Spatial and temporal dynamics of soil moisture for surfaces with a change in land use in the semi-arid region of Brazil

Abstract

The semiarid region of Brazil undergoes constant changes in land use due to the process of deforestation and high water seasonality, represented among other variables by a decline in the soil moisture content. In this study, the influence of land use on the soil moisture dynamics is analysed. For this purpose, three experimental sites were evaluated, characterised by different types of land use: an agroecosystem of forage cactus, a deforested environment and an area of Caatinga vegetation, located in the semiarid central hinterlands of Brazil. Calibration of the Diviner2000® capacitive probes was carried out using the relationship between the soil volumetric moisture content (θv), obtained by the gravimetric method, and the relative frequency readings of the probes. Weekly measurements of the moisture content were taken at 22 points at depths of 0.00–0.60 m from November 2014 to October 2017, providing 157 sampled days. The mean values and standard deviations of the θv variable for the three surfaces were analysed over twelve periods, and grouped according to the local water regime into rainy, dry and transitional. To analyse the temporal stability of θv, the relative difference and the Spearman correlation matrix were calculated. Finally, multivariate principal component analysis (PCA) was used to identify associations between the mean values of θv and physical properties in the different soil layers. Based on the results, the local calibration curves presented significant coefficients of determination, showing that the equations can be used for reliable estimates of θv. The θv was higher in the Caatinga (0.086 m3 m−3), intermediate in the forage cactus (0.064 m3 m−3) and lower in the deforested area (0.045 m3 m−3). Changes in land use influence the spatial variability of θv in the soil layers, which responded to the local rainfall regime, especially in the upper layers (0.05–0.25 m). Significant differences in θv between soil surface coverage occurred during the dry sub-periods and the transition periods, when the Caatinga was superior to the other areas. The temporal stability analysis identified the forage cactus area as the most representative location for θv estimates, while the Spearman correlation matrix detected the more-unstable areas of moisture, with persistent moisture patterns between the dry sub-periods and transition periods only. Associations between moisture and different soil properties were affected by soil surface coverage and soil layer depth. All the proprieties explained moisture distribution along the profile. It can be concluded that converting the Caatinga vegetation into other soil surface coverages changes soil-moisture distribution patterns, resulting in a reduction in θv of 26% and 47% for forage cactus and deforested areas, respectively. Therefore, the establishment of forage cactus in current deforested areas of the Brazilian semiarid may attenuate the temporal variability and reduction of soil moisture compared to bare soil surfaces.