Accuracy of daily PM-ETo estimations with ERA-Interim reanalysis products

Paredes P, Martins DS, Cadima J, Pires J, Pereira LS
European Water 59: 239-246, 2017,

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This study assesses the accuracy of estimates of FAO Penman-Monteith daily grass reference evapotranspiration (PM-ETo) using ERA-Interim reanalysis products with 0.75o x0.75o spatial resolution. PM-ETo requires daily maximum and minimum air temperature, net radiation or sunshine duration, relative air humidity or dew point temperature, and wind speed data. For that, local ETo estimates from 1979-2013 over 37 weather stations distributed across Continental Portugal were compared with those issued from the nearest reanalysis grid-point to each station. Accuracy was assessed using several statistical indicators. Results show that the PM-ETo computed with raw reanalysis data clearly underestimates (regression slope b0?1.05) with RMSE exceeding 1.0 mm day-1 in nearly 30% of cases, but with EF generally above 0.50. Using a monthly bias correction improved the performance of estimates, with b0 usually close to 1.00, RMSE rarely exceeding 0.80 mm day-1 , and EF generally above 0.80. These results show that when full local data sets are not available, the PM-ETo may accurately be computed using daily reanalysis products, thus keeping the ability for the understanding of the physics processes.