Open water bodies also experience high evaporation losses depending on the prevailing climatic conditions within their locality with very high values registered in the dry seasons. Lake Victoria experiences very high values in the months of January – February and July – August during which time, low rainfall amounts are registered in and around the lake. Similarly, Lake Tana registers very high evaporation losses in the months of January – May during which period, low rainfall amounts are registered in and around the lake.
ET represents a certain amount of water lost from the catchment area. By understanding how much water is available after such losses, it is possible to plan for efficient and effective usage of the existing resource, especially where water scarcity and drought are important issues. Evapotranspiration is also an indicator of climatic trends, as in periods of depressed rainfall there will be a tendency towards lower evapotranspiration values.