Abstract:
The effect of evapotranspiration on river catchments needs to be consistently monitored as water availability and usability affects the livelihood and health of all living organisms. The consistent monitoring of water quantity and quality at scale using ground based methods can however be costly and unsustainable. Alternative methods of monitoring such as Remote Sensing thus need to be explored and tested. The main aim of this study was to assess the seasonal water loss and variation in water quality in the Modder River due to evapotranspiration by using remote sensing techniques. Landsat 8 satellite images and data were used to compute remote sensing indices, such as the Normalised Difference Water Index and the Normalised Difference Vegetative Index, seasonally over three years (2017–2019). The remote sensing indices were used to evaluate the surface water area across the Mockes and Krugersdrift dams which are part of the Modder River catchment. The evapotranspiration values were then assessed against the surface water area. Seven water samples were taken across several points on the Modder River and the chemical and physical parameters of pH, electrical conductivity, total dissolved solids, calcium cations, magnesium cations, sodium, potassium, chlorine, nitrates, and sulphates, were used for calculating the seasonal Water Quality Index. The seasonal Water Quality Index was analysed against evapotranspiration, as well as the Normalised Difference Vegetative Index. The Pearson regression test results gave a strong r value of -0,9 between evapotranspiration and water quality relationship as well as a weak r value of -0.22 between evapotranspiration and surface water quantity, respectively. The spring and summer seasons had the maximum and minimun seasonal evapotranspiration average values of 10.1 mm and 7.8 mm respectively with corresponding poor water quality indices of 62.6 and 53.5 respectively. The average seasonal Normalised Difference Vegetative Index values for the Summer, Autumn, Winter, and Spring seasons across the 3 years were 0.34, 0,38, 0.22, and 0.26, respectively. The study outcome showed that potential evapotranspiration does have a significant effect on water quality but showed a weak correlation with surface water quantity. Furthermore, the study proved that remote sensing techniques can be used in assessing surface water quality and quantity parameters.
