**l0**

Nutrient loads from fertilized agriculture in thc Marnna watersllcd in Estonia is accelerating the eutrophication of Matsalu Bay. Krysanova ct. al. (1989) developed a model to evaluate alternative management practices in the river basin. A series of four differential relationships were specified to: capture nutrient transfer from the drainage area, water movement through the soil, nutrient balance in the soil, and nutrient concentration and movement in the river. T'he model was used to simulate the effects of fertilizer application rate, application method, application timing, and acreage of fertilized crop planted on nutrient loads into the bay.

Wastewater, sewage, storm runoff, and irrigation return flows pollute the Tong Hlu] and the Liang Shui Rivers in Beijing, China. As a result, water quality is below drinking standards, fish and wildlife habitats are threatened, and recreational uses are severely limited. To detenrmine the effectiveness of sewage treatment on water pol!ution levels, a simulation model was developed. For modeling purposes, the river system was divided into discrete sections and defined in terms of its boundaries and sewage loading, sewage treatment, biological oxygen demand and dissolved oxygen within each section. Mass and energy conservation were assumed. Sewage treatment measures were grouped into "plans" representing various levels of control. The treatment plans were simulated, and the benefits in terms of savings from improved water quality were compared to the costs. The plans were ranked based on their estimated net present value under various discount rates (Hufschmidt, 1 986a).

After completion of the Uboiratana Dam on the Nam-Chi River in Thailand, rapid settlement into the upper watershed area resulted in heavy logging, slash and burn cultivation, and consequently, erosion and transport of pesticides and fertilizers into the reservoir. Sedimentation lowers the value of the reservoir by reducing its capacity to produce electricity, store water, provide flood protection, and serve as a fish habitat. A model was developed by Hufschmidt (1986b) to evaluate the net benefits from soil erosion management of the watershed over a 50 year period. Sedimentation rates were captured with a mass conservation model, and the cost of sedimentation was assessed as the value of lost reservoir productivity lost. Two alternatives were evaluated: employment of watershed managemen **t** o reduce erosion, and the status quo, no additional management and increasing erosion. Given the costs of watershed management and the small returns to the basin from reduced sedimentation, results showed that erosion control in the upper Ubolratana yielded no net benefit.

# **Recreational demand for water**

In arid regions, efficient allocation of scarce water resources to competing uses can yield large econornic benefits. In the Pecos River Basin in New Mexico, recreational uses compete with irrigated agriculture for available water, and competition is highest during the dry summer months. Ward (1989) estimated the implicit value of water in recreational uses for four reservoirs in Pecos. Assumning that the user value of a "site' (a reservoir with water) must sufficiently exceed the cost of travel to the site, the value of water in recreational uses was estimated statistically using survey responses from households in the surrounding area. Results showed that the value of water was four times higher in recreational than in agricultural uses, indicating that an "entry" fee to recreational uses could potentially provide sufficient funds for purchasing water from agnculture uses to keep the basins fill during the summner months.