Wetland Characterization and Implications on Agriculture in L. Victoria Basin

Abstract

Wetlands in flood plains can be characterised by significant variations in water transitions. The Ombeyi watershed in Nyando Basin hosts a number of wetlands that are characterised by competing water uses, especially between expanding irrigated agriculture and availability of wetland products. The seasonal contribution of water from various sources to the wetland water is a function of season of the year, geology, land use/cover, rainfall and upstream water-use activities. These influence the extent to which the wetlands function. An electrical resistivity tomography (ERT) mapping was done in June 2014 along four transects that coincided with the location of shallow wells whose water levels were being monitored. To further understand the relationship between surface and groundwater, water samples from rainfall, streams, wetlands and the shallow wells were collected at weekly intervals for a period of 6 weeks between May and June 2014 in the Ombeyi watershed. Measurement of stable isotopes oxygen-18 (O-18) and Deuterium (H-2) on these samples was carried out in July 2014 at UNESCO-IHE. The ERT imaging revealed permeable zones that surface water infiltrates into shallow groundwater. The profiles also showed subsurface features that could explain the surface hydrological behaviour. The results of stable isotope content showed that shallow groundwater is recharged from local rainfall but suffers evaporation during transit from the supplying rivers through wetlands to downstream rivers. The Deuterium excess of the local rainfall was 5.4. This surface water-groundwater linkage calls for conjunctive management of the water sources in the prevailing circumstances of expanding land under rice irrigation.