Spatiotemporal variability of evapotranspiration in Suterhland

Abstract

In a semi-arid and water scare country like South Africa with a significant number of consumers of water, it is pivotal to determine the rate of evapotranspiration with a high degree of certainty. Measurements of evapotranspiration can be useful in areas where the demand for water exceeds the supply thereof. Precise measurements of spatiotemporal variability of evapotranspiration are pivotal in disciplines of meteorology, agriculture and hydrology, especially in semi-arid and arid areas where water scarcity is becoming a hindrance on economic welfare and sustainable development. Evapotranspiration models have been developed with a combination of remote sensing and meteorological data inputs. After careful consideration of the various surface energy balance algorithm which include; surface energy balance index, surface energy balance system, simplified surface energy balance index, mapping evapotranspiration with internalized calibration and surface energy balance algorithm for land. The surface energy balance algorithm for land was selected because of its success rate of modelling evapotranspiration in semi-arid areas. The pre-packaged Surface Energy Balance Algorithm for Land model along with Landsat 7 satellite imagery, was used to determine the parameters of the energy balance model and the spatiotemporal variability of evapotranspiration. The spatiotemporal variability of the evapotranspiration quantified with Landsat 7 was validated with the American Society of Civil Engineers Reference evapotranspiration equation (ASCE ETSZ). The study was conducted for the 8th, 24th of December 2009 and the 9th of January 2010 in Sutherland. The SEBAL results showed a good agreement with the ASCE ETSZ results for the 24th of December 2009 with evapotranspiration rates of 0.52mm and 0.29mm, respectively. A good agreement between ASCE ETSZ and SEBAL evapotranspiration measurements was also obtained for the 9th of January 2010, with evapotranspiration readings of 0.44m and 0.37mm, respectively. However for the satellite image acquired on the 8th of December 2009 the evapotranspiration measurements were not in agreement with the point based measurement, with a reading of 0.9mm and 0.3mm, respectively. It was concluded that in order to achieve a high degree of remotely sensed evapotranspiration measurements, point-based instruments which cover a larger spatial area should be utilised.