Reference Crop Evapotranspiration and Climate Variability Study in a Mexican Irrigation District

In 1994, the Mexican Institute for Water Technology (IMTA) and the Mexican National Water Commission (CNA) initiated a joint program for technology transfer and technical training in the most important irrigation districts of the country. The goal of this project is to establish a real-time irrigation forecasting system to be used as a scheduling tool, which allows us to precisely determine the appropriate timing and quantity of irrigation required. A key system component is the generation of databases containing the weather variables necessary for the efficient and accurate estimation of crop evapotranspiration. Therefore, a project was established to build weather station networks throughout the country, which are operated by the water users' associations. To date, the IMTA has provided technical assistance in 10 irrigation districts, installing weather station networks in 6 Mexican states (totaling approximately 65 automatic weather stations). Technical assistance offered typically includes training in installation, operation, maintenance and calibration of the weather stations, as well as use of software and telecommunications equipment.

In 1995, a weather station network was established in the Santo Domingo Valley, Baja California Sur. Initially, two automatic weather stations were installed at the sites considered most representative of the whole irrigation district. In 1999, a study was undertaken to verify if it would be necessary to install more stations in order to achieve a better spatial representation of the district's climate variability. Analysis of the resulting data set confirmed that the percent-difference in reference crop evapotranspiration for the five sites considered in this study varies between 1.7 y 3.9 % - a range of variability too small to justify installing additional stations in the district if they are to be used for real-time irrigation scheduling. This paper presents details of the variability study and field-comparisons of sensors.