Assessment of Catchment Erosion, Sedimentation and Nutrient Export into Small Reservoirs from their Catchments in the Upper East Region of Ghana

Soil erosion is one of the most important forms of land degradation that threatens continued and sustained agricultural production in Ghana. The most severely affected areas are the three northern savanna regions, particularly the Upper East Region, where large tracts of land have been destroyed by water erosion leading to soil depth reduction, soil fertility decline and siltation of rivers and reservoirs. This study was carried out in the Upper East Region of Ghana to assess the rate of sedimentation and nutrient export into five small reservoirs and their implications. The five reservoirs studied were Dua, Doba, Zebilla, Kumpalgogo and Bugri. The rate of sedimentation was determined by bathymetric survey. The catchment soils and reservoir sediments from the various study sites were sampled and analyzed for their bulk density, particle size and nutrient content. The results, analyzed by Analysis of Variance (ANOVA), showed sediment deposit in the reservoirs, to range from 55,413 to 11480 Mg. The amount of sediment deposit followed the trend of Kumpalgogo> Bugri >Dua>Zebilla>Doba and varied significantly between the reservoirs. The annual rate of siltation (RS) ranged from 1276 to 6157 Mg y-1 for the reservoirs. Sediment yield varied significantly among the reservoirs with a range of 1299 to 6302 Mg y-1. The results showed that the reservoirs have lost the dead storage capacity designed to store sediment until their anticipated design life. This could adversely affect the long dry season occupation and the various benefits small reservoirs bring to the local inhabitants. The area specific sediment yield (ASY), taken as proxy for catchment erosion, ranged from 18.28 to 157.55 Mg ha-1y-1 with a mean of 64.74 Mg ha-1y-1. The magnitude of ASY was in the order of Kumpalgogo>Dua>Zebilla>Doba>Bugri with considerable variation among the reservoirs. The ASY in this study was higher than the tolerable soil loss of 2 to 18 Mg ha-1 y-1 for the tropics but fell within the range of 10 - 200 Mg ha-1 y-1 typical of savanna ecosystems. ASY increased as catchment area decreased. Although the large catchments may produce more total sediments than the small ones, they also provide a longer travel distance for sediment transport and opportune time for sediment entrapment and storage by surface roughness elements. Soil loss-induced reduction in soil depth in the various catchments ranged from 1.087 cm to 8.456 cm in the order of Kumpalgogo>Dua>Bugri>Zebilla>Doba and a mean of 3.82 cm. The loss in soil depth decreased the water holding capacity of the top 20 cm depth of the catchment soils by 5.44 to 42.28 percent. The assessment of nutrient concentrations showed the reservoir sediments to be richer not only in nutrients and organic carbon but clay and silt than the catchment soils with enrichment ratios greater than 1. The total amount of organic carbon (OC) loss ranged from 2383 to 19672 kg ha-1 equivalent to 3619 to 33894 kg ha-1 organic matter (OM). The magnitude of OC loss was in the order of Kumpalgogo>Dua>Bugri>Zebilla>Doba with a considerable spatial variability. The total amount of nutrient loss in kg ha-1 among the reservoirs ranged from 153 to 3048 for N, 3.15 to 42.59 for P, 41 to 290 for K, 17.6 to 184.2 for Na, 432 to 2158 for Ca, and 63 to 483 for Mg. The N, P, K removed by erosion from the catchment areas and deposition in the reservoir sediments represents a hidden cost to agricultural production. The cost in Ghana cedis per hectare of N, P, K losses expressed as sulphate of ammonia, single superphosphate and muriate of potash ranged from 437.10 to 8708.70, 36.00 to 487.80 and 131.20 to 928.00 GH¢/hectare respectively. Soil carbon sequestration is an important and immediate sink for removing atmospheric CO2 and mitigating global warming and climate change. From this study, the total organic carbon sequestered by the five reservoirs was 226.31 Mg y-1 with its CO2 equivalent of 830.56 Mg y-1. This is indicative of the potential of reservoirs as an important sink for OC in the global carbon balance. Nutrient export rates (NE) (in kg ha-1y-1 × 10-3) from the reservoir catchments ranged from 755 for OC, 104 for N, 2 for P, 16 for K, 9 for Na, 113 for Ca, 27 for Mg and 1300 for organic matter (OM). These rates were lower than those of other studies probably due to the low content of the nutrients in the catchment soils. Relationships established between NE and ASY showed NE to be positively correlated with ASY (R2 = 0.66 – 0.99). ASY therefore accounted for between 66 to 99% of the variations in the calculated NE. The empirical equations can be satisfactorily used to predict the amount of plant nutrients actually lost from the catchments and stored in the reservoir sediments. The use of sustainable land management practices to forestall erosion in the catchment areas and to reduce sedimentation of the reservoirs for enhanced livelihoods of the communities in the study area is very important.

Source: KNUST