Spatio-Temporal Distribution of the Calibration Coefficients of Hargreaves Equation for Estimating Potential ET in Mainland of China.

It is important to calibrate the potential evapotranspiration measured by the Hargreaves method (ET_{0, HG}) using the Penman-Monteith method (ET_{0, PM}).  However, the substantial temporal variations in the calibration coefficients (the ratio of the ET₀ measured by the two methods) have not been considered in its spatial analysis.  The objectives of this study were: (1) to investigate the spatio-temporal distribution of the calibration coefficient at the scale of the Mainland of China, and (2) to compare the accuracy of the calibrated ET_{0, HG} by this spatio-temporal calibration method with three alternative methods including the calibration coefficients over space and/or time.  One year was divided into 72 five-day periods in this study.  For each five-day period, quantitative models were used to describe the spatial dependence of the calibration coefficients, and the maps of the calibration coefficients were produced by the kriging method. The accuracies of the calibrated ET₀ by this spatio-temporal calibration method and other three methods were evaluated by the creteria including the absolute bias, relative bias, root mean square error, and mean bias error.  The results showed spatio-temporal changes in the parameters of the geostatistical models and the maps of the calibration coefficients of Mainland China.  The maps were characterized by their divisions into two regions along a contour of calibration coefficient of 1.0, and the directions of these contours showed obvious seasonal changes.  The accuracy of the calibrated ET_{0, HG} was highest when both the spatial and temporal variations of the calibration coefficients were considered simultaneously.