Joseph K. Sang

CLIMATE- AND LAND USE-INDUCED RISKS TO WATERSHED SERVICES IN THE NYANDO RIVER BASIN, KENYA

Climate change and land use change are two forces influencing the hydrology of watersheds and their ability to provide ecosystem services, such as clean and well-regulated streamflow and control of soil erosion and sediment yield. The Soil Water Assessment Tool, SWAT, a distributed, watershed-scale hydrological model was used with 18 scenarios of rainfall, temperature and infiltration capacity of land surface to investigate the spatial distribution of watershed services over the 3587 km 2 Nyando basin in Western Kenya and how it is affected by these two forces. The total annual water yield varied over the 50 sub-basins from 35 to 600 mm while the annual sediment yield ranged from 0 to 104 tons ha −1 . Temperature change had a relatively minor effect on streamflow and sediment yield compared to change in rainfall and land surface condition. Improvements in land surface condition that result in higher infiltration are an effective adaptation strategy to moderate the effects of climate change on supply of watershed services. Spatial heterogeneity in response to climate and land use change is large, and hence it is necessary to understand it if interventions to modify hydrology or adapt to climate change are to be effective.

A review of radiometric analysis on soil erosion and deposition studies in Africa

Abstract Soil erosion is one of the main soil degradation phenomena that threaten sustainable use of soil productivity thus affecting food security. In addition, it leads to reservoir storage capacity loss because of sedimentation. This not only affects water quantity but also water quality. Worldwide, annual loss in reservoir storage capacity due to sedimentation is 0.5 to 1%. Similarly, about 27% of land in Africa is largely degraded by erosion. As a result, there is need to minimize soil erosion and deposition through site specific estimation of soil erosion and deposition rates in the reservoirs. To achieve this, Fallout RadioNuclides (FRNs) are some of the methods in use. The most common radionuclides include; 137Cs, 210Pb and 7Be. Only few countries in Africa have exploited these FRNs. In these countries, 137Cs has been largely exploited but in some regions, it has been reported to be below minimum detection limit. Using 137Cs and 210Pb, maximum reference inventory in Africa is found to be 1450 and 2602 Bq/m2, respectively. However, there is minimal application of 7Be within the continent. Also, very little has been done in Africa to assess chronology and sedimentation rates of reservoirs using FRNs measured from sediment cores. In conclusion, a gap still exists on FRNs application in Africa in assessing soil erosion, deposition and reservoir sedimentation.

Assessment of Sedimentation Status of Ruiru Reservoir, Central Kenya

Reservoir sedimentation can adversely affect the storage capacity of water bodies; reduce flood attenuation, change water quality, damage valves and conduits amongst others. This study investigated sedimentation rate in Ruiru reservoir as a measure for monitoring water resource management using capacity survey method. Water depth was measured using a transducer fitted on the side of the boat which also recorded corresponding geographic positions with an inbuilt GPS receiver. About 5000 points covering approximately 36 hectare of the water surface were surveyed. A total of eleven sediment sampling points were identified in the reservoir in which five were strategically selected at location near entry from respective streams. The depth of sediments, which relate to the quantity of sediment deposition from the streams ranged between 600 mm and 2100 mm. Ruiru stream demonstrated the highest level of sediment influx into the dam while Ngeteti stream had the least. The reservoir volume was calculated to be 2,632,347m3 and estimate volume of the sediments 389,245 m3, which is a 13.1% storage capacity loss. A related recent study noted storage capacity loss of 11%, which shows that the reservoir has lost about 11-14% of its storage capacity in 65 years. Results further showed that the Area Specific Sediment Yield (ASY) was 38.84 Mg ha-1 y-1, which is higher than the tolerable soil loss of 2 to 18 Mg ha-1 y-1 for the tropics, but is within the range of 10 - 200 Mg ha-1 y-1 typical of savanna ecosystems. These findings are useful to water resource managers because they can help in computing the useful life of a reservoir.