Estimates of Evapotranspiration and Evaporation along the Lower Colorado River

Submitted by atripp on Wed, 05/19/2021 - 15:18

The Colorado River is the principal source of water for agriculture and riparian vegetation in Arizona, Southern California, and southern Nevada. In the Lower Colorado River Basin, the BOR accounts for water use in each State, verifies water conservation programs, and fulfills other water management information needs. To accomplish this, BOR monitors more than 3.5 million acres of agricultural land and riparian vegetation along the Lower Colorado River, from Hoover Dam south to the International Boundary with Mexico.

Four times per year, BOR performs supervised classifications on every field along the main-stem Colorado River, Imperial and Coachella Valleys, and Gila River. This task is accomplished using a combination of multispectral satellite data, aerial images, and ground-based field verifications. Roughly half of the collected verified fields are used to train the satellite imagery and determine the crop growing on each field for each time period. The other half of verified field data are used to assess the accuracy of each classification. Data about the crop type and acreage, along with evapotranspiration (ET) crop coefficients, are then used to estimate the ET from crops within each area. Total ET from agricultural irrigation is close to 3 million acre-feet per year. The most prevalent crops by acreage are alfalfa, lettuce, Sudan grass, small grains, and Bermuda grass. BOR also maintains riparian vegetation and open water datasets to determine ET and evaporation from these sources. Each year, these datasets are updated using the best possible imagery (satellite or aerial) by performing change detection analyses. Results from these efforts are reported annually in the report “Lower Colorado River Annual Summary of Evapotranspiration and Evaporation.” Annual ET from riparian vegetation is usually over 400,000 acre-feet and annual evaporation from open water sources is over 300,000 acre-feet.

This information assists the BOR in meeting its U.S. Supreme Court mandate to provide detailed and accurate records of diversions, return flows, and consumptive use estimates of water diverted from the main stem of the Lower Colorado River. This program is an example of implementing remote-sensing-based methodologies to meet the BOR’s water management needs.

The BOR uses remote sensing and geographic information systems (GIS) to map agricultural fields, riparian vegetation, and open water in order to estimate evapotranspiration within the Lower Colorado River Region (left). Examples of the riparian/open water databases near the Havasu National Wildlife Refuge, Arizona (top right), and the agricultural database near Yuma, Arizona (lower right), USA.

Author Name
Troy Wirth
Author Email