Hydro-electrical potential at risk und climate change: the case of Lake Malawi basin and the Shire River in Malawi / Southeast Africa

The hydroelectrical potential is derived from the hydraulic head and the available water discharge. In certain hydroclimatic regions, the water discharge is only a small part of the regional precipitation and the water cycle. This is particularly true when evaporation accounts for a large proportion of the regional water balance. Such conditions prevail, for example, in the catchment area of Lake Malawi and the Shire River in Malawi in south-east Africa. The country produces over 95% of its electricity from hydropower plants in the Shire River. This river is the outlet of Lake Malawi. Our study examines the sensitivity of regional water resources and hydropower generation to climate change, covering various aspects:

• Processing of rainfall, runoff and evaporation data for this tropical region, with particular attention to data scarcity.
• Hydrological simulation of the water balance and water level of Lake Malawi as well as runoff.
• Calculation of possible hydropower generation in the Shire River.
• Performing scenario calculations for climate change conditions and associated sensitivity analyses.

The most important results of these analyses are

• Between 1970 and 2013, meteorological droughts have increased in intensity and duration. This can be attributed to a decrease in precipitation and an increase in temperatures and evaporation rates.
• The hydrological system of Lake Malawi reacts to meteorological droughts with a time lag (up to 24 months), so that hydrological droughts can be predicted up to 10 months in advance by meteorological drought parameters.
• Despite the uncertainties in the regional climate projections, it is clear that the water level of Lake Malawi, as a residual of the catchment water balance, is very sensitive to changes in precipitation and evaporation.
• The discharge from the lake is a direct function of the lake's water level, and the combination of the projected decrease in precipitation and increase in temperature leads to a significantly lower flow in the Shire River.
• This suggests a future decline in annual hydropower production of between 1 % and 2.5 % (2021-2050) and 5 % and 24 % (2071-2100)
• Some projections even result that the outflow of Lake Malawi would temporarily dry up and the country's electricity supply would be interrupted.
• It is shown that regional evaporation and its changes are the key variable for assessing future water availability. This process is characterized by a particularly high degree of uncertainty.

The example of Lake Malawi basin shows that a careful hydro-climatic analysis is required to assess such sensitive hydro-systems. Global-scale analyses do not have sufficient predictive power and explanatory potential.