Analysis of the spatio-temporal variability of soil water dynamics in an arid catchment in South Africa

Abstract

The accurate quantification of soil water dynamics is critical as it is a key parameter in the soil-plant-atmosphere continuum. This study was conducted to analyse the spatio-temporal variability of soil water dynamics in dry areas. The Soil and Water Assessment Tool (SWAT) was used to estimate the dynamics of soil water balance. The model was calibrated and validated by the regionalization with physical similarity approach. Unlike the revap water (water drawn upwards from shallow groundwater tables), the soil water, percolation and lateral flow increased with precipitation. The Mann Kendall trend test also shows that there was no trend in both the annual and monthly soil water content (SWC) and percolation; however, there was a decreasing trend (1.091 mm per annum) in the revap water. The contribution of the revap water to evapotranspiration (ET) varied from 9% during summer to 32% during spring. The SWC also exhibited a higher spatial variation. Larger part of the catchment recorded a long-term annual mean soil water content varying between 0.1 and 0.2 mm mm−1. The soil water anomaly (SWA) consistently follows the trends of dry and wet seasons in the study area. Although the use of model-derived drought monitoring indices are relevant for farmers and decision makers in arid areas, it requires intensive verification over much larger scales to increase the reliability of model results.