|Using Water Efficiently: Technological Options (WB, 1993, 61 p.)|
|River basin management: When is low efficiency appropriate ?|
From the viewpoint of basin management, the following points are important:
i. Where there is little return flow or little recharge to be reused by downstream users, increasing WUE through technological and managerial improvements is recommended. For instance, near coastal areas, waters are discharged to the sea. In some areas, return flows enter saline groundwater or salt sinks, resulting in salinity and water quality problems for reuse. In neither case can the water be reused for irrigation, industrial or urban consumption without treatment. Under these circumstances, since the lost water cannot be recovered, increasing localized WUE results in an increase in water availability of a basin.
ii. However, the sole measurement of water availability is not enough to decide whether a local WUE should be increased and, if so, to what extent. One environmental dimension of situation i) is the problems of salinity and preservation of estuary ecosystems. An environmentally sound decision needs also to consider protection of aquatic life and wetlands in coastal deltas and estuaries. A minimal stream flow should be maintained in the rivers. An extreme example is the deterioration of the Aral Sea. The massive diversions of the Syr Dar'ya and Amu Dar'ya rivers, which originally flowed into the lake, took place since the 1960s to expand irrigated areas for cotton cultivation. As the rivers dried up slowly, the lake shrank by 66 percent. Fishery production collapsed. The lake became famous for its extremely high salt concentration (Levintanus, 1992). Even the basin climate changed as a result of the reduced surface of the lake, and the high soil salinity.
iii. Localized increases in WUE may have little effect on basin wide efficiency if there is a potential for seepage water or runoff losses to be reused elsewhere in the basin. This is even truer in cases where the return flows and runoff can be repeatedly used downstream. Under these situations, increasing agricultural production per unit of water used in the upstream areas of a basin may not serve the purpose of water conservation in the whole basin. Increasing WUE upstream, thus making more water available to upstream users, has to be traded off against lower water supplies to downstream users who depend on return flows.
iv. Increasing WUE upstream has a merit of improved water quality downstream, as illustrated earlier. That is, by releasing more fresh water to downstream areas, higher WUE in the upstream area has a favorable environmental impact on water quality.
Another technological dimension of water reuse is for conjunctive water use. In some places, water use efficiencies are intentionally kept low and irrigation canals are intentionally unlined. The purpose is to increase seepage recharging to groundwater for conjunctive operations, especially during low runoff years.
The criteria of technical efficiency should not be the only ones on which to judge water use. At the basin level, the concept needs to be expanded by an evaluation of economic efficiency, especially when high pumping costs are involved. The following factors should be considered in the evaluation: costs of physical improvements of water supply systems; benefits from production increments; and costs of water pumping and re-pumping. From the farmers' perspective, the financial returns are directly affected by benefits from water use, the prices achieved for crops, costs of high water use efficiency, water charges, and taxes. In addition, other factors such as groundwater table, salinity, water rights, water availability, and timing of delivery are also important. Together, these factors eventually determine optimal efficiency.