The most fundamental human needs for water are for drinking, cooking, and personal hygiene. The quality of the water used to meet these needs must pose no risk to human health. The quality of the water in nature also impacts the condition of all living organisms found in aquatic ecosystems that we depend upon for our own wellbeing. At the same time watersheds and their water bodies serve as convenient sinks for domestic, industrial, and agricultural wastes. Runoff from agricultural and urban lands containing excess nutrients, oils, and solid wastes together with direct point source discharges of wastewaters into water bodies degrades the quality of those water bodies. Water resources management involves the monitoring and management of water quality as much as the monitoring and management of water quantity. Various models can assist in predicting the water quality impacts of alternative land and water management policies and practices. This chapter introduces some approaches to water quality modeling, leaving descriptions of more advanced methods to textbooks devoted solely to this subject.
IntroductionWater quality management is a critical component of overall integrated water resources management. Most users of water depend on adequate levels of water quality. When these levels are not met, water users must then either pay for water treatment or incur increased risks of using lower quality water. As populations and economies grow, more wastewater pollutants are generated. Many of these are discharged into surface and ground water bodies. Increasingly the major efforts and costs involved in water management are aimed at water quality protection and management. Conflicts among various users of water are increasingly over issues involving water quality.Natural water bodies are able to serve many uses. One of them is the transport and assimilation of many waterborne wastes. As natural water bodies transport and assimilate wastes, their quality changes. If the quality of water drops to the extent that other uses are adversely impacted, the assimilative capacities of those water bodies have been exceeded with respect to those impacted uses. Water management measures are actions taken to ensure that the total pollutant loads discharged into receiving water bodies do not exceed the waste assimilative capacity of those water bodies and that the quality meets the quality standards set for those waters.What uses depend on water quality? Almost all one can identify. As everyone knows, all living organisms require water of sufficient quantity and quality to survive. Different aquatic species can tolerate different levels of pollutant concentrations that impact water quality. In much of the developed world it is no longer ''safe'' to drink natural surface or ground waters. Treatment is usually required before these waters are safe for humans to drink. Treatment is not a practical option for improving the quality of water found in
