Methodology for the spatialisation of a reference evapotranspiration from SRTM data

The aim of this work was to develop a methodology for the spatialisation of mean monthly reference evapotranspiration for the State of Ceará, using the Hargreaves and Samani equation, from estimated air-temperature data based on digital elevation models. The study area is located in northeastern Brazil, between the latitudes 2.5º and 8º South and longitudes 37º and 42º West. The data used in the study are from the network of meteorological stations of the National Institute of Meteorology. The equation proposed by Hargreaves and Samani was used to estimate ET, being spatialised in the form of maps using the Idrisi Andes© software for the manipulation of digital data of latitude, longitude and altitude in order to obtain the variables of the equation. The study used the SRTM data as an altitude map. To compare the results obtained by the Hargreaves and Samani equation to temperatures maps calculated from the digital terrain model, the value of ET was determined for each location from weather data with the help of the REF-ET program. The coefficients of determination of the temperature models generated varied between 0.88 and 0.98, which showed a good correlation between the air temperature and the geographic data. The proposed methodology for the spatialisation of ET from temperature maps obtained by SRTM, proved to be a viable alternative, given the results of statistical analysis when compared to the standard method.