Dynamics of MODIS evapotranspiration in South Africa

Abstract

This paper describes the dynamics of evapotranspiration (ET) in South Africa using MOD16 ET satellite-derived data, and analyses the inter-dependency of variables used in the ET algorithm of Mu et al. (2011). Annual evapotranspiration is strongly dependent on rainfall and potential evapotranspiration (PET) in 4 climatically different regions of South Africa. Average ET in South Africa (2000–2012) was estimated to be 303 mm·a-1 or 481.4 x 109 m3·a1 (14% of PET and 67% of rainfall), mainly in the form of plant transpiration (T, 53%) and soil evaporation (Soil E, 39%). Evapotranspiration (ET) showed a slight tendency to decrease over the period 2000–2012 in all climatic regions, except in the south of the country (winter rainfall areas), although annual variations in ET resulted in the 13-year trends not being statistically significant. Evapotranspiration (ET) was spatially dependent on PET, T and vapour pressure deficit (VPD), in particular in winter rainfall and arid to semi-arid climatic regions. Assuming an average rainfall of 450 mm·a-1, and considering current best estimates of runoff (9% of rainfall), groundwater recharge (5%) and water withdrawal (2%), MOD16 ET estimates were about 15% short of the water balance closure in South Africa. The ET algorithm can be refined and tested for applications in restricted areas that are spatially heterogeneous and by accounting for soil water supply limiting conditions.

Keywords: MOD16, potential evapotranspiration, soil evaporation, transpiration, water balance

Erratum for Water SA 41 (1) 79–90, originally published in January 2015:

Page 79, line 4 of abstract:
'481.4 x 109 m3∙a‐1' should be replaced with '371.6 x 109 m3∙a‐1'.

Page 83, left column, last paragraph should read (please note the change in units):
'In absolute terms, MOD16 ET estimated an average water loss to the atmosphere of 371.6 x 109 m3∙a‐1 for the country (Table 1). Assuming an average rainfall of 450 mm∙a‐1, corresponding to 553 x 109 m3∙a‐1, it was calculated that 67% of rainfall water evaporates. It was estimated that runoff is about 50 x 109 m3∙a‐1 or 9% of rainfall (DWAF, 2004) and that groundwater recharge is about 30 x 109 m3∙a‐1 or 5% of rainfall (Vegter, 1995; DWAF, 2005). Frenken (2005) estimated water withdrawal in South Africa to be 12.5 x 109 m3∙a‐1 in 2000.'

Page 87, right column, last paragraph (please note the change in units):
'371.6 x 109 mm∙a‐1' should be replaced with '371.6 x 109 m3∙a‐1'.