Determination of sediment, water quantity and quality for swat modelling of sedimentation in the Makoye reservoir, southern province, Zambia.

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Muchanga, Manoah
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University of Zambia
Reservoir sedimentation is one of the temporally and spatially distributed challenges facing managers of small reservoirs today. Fluvial system formed the geomorphological plinth of the study. The study was motivated by the problem of sedimentation in the Makoye Reservoir (about 60500 m2), which had been affecting 474 pastoralist households rearing over 10,000 cattle. The objectives of the study were to: (i) determine the bathymetry of the Makoye Reservoir at different temporal scales; (ii) measure the long-term quantity of sediment deposited in Makoye Reservoir; (iii) determine the short-term real time sediment settling rate in the Makoye Reservoir; (iv) examine concentration levels of selected physical and chemical parameters of water for livestock in Makoye Reservoir; (v) evaluate the efficiency of Soil Water Assessment Tool in simulating sedimentation in the Makoye Reservoir; and (vi) to develop a conceptual model for understanding sedimentation process in small reservoirs in Zambia. The study used Critical Analytical Experimental Research Design implicitly inspired by Critical Empirical Analytic Paradigm. Five bathymetric surveys were conducted using a Remote Controlled Hydrographic Survey Boat with the aid of an inflatable boat. Sediment pits (195) were dug across the dry reservoir bed with aid of picks, ranging poles, measuring tape, Differential Global Position System and iron pegs. Real time sediment depth was measured using SediMeter SM3A, whose 36 Optical Backscatter Detectors captured sediment depth with resolution of 0.001 mm. The data inputs for Sediment simulation included the 90m Shuttle Radar Topography Mission Digital Elevation Model (SRTM DEM) , weather data, soils and landuse maps. Three soil samples (50cm) were collected using augers, and suspended and settled sediment samples were collected using grass carpets and coring. Bathymetric data was analysed using 3D Spatial Analysts Tools (3DSATs) in ArcGIS 10.3 and spreadsheet Microsoft Excel. This enabled determination of volumes, surfaces areas and development of hypsometric curves showing relationship among water depths, volume and surface areas. Real time sediment data was analysed using descriptive statistics and time series. Simulated sediment data was analysed using SUFI-2 in SWATCUP 2012. Soils, sediment and water physico-chemical analysis were done in the Soils Sciences and Environmental Engineering laboratories at the University of Zambia, respectively. Seasonal comparison of reservoir's bathymetries and water volumes showed drastic changes in average depths and volumes of water (24,830.93 m3 to 75,974.21 m3). This supply was below the water demand for cattle due to diverse physical processes (weather conditions, drainage hydro-geomorphology and mainly, sedimentation). On average, the real time daily sediment settling rate was 0.0003 m/day. Between 1988 and 2017, the average rate of long term sedimentation was 5,834.12 tonnes/year. SWAT efficiently simulated sediment with both r2 and NSE at 0.77 and 95PPU at 57 percent. Sediment was sourced from Agricultural land (35%) grazing land (26%), deciduous forest (22%) and range-brush land (17%). Water quality was influenced by sediment upstream and 80 percent of its chemicals parameters were within Maximum Permissible Limits for cattle. The study designed a conceptual model on understanding and simulating sedimentation by integrating sediment depths from Sedimeter SM3A and regression model, which can be adapted to different spatial and temporal contexts. Conclusively, the reservoir was highly silted with about 54 percent of its capacity reduced. Community and government agencies awareness on how to reduce sedimentation in the catchment is highly recommended. Key Words: Bathymetry, Geomorphology, Sedimentation, Sedimeter SM3A, SWAT Modelling
Reservoir sedimentation , Makoye Reservoir--Southern, Zambia