Non-point source pollution processes and connectivity modelling in the Mkabela Catchment, South Africa.
Kollongei, Kipkemboi Julius.
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In South Africa water resources and water quality issues are becoming increasingly important as the country manages its scarce water resources. The National Water Act of South Africa Act 36 (1998) stipulates that water resources must be shared in a sustainable fashion among humans, environment and economic land uses. Deterioration of water qualities in rivers are not only unique problems to South Africa. Indeed the degradation of water quality by nutrients originating from agriculture through excessive use of fertilizers (NO₃ and P) and erosion (sediments) is an international environmental concern. Rivers passing through agricultural areas experience high pollution levels from non-point sources resulting from these agricultural activities. Dealing with this issue is not straight forward because the agricultural contribution to diffuse pollution varies widely as a complex function of soil type, climate, topography, hydrological connectivity, land use and management. This creates widespread, intermittent, and poorly defined contaminant sources that degrade water quality in a way that makes their control difficult. In recognition of the accelerated degradation of water bodies from agricultural Non-Point Source (NPS) pollutants, watershed models have evolved from traditional hydrology models to more comprehensive water quality models. Diminished use or even loss of the water resource for other beneficial uses has resulted from over burdening of the receiving waters with waste fertilizers from agriculture. For example, many surface and groundwater bodies used as a water supply have lost their utility due to agricultural pollution. Upstream users of land do not feel the economic impact of their action on the downstream users who must use water from other sources because the water quality of their source was rendered unusable. Farmers respond to their economic realities, i.e. they want to make at least some profit or at least survive during harsh economic situations, both of which can be accomplished by increased crop yields. Without some intervention in the farmers’ economic reasoning, the potential water quality problems far downstream is not a part of the farmers’ decision making process with regards to how much fertilizers they will use on their land or how to dispose of their animal waste. The new ACRU-NPS (Agricultural Catchment Research Unit- Nitrogen, Phosphorous and Sediments) model was developed to try to address these challenges. The model was configured using a nested approach, which requires an effective description of all relevant components of the system and an understanding of the processes and feedbacks taking place within and between different scales and hydrological processes response zones. The concept of connectivity in the model was introduced to improve simulations. Connectivity defines the physical coupling of landforms (e.g. hillslope to channel) within a catchment where the passage of water from one part of the landscape to another is expected to generate a catchment runoff response that may carry along with it dissolved pollutants, sediments and any contaminants that they may carry through the drainage basin. The connectivity of the river (drainage) network in the Mkabela Catchment (near Wartburg, KwaZulu-Natal) was assessed on a sub-catchment basis and was linked to in-stream controls that included farm dams, wetlands and buffer zones where the fate and transport of dissolved N and P, sediment and associated adsorbed P were studied. A new method of calculating crop yield, different from that used in the SWAT (Soil and Water Assessment Tool) model, was developed and incorporated in the ACRU-NPS model. In the new method water and nitrogen stresses were used to limit crop growth on a daily time step. This enabled consequences of subsequent nutrient and sediment loads in streams to be studied. The major limitation to long-term use of SWAT in South Africa is the lack of long-term nutrient and suspended solids data for calibration and validation. This study utilized the new ACRU-NPS modelling approach to study pollutants emanating from the Mkabela Catchment in South Africa. The developed ACRU-NPS model included sufficient process details to allow for the implementation of controls such as wetlands, dams and buffer strips. Successful simulation of crop yields, nutrient and sediment production, together with the fate of NPS pollutants, for various land uses was thus achieved. The major contribution of this study however was to link hydrology and NPS pollution processes by describing and defining pathways through which pollutants moved in the catchment. This was achieved through studying the dynamics and connectivity of water, sediments and nutrient fluxes by combining hydrometric, hydropedological, geophysics and stable water isotope techniques to interpret the field and laboratory data. Suggestions for future improvements on the ACRU-NPS model were given based on the understandings gained from the different observations and sampling done in the Mkabela Catchment.
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