The influence of vegetation on the water resources of the Naro Moru catchment A Water Balance Approach

Abstract

On the slope West and North of Mt. Kenya, population increase has lead to rapid land use change and increased demand for water resources. In this area, Water resources are limited and fragile. To achieve socio economic development, advisory and utilisation tools are required to guide sustainable development of water resources. As a contribution to this challenge, the influence of vegetation on the water resources of the Naro Moru river was investigated through a water balance approach. The Naro Moru river, an important water source within the upper Ewaso Ngiro basin, extends from the peak region of Mt. Kenya (5200 meters above sea level) and moves out Westwards traversing five ecological zones to the confluence with the Ewaso Ng'iro at 1800 meters above sea level. The study covered the Alpine, Moorland, Forest and Foot zones of the catchment. Climatic and soil water measurement were made during the study period. Measurement of rainfall, windspeed, temperature, humidity and sunshine hours were made on daily basis and evapotranspiration calculated from the climatic data using the Penman method. The climatic water balance was hence calculated as the difference between rainfall and evapotranspiration. Soil water content was determined gravimetically on monthly intervals in the Alpine and Moorland zones and using a neutron probe on weekly intervals in Forest and Footzone. From the soil water measurements, the available soil water for different vegetation systems and its change with time and space was evaluated. The influence of vegetation, on the water resources was hence investigated by evaluating the climatic water balance, the available soil water and the vegetation water balance. The climatic water balance of the four zones was found to be significantly different in the five zones. In the 1stwet and dry periods, the climatic water demand was met only in the Alpine and Moorland zones. In the 2nd wet period, the climatic water demand was met in all the zones except in the Footzone. The available soil water for different vegetation, ecological and slope conditions was found to be significantly different. The Potato and grass vegetation showed higher and less varied soil water compared to the natural Forest vegetation and cypress plantation vegetation. The Moorland zone showed higher available soil water content compared to the Alpine zone. The lower Forest showed higher available soil water compared to the upper Forest and Footzone. From the vegetation water balance analysis, the vegetation water requirement was higher for the natural Forest and Cypress plantation compared to the Potato crop and Grass vegetation. In the Alpine and Moorland zone, the latter vegetation showed high water use. Weather conditions wet and dry period) were found to be important for the differences observed in the vegetation water balance. Vegetation water requirement was generally higher than water use in the dry period and lower than water use in the wet periods. It is recommended that when planning for water resources, weather, ecological and vegetation differences should be considered. Further, future research activities should aim at developing practical water balance models which could be transfered to other similar areas. This could be achieved through field measurements of soil water, deep percolation and surface runoff.