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Reference evapotranspiration (ETo) assessment at regional scale is a challenge to agricultural and hydrological research requiring the characterization of vast areas to obtain accurate meteorological variables. To account for this, a new approach considering a modified version of the Makkink equation and based on a combination of remotely sensed solar radiation (R-s) considering the MSG satellite, and numerical weather forecast of near surface air temperature (T-2m), was developed. The new approach is referred as Makkink-Advection equation (MAK-Adv).

Once R-s and T-2m were validated for the semiarid conditions registered in Southern Spain with very satisfactory results (RMSE = 1.6 MJ m(-2) d(-1) and 1.4 degrees C respectively, with linear regressions with slope = 0.99 and R-2 = 0.97), the MAK-Adv approach was calibrated and validated with ETo measurements from a weighing lysimeter under near-reference conditions in Southern Spain. The new approach has provided accurate daily ET estimations compared with lysimeter data (RMSE = 0.50 mm d(-1), RE = 12.3% and MAE = 0.39 mm d(-1)), and remains accurate during summer time (RMSE = 0.62 mm d(-1) and RE = 9%), the critical period when accurate ETo data are essential for a correct agricultural water management.

Once calibrated and validated, and thanks to the use of remotely sensed data, the MAK-Adv approach allowed the ETo assessment at regional scale. For a semi-arid region located in southern Spain with an area of 87,300 km(2), a detailed spatial ETo assessment was performed, determining ETo values for the totality of the area with a resolution of about 9 km(2) allowing to evaluate the spatial and temporal variability at basin/region scale. This new approach gets over the serious limitations for a correct ETo assessment when traditional methodologies are not able to provide these data for vast areas, even with the modern weather station networks. Thus, MAK-Adv approach is especially useful for those areas with missing or inaccurate meteorological in situ observations. (C) 2014 Elsevier B.V. All rights reserved.