Estimation of Spatio-Temporal Variability of Actual Evapotranspiration Using the Sebs Model in the Semiarid Barotse Basin, South-Western Zambia

W.K. Phiri; I.A. Nyambe; J. Kabika; Z. Vekerdy; T. Woldai

doi:10.53974/unza.jonas.1.2.352

W.K. Phiri University of Zambia, IWRM Centre, C/O School of Mines, P.O. Box 32379, Lusaka, Zambia
I.A. Nyambe University of Zambia, IWRM Centre, C/O School of Mines, P.O. Box 32379, Lusaka, Zambia
J. Kabika University of Zambia, IWRM Centre, C/O School of Mines, P.O. Box 32379, Lusaka, Zambia
Z. Vekerdy Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, The Netherlands
T. Woldai Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, The Netherlands

DOI: https://doi.org/10.53974/unza.jonas.1.2.352

Keywords: Actual evapotranspiration, Spatio-temporal variability, Aerodynamic Roughness, SEBS, MODIS

Abstract

vapotranspiration (ET) is a dominant hydrologic loss flux in the water budget of arid and semi-arid areas. Thus, accurate estimation of its dynamics is critical for assessing water availability and improving water resources management in such areas. In this study the physically-based Surface Energy Balance System (SEBS) model was applied to estimate spatio-temporal variability of actual ET (AET) in the semiarid Barotse basin, South-Western Zambia. The model was run using atmospherically rectified MODIS satellite imagery on clear-sky warm-wet, cool-dry and hot-drydays. Furthermore,based on sunshine hours and daily AET, monthly fluxes were generated. The outputs were evaluated against potential ET (PET) and independently modelled AET from the global circulation model of the European Centre for Medium-range Weather Forecast (ECMWF). It was observed that the ratio of AET to PET at the reference station was in the order of 1.04, 0.64 and 0.30 on warm-wet, cool-dry and hot-dry days respectively. Systematic lack of physical agreement on warm-wet days suggested that SEBS estimates were not necessarily implausible but that assumptions on which PET isbased differed from surface conditions. ECMWF estimates were in better agreement with SEBS at daily and monthly time-steps at Sesheke station than at Kamanga. This was ascribed to input data and vegetation index-based roughness parametrisation. Sensitivity analysis of the model to landuse-based versus NDVI aerodynamic roughness revealed a reduction of fluxes of up to 1.5 mm day-1 on forests using the latter. Flux analysis showed that water bodies and regularly flooded vegetation had the highest rates of 6.9 and 5.9 mm day-1 on warm-wet days respectively. The lowest occurred on croplands and grasslands with a high variation between warm-wet and hot-dry days of up to 64.1 and 71.1% respectively. It is concluded that SEBS model can accurately estimate AET in heterogeneous areas with spatial input data and robustly determined roughness values.

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