Abstract
Abstract. Actual Evapotranspiration (AET) is a key component of the water and energy balance and hydrological regime of catchments. A land surface energy balance system model (SEBS) was used to estimate the AET of the 160100-km² Medjerda river basin in Northern Tunisia. This model uses satellite data in combination with meteorological data. In this study, we investigated the sensitivity of the AET model output to five major input variables: the 30-minute Downward Surface Shortwave solar radiation fluxes (DSSF), and Land Surface Temperatures (LST), the roughness height for momentum transfer z0m, and the influence of the spatial resolution of satellite-based Leaf Area Index (LAI) and fraction of Vegetation Cover (FVC) estimates. The DSSF product was validated using a comparison to solar radiation estimates by the Angstrom formula based on in-situ station data. Gaps in the 15-min satellite-based land surface temperature time series were filled using a sinusoidal model on pixels containing meteorological stations. One-half to two standard deviations of the errors of the regression curves were applied to analyse the sensitivity of the SEBS output. Two methods to estimate the near surface aerodynamic parameter z0m were applied and compared. Maps of LAI and FVC derived from two sensors alternatively applied as an input to the SEBS model. A sensitivity analysis, performed in the first decade of May 2010, showed that SEBS model parameterization is quite sensitive in the forestland cover type. The difference can be up to 0.3 mm day−1. For agricultural land areas, representing an important percentage of the Medjerda basin, AET estimations based on the SEBS model proved to be used to satisfy the actual evapotranspiration estimates.
Highlights
Accurate quantification of the amount actual evapotranspiration (AET) is a key element for solving water balance and water use issues in hydrology, agronomy, meteorology and for monitoring the state of the environment such as drought occurrences and water management
To fill gaps of land surface temperature (LST) time series, a sum of sinus functions was fitted for 10 pixels containing meteorological stations of the study area using least squares method
Concerning Downward Surface Short waves Flux (DSSF), three levels of uncertainties of ± onehalf standard deviation and ± one standard deviation and ± two standard deviations, are applied.The results show that changes of AET reach 1 mm day-1 for changes of 1/2 Std in LST in the 10 pixels
Summary
Accurate quantification of the amount actual evapotranspiration (AET) is a key element for solving water balance and water use issues in hydrology, agronomy, meteorology and for monitoring the state of the environment such as drought occurrences and water management. To estimate the aerodynamics parameters (canopy height, roughness length for momentum transfer and zero displacement height) of the SEBS model, Normalized Difference Vegetation Index (NDVI) and Leaf Area Index (LAI) maps are required. The latter along with the fraction of vegetation cover (FVC) are inputs to estimate the resistance term kB-1 in order to assess roughness length for the heat transport z0d as proposed by Su et al (2001). Fraction of Vegetation Cover (FVC) might be used in SEBS for the surface emissivity (Valor and Caselles 1996), and the soil heat flux estimation (Su 2002)
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