2-dimensional) Recharge and discharge are best scientific ESTIMATES Aquifer properties are best scientific ESTIMATES

Using the Model (Scenarios)

Basic Goals of Scenarios Discriminating effects of specific activities or events e.g. What is the effect of ground water pumping on spring discharge ? Prediction (Future Conditions) e.g. What will aquifer water levels and spring discharge be in 20 years if we continue our current practices and have average weather ?

Discriminating Effects Time Spring Depletion Pumping Starts 0 Results show only the effect of specified pumping on a specific reach

Prediction Time Spring Discharge Past Future Requires that we estimate future weather conditions

Numerical Superposition Effects of individual events can be summed to determine total effect Model can be used to evaluate effects of changes in recharge and pumping, without consideration of other events Example: effect of pumping 100 AF from the aquifer is the same whether it is a wet year or a dry year Analogy: going up 1000 feet in altitude drops the temperature by 3.2 degrees whether you are at 3000 feet altitude or 6000 feet altitude

Superimposing Effects (Superposition) Time Spring Accretion Pumping Stops Time Spring Discharge Past Future With Pumping Without Pumping

Bottom Line: What the model can do The model will provide best estimates of –Changes in aquifer water level in a region –Changes in spring discharge and river gains and losses for a reach along the Snake River Given: Estimates of changes in recharge or discharge

What the model won’t do Automatically determine changes in recharge and discharge associated with some change in land or water management –Can be lots of work –Wrong input yields wrong answer Provide “point specific” estimates of aquifer water level and spring discharge Provide exact solutions Assess injury in a legal sense Assess economic impact of alternatives

How to contact us Gary Johnson (208) Donna Cosgrove (208) Idaho Water Resources Research Institute (Idaho Falls)