Improving Evapotranspiration Maps Obtained from the Remote Sensing of Evapotranspiration (ReSET) Model
The ReSET model uses an energy balance algorithm to estimate evapo-transpiration (ET) from a set of satellite images. Reclamation funded the development of the ReSET as an alternative to methods that use crop maps and crop coefficients to estimate water use. ReSET has advantages over the conventional crop coefficient method of estimating ET, chief among them:
* There is no need to map crop types
* ReSET can measure reduced ET from crops with a partial water supply.
Reclamation has begun using ReSET in the Upper Colorado River basin. Through the use of the model, numerous questions have arisen, including:
* Of the many user-supplied inputs to the ReSET model, which have the greatest impact on the output ET estimates?
* Are there better data sources for the requred model inputs?
* How do model results compare with estimates from conventional methods?
This Science and Technology (S&T) Program research project addresses those questions.
Need and Benefit
Currently, Reclamation staff rely almost exclusively on the crop coefficient method to calculate consumptive water use by agriculture and riparian vegetation. Crops and riparian vegetation are mapped using remote sensing, and the results are placed in a Geographic Information System (GIS). Every vegetation type in the map is assigned a "crop coefficient," which estimates the proportion of water used by that vegetation type compared to a reference crop (usually grass or alfalfa) under the same weather conditions. Reference crop ET is predicted from local weather data using American Society of Civil Engineers (ASCE) standardized equations. Total water use for any crop for any time period is determined by multiplying the reference crop ET for that time period by the crop coefficient. Total water use is determined by multiplying the water use value for each crop type by the acreage of the crop.
The crop coefficient method is widely accepted, but has the unrealistic assumptions of:
* Identical phenological development for all members of a given vegetation class
* Ideal growing conditions
The first assumption is never met due to variability in planting dates and natural variation in crop development, but many of the errors may be in different directions and, thus, cancel one another. Errors resulting from the assumption of ideal growing conditions are a more significant problem. Conditions such as reduced water supplies, pest and disease damage, inadequate nutrients, salt buildup, or other such factors can reduce actual vegetation ET from the theoretical value. In the Upper Colorado River Basin, Reclamation's Water Resources Planning and Operations Group in Denver estimates that over 20 percent of the more than 1.5 million irrigated acres in the Basin does not receive a full water supply in a typical year.
The ReSET model uses an energy balance model to estimate ET.
R plus G plus H plus LE = 0
R = net radiation flux at the surface
G = sensible heat flux to or from the ground
H = sensible heat flux to or from the atmosphere
LE = latent heat flux (ET)
ET is estimated as the residual of the energy balance equation after R, G, and H have been measured or estimated using satellite imagery and physical models. 24-hour estimates of ET generated from the satellite imagery are interpolated temporally using local weather station data to "fill in the gaps" between satellite overpasses.
ReSET does not make the assumptions of uniformity and ideal growing conditions, and uses satellite imagery to solve the energy balance equation for every pixel in the image. The result is a continuously varying map of ET that offers more information to farmers and water managers than a similar product produced using crop coefficients.
Reclamation has a clear interest in developing new, potentially cheaper and more accurate methods of measuring ET. Reclamation has also showed its desire to develop an in-house capability for this work to take advantage of in-house knowledge and expertise. Reclamation's Middle Pacific (MP) Region funded the development of the ReSET model as an alternative to the crop coefficient method, and the Upper Colorado Region funded a pilot study to investigate its application within the Upper Colorado Basin.
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