No water budget would be complete without accounting for evaporation and related processes, such as transpiration and sublimation. Evapotranspiration, or "ET," refers to the combined flux of plant transpiration and evaporation from the adjacent soil. It is especially important for understanding water used by irrigated crops, and is related to crop productivity. Consumptive water use for irrigation represents the largest percentage across all water-use categories and is concentrated in the more-arid western U.S. where precipitation is not sufficient to grow many crops.

These fundamental mechanisms are a major part of the water cycle and have an important influence on water availability. Being able to quantify water lost or used through these processes can even have implications for administration of water rights and river basin compacts. Therefore, several studies are underway to improve techniques for quantifying these processes as part of the National Water Census. This will enable water managers to more accurately account for water through its entire cycle, improving the accuracy of water budgets.

Remote Sensing Research on ET

Historically, scientists have used empirical formulas calibrated with specialized instruments to measure crop-specific ET or reference crop ET to estimate other areas where crops are grown. The downside of this approach is that it results in local calibrations of crop-specific ET that may or may not be accurate to other areas with different climate, irrigation and conservation practices.  This has made it difficult to quantify ET over broad areas such as irrigation districts, river basins, or States – which is commonly the scale at which modern water management decisions are coordinated or negotiated. Hence, there is a need to ensure accurate estimates of ET at the landscape scale.

In response to this challenge, the National Water Census has developed methods to use remote sensing as a method of estimating ET. USGS EROS has produced evapotranspiration estimates using satellite land surface temperature imagery using the Simplified Surface Energy Balance (SSEB) described in Senay et al. (2010), Senay et al., (2008), and Senay et al. (2007) (all references can be found on the Data and Products page). Monthly actual ET (ETa) estimates at 1 km2 resolution are available for 2000 to present using MODIS thermal imagery for the conterminous U.S. Additional ETa products at 90 meter resolution are available using USGS Landsat data and has been applied to National Water Census Focus Area Study locations and as a pilot for a single year (2015) for the conterminous U.S. The MODIS results (1 km-resolution) are also used to produce anomaly maps as data become available, as part of an early warning system for droughtOpens page in a new window. Due to the availability of satellite imagery that is repeated on a regular scale (depending on resolution), the SSEBop method results in ETa estimates that incorporate the spatial and temporal variation that occurs naturally throughout the growing season. This national coverage of ETa is scientifically defensible, is consistently generated on the basis of remotely sensed data, and can be used as a foundation for other projects, most notably the 2015 compilation of irrigation consumptive use. This analysis has shown the importance of the addition of both a detailed GIS layer of irrigated lands throughout the United States and implementation of the increased spatial resolution provided by the use of LANDSAT thermal-band imagery (90-meter) for future project efforts.

In addition, USGS is also researching remote sensing approaches to better quantify crop type, including fallowed lands, and crop water productivity.

Center-Pivot Irrigation, north of Hereford, Deaf Smith County, Texas, photo by Alan Cressler, USGS