Use of satellite application facility on land surface analysis products to improve an operational method for monitoring forest evapotranspiration

We propose a modification of a water balance method (NDVI-Cws) aimed at improv- ing the estimation of actual evapotranspiration in forest areas. The improvement concerns the calibration of the Cws meteorological factor, which regulates the sen- sitivity of forest ecosystems to short-term water stress. Such calibration is based on Satellite Application Facility on Land Surface Analysis evapotranspiration products, which are informative on the actual impact of water stress on plant evapotranspira- tion. The original and calibrated versions of NDVI-Cws are applied in two environ- mentally diversified forest areas in Central Italy, where water fluxes were measured by the eddy covariance technique. The first site corresponds to a Mediterranean coastal pine forest (San Rossore) and the second to a mountain beech forest (Collelongo), where water fluxes were measured for the years 2001 to 2005 and 2000 to 2009, respectively. The calibration performed leaves unchanged the model setting for San Rossore while it induces a reduction of the model sensitivity to water stress for Collelongo. The calibrated NDVI-Cws version yields optimal evapotran- spiration estimation accuracies for both study sites; the determination coefficient is around 0.90, the root mean square error is lower than 0.30 mm day −1 and the mean bias error is around 0.01 mm day −1 . These findings indicate that the modi- fied water stress factor is a realistic descriptor of the soil–vegetation–atmosphere response of forest ecosystems to water shortage.