Spatial Interpolation of Daily Evapotranspiration Data in the Texas High Plains.

The Texas High Plains Evapotranspiration (ET) Network collects meteorological data from grass referenced weather stations at hourly intervals and estimates hourly and daily reference ET using the American Society of Civil Engineers (ASCE) Standardized Reference ET equation. Producers in the Texas High Plains have accessed the daily reference ET for any weather station of their interest. However, one concern in using these data is to determine which reference weather station best represents climatic conditions similar to those in their specific irrigated fields. Availability of accurate daily reference ET maps for the Texas High Plains is expected not only to relieve producers from this concern but also to assist in attracting more producers to adopt reference ET-based irrigation scheduling. Therefore, the main objective of this effort is to provide accurate daily reference ET maps for the Texas High Plains. A set of computer programs were written to develop daily grass and alfalfa reference ET maps as well as maps for the weather parameters such as solar radiation, wind speed, air and dew point temperatures. The programs were written in R statistical programming language and outputs were linked to a Water Management Website using Java Programming Language. Since it is difficult to use one predetermined interpolation technique and/or fitting model for interpolating and mapping reference ET over the region, a set of interpolation techniques including inverse distance weight (IDW), spline, kriging and co-kriging with linear, exponential, Gaussian, and spherical models to choose the optimum variogram model were implemented. Comparison of the interpolation methods indicated the spline method consistently produced more accurate daily reference ET maps followed by co-kriging method with wind direction as a co-kriging variable.