Overview

The University of Idaho/Idaho Water Resources Research Institute, in collaboration with the U.S. Bureau of Reclamation and the University of Wyoming, has developed a tool for calculating recharge and discharge for MODFLOW ground water models using spatial data. The tool was developed as part of the upgrade of the eastern Snake River Plain Aquifer model. The GIS Recharge Tool uses a combination of geographical information system (GIS) technology and a Fortran program to process spatial data to calculate recharge and discharge to each model cell for a ground water model.

The GIS Recharge Tool is independent of model grid or aquifer basin. The tool was designed for use in basins with arid irrigation, however can easily be adapted to basins with no irrigation. Spatial data inputs to the tool include the model grid, precipitation, evapotranspiration, recharge on non-irrigated lands, land use, soil type, and irrigation. Additionally, line source and point source data such as canal seepage, river seepage, tributary underflow and municipal pumping are entered as spatial data. The GIS part of the tool intersects each component of data with the model grid and generates ASCII output files which are input to the Fortran program. The figure below shows schematically how the GIS part of the tool functions.

The Fortran program uses the cell-by-cell information to calculate net aquifer recharge/discharge for each model cell for each stress period and generates the MODFLOW input files. The figure below shows conceptually the individual components of aquifer recharge and discharge which are accounted for by the GIS Recharge Tool.

The tool was developed primarily for use with the upgraded eastern Snake River Plain Aquifer Model, but was designed to be applied in other basins as well. The tool design divides functionality between spatial data analysis (the GIS part) and background computer processing (the Fortran part). This split of functionality allows the analyst maximum flexibility in the processing of the aquifer recharge/discharge. By segregating the background computer processing into a separate Fortran program, the recharge tool can be used with a parameter estimation package such as PEST enabling dual calibration of aquifer model parameters (transmissivity and storativity) with model recharge.

Eastern Snake River Plain Aquifer Model Background

There is a current effort underway to upgrade the eastern Snake River Plain Aquifer Model. The original model was built in the 1970s by the University of Idaho, the Bureau of Reclamation and the Idaho Department of Water Resources for management of the aquifer resource. The upgrade to the model includes extension of some of the model boundaries, conversion to a smaller model grid size (1 mile by 1 mile), improved representation of the river/aquifer interconnection and calibration to a longer time period (1980-2001). One of the more difficult tasks in development of a numerical ground water model is the calculation of model recharge and discharge. The GIS Recharge Tool was originally developed to facilitate calculation of aquifer recharge and discharge. However, as the tool development progressed it became apparent that the tool had much broader application than just in support of the Snake Plain model.

GIS Component

The GIS Component of the tool was developed by Mr. Jim Oakleaf of the University of Wyoming. The work was funded by the U.S. Bureau of Reclamation. The figure below shows the user interface for the GIS component of the tool.

The tool allows creation of a new simulation and scenario or creation of a simulation based on an existing simulation. As the user builds a recharge data set, the GIS component provides the user with pull-down menus for selection of desired data files or data fields within a file. The figure below shows the user interface for the GIS component of the tool.

The tool allows creation of a new simulation and scenario or creation of a simulation based on an existing simulation. As the user begins building a recharge data set, the GIS component provides the user with pull-down menus for selection of desired data components. The figure below shows a sample of the data components available for analysis. Once the user has selected the component to be analyzed, the GIS interface steps the user through selection of data files and data fields within the files. Once the data sources have been identified, the user selects the RunAnalysis button and the GIS component processes the selected data, generating the input files for the Fortran component.

The following figures show examples of user data selection.

The GIS component is designed in a modular fashion, so that part of a recharge data set can be built or modified independently of the rest of the data set. The user can build part of a data set, close the tool, and resume processing of the same data set at a later time. The tool also allows easy modification of an existing data set, for scenario generation. The tool was built for maximum flexibility in generation of model recharge/discharge data.

Fortran Component

The Fortran Component of the tool was developed by Dr. Donna M. Cosgrove of the Idaho Water Resources Research Institute at the University of Idaho. The Fortran component reads all of the data files created by the GIS Component which contain recharge and discharge information for each model grid cell.

The Fortran component calculates recharge and discharge for each individual ground water model cell for each stress period, based on the input data from the GIS component. Outputs of the Fortran component include the MODFLOW well file (for direct input to the ground water model) as well as output of intermediate recharge variables such as evapotranspiration, precipitation, or applied irrigation water for each model cell for each stress period, which can be viewed using GIS software. This allows the analyst to graph the components of recharge spatially or to graph totals through time, enabling full analysis of the recharge data for error-checking as well as for hydrological analysis.

The Fortran Component can be run solely to calculate recharge and discharge values for a single model scenario, or it can be run in conjunction with parameter estimation sofware such as PEST. The next figure shows a flow diagram of how the Fortran Component would be used for generation of recharge/discharge for a single model scenario.

The figure below shows how the Fortran Component would be included in a parameter estimation loop using PEST.

Inclusion in a parameter estimation loop would enable the user to estimate model recharge parameters such as tributary underflow, percentage of canal leakage or aquifer recharge on non-irrigated lands along with the traditional model parameters of transmissivity and storativity.

Tool Use

The GIS Recharge Tool provides an analyst with a powerful tool for generation of model scenarios and different conceptual representations of aquifer recharge and discharge. The feature of the tool which allows a scenario to be based on an existing scenario enables the user to retain most of the input variables constant, varying only selected items, in order to test different conceptual models of recharge and discharge. This allows rapid generation of complex recharge/discharge scenarios for a ground water model, which is particularly useful during both model parameterization and model use.

In addition to generation of recharge and discharge for ground water models, the GIS Recharge Tool can be used to estimate the impacts to a regional aquifer from large-scale changes such as climate changes or changes in land use. By modifying the spatial inputs to the GIS Component, the user can create scenarios which represent significant changes in land use or natural precipitation or evaporation, then linking with the Fortran Component to predict the changes to aquifer recharge/discharge caused by these basin changes. This makes the GIS Recharge Tool a powerful tool for assessing natural and management changes of a groundwater basin.