Fresh water availability is fast becoming the top environmental concern in large areas of the U.S. As climate change and growing populations take their toll, many states are experiencing water shortages. The state of Idaho is both arid and largely agricultural and therefore is faced with making critical decisions about water use. Irrigated agriculture accounts for more than 90 percent of water use in the state.
Evapotranspiration is the water evaporated from soil and transpired from vegetation. It represents the amount of water consumed by irrigated agriculture and other land uses as opposed to water that is diverted or pumped onto land but then quickly returned to its source. Evapotranspiration data is essential for water management, conservation efforts, settling water resource conflicts, and transferring water among users. However, traditional, ground-based methods of measuring evapotranspiration are unreliable, expensive, and take a long time to compute so that one is always using old data.
In collaboration with the University of Idaho, Idaho’s Department of Water Resources is the first government agency in the nation to develop and use satellite-based evapotranspiration imagery to analyze water usage in the state. Researchers developed a computer model that uses visual and thermal images from NASA's Landsat satellites. The METRIC (Mapping EvapoTranspiration at high Resolution with Internalized Calibration) model is able to calculate evapotranspiration data on a daily, monthly, or seasonal basis.
Individual Landsat images contain 30 million pixels in which each pixel is 30 square meters. This allows for the accurate and repeatable mapping of water usage for areas as large as 10,000 square miles and as small as a single 40-acre farm field. Previous calculations for quantifying water usage were limited to regional maps with no capability for historical comparison. By tracking usage on a field by field basis, the state can more effectively understand and regulate water use and compare it to past archived usage data. In addition, it is estimated that satellite mapping costs taxpayers one-tenth of previous methods.
It was anticipated that METRIC would have a number of uses, such as determining a water balance for hydrologic modeling, monitoring irrigation water use, and resolving water rights conflicts. However, satellite-based mapping has proven to have a variety of unforeseen applications. These include estimating water use for endangered species and computing evapotranspiration by land use class to help predict trends in future water use. The state’s lawyers are even using the evapotranspiration data to help defend water rights decisions. Residents using water in excess of their rights may be more easily tracked and regulated.
Because Landsat images are readily and freely available from the US Geological Survey, the opportunities for other states and even countries to employ this technology are virtually unlimited. Other states have already begun making the investment to train staff on the highly complex METRIC model. And, organizations that are not ready to fully commit can initially purchase a processed evapotranspiration image for evaluation.