Interstate Water Compact - Republican River Basin
  
In November 1999, the U.S. Supreme Court appointed a Special Master to oversee resolution of a dispute regarding allocation of water from the Republican River and allegations that Nebraska had violated the Republican River Compact. The Compact, signed in 1943 by the three basin States of Kansas, Colorado and Nebraska, allocates the water supply of the Republican River, with the total allocation given to each State derived from listed tributaries, and for Nebraska and Kansas, from the main stem of the Republican River. SSP&A was retained by the State of Kansas to provide technical support in quantifying the nature and magnitude of streamflow accretions and depletions from the ground water system in the Republican River Basin.

Early work on the project was completed by technical consultants to three states - Kansas, Nebraska, and Colorado - largely independently of each other. Concurrently, the USGS undertook work through a cooperative agreement in Nebraska to develop a model of the Republican River Basin. During this phase of work, consultants and the USGS reviewed existing models of the region; built relational databases of historic groundwater elevations, pumpage and streamflows throughout the basin; and developed conceptual and numerical models. During 2001 the parties began settlement negotiations and a settlement in principle was reached in December 2002.

The settlement terms included the cooperative development by the parties of a groundwater model of the basin that would be used to quantify historical streamflow depletions due to groundwater pumping, and to manage future groundwater pumping. Depletion estimates from the model would be incorporated into the compact accounting and allocation process that determines each state’s share of the water supply. SSP&A, along with consultants for the other states, completed a collective analysis of data for the period 1900 to the present, including:

• Estimation of consumptive use for municipal & industrial ground water diversions;

• Estimation of irrigation efficiencies for surface water diversions and groundwater extractions;

• Delineation of soil type and land use throughout the basin;

• Delineation of canals, laterals, and surface water returns, including impacts on the North Platte River located on the northern margins of the Republican River Basin;

• Analysis of baseflows through application of various baseflow separation techniques;

• Estimates of varying phreatophyte area, evapotranspiration (ET) rate and extinction depth;

• Analysis of the role of ET-salvage in the river basin water-budget.

Over a period of about 4 years, a regional-scale basin groundwater flow model was constructed, parameterized, and calibrated by the three states. Calibration targets for the transient model included nearly 100 years of record of water levels at over 1100 wells, and stream flows at nearly 100 stream gages. SSP&A made several modifications to the MODFLOW code to facilitate the application of the code to specific conditions within the basin. The completed model was used to simulate the period from 1900-present to identify the location, timing, and causes of significant surface water depletions. SSP&A met several times with representatives from Colorado and Nebraska, and collaborated closely over 18 months with consultants from all parties to develop calibration data, databases, software, and pre-and-post-processing programs to assist with depletion calculations using the constructed model.