Lower Colorado River Water Accounting
History and Types of Reports
The Colorado River is the principal source of water for irrigation and domestic use in Arizona, southern
California, and southern Nevada. The Consolidated Decree of the U.S. Supreme Court in Arizona v California requires the Secretary of the Interior to provide detailed and accurate records of diversions, return flows, and consumptive use of water diverted from the mainstream of the Colorado River below Lee Ferry (lower Colorado River).
Decree Accounting Reports
The Bureau of Reclamation provides these records annually in a report called "Compilation of Records in Accordance with Article V of the Decree of the Supreme Court of the United States in Arizona v. California Dated March 9, 1964". These reports, generally referred to as "Decree accounting reports", tabulate measured diversions, measured returns and consumptive use of each user taking water from the lower Colorado River. Beginning with the 2002 Decree Accounting Report, an estimate of "unmeasured" return flows -- flows that return to the river through the groundwater system -- for each water user is included. These unmeasured returns are calculated as a coefficient multiplied by the measured diversion amount.
These reports have been variously titled:
- Compilation of Records in Accordance with Article V of the Decree of the Supreme Court of the United States in AZ vs. CA dated March 9, 1964;
- Accounting for Colorado River Water Use within the States of Arizona, California, and Nevada; and
- Colorado River Accounting and Water Use Report - Arizona, California, and Nevada.
They were all published to comply with Article V of the 1964 Supreme Court Decree, and can be collectively and individually referred to as the annual Water Accounting Reports. Beginning in 2002, the reports have sections, supplemental to the requirements of the Supreme Court, which contain information on such programs as interstate banking, the Colorado River Water Delivery Agreement, Inadvertent Overrun and Payback Policy, and Intentionally Created Surplus.
Lower Colorado River Accounting System
The Lower Colorado River Accounting System (LCRAS) was developed to refine estimates of agricultural consumptive use, based on evapotranspiration and a water balance (inflows minus outflows) of the lower Colorado River from Hoover Dam to Mexico. The "LCRAS Demonstration of Technology" reports document the consumptive use results that LCRAS can provide for the Decree accounting reports, and the data and calculations LCRAS uses to produce those results.
In 2004, Reclamation chose to continue to use the traditional methods from the Decree accounting reports, and to use LCRAS evapotranspiration calculations to monitor water use by agricultural and riparian vegetation below Hoover Dam. The LCRAS calculations will also be used to improve the unmeasured return flow coefficients used in the Decree accounting method, and to help determine beneficial use estimates for Reclamation's water conservation program. The 2003 LCRAS report and future LCRAS reports will be published under the name "LCRAS Evapotranspiration Calculations."
Updated: May 2011