Water is a renewable resource which is intercepted (or "appropriated") by man as it moves through the hydrologic cycle. This cycle, which is ultimately driven by solar energy, carries water from the atmosphere to the land and oceans and back again. The largest flux appropriated for human use is the portion of natural evapotranspiration emanating from harvested forests and dryland agriculture. The global long-term average is about 18.2 x 103 km3/yr. This is about three times the annual flow of the Amazon river. Most of the remaining appropriations are for irrigated agriculture, industry, and domestic use, totaling about 4.4
x 103 km3/yr (Global water budget data cited in this section are from Postel et al.1 and Gleick2). These are satisfied primarily by direct withdrawals from "accessible runoff" defined as the accessible flux of water flowing from the land to the ocean as either river water or groundwater. The remainder is satisfied by nonrenewable extractions from water stored in lakes and ground water aquifers. Direct withdrawals account for about 35% of the current accessible runoff of 13 x 103 km3/yr. If "instream water uses" such as navigation, maintenance of riparian ecosystems, and dilution of contamination are treated as claims on accessible runoff, this percentage increases to about 50%.
A global analysis provides useful context, but it only tells part of the story. Hydrologic fluxes vary greatly over time and space, and many regions use a much larger percentage of available water than the global average of 35-50%. An additional complication is the effect of water quality on accessible runoff. As water moves through the hydrologic cycle it picks up, transports, and deposits a wide range of substances, both natural and man-made, which influence its suitability for human uses. Domestic uses are typically the most demanding, since even trace amounts of certain contaminants can have adverse effects on human health. But, the suitability of water for industrial and agricultural uses is also dependent on the levels of solutes, especially salts, and on suspended sediment loads. In some areas, shortages can occur because poor quality renders a portion of accessible runoff unsuitable for particular uses. Although contaminants can usually be removed at the point of end use, this increases the cost of water, sometimes enough to make it unsuitable for price-sens- itive industrial or agricultural uses. In a sense, degraded water quality can be viewed as a depletion of accessible runoff. A variant on this theme is the concept that some portion of accessible runoff needs to be reserved to dilute natural and manmade wastes. As noted above, this has the effect of increasing the percentage of accessible runoff needed to satisfy human needs.
Spatial and temporal fluctuations in precipitation, evaporation, runoff, and water quality create regions of varying water abundance and scarcity. This has the effect of making water resource management a regional rather than a global enterprise. The spatial variability of water resources can be described in many ways. As a rough indication, it is revealing to note that the runoff available per capita varies from 300 m3/yr in arid areas of Africa and Asia to 100,000 m3/ year in Canada.2 This variability has serious implications for food production in arid and semi-arid regions which depend heavily on irrigation. Irrigated agriculture requires 1000 m3/yr of diverted runoff to provide enough food to feed one adult for a year. Although only about 18% of all cropland is irrigated, irrigation accounts for 25% of all agricultural land in India, 35% in Indonesia and Iran, nearly 50% in China and Iraq, over 80% in Pakistan, and 100% in Egypt. In some of these areas there is simply not enough runoff to feed the local population. As a result, an undetermined (but probably large) fraction of irrigation water is obtained from unsustainable depletion of groundwater reserves. Once these reserves are exhausted, withdrawals will have to decrease.
Water shortages will also become more common in urban areas as populations grow and traditional supplies are jeopardized by degrading water quality. However, urban water users are in a better position to invest in conservation and treatment technology than subsistence farmers. To put the issue in perspective, the minimal amount of water required for domestic use is somewhat less than 10 m3/person/year. This is typical of consumption levels in homes which lack running water. The global average domestic consumption level is about 50 m3/person/year while per capita domestic water use in the United States is nearly 300 m3/person/year. These wide ranges suggest that improved conservation and treatment technology could absorb much of the population-driven increase in urban water demand. The potential for urban water conservation is demonstrated both by the success of voluntary and imposed reductions in domestic use during droughts and by the substantial increase in water recycling by industrial users over the last few decades.
There is also room for water conservation in irrigated agriculture. Since most water withdrawals are used for irrigation, a small improvement in agricultural efficiency could provide enough extra water to satisfy growing urban demands. This argument has been convincingly applied to the California water situation, where large urban areas and productive agricultural regions compete for water from the same sources.3 Although more efficient irrigation could resolve supply problems in some areas, it may have only a modest effect in others. This is because the improvements in efficiency which are technically and economically feasible may not be sufficient to satisfy the projected increase in demand, particularly in arid and semi-arid parts of the developing world. Improvements in irrigation efficiency require investments in expensive technology (e.g., drip irrigation). Commercial farmers can afford such investments only if their operations remain profitable. Subsistence farmers are generally dependent on free or low cost local sources, and have little or no cash to invest in more efficient technology. Also, there are inherent lower limits (other than plant evapotranspiration) on the amount of water required for productive cultivation of crops. One of the most important is related to the use of water for leaching salt from the root zone in situations where the applied irrigation water is moderately saline. If the irrigation is very efficient and all the applied water is taken up by the crop, salt will accumulate in the root zone and yield will eventually start to decline.
Our analysis of current and projected demands for water repeatedly reveals the critical role of irrigated agriculture in developing countries. Overall, we feel that the most serious problems of water scarcity and quality will be associated with food production in developing countries rather than with domestic and industrial use in urban areas. It will become increasingly difficult for arid and semi-arid regions in Africa and Asia to be self-sufficient in food production as their populations increase, their groundwater reserves are exhausted, and per capita runoff availability declines. This loss of self-sufficiency could be managed if:
1. Production in water rich areas of the major grain exporting countries (e.g., Canada and the United States) increases sufficiently; and
2. Local populations have the economic resources to buy food on the international market.
After all, many developed countries, even those with abundant water, are not self-sufficient in food but rely on imports to satisfy their needs. Although there is considerable room for expansion of production in the exporting countries, subsistence farmers in many arid regions do not have the cash income required to participate in the international grain market. In some developing countries, a sizable portion of the gross domestic product comes from vulnerable irrigated agriculture. The economies of these areas will have to change dramatically before their food needs can be met by imports.
We can anticipate that water problems will be especially serious in areas where:
1. Demands approach or exceed available fluxes;
2. Stress on limited water resources is degrading the quality of supplies; and
3. The population is highly dependent on food grown locally.
As the consequences of unsustainable depletion and degradation of water resources begin to be felt, current practices will inevitably have to change. The required changes may involve transitions from dependence on local subsistence agriculture to dependence on imports, investment in improved water resource infrastructure, adoption of new technologies, development of new policies which regulate depletion or insure that water is properly valued, or a combination of all of these. Careful planning and preparation will ease difficult transitions while maintaining food security and public health.
Was this article helpful?