Evapotranspiration (ET) links the cycles of water, energy and carbon. It is an important indicator of vegetation health and water scarcity, because when vegetation is experiencing water stress it is not transpiring at its potential rate, which only depends on available energy. For this reason, monitoring ET is essential for managing agriculture and forestry in a context of climate uncertainty, especially in mountainous regions, which are particularly vulnerable to climate change because temperature is increasing faster than the global land average.
Remote-sensing based procedures have been developed for modeling ET from thermal imagery. However, due to the lack of high resolution thermal data in the Alps, at present it is difficult to reach a resolution above 1 km. For this reason, in addition to exploiting thermal-based algorithms (TSEB ALEXI/DisALEXI) at moderate resolution, an alternative method is implemented, based on optical data available from the Sentinel-2 MSI radiometer at a resolution of 20 m.
The ET product, which is under development at present, will consist of maps of daily ET (mm day-1) for South Tirol in correspondence of every Sentinel-2 overpass, for cloud-free pixels. ET is modelled by the dual crop coefficient method (Allen et al., 1998, Allen et al., 2009), modified for allowing its application in wide areas, characterized by natural or managed environment, in presence or absence of water stress (Maselli et al., 2014). Actual ET is calculated by multiplying a weather-based estimate of potential ET, ETo, by a function of vegetation indices, capturing the effect of crop growth stage on evaporative demand. Vegetation indices, like NDVI and SAVI, are derived from Sentinel-2 MSI radiometer surface reflectance in the visible and near infrared. The effect of short-term water stress is considered through a balance between cumulated precipitation and potential ET.
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. FAO, Rome, 300(9), D05109.
Allen, R. G., & Pereira, L. S. (2009). Estimating crop coefficients from fraction of ground cover and height. Irrigation Science, 28(1), 17-34.
Maselli, F., Papale, D., Chiesi, M., Matteucci, G., Angeli, L., Raschi, A., & Seufert, G. (2014). Operational monitoring of daily evapotranspiration by the combination of MODIS NDVI and ground meteorological data: Application and evaluation in Central Italy. Remote Sensing of Environment, 152, 279-290.