Basin water use efficiency

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).

Impact of increasing local WUE of a basin

Some studies argue that a high basin water use efficiency leaves little room for conserving water by simply increasing efficiencies at local levels (Keller, 1992; Frederiksen, 1992). This implies that localized increases in WUE may have little effect on basin wide efficiency if there is potential for reuse of the seepage and runoff losses within the basin.

However, the evaluation of whether a certain level of local WUE is undesirable or appropriate, or of whether only basin efficiency matters, should be related to an evaluation of a basin's hydrogeological features and the pattern of its water resources utilization and development.

A simple example is given below to illustrate the impact--both favorable and unfavorable--of increasing localized WUE in the context of a basin. The concept underlying this example is simple to grasp. A detailed numerical simulation is given in Annex II.

Figure 5 Impact of Increasing Water Use Efficiency

Let us assume that the source (e.g., a reservoir) provides 300 units of water to various users in the basin (Figure 5). Of this, 100 units are diverted through conveyance and distribution canals for irrigation to Area-1, the remainder flows downstream to Area-3. An intermediate section, Area-2, does not receive water directly from the source (as do Area-1 and 3). Instead, it relies on return flows from Area-l, after using the water for irrigation. It is also assumed that Area-1 has an initial irrigation network efficiency of 60 percent (i.e., of the 100 units diverted from the source, only 60 reach the field). What happens if we raise the efficiency level to 70 percent? Let us examine alternative water use configurations in the basin:

i. The irrigated land in Area-1 is either expanded, or kept constant to make a larger volume of water available to reach downstream users (e.g. Area-3).

ii. Since Area-2 depends on return flows from Area-1, a higher efficiency of 70 percent in Area 1 in either of the above cases would result in a decline in water availability in Area-2.

iii. There is, therefore, a trade-off among Area 1, 2 and 3. Production levels can be maintained or increased in Area-1, and will fall in Area-2, and will either increase or be maintained in Area-3 depending on the choice made for Area-1. The resolution of this trade-off depends on the socio-economic valuation of activities in each area of the basin.

iv. Let us assume that some of Area-3's water supply is also derived from return flows from Area-2. The increase in efficiency in Area-1 could either lead to expanding the irrigated area and reducing return flows to Area-2, and by extension to Area-3, or result in increased water savings and increases in direct water supplies to Area-3. An obvious benefit of the latter is improved water quality downstream due to increased direct water flows in the river or canal, as opposed to return flows. Water lost due to seepage, percolation, spills and runoff during each use-cycle can be reused as long as its quality is not severely degraded. As water is progressively reduced by EV/ET during each use-cycle, the salt concentration and pollutants in reused water increase. This deteriorates water quality. Again, the resolution of the trade off among the three areas depends on the economic valuation of activities in each area, and on environmental and water quality requirements in the basin.

Technological, environmental and economic considerations

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.