Nitrate-Nitrogen Pollution and Attenuation Upstream of the Okavango Delta in Angola and Namibia

Abstract

Development plans in the Okavango River Basin in Angola and Namibia will abstract 18.13% of mean annual flow for urban water supply and irrigate 108,992 hectares of infertile Kalahari sandy soils requiring high fertilization rates. This alludes to high leaching and hence nutrient pollution of the river system. The loads of nitrates leached from Namibia’s existing 4,000-hectare irrigated area into the river’s reaches are unknown. Hence, a unique simulation model was developed by combining a gross nitrogen balance, river nitrate transfer and CROPWAT 8.0 models, for estimating quantities of nitrates in agricultural leachate and urban effluent, nitrates transported into the river’s mainstream and river reach nitrate attenuation and transfer and to predict future mainstream nitrate levels. The model showed that 7,000 m3/ha/year was leaching 23% and 18% of applied N fertilizer in wheat and maize crops, respectively, which had negligible impact on river nitrate levels. Irrigating at most 15,659 hectares in Namibia will increase mode nitrate levels from the long-term 0.5 mg/L to 0.6 mg/L. Okavango Delta inflow nitrate levels will increase above 1.0 mg/L if 17.49% of the mean annual flow is abstracted for irrigation and 60 million m3/year is abstracted for Namibian urban water supply.