Dynamics of MODIS evapotranspiration in South Africa

Abstract

This paper describes the dynamics of evapotranspiration (ET) in SouthAfrica using MOD16 ET satellite-derived data, and analyses theinter-dependency of variables used in the ET algorithm of Mu et al.(2011). Annual evapotranspiration is strongly dependent on rainfall andpotential evapotranspiration (PET) in 4 climatically different regionsof South Africa. Average ET in South Africa (2000-2012) was estimated tobe 303 mm.a(-1) or 481.4 x 10(9) m(3).a(1) (14% of PET and 67% ofrainfall), mainly in the form of plant transpiration (T, 53%) and soilevaporation (Soil E, 39%). Evapotranspiration (ET) showed a slighttendency to decrease over the period 2000-2012 in all climatic regions,except in the south of the country (winter rainfall areas), althoughannual variations in ET resulted in the 13-year trends not beingstatistically significant. Evapotranspiration (ET) was spatiallydependent on PET, T and vapour pressure deficit (VPD), in particular inwinter rainfall and arid to semi-arid climatic regions. Assuming anaverage rainfall of 450 mm.a(-1), and considering current best estimatesof runoff (9% of rainfall), groundwater recharge (5%) and waterwithdrawal (2%), MOD16 ET estimates were about 15% short of the waterbalance closure in South Africa. The ET algorithm can be refined andtested for applications in restricted areas that are spatiallyheterogeneous and by accounting for soil water supply limitingconditions.