Impact assessment of global change on wetland-catchment interactions in a tropical East African catchment

Abstract

Water is the key to sustainable development, especially in sub-Saharan Africa (SSA), where a large part of the population lives on subsistence farming. Reliable knowledge of available water resources therefore is an indispensable component of sustainable water resource management. An important tool for the management of water resources is hydrological modeling, which, depending on the model type, is capable to quantify water quantities spatially explicit and to predict water availability under changing conditions. The bottleneck for these simulations often is the lack of data availability, especially in sub-Saharan Africa. In recent decades, however, satellite data sources have been developed for hydrological modelling on different scales. The aim of this work is to develop a modeling framework for a meso-scale catchment area in Tanzania based on locally collected data and freely available satellite data sets. This model system should serve to better understand the hydrological processes in the catchment area with an emphasis on wetland-catchment interactions. At the same time the model should be able to estimate the availability of water resources under changing environmental conditions for the catchment area. <br /> The Soil and Water Assessment Tool (SWAT) was applied to the Kilombero Catchment area in Tanzania, which, like many other East African catchments, is characterized by a general data shortage. Due to the lack of current discharge data, the model was calibrated for the period 1958-1965 (R² = 0.86, NSE = 0.85, KGE = 0.93) and validated from 1966-1970 (R² = 0.80, NSE = 0.80, KGE = 0.89) with the sequential uncertainty fitting algorithm (SUFI-2) at a daily resolution. The model results show the water-related dependency of the floodplain in the center of the catchment area on the base flow of the surrounding highland forests and savannas, especially in the dry season. <br /> In addition, this study investigates the influence of climate change on water resources in the catchment area. To account for these changes, regional climate models of the Coordinated Regional Downscaling Experiment (CORDEX) Africa project were applied to investigate changes in climate patterns up to 2060 according to the RCP4.5 (representative concentration paths) and RCP8.5 scenarios. The SWAT model was used to investigate the impacts of climate change on water resources under different scenarios and model combinations. The climate models show a clear temperature increase, especially in the hot dry season, which further reinforces the pronounced differences between the dry and rainy seasons. This, together with changing precipitation patterns, leads to an intensification of hydrological extremes in the catchment area, e.g., more pronounced flooding in the rainy season and decreasing low flows in the dry season. Overall, the annual average values of water yield and surface runoff within the simulations increase by up to 61.6% and 67.8%, respectively, by 2060 compared to the historical simulations. The changes of the hydrological processes show a heterogeneous spatial-temporal pattern within the catchment area.<br /> In many parts of sub-Saharan Africa and also in the study area, natural systems are being converted into agricultural land in order to feed the growing population. Therefore, this study additionally examines historical land use and land cover patterns as well as potential future land use and land cover patterns and their impacts on water resources in the catchment area. The Land Change Modeler (LCM) is used for the analysis and projection of land use patterns until 2030 and the SWAT model is then utilized to simulate the water balance under changing conditions. The results show that the low flow in the land use/land cover scenarios decreases by 6-8%, while the high flow in the combined land use/land cover scenarios increases by up to 84% considering also the climate scenarios. The impacts of climate change are therefore more pronounced than the impacts of changing land use/land cover patterns, but also contain higher uncertainties and show different patterns in the climate model combinations applied in this study. <br /> Within this study, a methodological approach was developed to quantify the impacts of land use/land cover patterns and climate change for data-scarce regions. The results and the methodology from this study thus contribute to the sustainable management of the investigated catchment area, as they show the effects of environmental changes on hydrological extremes (low flows and high flows) and additionally identify particularly sensitive subcatchments that are of essential importance for the preservation of the social-ecological system.