Assessment of freshwater is becoming of increasing importance with regard to the growing pressure on this scarce resource. The fresh water resources of Ethiopia have been estimated by several river basin master plan and other specific studies. However, apart from estimating irrigation water requirements from climatic and agronomic data no study has been carried out to estimate irrigation water use. Assessing the impact of irrigation on water resources requires an estimate of the water effectively withdrawal for irrigation, i.e. the volume of water extracted from rivers, lakes and aquifers for irrigation purposes. Irrigation water withdrawal normally far exceeds the consumptive use of irrigation because water is lost in its distribution from its source to the crops.

Except estimation of water abstraction in some large scale irrigation schemes, water withdrawal for irrigation remains unknown because flow measuring structures or devices are not incorporated in most irrigation systems particularly in those owned and managed communities For this reason, how much water is abstracted at the diversion point and what portion of the abstracted water is effectively used for crop production remains unknown. In the absence of this vital information, it is difficult to properly plan water resources development programmes and projects. On the other hand, there is a need to have at least a crude estimate of annual agricultural water withdrawal for and compared with the renewable water resources. Assessing the impact of irrigation on water resources, the potential for future development and the overall impact of human activities on the water cycle requires an approach by river basin. Cognizance to the reality of lacking water measuring facilities, annual agricultural water withdrawal at country, river basin or lower level needs to be estimated using the methodology outlined below