Application of GIS and Remote Sensing techniques to evaluate the impact of land cover and land use changes on the hydrology and water resources of Luvuvhu River Catchment in Limpopo Province, South Africa.

Abstract

Luvuvhu River Catchment (LRC) exhibits diverse land use and land cover patterns that are influenced by seasonality and socio-cultural practices of the local communities. From 1950, the catchment has been undergoing land cover changes caused by expanding villages, new urban centres and clearing forest land for agriculture. Conversion of natural landscape for agricultural and urban purposes degraded the catchment by negatively affecting the hydrologic processes. This study was therefore conducted to evaluate the impact of land cover and land use change on the hydrology and water resources of LRC. Geographical Information Systems (GIS) and remote sensing techniques were applied to evaluate the impact of the changes on the catchment. Remotely sensed imagery was used as the primary sources of data for classification and detection of changes. Digital Elevation Models (DEMs) were used for hydrologic and geomorphic modeling in combination with information from remotely sensed imagery. Field data sets for soil and meteorology were obtained from selected sampling segments, based on the area frame sampling. The method of direct expansion was used to quantify land use classes. Flood frequency was analysed using probability distribution methods at recurrence intervals of 2, 5, 10, 20, 25, 50, 100, and 200 years. The FAO CROPWAT software based on Penman-Montheith equation was used to assess the impact of land cover changes on evapotranspiration regimes. To study the hydrological response of land cover change in the catchment, the Soil Conservation Services-Curve Number (SCS-CN) method was first used independently to simulate surface runoff and investigate the impact of land use change on runoff under historical land cover regimes. The Soil and Water Assessment Tool (SWAT) model was then applied in the Tshakhuma-Levubu subcatchment to assess the impact of land management practices on the soil and water bodies in the catchment. The results indicated that changes were having negative impacts on the hydrology of the catchment. The impact of land use and land cover change on hydrology of LRC was manifested in stream flow, surface runoff, suspended sediment and flood frequency and magnitudes. There was significant land cover and land use change from forestland, woodland and open grassland to medium size farms, subsistence agriculture and built-up land. These developments were concentrated on hillsides and hilltops in the catchment and they were of concern as they were ix impacting on the hydrological processes. Throughout the 2000’s, land use change revealed a decrease in natural forest from 32.15% to 20.67%, giving rise to agriculture which rose to 38.57% in 2010. Runoff was observed to be highly variable during the month of February with maximum runoff records of 1.63 m3 and 3.84 m3 upstream and downstream, respectively. Flood frequency results showed that an increase in the peak discharges was to be expected, especially for the discharge range corresponding to smaller and medium flood magnitudes. The use of imagery and DEMs within GIS was found to efficiently represent ground surface and allow automated extraction of features, thus bringing advantages in terms of processing efficiency, cost effectiveness, and accuracy assessments. This technique could therefore be adopted to improve land use planning, water management, and rapid identification of slopes and elevations in consideration for their functional and structural requirements. Analysis showed that the SWAT model was suitable for predicting the location and extent of pollution in the catchment. It assumed sheet and rill erosion as the dominant erosion type contributing to siltation and water pollution in rivers. The study recommends close monitoring and sustained enforcement of the rural land use regulations to prevent the conversion of land to urban land use.