The deterioration of water quality due to human-driven alternations has an adverse effect on the environment. More than 50% of surveyed surface water bodies in the United States (US) are classified as impaired waters as per the Clean Water Act. The pollutants affecting the water quality in the US are classified as point and non-point sources. Pollutant mitigation strategies such as the selective implementation of best management practices (BMPs) based on the severity of the pollution could improve water quality by reducing the amounts of pollutants. Quantifying the efficiency of a specific management practice can be difficult for large watersheds. Complex hydrologic models are used to assess water quality and quantity at watershed scales. This study used a Soil and Water Assessment Tool (SWAT) that can simulate a longer time series for hydrologic and water quality assessments in the Yazoo River Watershed (YRW). This research aims to estimate streamflow, sediment, and nutrient load reductions by implementing various BMPs in the watershed. BMPs such as vegetative filter strips (VFS), riparian buffers, and cover crops were applied in this study. Results from these scenarios indicated that the combination of VFS and riparian buffers at the watershed scale had the highest reduction in sediment and nutrient loads. Correspondingly, a comparative analysis of BMP implementation at the field and watershed scale showed the variability in the reduction of streamflow, sediment, and nutrient loads. The results indicated that combining VFS and CC at the field scale watershed had a greater nutrient reduction than at the watershed scale. Likewise, this study investigated the soil-specific sediment load assessments for predominant soils in the YRW, which resulted in soil types of Alligator, Sharkey, and Memphis soils being highly erodible from the agricultural-dominant region. This study also included the effect of historical land use and land-cover (LULC) change on water quality. The analysis revealed that there was a significant decrease in pastureland and a simultaneous increase in forest and wetlands, which showed a decreasing trend in hydrologic and water quality outputs. Results from this study could be beneficial in decision-making for prescribing appropriate conservation practices

New York City's municipal water supply system provides about 1 billion gallons of drinking water a day to over 8.5 million people in New York City and about 1 million people living in nearby Westchester, Putnam, Ulster, and Orange counties. The combined water supply system includes 19 reservoirs and three controlled lakes with a total storage capacity of approximately 580 billion gallons. The city's Watershed Protection Program is intended to maintain and enhance the high quality of these surface water sources. Review of the New York City Watershed Protection Program assesses the efficacy and future of New York City's watershed management activities. The report identifies program areas that may require future change or action, including continued efforts to address turbidity and responding to changes in reservoir water quality as a result of climate change.

For an agricultural watershed, best management practice (BMP) is a conservational way to prevent non-point source pollution, soil and water loss and mitigate groundwater declination. In this dissertation, several BMPs of tail water recovery system, conservation tillage system and crop rotation were selected and evaluated in order to demonstrate the impacts of those activities on stream water quality and quantity. Besides, a land use change scenario was also evaluated. In order to evaluate the scenarios comprehensively, Soil and Water Assessment Tool (SWAT) and Annualized Agricultural Non-point Source Pollution (AnnAGNPS) were applied to simulate surface hydrology scenarios, and Modular flow (MODFLOW) models was used to simulate groundwater level change. This dissertation contains several novel methods regarding to model simulation including (i) using satellite imagery data to detect possible tail water recovery ponds, (ii) simulating surface and groundwater connected, (iii) selecting land use change area based on local trend and spatial relationship, (iv) comparing scenarios between two models. The outcomes from this dissertation included scenarios comparison on surface water quantity and quality, groundwater level change for long term simulation, and comparison between surface water models.

For an agricultural watershed, best management practice (BMP) is a conservational way to prevent non-point source pollution, soil and water loss and mitigate groundwater declination. In this dissertation, several BMPs of tail water recovery system, conservation tillage system and crop rotation were selected and evaluated in order to demonstrate the impacts of those activities on stream water quality and quantity. Besides, a land use change scenario was also evaluated. In order to evaluate the scenarios comprehensively, Soil and Water Assessment Tool (SWAT) and Annualized Agricultural Non-point Source Pollution (AnnAGNPS) were applied to simulate surface hydrology scenarios, and Modular flow (MODFLOW) models was used to simulate groundwater level change. This dissertation contains several novel methods regarding to model simulation including (i) using satellite imagery data to detect possible tail water recovery ponds, (ii) simulating surface and groundwater connected, (iii) selecting land use change area based on local trend and spatial relationship, (iv) comparing scenarios between two models. The outcomes from this dissertation included scenarios comparison on surface water quantity and quality, groundwater level change for long term simulation, and comparison between surface water models.

Watershed modeling is at the heart of modern hydrology, supplying rich information that is vital to addressing resource planning, environmental, and social problems. Even in light of this important role, many books relegate the subject to a single chapter while books devoted to modeling focus only on a specific area of application. Recognizing the

The effectiveness of conservation practices depends on their placement on the fields within the watershed. Cost-effective placement of these practices for maximum water quality benefits on each field requires comparing a very large number of possible land-use scenarios. To address this problem, we combine the tools of evolutionary algorithm with the Soil and Water Assessment Tool (SWAT) model and cost data to develop a trade-off frontier of least cost of achieving nutrient reductions and the corresponding locations of conservation practices. This approach was applied to the Raccoon River Watershed, which drains about 9,400 km2 of an intensive agriculture region in west-central Iowa. Applying genetic algorithm to the calibrated SWAT modeling setup produced multitudes of optimal solutions of achieving nutrient reductions in relation to the total cost of placing these practices. For example, a 30% reduction in nitrate (and a corresponding 53% reduction in phosphorus) at the watershed outlet can be achieved with a cost of $80 million per year. This solution frontier allows policymakers and stakeholders to explicitly see the trade-offs between cost and nutrient reductions.