In any given river basin hydro-system reservoirs for irrigated agriculture can be planned/implemented upstream of hydro power plants. Depending on the location of resources in a given hydro-system, irrigation and hydro power can be computing needs for the river basin system. Such situation demands proper planning of resources in which reservoir operation is one of the important tools to effect resource planning exercises. Reservoir operation works are basic tools to establish operation rules, in which water balance simulation of the reservoir resources is a pre- requisite. Water balance simulation works for natural lakes like Tana is a night mare. This book, practically indicates the techniques of reservoir operation and water balance simulation works for a natural lake. It also indicates the techniques on how to establish operation rules at different scenarios.