Water Resources Assessment and
Water Rights Adjudication
Navajo Nation, Arizona


Development of groundwater resources for Indian Tribes in the West revolve around Winters Doctrine rights and the McCarran Amendment, which established Practical Irrigable Acreage (PIA) as the yardstick to quantify water rights. PIA claims are a complex interrelationship among economic, hydrogeologic, market, agricultural, and social factors. The hydrogeologic challenge is to work closely with tribal leaders, economists, agronomists, and soil scientists to jointly develop appropriately scaled and economically viable irrigation projects.


We were retained to conduct several groundwater resource assessments and impact studies within the Little Colorado River Basin of Arizona, a 27,000 square mile watershed. The development of groundwater resources from aquifers in the Dakota, Navajo, and Coconino Sandstones was to be investigated. Optimal resource development scenarios required working closely and interactively with the economists and agricultural specialists.

Investigation and Results

A large–scale hydrogeologic and geochemical field investigation was conducted throughout the Basin. The investigation included geologic mapping, inventorying springs and wells, aquifer testing, and geochemical and isotopic sampling and interpretation. Analytical and numerical models were developed to evaluate groundwater response to proposed agricultural and Tribal water supply developments. Basin–wide numerical models were developed of all three major aquifer systems to evaluate such diverse goals as sustainable development and impact on spring discharges in the Grand Canyon and other critical habitats. More than 20 project reports describe the results and interpretations of the various tasks. We are providing continuing expert witness support in the adjudication process. Completed tasks include:

• Numerical simulation and water quality evaluation of a proposed agricultural development using brackish to moderately saline water.

• Prediction of impacts of industrial groundwater developments (power plants) on groundwater resources, spring flow, and stream flow.

• Basin–wide isotopic assessment including recharge and groundwater age determinations.

• Projections of the impacts of population changes on Tribal lands on Tribal water resources.

• Development of an agricultural wellfield cost model.

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