Drought Analysis of the Lake Manyara Catchment: Meteorological Drought Occurrence, Influence of Atmospheric Teleconnections and Impact on Lake Manyara

Abstract

Lake Manyara is a terminal lake in northern Tanzania. As a destination of wildlife and part of a national park, it provides a booming tourism industry in the region. The lake also reflects water availability in the catchment used e.g. for agriculture. The catchment experiences a bimodal precipitation pattern, with a long rainy season in March-May and a short rainy season in OctoberDecember. Fluctuating lake levels, especially shrinkage of Lake Manyara during dry periods, raise concerns about the (future) water availability and potential disappearance of the lake. Possible mechanisms driving these fluctuations are: land use and land cover changes, sedimentation of the lake, irrigation water abstractions and climate change/drought. The aim of this study was to determine the impact of drought on the dynamics of Lake Manyara. The occurrence of drought over the past century in the Lake Manyara catchment (LMC), the influence of atmospheric teleconnections on LMC rainfall and the impact of drought on Lake Manyara are explored. The occurrence of meteorological drought and characteristics (duration, severity and frequency) was studied using the Standardized Precipitation Index (SPI) and Standardized Precipitation Evapotranspiration Index (SPEI) with periods of 3 (short-term droughts) and 12 (long-term droughts) months. A literature study explored the relevant teleconnections influencing East African rainfall: the El Niño-Southern Oscillation, Indian Ocean Dipole, Madden-Julian Oscillation, Southern Annular Mode and North Atlantic Oscillation. Monthly, seasonal, lead-lagged, sliding and partial correlations between rainfall and teleconnections were applied. To determine the impact of drought on Lake Manyara available observations regarding its size were linked with meteorological variables and the drought indices.

In general, a drying trend was found in the LMC over the last century. The short rainy season does not show a clear drying or wetting trend. Precipitation of this season was found to be related to atmospheric teleconnections. Though overall correlations with the IOD, ENSO and MJO are quite strong, the relations between rainfall and teleconnections changed through time. These changes correspond with changes in the strength of Walker circulation cells of the Indian, Pacific and Atlantic Ocean. The long rainy season shows a drying trend. Rainfall during this season showed only weak influence by atmospheric teleconnections. Combined rainfall and evapotranspiration showed largest impact on changes in the volume of Lake Manyara (r=0.56). Though the drought indicators did not show strong relation, the strong influence of P-PET on changing lake volume does imply the impact of drought occurrence. This would decrease the value of P and P-PET, which can cause shrinkage of Lake Manyara. Models predict further increase in temperature, and thus evapotranspiration, in the future. This implies (long-term) shrinkage of the lake. However, future change in P is uncertain. Improvement of the models simulating future precipitation are important to estimate the magnitude of effort required to improve future water resource management in the Lake Manyara catchment.