Exploring the Potential Impact of Reforestation on the Hydrology of the Upper Tana River Catchment and the Masinga Dam, Kenya

The Upper Tana River Basin is strategically one of the most critical resource areas of Kenya. The Masinga Reservoir, at the outlet of the basin, provides water and hydroelectric power for 65% of the Nation. Unregulated deforestation and expansion of cultivation practices onto marginal soils in this critical river basin has resulted in significant reservoir siltation, reduced ecosystem function and more erratic downstream flows. Using a participatory process, collaborating technical policy analysts working for key government institutions in Kenya identified the need to assess the impact of meeting a national goal for reforestation of 30% of deforested lands with the infusion of new agro-forestry technologies and land tenure laws through the consideration of population expansion to 2015. Using a rapid rural appraisal methodology, it was determined that reforestation below 1,850 m would be difficult to achieve. However, reforestation at elevation increments of 2,000 m, 1,950 m, 1,900 m and 1,850 m would represent a 30 to 55% increase in reforested area in the Upper Tana River catchments. In addition, the results of this analysis show that full implementation of reforestation to 1,850 m would result in a 7% decrease in sediment loading in the Masinga Reservoir. Runoff yields would be similar to baseline conditions but peak annual flows would increase approximately 3% with less inter-annual variability, resulting in greater stability of water levels in the reservoir. However, replacement of tea plantations with forest did not result in hydrologic benefits to the system, thus tea plantations should remain in place. Based on these findings it has been determined that reforestation would allow the government to pursue reduction of lake water levels to stabilize fluctuation in vegetation, thicken the epilimnion with greater nutrient mixing, and greater downstream delivery of more stable water flows and coastal nutrient loading. In addition, priority subbasins were identified for reforestation based on costs of dam sediment management relative to sediment yields which will allow decision makers to sequence reforestation efforts in a more cost effective manner. Sediment reduction costs varied significantly among the subbasins as varied landscapes and land uses were identified under the baseline conditions. In subbasins where reforestation would replace marginal, erosive prone lands, sediment reduction costs were less than $1/Ton; in subbasins with high valued crops (tea and coffee) that contribute little to sedimentation, the costs were well over $100/Ton. Finally, based on evaluations of current activities and the findings of this research, recommendations on government actions have been made. These recommendations include greater enforcement of illegal logging and illicit drug plantations, community based tree nursery and reforestation activities, improved land tenure laws, greater efforts in soil conservation on croplands, patterning reforestation to preserve biodiversity and strategy promotion of tea plantations in key elevational areas.