LSA SAF evapotranspiration and its potential use in hydrological modelling
Evapotranspiration (ET) accounts for the flux of water evaporated at the Earth-atmosphere interface (soil + vegetation + water bodies) and transpired by vegetation through stomata in its leaves as a consequence of photosynthetic processes. ET plays a crucial role in the recycling of precipitation, in soil water availability and in modelling the hydrological cycle components, especially in arid regions.
In hydrological models, ET is one of the key input parameters in addition to precipitation. The output of the hydrological system are forecasted time series, i.e. of river discharges or groundwater levels, for several days ahead with daily or more frequent forecast updates.
In hydrology modelling, ET may be assessed from different sources. Some hydrological models use climatological values of potential ET. A significant improvement to this can be achieved by using actual values of ET, e.g., by using LSA SAF half-hourly or daily actual ET products (LSA-311 and LSA-312, respectively) based on Meteosat geostationary satellite data.
In the figures below, total monthly ET as based on LSA SAF daily evapotranspiration (DMET) data is illustrated displaying patterns of regional variations. Monthly ET totals are shown for May 2019 and May 2020 (Fig. 1-2).
Figure 1: Total monthly ET (in mm) as based on LSA SAF DMET, May 2019.
Figure 2: Total monthly ET (in mm) as based on LSA SAF DMET, May 2020.
In May 2019, total monthly ET was much lower than climatological values over most of Europe with the exception of Iberian peninsula and Greece (Fig. 3). Inclusion of climatological ET values in hydrological models in such situations would result in significant systematic errors of forecasted river discharge. In May 2020, total monthly ET was closer to normal in Central Europe, below normal in parts of Eastern Europe and above normal in the Iberian peninsula (Fig. 4).
Use of actual ET in hydrological modelling has an impact on the final results of forecasted river discharge. LSA SAF provides a suite of satellite based ET products. Particularly in cases when actual ET significantly deviates from climatological values, satellite based ET data should significantly improve the results of hydrological models.
Figure 3: Monthly accumulation anomalies of ET (in mm) for May 2019, reference years are 2011-2018.
Computed as May 2019 - reference years May. Higher values indicate that ET in May 2019 was higher than average ET of reference years.
Figure 4: Monthly accumulation anomalies of ET (in mm) for May 2020, reference years are 2011-2018.
Computed as May 2020 - reference years May. Higher values indicate that ET in May 2020 was higher than average ET of reference years.