| dc.description.abstract | A study to assess the impact of climate and land use/cover on hydrological regime in River Gucha Catchment was conducted with the principal objectives of (a) assessing trends in land use/cover change for the past 34 years (b) assessing trends in rainfall, temperature and stream. Flow regimes over the past 34 years ( c) determining the relationship between land use/cover change rainfall amounts and temperature patterns with stream flow regime (d) assessing the performance of the HEC-HMS hydrologic model in predicting stream flow and (e) simulating effect of land use/cover change on stream flow of River Gucha using HEC-HMS model. Satellite images of 1976, 1993 and 2010 of the study area were classified and percentage area changes of land use/cover types determined for the period between 1976- 1993 and 1993-2010.
Rainfall, temperature and stream flow monthly data trends of the study area for the year 1976, 1993 and 2010 were analyzed and their percentage changes calculated over the period. Regression scatter diagrams were computed and coefficient of determination (R2) determined using data on total annual rainfall, average annual temperatures, and area percentages of land use/cover types against total annual stream flow for the period 1976- 2010. HEC-HMS model was evaluated using nine selected rainfall and observed stream flow events for the year 1976, 1993 and 2010. Based on the HEC-HMS model evaluation parameters, curve number grid maps (1976, 1993 and 2010) and one selected rainfall event, stream flow hydro graphs were simulated to quantify the change in outflow due to land use/cover change over the period.
Forest cover decreased by 62.94 and 68.49%, agricultural land increased by 30.36 and 7.53% and residential area increased by 7.35 and 32.89% of the original area for the period between 1976-1993 and 1993-2010 respectively. The reduction of forest cover and increase of residential area could be attributed to clearing of forests to give room to cultivation and settlement. Total annual rainfall increased by 17.9 and 212.5 rnm, average annual temperature increased by 0.23 and 1.08 °C and total annual stream flow increased by 3468.51 and 670.06 m3/s for the period between 1976-1993 and 1993-2010 respectively.
The observed increase in average annual temperature could be in some part due to greenhouse warming. Stream flow showed a higher relationship with the land use/cover change (R2 of 0.8440) than with the temperature (R2 of 0.5564) and rainfall (R2 of 0.4595). The higher correlation of land use/cover with stream flow could be due to expansion of agriculture and reduction of forest cover hence reducing evapotranspiration which cause soils to be wetter and therefore more responsive to rainfall. If rainfall and temperature were held constant, a significant increase in stream flow was expected as a consequence of expansion of agriculture and reduction of forest. The shape of simulated hydro graphs generally followed the observed hydro graphs during the HEC-HMS evaluation process.
Modeled flows against observed stream flows attained R2 values ranging from 0.7604 to 0.9987. The performance of HEC-HMS was considered satisfactory. Lag time decreased by 16.67 and 16.67%, base flow decreased by 4.0 and 4.17% and peak flow increased by 30.4 and 7.36% for the period between 1976-1993 and 1993-2010 respectively. The reduction of base flow over the period could be due to replacing natural vegetation with exotic vegetation. Understanding how these land use activities and climatic factors influence stream flow will enable planners to formulate policies towards minimizing undesirable effects of future landuse changes on streamflow patterns. | en_US |
