Assessment of the Potential Evapotranspiration MODIS Product Using Ground Measurements in the Pampas

Evapotranspiration is the hydrological variable of greatest relevance in the Argentina Pampas Region (APR). The estimation of potential evapotranspiration (PET) in this area becomes essential since primary productivity is directly linked to water availability. In order to evaluate the MOD16_A2 product of evapotranspiration (ET), a comparison with in situ measurements was conducted. We used ET data provided by the Oficina de Riesgo Agropecuario, corresponding to 24 stations placed in the region covering all seasons for the years 2012 to 2014. Results show an overestimation of 86% and 52% in Autumn-Winter and Spring-Summer, respectively. Mean Absolut Error (MAE) range between +/- 0.9 and +/- 2.1 mm day(-1) in Autumn-Winter, and between +/- 1.6 and +/- 2.6 mm day(-1) in Spring-Summer, with R-2 values close to 0.7 for both data sets, with no seasonal differences at this point. Overestimation in PET was found to be partly due to the empirical model used in the MOD16_A2 product to determine the downward longwave radiation (Rl(down arrow)) term. A specific application in our study sites showed that the term of Rl(down arrow) produces in an overestimation of 1 mm day(-1) in the calculation of PET. Although further research is still required to find out the reasons for these discrepancies, these results advice to be cautious when using PET data of the MOD16_ A2 product in the APR.