Impacts of Rainfall and Forest Cover Change on Runoff in Small Catchments: A Case Study of Mulunguzi and Namadzi Catchment Areas in Southern Malawi
The impacts of climate change on water resources have received much attention globally especially in the last 30 years. Rainfall, the main driver of the hydrological cycle, has been varying in parts of the world in various ways. The picture is more complicated if impacts of land cover changes on water resources are also taken into consideration. These two pose challenges which require an integrated approach to address. Not many of such studies have been conducted in Malawian catchments and other tropical regions. In this study, annual, seasonal and monthly series of rainfall and river discharge of the Mulunguzi and Namadzi River catchments, two small sub-catchments in the Lake Chilwa catchment area, were analysed for trends using the non parametric Mann-Kendall statistic and Sens slope estimator. Further, Linear regression and the RainRU model were applied to establish whether the relationship between rainfall and runoff in the two catchments has changed. Furthermore, linear regression was used to establish how increased forest cover has influenced river flows in the two catchments. The results suggest that rainfall in the Mulunguzi catchment has decreased significantly at all scales and this has also led to reduced river flows. Increased forest cover since the pre 1950s has also resulted in reduced flows but this is not as significant as the rainfall decrease. In the Namadzi catchment, the rainfall trends suggest a varying pattern with no obvious straight trends. At annual and some months timescale, the rainfall has increased significantly. The river flow on the other hand suggests an overall declining pattern. This pattern is well linked with significant forest area increments which have occurred since 1995. It is therefore important that more detailed studies should be conducted to gain further insight to these relations as both catchments are important socioeconomically.
Keywords: Land use; land cover; river flow; rainfall; climate change.