|Using Water Efficiently: Technological Options (WB, 1993, 61 p.)|
|River basin management: When is low efficiency appropriate ?|
Improving WUE can often offer opportunities for conserving water and increasing water availability. Therefore, governments have made great efforts and investments to improve water resources management through the application of technologies in the urban and agricultural sectors. Such investments are intended to reduce water losses and to increase water availability at local levels. However, when entire river basins are considered, the issues become more complex.
In a river basin, how will increased local water use efficiency affect the availability of water for other users? From a basin point of view, how much water is actually saved by using better technologies such as lining, pipes, sprinkler and drip systems? WUE may be viewed differently for farmers, management of an irrigation project, or a river basin authority. The answer is usually positive at project, irrigation network or farm levels. At the level of an entire basin, however, the answer depends on specific basin hydrogeological and socio-economic characteristics.
The hydrological processes of a basin provide downstream users with return flows from upstream uses. For any given level of water use efficiency, E, we define the 'loss' by (1-E). The lower E is, the greater is (1-E). However, much of (1-E) in the upstream areas may be reused downstream. The sequential location of irrigation projects from the upper reaches down to the basin tributaries and rivers allows for the recovery and reuse of most water 'lost' through low project efficiencies at different levels upstream. Thus, within a basin, when water is 'lost' through one use but can be reused downstream, it is not actually lost.
The interrelationship between water diversion by users upstream and users and aquifers downstream leads to another important concept--the WUE at a basin level. Basin water use efficiency, Eb, is the ratio of the amount of water beneficially consumed in the basin to the amount of utilizable water resources entering the basin.
For example, using the overall water balance in the Nile Basin in Egypt, the basin efficiency is estimated at 89 percent (Keller, 1992), although the WUE of individual irrigation projects are generally lower, as discussed previously. Similarly, for the UPRIIS project in the Philippines, only a small amount of water leaves the downstream part of the Upper Pampanga Basin. The basin efficiency is high due to reuse of water, despite relatively 'low' efficiencies of individual schemes (Israel, 1990).