Abstract
Understanding the potential impacts of climate variability/change on soil moisture is essential for the development of informed adaptation strategies. However, long-term in-situ soil moisture measurements are sparse in most countries. The objectives of this study were to develop and analyse the temporal variability of a long-term soil moisture data set in South Africa. In this study, a water balance model was used to reconstruct long-term soil moisture data sets from 1980 through 2018, in three sites that represent the diverse agroclimatic conditions of South Africa. Additionally, long-term changes and variability of soil moisture were examined to investigate the potential impacts of climate variability on soil moisture. The results of the Mann–Kendall test showed a non-significant decreasing trend of soil moisture for inland stations at a rate between -0.001 and -0.02 mm per annum. In contrast, a statistically significant (at 5% level of significance) increasing trend of soil moisture for a coastal station at a rate of 0.1131 mm per annum was observed. The findings suggest that the Bainsvlei and Bronkhorstspruit stations located in the inland region are gradually becoming drier as a result of decreasing rainfall and increasing air temperature. In contrast, the Mandeni station located in the coastal region is becoming wetter as a result of increasing rainfall, despite the increase in air temperature. The findings indicate that climate variability is likely to change the soil moisture content, although the influence will vary with region and climatic conditions. Therefore, understanding the factors that affect soil moisture variability at the local scale is critical for the development of informed and effective adaptation strategies.Significance:
 
 Long-term modelled estimates were used to investigate the potential impacts of climate variability on soil moisture in three different agroclimatic conditions of South Africa.
 Results show that inland regions are gradually becoming drier as a result of decreasing trends of rainfall and increasing air temperatures while coastal regions are becoming wetter as a result of increasing trends of rainfall.
 This study indicates that climate variability is likely to change soil moisture, although various regions will be affected differently.
 The development of informed adaptation strategies at the local scale is critical to cope effectively with climate variability.
Highlights
Soil moisture plays a critical role in the partitioning of energy fluxes between the land and the atmosphere through its influence on soil reflectivity, emissivity and thermal capacity.[1,2,3] Soil moisture plays a critical role in the partitioning of rainfall into different components of the water balance, such as runoff, drainage and soil evaporation through its influence on infiltration rate.[2]
Soil moisture is a critical parameter in the forecasting and assessment of weather-induced extreme events such as heatwaves, droughts and floods, which are likely to increase in both frequency and intensity as a consequence of the projected climate change in southern Africa.[10,11,12]
The findings of our study agree with the findings of Pan et al.[18], who reported that soil moisture peaked in February and was minimal in July in the summer regions of South Africa
Summary
Soil moisture plays a critical role in the partitioning of energy fluxes between the land and the atmosphere through its influence on soil reflectivity, emissivity and thermal capacity.[1,2,3] Soil moisture plays a critical role in the partitioning of rainfall into different components of the water balance, such as runoff, drainage and soil evaporation through its influence on infiltration rate.[2]. Soil moisture is a critical parameter in the forecasting and assessment of weather-induced extreme events such as heatwaves, droughts and floods, which are likely to increase in both frequency and intensity as a consequence of the projected climate change in southern Africa.[10,11,12] Analysis of the trends and variability of the long-term soil moisture data set could be used to detect changes in the water cycle associated with climate change and could support climate change modelling and forecasting.[3,4,13,14,15,16,17,18] the long-term soil moisture data set is critical for sustainable agricultural productivity, and efficient management and sustainable use of natural resources within the context of climate change adaptation.[1,16,19,20]
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