Estimation of Evapotranspiration through remote sensing. Comparison between three different resolution sensors at Barrax field, Spain.

Abstract: Nowadays importance of water balance regarding measurements and water cycle pathways are of prime interest in a new perspective. Global issues, like Earth warming, or regional planned uses of water, e.g. irrigation, demand an increase in accuracy and coverage of different components of the water balance. Precipitation and the water loss from vegetation and soil, called Evapotranspiration (ET), are main components in the water balance equation. While precipitation, the main input in water balance models, is easily measured, ET, the major loss, is not. Current and most used methods are accurate but limited in time and mostly in space. Extended ET measurements are needed to manage water resources at regional or broader scales with reasonable accuracy. The use of remote sensing has been tested for last two decades and is still subject to extended research to overcome definitively this problem. Present work compares results of application of a simplified version of residual of energy budget, Roerink´s S-SEBI model, to calculate ET using three sensors with high (AHS), medium (ASTER) and low resolution (NOAA´s AVHRR) at Barrax field, Spain, on 18th July 2004. Its application basically stands for energy balance equation using visible and near infra-red radiation measurements coming in and out of the surface-vegetation-atmosphere system. Incoming short and long wave radiation were measured on field at ?Las Tiesas? lysimetric station, which was used as reference, and the remaining variables were measured instantaneously at sensors overpasses and then extrapolated to daily values. Jackson empirical model is also used to contrast results. Albedo, temperature and emissivity were retrieved from sensors and used to calculate Evaporative Fraction, net radiation, geothermal flux and vegetation indices NDVI and MSAVI. Once all variables are calculated the S-SEBI is applied and the instantaneous ET value is extrapolated to daily value in mm/day at the chosen area. Results showed a good correlation, for most of variables, between AHS and ASTER and acceptable error (around 10%) for the higher resolution sensor. Meanwhile AVHRR did not show good results but, anyhow, it could still represent some potential use in large and homogeneous areas with less accuracy demand assessments or as complementary information for water resource management at regional or even global scales.