Mapping reference evapotranspiration from meteorological satellite data and applications

Reference evapotranspiration (ETo) is an agrometeorological variable widely used in hydrology and agriculture. The FAO-56 Penman-Monteith combination method (PM method) is a standard for computing ETo for water managements. However, the application of this scheme is limited to areas where climatic data with good quality are available. In this study, maps of 10-day averaged ETo at scale of 5km × 5km for Taiwan region were produced by multiplying pan evaporation (Epan), derived from ground solar radiation (GSR) retrieved from satellite images using Heliosat-3 method, by a fixed pan coefficient (Kp). Validation results indicated that the overall mean absolute percentage error (MAPE) and normalized root-mean-square deviation (NRMSD) were 6.2% and 7.7%, respectively, when comparing with ETo computed by the PM method using spatially interpolated 10-day averaged daily maximum and minimum temperature datasets and GSR derived from satellite as inputs. Values of land coefficient (K_L , based on the derived ETo estimates and long term latent heat flux measurements, were determined for the following landscapes: Paddy rice (Oryza sativa), subtropical cypress forest (Chamaecyparis obtusa var. formosana and Chamaecyparis formosensis), warm-to-temperate mixed rainforest (Cryptocarya chinensis, Engelhardtia roxburghiana, Tutcheria shinkoensis, and Helicia formosana), and grass marsh (Brachiaria mutica and Phragmites australis). The determined land coefficients are indispensable to scale ETo in estimating regional evapotranspiration.