Assessment of the impact of landuse changes on the water quality of Incomati River, Southern Mozambique.

Abstract

The growth of socio-economic activities such as irrigated agriculture and urbanization in the Lower Incomati River Basin in Mozambique is putting pressure on water quality in the catchment. The area between Xinavane and Marracuene is covered with high density of aquatic weeds and signals of eutrophication in the river and related lagoons. This could be caused by the excessive use of fertilizers, disposal of untreated wastewater from settlements and sugar industry. This study aims to assess the impact of landuse changes on water quality based on GIS and remote sensing techniques. The study used historical data on water quality from 2000 to 2015 and primary data collected in seven established sampling sites from January to March 2016. Images with less than 10 % cloud cover were downloaded from http://glovis.usgs.gov/, for 2000, 2005, 2010 and 2015, for the off rainy season (July-September). The images were classified in a GIS environment, based on the maximum likelihood classifier algorithm and future projections were done using the Markov Chain analysis through the Terrset software. Results show that EC, nitrates and turbidity are the most variant parameters; with EC varying from 191 to 924 μS/cm, nitrates from 0.08 to 88.63 mg/l and turbidity from 0.5 to 53.3 mg/l. The spatial variation in the water quality results revealed that the sampling site at the drainage of Três de Fevereiro Churamati (SS5) presented the highest parameter values and the sampling site control located in Chinhanguanine (SS1) had the lowest values. The land cover changed significantly from 2000 to 2015 with an increase in settlements by 185 % and irrigation by 162 %. There was a considerable decrease in forestland by 58 % during the same period. Between 2015 and 2030, there is a predicted increase in irrigation by 58 % and settlements by 61 % and a decrease in forestland by 98 %. The study revealed that there is significant relationship between water quality (nitrates) and forestland (-0.97), grassland (-0.95), settlements (0.98) and irrigation (0.65) between 2000 and 2015, showing that these changes in landuse could have caused a deterioration in water quality in the Lower Incomati River Basin. At the same rates of increase, in 2020 the concentrations of pH, nitrates and turbidity are expected to be 8.32, 20.9 mg/l and 11.0 mg/l, respectively; and for 2030 the concentrations of the same parameters are expected to be 8.43, 21.3 mg/l and 11.3 mg/l, respectively. The study concluded that there are significant relationships between landuse and water quality, and it is therefore recommended that there should be an improvement on the monitoring network by increasing the sampling frequency from quarterly to monthly, and by increasing the key parameters which are TSS, TDS and COD. Best practices in irrigated agriculture are encouraged and the implementation of proper environmental management plans for the urban area in order to improve water quality is also recommended.