Ground and satellite-based assessment of hydrological responses to land cover change in the Kilombero River Basin, Tanzania.

Abstract

Changes in land use and land cover are a global issue of concern, especially with regard to possible impacts on biophysical processes which affect the hydrological functioning of a system. Tanzania is no exception to this concern. This study, therefore, addresses the implications of land use alterations on the hydrodynamics of the Kilombero River Basin, specifically with regard to the Kilombero Valley’s wetlands and water resources, which have been altered and exploited to a great extent. As its starting point, the study embarked on mapping the current land cover in the Kilombero Basin and the quantification of the historical changes. The study revealed significant changes and, in recent years, increased rates of clearing natural vegetation cover and conversion to agricultural land. The most affected area of the Basin was the Kilombero Valley, a Ramsar Site and formerly extensively inhabited by wildlife, but which now has 62% of its area converted into agricultural and/or human settlements. In line with this observation, the study used two approaches for the impact analysis, a regional scale and a local scale approach. Plant physiology, soil moisture and micro-meteorological measurements were undertaken to quantify the impact of land cover change at local scale. Sensing techniques were then applied to assess the spatial extent of the changes and the basin scale (regional) impact thereof. Investigation of hydrological processes at a local scale placed emphasis on the implications of forest conversion from indigenous Miombo woodland to exotic Teak (Tectona grandis) forests. Field measurements showed the distinctive nature of Teak trees consumptive water use, both in quantity as well as in regard to the seasonal variation as compared to the native Miombo woodland forests. Teak was found to have higher transpiration rates, both during the rainy season (where the rates were approximately 10-fold higher than that of Miombo) and the period immediately after the cessation of rainfall, with consumptive water use rates being four-fold higher than that of Miombo. This contrast in water use was further observed in the measured soil water fluxes which evidenced a large difference in the components of the soil water balance. Less recharge was observed in the Teak forests suggesting significant impacts on the replenishment of groundwater resources in the study area. Assessment of the basin scale impacts of the land cover changes on the evapotranspiration (ET) regime was undertaken using the Surface Energy Balance System (SEBS) remote sensing model. Validation was provided by the Teak field sites and through the monitoring of ET from sugar cane using a Large Aperture Scintillometer (LAS). Results suggest a decrease in ET during the dry season. There is a clear transition of ET that follows the land cover transition from the natural and more adaptable vegetation, to rain-fed dependent crops and bare lands, where minimal ET is observed during the dry season. Similar seasonal leafing, and therefore a similar ET pattern, is observed with the conversion of natural forests to deciduous plantation forests. Irrigated crops, on the other hand, were found to have persistently higher ET throughout the year regardless of rainfall variability. This implies that land cover change in the Kilombera Valley is resulting in higher water use and less recharge in the wet season and a correspondingly lower ET (and possibly lower river flow) in the dry season than would occur under natural conditions. This research provides valuable information relevant to all stakeholders in the Kilombero River Basin (i.e. both smallholders and commercial sugarcane farmers, the forestry industry, Basin Water Authorities etc.). This information will help to inform decision-making around the sustainable management of the water resources in the Kilombero Valley for food security as well as for sustaining livelihoods and ecosystems.