What does OpenET measure?

OpenET provides satellite-based estimates of the total amount of water that is transferred from the land surface to the atmosphere through the process of evapotranspiration (ET). This is also referred to as ‘actual ET’, since it represents an estimate of the actual amount of ET that occurred. OpenET also provides access to data for grass reference ET (ETo), which is the evapotranspiration from a uniform grass surface that is well watered. ETo is calculated from weather data and widely used as an input to irrigation scheduling, and is included as an important benchmark to evaluate and interpret the satellite-based ET. However, it is the actual ET, estimated using satellite-driven models, that quantifies the amount of water that is removed from the water system through ET.

The crop type information available on this site is from publicly available datasets, and does not affect the accuracy of the ensemble ET value, or the five models that use thermal infrared data as an input for calculation of ET

Crop type information available through the Data Explorer is not generated by OpenET, and is not used as an input for the five energy balance models on the OpenET platform (ALEXI/DisALEXI, eeMETRIC, geeSEBAL, PT-JPL, SSEBop). As such, crop type information does not meaningfully affect the accuracy of the ensemble ET value. Crop type information is derived from publicly available datasets, which often lag 6-12 months or more behind the current growing season, and thus may not always accurately reflect the current crop. Crop type information is provided to assist with interpretation of ET values. Outside of California, all crop type information is derived from the USDA Cropland Data Layer. Crop type information for California for 2016 and 2018 is based on the CA Department of Water Resources (DWR) Land Cover database developed by LandIQ. Data for annual crops in California for all other years are from the USDA Cropland Data Layer; this data has lower classification accuracies for many of the specialty crops grown in California. 

The SIMS model does use crop type information to separate annual and perennial crops, and to adjust model parameters for stomatal resistance and maximum crop height for perennial crops. However, errors in crop type classification for annual crops don’t affect the SIMS ET calculations, and the crop-specific adjustments for perennial crops usually have a small influence on SIMS ET relative to other inputs. In cases where crop type errors do have an effect on SIMS ET data (e.g., misclassification of a perennial crop as an annual crop), SIMS is typically flagged as an outlier and removed from the ensemble prior to calculation of the ensemble ET value.    

Many factors affect ET, complicating comparisons across fields or between regions

When evaluating data from OpenET, it is important to consider the many factors that may lead to variations across fields and regions, even for the same vegetation or crop type. These include: soil type and texture; salinity management; ground cover; the age of an orchard or vineyard; irrigation system type and distribution uniformity; production goals; fertilizer, pest and pathogen management considerations, and other factors.

The relationship between ET and crop water requirements can be complex

For many crops, ET is a key measure of the minimum amount of water that must be replaced during the growing season through irrigation or precipitation to ensure a well-watered crop with maximum yields. OpenET provides data that can help growers track the amount of water that is transferred from the root zone to the atmosphere through ET and manage irrigation to maintain sufficient soil moisture levels. However, in addition to accounting for ET, growers and irrigators must also budget water for crop planting and germination, salinity management, frost protection, cover crops, variability in the soil type, the distribution uniformity of the irrigation system, the timing of availability of water for irrigation, and other factors. As a result, in many situations the total amount of water diverted or pumped and applied for irrigation must equal or exceed ET, even in highly efficient irrigation systems. Conversely, for some crops, deficit irrigation for key periods during crop development can also be important to maximizing crop quality or crop yields. And in some regions, deep rooted perennial crops can also access stored soil moisture or shallow groundwater deep in the root zone, resulting in high yields even when the total amount of water applied for irrigation is well below total ET over the growing season.

General limitations of OpenET

Direct measurements of ET in the natural environment, such as from weighing lysimeters, are costly, difficult to obtain, and therefore usually limited to a few research sites across the western US. ET data from OpenET do not represent direct measurements of ET; rather they are estimates based on satellite, meteorological, soil, and vegetation datasets that are used within state-of-the-art ET models. While they are generally accurate and consistent, these data and models have limitations, particularly during times of consistent and dense cloud cover and within complex topography.