The Status of Wetlands and their Influence on Stream Flow and Sediment Yield in Maragua Watershed, Murang’a County, Kenya

Abstract

Wetlands are areas on the surface of the earth that are either seasonally or permanently saturated with water or have soil moisture higher than the surrounding uplands. Wetlands attenuate peak flows, reduce sediment loads in surface water bodies, recharge ground water and are home to a wide range of biodiversity. Wetlands in Murang‟a County are under stress due to agricultural intensification and urbanization, which have resulted in the shrinkage of wetlands. This study aimed at evaluating the status of wetlands in Maragua watershed and their effect on stream flow and sediment yield. Landsat images and Digital Elevation Model (DEM) were used to identify and map the wetland conditions in 1987, 1999 and 2018. Index-based classification method was adopted for wetland identification using Normalized Difference Vegetation Index (NDVI), Normalized Difference Water Index (NDWI) and Topographic Wetness Index (TWI) indices and the ERDAS Imagine software. Interviews, a focus group discussion, a stakeholder workshop, storytelling events and observation were used to gather data on community‟s perceptions of wetlands and the challenges facing wetland conservation. Stream flow data for Githanja catchment was generated using the Australian Water Balance Model (AWBM), a hydrological model with Karurumo catchment as the donor catchment while the GeoWEPP model was used to determine the effect of land use in wetland on stream flow and sediment yield in Githanja catchment. Three modeling scenarios were done in the study: (i) Cultivated wetlands on 9.5% of the catchment area (CULT9.5%); (ii) Wetlands with tall grass on 9.5% of the catchment area (GRASS9.5%); and (iii) Wetlands with tall grass with on 16.6% of the catchment area (GRASS16.6%). AWBM simulated stream flow data and TSS data from WRA was used to calibrate and validate GeoWEPP model. Between 1987 and 2018, the area under wetlands decreased by 58% from an area of 24.1 ha. Wetland cultivation, planting of exotic tree species and fodder crops were the main human activities taking place within the wetlands. However, residents were not aware of the adverse effects their activities have on wetlands. Also, conflict in existing land ownership policies and wetland regulations and inadequate community awareness creation were the main impediments against wetland conservation. The simulation study showed the annual average runoff in Githanja catchment for CULT9.5% was 190,121m3/year while the annual average runoff for GRASS9.5% was 161,886m3/year which worked out to about 15% reduction in runoff. For 16.6%GRASS, the annual average runoff 134,817 m3/year which was 29% reduction compared to CULT9.5%. Furthermore, the average annual sediment yield in Githanja catchment for CULT9.5% 2,201Tonnes/ha/year while for GRASS9.5% it was 1,423Tonnes/ha/year, which works out to about 35% less sediment yield. For the 16.6%GRASS average annual sediment yield was 1,103Tonnes/ha/year which is about 50% the sediment yield under CULT9.5%. Transforming wetlands from cultivated land units to conserved grassland attenuates peak flows and reduces sediment yield. Finally, reclaiming lost wetlands will further reduce sediment yield and runoff.