9 maximized returns to watcr uses in thc rivor basin under various river flow scenarios. A detailed dcsciiption of **the** physicntl model appears in Lee ct al. (1993). The model was solved witli nonlinear and stochastic programming mcthodls. Oamek (1990) and Booker and Young (1993) e ;amincd thc economics of intcrbasin water transfcrs from upper basin a,griculturc to lower basin urban uses to meet iising urban dcmands as a lower cost altcmativc to construction of new waler projects. Booker and Young presented an institutional model of water usc, flows, quality, and phiysical and institutional constraints. A rangc of 'cfficicnt" transfcr quantities was ascertaincd under altcrnativc specifications of' thc bencrit function.

The Trent River in the U.K. provides irrigation water, navigation, municipal and industrial watcr supply, hydroelectric power, flood protection, commercial fishery, and rccrcational uses to uscrs in England and Wales. Due to rising pollution levels and increasing demand for clean water, a plan of action and a series of development projects were proposed. Newsomc et. al. (1972) developed a model to determine the minimum capital investment required to meet rising demands. Their model included: water transfers, pumped reservoir storage, estuary storage, and sewage treatment. Brewin et. al. (1972) developed a model to predict changes in Trent River water quality from developmcnt, efluent loads, river retention, increased diversions, and power generation. Water quality characteristics considered were: biochemical oxygen demand, temperature, and acration. Within the model, the river was characterized as a series of discrete states, and the state of the river was characterized as a function of effluent emissions at or below that required to achieve minimum water quality star.dards. Dynamic programming was applied to determine the least cost method of achieving each state. The global optimum was defined as the river state that could be achieved at the lowest total cost.

# **Water** quality

For the Axios River Basin in Greece, water quality is important to consumptive uses, recreation, fishing, and maintenance of natural reserve areas. van Gils and Argiropoulos (1991) specified a water quality model to replicate the concentration of pollutants in the river. The model allowed for sources and sinks, dispersion, degradation, transport, and chemical and physical processes. Altemative management strategies included: a<sup>**5**</sup> **0%o** reduction in polution levels, a reduction in agricultural acreage, and a 100%/o increase in municipal and industrial waste loads and disposal of partly treated wastewater. With a cumulative frequency distribution derived from monthly flow records, the model was used to determine the probability of violating water quality standards under each alternative strategy.

The Densu River in Ghana provides water for approximately 2 million people, and also serves as the pnimary recipient of all domestic and industrial waste in the basin including a food cannery. The warm tropical waters tend to be polluted all year around. Lamnie et. al. (1989) specified a steady-state hydrological model to capture physical, chemicaL and biochemical activity from point and nonpoint pollution loads and their effect on the water quality in the river. Water quality constituents included: chlonide, dissolved oxygen, and BOD. Using monthly flow and quality data the model was parameterized for the wet and dry seasons of the year.