As  Lippy has said, it is ...

As  Lippy has said, it is usual for dams to be designed and operated to fulfill multiple objectives. In such cases it is common practice to develop  set of rules based on past experience. In your specific case, where the concerns are both floods and droughts, a typical solution is to keep the dam only partly full, for flood limitation, during the most of the rainy part of the year and allow it to fill completely toward the end of the rains, to have water for irrigation. This is delicate balancing act; if the rains finish early, the dams are not full for irrigation, if the rains are late the dam could be full during a flood. Kenya also has the added complication of twp rainy seasons in much pf the country. Each case has to be studied based on past experience. It depends on the costs and frequency of flood damage and the value of the irrigated crops.