Gila River Basin Water Rights Adjudication, Arizona

Client: U.S. Department of Justice/Stetson Engineers, Inc.
Project: Gila River Basin Water Rights Adjudication, Arizona

As a part of the Practicable Irrigable Acreage study for the Gila River Indian Community (GRIC) in Arizona, NRCE conducted a surface water supply study of the Gila River Basin upstream of the northwestern corner of the GRIC near Phoenix, just below the confluence of the Gila and Salt Rivers. The study area covers 43,564 square miles in Arizona and New Mexico and includes the Gila, San Pedro, Santa Cruz, Salt, and Verde River basins. To estimate the undepleted flows at different points in the study area, NRCE conducted both precipitation-runoff modeling and water budget analyses.

Precipitation-runoff modeling was performed using the model Hydrological Simulation Program Fortran (HSPF), developed by the U.S. Environmental Protection Agency. To apply this model, NRCE collected and analyzed large amounts of climatic data and developed complete daily data sets for precipitation, temperature, dew point temperature, wind speed, solar radiation, and potential evapotranspiration at over eighty locations in the study area. In addition, NRCE developed Geographic Information Systems (GIS) coverages of hydrology, topography, soils, land use, and land cover, which were used to divide each sub-basin into land segments of uniform hydrological properties. Each segment was then assigned a set of parameters that collectively control the precipitation-runoff relationship. The functional dependence of these parameters on the soil type, land use, and other factors was established through the process of calibration, which was carried out on several selected sub-basins in which the measured flows are approximately undepleted. Transferability of this dependence was verified for several other selected sub-basins, in some of which the measured flows are depleted.

For sub-basins used in the process of verification in which the recorded flows are depleted, it was necessary to perform a water budget analysis in order to estimate the undepleted flows to which predictions of the model can be compared. Each of 28 sub-basins in the study area was modeled using HSPF, while selected sub-basins were also analyzed by the water budget method for the purpose of verification. Subsequently, NRCE estimated channel transmission losses and developed a model to route the simulated natural runoffs through the stream network. This enabled the undepleted flows to be simulated at downstream locations and permitted additional verification against water budget estimates at those locations.