Analysis of monthly MOD16 evapotranspiration rates at sites with different climatic characteristics; Heuningnes and Letaba catchments in South Africa
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Evapotranspiration (ET) is an important component of the water cycle that is estimated to return about 60% of precipitation back to the atmosphere. Actual ET can be estimated using remote sensing techniques and ground-based measurements. In recent years, a remote sensing product MOD 16 ET has been developed. The limited validation of this product done in South Africa showed that ET was underestimated at some sites. A comprehensive analysis of historic and seasonal trends in MOD 16 ET data in different climatic regions of South Africa has not been done. This study has the objective of evaluating if MOD 16 evapotranspiration estimates realistically represent the seasonal variations of ET on different land cover types in two different climatic regions; Mediterranean (Heuningnes catchment in Western Cape) and Sub-tropical (Letaba catchment in Limpopo) regions. Monthly MOD 16 ET maps for 2000-2012 for the Letaba catchment and Heuningnes catchment were created using ArcGIS. The results suggested that during the 2000 - 2012 period, ET was 438 - 753 mm/ year in the Letaba catchment and 432 - 458 mm/year in the Heuningnes catchment. The accuracy of MOD 16 ET was evaluated using estimates of actual ET from scintillometer data in Elandsberg (Western Cape) and flux tower data in Malopeni (Limpopo) and Skukuza (Limpopo). Monthly ET estimated using scintillometer and flux tower were calculated to coincide with the monthly MOD 16 ET data for a period of 1 year from Nov 2012- Oct 2013 in Elandsberg, 1 year 1 month from Mar 2009- Mar 2010 in Malopeni and 13 years from 2000- 2012 in Skukuza. In Elandsberg, the results showed that MOD 16 underestimated ET (R2 = 0.16, RMSE = 28.30 mm/month). In Malopeni, the results suggested that there is a strong relationship between ET estimated from flux tower data and MOD 16 ET (R2 = 0.77), but MOD 16 slightly overestimate ET (RMSE = 8.6 mm/month). MODIS ET estimates for Elandsberg had a poorer comparison with the results obtained at Malopeni and Skukuza.