One of the main targets of dam construction is to satisfy downstream water requirements in a watershed.

Nutrients emerged from point and non- point sources deteriorate the water quality of the reservoir due to

the eutrophication phenomena. In order to reduce the watershed nutrient load, Best Management Practices

(BMPs) can be used to allocate the Total Maximum Daily Load (TMDL) which maintains the water quality

at the standard level.

In this paper, the Soil and Water Assessment Tools (SWAT) model is applied to evaluate the impact of

BMPs on phosphorous load reduction at the watershed level.

The methodology is developed to reduce the eutrophication of Satarkhan reservoir in Aharchai river

watershed and supply the downstream demand with least cost of BMP implementation. In order to assess

the impact of BMPs, different scenarios are defined including point source reduction like wastewater treatment

and non-point source control including detention ponds, grassed waterways and filter buffer strips.

In each scenario, the total phosphorous load input to the reservoir is estimated and its impact on the reservoir

phosphorous concentration is simulated using the Chapra's model. The hedging rule model for reservoir

operation is used to evaluate the downstream demands considering the inflow to the reservoir. The

developed scenarios are ranked based on the execution cost, reliability of water supply and the phosphorous

concentration in the reservoir as the eutrophication index.