Quantifying Ecological Productivity and Value of Watershed Ecosystem Services in Chania River Basin, Kenya

Abstract

The Millennium Ecosystem Assessment indicates that most ecosystems and the services associated are declining globally. This has triggered a wide range of research on innovative ways to characterise, quantify, and value –these ecosystem services across a range of scales for better and sustainable management. Ecosystem services are often related to biophysical features of a catchment or watershed (Precipitation, evapotranspiration, topography, soil, biomass productivity). According to Global Water Partnership, the biophysical provisioning ecosystem services either at watershed, continental, sub global or global scale is constrained by data availability. This study was carried out in the Chania River catchment in Kenya with the objective to quantify the watershed ecosystem services and their related values. Geographic Information System (GIS) and Remote Sensing (RS) technology were used to quantify biomass at spatial-temporal scales using 30-m resolution Landsat images. Normalized Difference Vegetation Index (NDVI) was used as a proxy for determining above ground biomass (AGB) while Thornthwaite model was used to determine actual water use (ETa). Predictive equations relating NDVI and AGB provided good correlations with R2 values ranging between 0.66 and 0.90. The estimated AGB compared well with measured AGB, with 62% of the validation points giving deviations of less than 15%, and maximum deviations of below 31%. Forests and wetlands had the highest annual ETa values of 921.8mm and 919mm, while coffee and mixed crops had the lowest values of 819.5 and 801. 9mm.Young forests, mature forests and tea had the highest value of biomass water productivity 0.085, 0.025, 0.018 kg/m3 and shrub and maize had the lowest at 0.002 kg/m3. The results revealed a substantive decline of forest cover by 7.78% in 11 years with a steep increase in built up areas, areas under tea, coffee and maize. The precipitation in the catchment showed a negative trend which was attributed the reduced forest cover.