Common Calibration of Solar Radiation and Net Longwave Radiation Is the Key to Accurately Estimating Reference Crop Evapotranspiration over the Tibetan Plateau

Reference crop evapotranspiration (ET0) is crucial for water management. Although the FAO56-PM method is widely used to estimate ET0, its input variables of solar radiation (Rs) and net longwave radiation (Rnl) are not readily available. Currently, mere calibration of the formula for Rs is assumed to be effective in improving FAO56-PM’s performance. To test this hypothesis, all input variables for FAO56-PM were measured in Lhasa, Linzhi, and Bange over the Tibetan Plateau (TP) to assess how different calculation methods affect ET0 estimates. Compared to the original FAO56-PM, calibration of both Rs and Rnl formulas yielded the best model performance. Mere calibration of the formula for Rnl notably improved ET0 estimation accuracy, while mere calibration of the formula for Rs reduced its accuracy. The general calibration model was slightly less effective than calibration of both Rs and Rnl formulas but obviously outperformed the original FAO56-PM. This model showed that ET0 increased from east to west, ranging from 569.4 mm/year to 1118.5 mm/year. Trend analysis indicated rapid increases in ET0 in the eastern region and significant decreases in the western region of the TP over recent decades. The findings are useful for the regional application of FAO56-PM to achieve sustainable development in Tibet.