Hydro-Management on Lake Chicamba in Mozambique
——Improving multi-use reservoir management with forecasts

Principle 5: Potential Impacts
In order to know what the impacts of the projected rainfall might be, Filipe needs to assess the current situation. It is approaching the beginning of the rainy season. For the past nine months the region has measured less than 50% of its normal rainfall. Although the year before that was relatively wet, the level of the rivers downstream is low. Filipe is also concerned with the declining level of Lake Chicamba.

Filipe's choices combined with possible climate outcomes can be summarized as:

1) Business as usual: keep 40% in reservoir

a) Rainfall ends up below normal
b) Rainfall is close to the long term average
c) Rainfall is above normal

2) Conservative action: withhold 75% behind the dam

a) Rainfall ends up below normal
b) Rainfall is close to the long term average
c) Rainfall is above normal

3) Compromise action: withhold 60% in reservoir, and monitor closely

a) Rainfall ends up below normal
b) Rainfall is close to the long term average
c) Rainfall is above normal

If Filipe manages the dam as he would under normal circumstances (decision 1), he runs a slightly higher risk of running out of water, given that the forecast suggests enhanced probability of below normal rainfall over the watershed (scenario 1a). This is especially true given the already low levels currently in the lake. The societal impacts of running low on water are

• The fishing communities around Lake Chicamba might suffer as fish populations decline
• Hydropower generation during next year's dry season may have to be curtailed, leading to power rationing in the surrounding towns.

But remember, there is a competing goal of minimizing risk of floods. Although the forecast states that there is an increased chance of normal to below-normal rainfall, there is still a 25-30% chance of above-normal rainfall (scenario 1c). If Filipe withholds too much water behind the dam (decision 2) and they get a large storm event Scenario 2c), it is possible he will have to release a lot of water quickly, potentially causing the Revue or Buzi Rivers to overflow their banks, endangering riverside villages. Some compromise level may be best (decision 3).