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Managing water scarcity

Congress: 2008

Keyword(s): water balance, scarcity, river basin, consumption, development
AbstractAnalysis of the issue The only source of water to a river basin is precipitation. Water used for development is “taken” either directly from this precipitation or from storages convenient for abstracting water. Part of the used water is consumed to sustain life or a related economic activity; the remainder is discharged to downstream water storages. These storages, in turn, serve as the water source for downstream users. If water is used, however, it always has a higher concentration of chemicals and/or sediments, or it changes in temperature. In the interests of sustainable development, this concentration must remain below critical values for downstream use to remain effective. Economic development whereby the upstream consumption of water increases, always results in a decrease in water resources for downstream use. This causes problems in river basins where almost all water is already consumed. In such river basins, the downstream wetlands are the first to suffer. Well-known examples are the Aral Sea basin and the Yellow River, while many smaller basins where water is scarce, such as the Tunuyan in Argentina, the Kouris in Cyprus and the l’Oum er Rbia in Morocco, fail to make international news. For river basins, the level of water scarcity can be quantified by the drainage ratio: For a river basin, this ratio equals the discharge into the sea over total precipitation. We may qualify water scarcity as shown in Table 1. This paper focuses on basins where decision makers have placed water on the political agenda. Mostly, these are basins where the drainage ratio is less than 0.2. Table 1. Classification of water scarcity at river basin level Drainage ratio Water scarcity greater than 0.33 no scarcity 0.2 to 0.33 moderate scarcity 0.1 to 0.2 water is scarce 0 to 0.1 very scarce (severe sustainability problems) less than 0 extreme scarcity (not sustainable) Some findings Water flows from one water-user to the next downstream user. Thus, the non-consumed part of the used water will be reused again. In the case of extreme water scarcity, this continues until drainage into the sea is zero. Under these conditions, all water that leaves one user system (e.g. leakage, spills, etc.) is reused by the next. Hence, increasing the efficiency of water use within one user system does not create “new” water for the next downstream user system. For example, if the efficiency of irrigation water use increases from 40% to 50% while the diverted volume of water remains the same, downstream water availability is reduced by 10% of the diverted volume. Thus, major technical and institutional investments are at the cost of downstream water users, who lose their water resources. This example is less exceptional than it appears, because the water rights of users are commonly based on the right to divert water, not on the right to consume water. From the perspective of the water right holder, the introduction of improved water use technology can only be justified economically because it makes more irrigation water available for crop production. As mentioned above, improving the efficiency of water use does not create “new water”. In water-scarce river basins, the most important water management questions should be: • To which user subsystem should we allocate water? • And which allocation criteria are to be used? Using satellite RS seems obvious to answer these challenging questions. To illustrate this water allocation challenge, Table 2 shows the actual evapotranspiration for three land uses in the Roxo basin, Portugal, as quantified through the energy balance of the pixels of Landsat and Modis images. ETa is given for the full year and not just for the crop growing season, because water also evaporates when there is no crop in the field. Table 2 shows the annual ETa from fallow land as being 380 mm/year. This is 155 mm/year less than the precipitation of 535 mm/year. This non-consumed water either runs off into streams or recharges the groundwater basin. Fallow land, however, does not yield a crop and is not appealing from a tourist point of view. Eucalyptus forest performs better on both counts. As shown, it consumes 660 mm/year, being 125 mm/year more than the rainfall. The forest thus takes water out of storage (from the groundwater basin). Contrary to common belief, planting forest is not a water conservation activity in this climate. Table 2. Actual evapotranspiration for three alternative land uses in the Roxo basin (approximately 38˚N, 8˚W), Portugal (mm/year). Land use Precipitation Actual ET Discharge for downstream use Fallow land 535 380 + 155 Eucalyptus forest 535 660 – 125 Irrigated maize 535 790 – 255 For irrigated maize, water consumption increases to 790 mm/year, being 255 mm/year more than precipitation and 410 mm/year more than the consumption for fallow land. Consumption is about the same if winter wheat is grown before an irrigated crop on the same field. The percentage distribution of these (and other) land uses in the river basin thus influences the volume of water flowing from the basin. If we consider the water balance of the entire basin for a “long” period, we can write: precipitation – actual evapotranspiration = flow from basin In this context, “long” means that changes in water storage within the basin are small with respect to precipitation. The above equation implies that the flow from a river basin (or sub-basin) can be estimated if data on precipitation are available and if the actual evapotranspiration can be quantified. Conclusions In many river basins there will not be enough fresh water to meet the demands of all users. In another generation, about 60% of the world population will be living in these basins! Economic development that consumes more water than the local precipitation reduces water for downstream use. of water-scarce river basins The water balance should be the leading criteria for all economic development in water-scarce river basins.
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