Richard Dh Bugan

Rainfall, Soil Water Content, and Groundwater Levels at the Riverlands Nature Reserve (South Africa)

Quantified medium- and long-term hydrological datasets are scarce in South Africa, yet they are essential to gain understanding of natural systems, contribute to ecosystem conservation, and ultimately quantify water balance processes accurately. A hydrological experiment was carried out at Riverlands Nature Reserve (Western Cape, South Africa) in order to quantify the components of the soil water balance at experimental sites occupied by endemic and invasive vegetation. In two separate follow-up projects, five-year time series were collected in three treatments, namely, endemic fynbos vegetation, bare soil, and land invaded by Acacia saligna. Rainfall was recorded daily with a manual rain gauge. Groundwater levels were logged hourly at 14 boreholes. Volumetric soil water contents and soil temperatures were logged hourly at different depths in the soil profile. Groundwater levels and soil water contents responded to rainfall with very clear seasonal trends. The data can be applied in water balance and evapotranspiration studies, unsaturated flux studies, soil temperature profile studies, and rainfall-groundwater level response analysis and for calibrating and validating a wide range of hydrological models.

Hydrosalinity fluxes in a small scale catchment of the Berg River (South Africa).

The occurrence of dryland salinity is widespread throughout semi-arid regions of the world. The sources of salts may be either rock weathering or rain deposition. Clearing of natural scrubland to make way for cultivated crops and pastures may also change the water balance, trigger salt mobilization and increase the salinity of water resources. These processes are suspected to be the main cause for salinization of the Berg river catchment (South Africa). The objective of this study was to determine the hydrosalinity fluxes associated with overland and subsurface (vadose zone) flow for different soils and land uses. For this purpose, the following data were collected during 2006 in a typical small scale catchment (~40 ha) located near the town of Riebeek West: weather data, hydrological and water quality measurements, soil water contents and chemistry. The climate of the area is Mediterranean with winter rainfall of about 350 mm a. The chemical speciation of water and soil solution in the catchment is conservative, with Na and Cl being the dominant ions. Soil water and salt contents varied seasonally. Due to the typical low intensity of rainfall, the fluxes of salts during individual runoff events were steady. Fluctuations in salinity due to local processes were buffered at a catchment scale. Uncultivated (bare) soil produced more runoff and higher salinity compared to pasture land. Overland flow varied between 3 and 18% of rainfall, mobilizing up to 24 g m of salts during 2006, depending on soil properties, slopes, rainfall intensity and duration, and antecedent moisture conditions. Subsurface fluxes of water and salts were estimated with the HYDRUS-2D model. Management practices at farm scale are required in order to reduce salt mobilization and salinization at catchment scale.

Validation and downscaling of Advanced Scatterometer (ASCAT) soil moisture using ground measurements in the Western Cape, South Africa

Satellite-based remote sensing of soil water content (SWC) is a promising technology for hydrological applications to overcome large spatiotemporal variabilities of SWC. This study investigated the performance of the Advanced Scatterometer (ASCAT) soil moisture product on METOP satellite (∼12.5 km and downscaled to ∼1 km resolution), against ground measurements of SWC taken with a Hydrosense II probe along transects of 360–820 m on agricultural and natural land at locations in the Western Cape. The ASCAT products estimated fairly accurately seasonal trends of SWC; performance was better on lower slopes (R2 = 0.66) and uniform vegetation. ASCAT 12.5 km performed better in estimating SWC than the downscaled product (average concordance coefficient = 0.60 and 0.39, and R2 = 0.84 and 0.74, respectively). ASCAT 12.5 km was more responsive to rainfall events, whilst the downscaled product was more sensitive to vegetation characteristics (normalised difference vegetation index and land surface temperature). In situations with ground measurement networks and data availability constraints, remote sensing could be a feasible alternative to monitor SWC for hydrological applications at the meso-scale (regional scale).

Generation and size distribution of sediment eroded in a small-scale catchment of the Western Cape (South Africa)

Rainfall, runoff (overland flow), total sediments mobilized and their size distribution were measured for a number of events over three years (from 2006 to 2008) in a small-scale catchment of the Berg River (Western Cape). The aim was to develop local-scale rainfall/runoff, sediment generation and particle size distribution relationships that could be incorporated into existing hydrological catchment models for predicting erosion, transport and deposition of sediments with sorbed contaminants. The results indicated that rainfall intensity was weakly correlated to rainfall amounts (R 2 = 0.54), whilst runoff (between 12.4% and 18.2% of rainfall) was weakly correlated to the product of rainfall amount and peak intensity at two sites with different soil properties and slopes (R 2 = 0.29 and 0.44), depending on antecedent moisture condition, land use and management practices. Sediment concentration was inversely correlated to runoff volume. However, more runoff produced larger total sediment loads. The bulk of sediment particles mobilized via runoff water had diameters in the range between 4.76 and 15.95 μm (measured with a Saturn DigiSizer 5200 particle size analyzer). Sediment size distribution was relatively uniform for all events recorded, did not depend on sediment concentration, runoff, rainfall amount and intensity, and it was similar to the texture of the soil A-horizons (source of sediments). Spatial information on soil A-horizon textural properties could therefore be used to infer the nature of sediments mobilized in the catchment.

A Comparative Analysis Of The PRMS And J2000Hydrological Models Applied To The SandspruitCatchment (Western Cape, South Africa)

The applicability of distributed hydrological models to the semi-arid conditions in the Western Cape was investigated through the application of PRMS and J2000 in the Sandspruit Catchment. The Sandspruit is an annual river, with the catchment receiving 300-400 mm/a of rainfall. The catchment exhibits shallow soils, with the dominant land uses being cultivated lands and pastures. To optimise the parameterisation of the models, 21 boreholes were drilled throughout the catchment for data collection and to get a better conceptual understanding of the catchment’s hydrologic conditions. Field evidence suggests that subsurface flow is the dominant contributor of streamflow and thus the models were calibrated accordingly. The models were run for a 20 year period. Both models were able to match the timing of seasonal hydrograph responses, however they were not able to match annual discharge volumes. Annual discharge was overestimated in certain cases and underestimated in others. Both models exhibited daily Nash-Sutcliffe Efficiencies of below 0.4. As the models were parameterised and calibrated manually, the feasibility of using automatic techniques needs to be investigated.