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Abstract
Increasing nutrient loads from land use and land cover (LULC) change degrade water
quality through eutrophication of aquatic ecosystems globally. The Vaal River Catchment
in South Africa is an agriculturally and economically important area where eutrophication
has been a problem for decades. Effective mitigation strategies of eutrophication
in this region require an understanding of the relationship between LULC change and
water quality. This study assessed the long-term impacts of LULC changes on nitrate
(NO3-N) and orthophosphate (PO4-P) pollution in the lower Vaal River Catchment between
1980 and 2018. Multi-year LULC was mapped from Landsat imagery and changes were determined.
Long-term trends in NO3-N and PO4-P loads and concentrations in river water samples
were analysed, while multi-year LULC data were ingested into the Soil and Water Assessment
Tool (SWAT) to simulate the impacts of LULC changes in NO3-N and PO4-P loads. Main
LULC changes included an increase in the irrigated area by 262% and in built-up area
by 33%. This occurred at the expense of cultivated dryland fields and rangelands.
In situ data analysis showed that at the catchment inlet, PO4-P concentration and
loads significantly increased, while NO3-N concentration and loads decreased between
1980 and 2018. At the catchment outlet, only PO4-P loads increased, while NO3-N loads
and concentrations remained the same. SWAT simulations at the Hydrologic Response
Unit scale showed that irrigated land was the largest contributor to NO3-N leaching
per ha. Aggregation of nutrient loads by LULC type showed increased nutrient loads
from irrigated and built-up areas over time, while loads from dryland areas decreased.
At catchment scale, dryland remained an important contributor of the annual nutrient
loads total because of its large area. In future, research efforts should focus on
crop management practices to reduce nutrient loads.