Abstract:
The main objective of the study was to determine how river discharge and sediment yield in the basin is influenced by variations in rainfall and land use land cover change. The study was based on the analysis of hydrological and meteorological data archived by the Water Resources Authority (WRA) and the Kenya Meteorological Department (KMD), respectively. The river discharge, Total Suspended Sediment Concentration (TSSC) and sediment yield data was collected from established river gauging stations in the basin, namely Sagana (4AC03), Maragua (4BE01), Saba Saba (4BF01), Mathioya (4BD01), North Mathioya (4BD07), Irati (4BE03), Thiba (4DD01), Thika (4CC05) and Gikigie (4BE08),for the period 2010-2012. Rainfall data was for the period 1980- 2012, river discharge data was for the period 1980-2012 while sediment yield data was from the period 2010-2012.The study used Landsat 5 and 7 satellite data for Land Use Land Cover (LULC) change detection analysis and Geographical Information System (GIS). The statistical methods of data analyses applied in this study included regression analysis, correlation analysis, measures of central tendency, coefficient of determination (R2), and the analysis of variance (ANOVA). The study also applied Soil Water Assessment Tool (SWAT) Model to simulate the relationship between sediment yield, river discharge and rainfall in NWUT catchment. Nash Sutcliffe Efficiency (NSE) was used to test the efficiency of the SWAT model in predicting these parameters.This study found a positive relationship between river discharge and sediment yield in the NWUT catchment with (r) of 0.74 and (R2) of 0.55. The maximum Total Suspended Sediment Concentration (TSSC) was 1,433 mg/l at Saba Saba (4BF01) and the peak river discharge was 170.4 m3/s at Sagana (4AC03). Water yield and sediment yield from NWUT basin was 327,638,974 m3/month and 590,637.4 tons/month respectively. Sediment production rate from the basin was 59.55 tons/km2/month in the period O2010-2012. Land Use Land Cover Change (LULCC) detections analysis for the period 2000- 2014 found out there was increase on bare-land (from 6.5 to 9.7%), increase in build-up areas (from 0.2% to 0.8%), decrease in forest cover (from 32.3% to 21.7%), decrease in rangeland (from 17.0% to 12.7%), increase in plantation (from 12.7% to 15.0%), increase in silted water bodies (from 0.1% to 0.7%) and reduced waterbody (1.5% to 0.5%). SWAT modelling application in the simulation of river discharge and sediment yield was good with NSE above 95%. The study puts forward various recommendations for soil and water management in the basin. Some of the recommendations for land management include application of soil and water conservation measures, Payment for Ecosystem Services (PES), Integrated Water Resources Management (IWRM), Eco-Hydrology and reforestation to improve high ediemnt yield from the basin.