INTRODUCTION OF WATER RESOURCE SYSTEMS :
Water resource systems have beneﬁted both people and their economies for many centuries. The services provided by such systems are multiple. Yet in many regions of the world they are not able to meet even basic drinking water and sanitation needs. Nor can many of these water resource systems support and maintain resilient biodiverse ecosystems.
Typical causes include inappropriate, inadequate and/or degraded infrastructure, excessive withdrawals of river ﬂows, pollution from industrial and agricultural activities, eutrophication resulting from nutrient loadings, salinization from irrigation return ﬂows, infestations of exotic plant and animals, excessive ﬁsh harvesting, ﬂood plain and habitat alteration from development activities, and changes in water and sediment ﬂow regimes. The inability of water resource systems to meet the diverse needs for water often reﬂect failures in planning, management, and decision-making— and at levels broader than water.
Planning, developing, and managing water resources to ensure adequate, inexpensive, and sustainable supplies and qualities of water for both humans and natural ecosystems can only succeed if we recognize and address the causal socioeconomic factors, such as inadequate education, corruption, population pressures, and poverty.
The planning of human activities involving rivers and their ﬂoodplains must consider certain hydrologic facts. One of these facts is that surface water ﬂows and aquifer storage volumes vary over space and time. They are also ﬁnite. There are limits to the amounts of water that can be withdrawn from them.
There are also limits to the amounts of pollutants that can be discharged into them. Once these limits are exceeded, the concentrations of pollutants in these waters may reduce or even eliminate the beneﬁts that could be obtained from other users of the resource.