It is estimated that global water c

It is estimated that global water consumption will increase by ≈3,800 km3/yr by 2025, and much of this water will have to be obtained from natural systems. This consumption increase will cause substantial additional depletion of river flows in many areas, with substantial environmental consequences. As a rough guideline, at least 30% of the average annual flow of a stream must remain in place if the ecological health of the stream and related ecosystems is to be maintained. Even today, flows at or above this threshold level are not maintained in a significant number of rivers. Additional diversions of the magnitude necessary to meet increases in direct human use will result in the depletion of flows below the 30% threshold in many more rivers and will create adverse environmental consequences on a massive scale. Based on current trends in population and water-use patterns, it is estimated that both China and India will need all of their runoff to meet urban and agricultural needs within the next 20 years. It is not at all clear how water will be found to maintain the environmental amenities and services that derive from healthy aquatic ecosystems in these countries and other water-stressed areas around the world.
Projections of global water needs are worrisome enough when the water demands arising from future population and economic growth are compared with current estimates of developed and developable supplies. However, the reliability of current supplies is also in question. The fact is that there are trends and circumstances which will almost certainly reduce available supplies in the face of sharply escalating water demands world-wide. Ground water overdraft, a condition in which the rates of extraction from an aquifer exceed the rates of recharge by water percolating from above, occurs in almost every region of the world. China and India today are estimated to be feeding nearly 400 million people through irrigation supported only by the persistent overdrafting of aquifers, and they are not alone in this practice. Because aquifer capacity is finite, ground water overdraft is always self-terminating. In the absence of effective management, water tables are drawn down to the point where it is no longer economical to pump them, and extractions diminish to levels that balance recharge or even cease entirely because they are too costly. This economic exhaustion is the final outcome of persistent overdraft.