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Changing climates, human population growth, and aging infrastructure threaten the availability and quality of one of life's most vital resources, water. Hydrological ecosystem services are goods and benefits derived from freshwater that include flood damage mitigation, water for agricultural and commercial use, swimmable and navigable waters, and healthy aquatic habitats. Using computer algorithms inspired by biological and ecological processes known as evolutionary algorithms and on-site stormwater management practices such structural best management practices (BMPs) and green stormwater infrastructure (GSI), this research aims to maximize hydrological ecosystem services at the watershed-scale in both agricultural and urban environments by integrating these algorithms with the watershed model Soil and Water Assessment Tool (SWAT), and the hydraulic model Storm Water Management Model (SWMM). This dissertation first develops an information theoretic approach to global sensitivity analysis for distributed models, demonstrated using SWAT, and later uses the sensitive model parameters in a multi-objective automatic calibration scheme using multi-objective particle swarm optimization (MOPSO). Multiple alternative watershed-scale BMP designs (parallel terraces, detention/infiltration ponds, field borders, and grade stabilization structures) that help minimize peak runoff and annual sediment yield were simultaneously identified using SWAT coupled with the species conserving genetic algorithm (SCGA). Finally, using recently developed economic estimates called triple bottom line (TBL) accounting, watershed-scale GSI designs are identified that reduce combined sewer overflow volumes in an urban setting while maximizing the net benefit across social, economic, and environmental categories. Overall, this dissertation research provides useful and relevant computer-based tools for water resources planners and managers interested in maximizing hydrological ecosystem services benefits.

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

The natural movement of water is known as the hydrological cycle. Many different factors affect the hydrological cycle and the availability of freshwater. The availability of freshwater is essential for many human activities. Three of the UN SDG’s (Sustainable Development Goals), number 2, 6 and 7, directly or indirectly relate to freshwater supplies. SDG number 2, zero hunger; a part of fulfilling this goal is to have enough available freshwater to irrigate crops. SDG number 6 is about access to clean water and sanitation. It is possible to desalinate saline water, treat unclean water or transport water long distances, but these are often costly and energy demanding processes. Therefore, it is important to manage the freshwater supplies that are available wisely. Access to clean and affordable energy is SDG number 7. One source of clean and affordable energy is hydropower, in order to produce electricity in a hydropower plant, plenty of water and a change in potential energy is needed . To be able to fulfil all these goals, it is important to include all of the perspectives when making policies regarding agriculture, building hydropower plants or making big infrastructure changes. The complex relationships between the different areas of interest can make it complicated to analyse the effects of a change made in any area. The concept of connecting different perspectives is called Nexus. This aim of this thesis was to evaluate different computer-based hydrological modelling tools, and how they can be used for Nexus assessments. This was done by performing a market research, choosing two tools for further assessment, creating evaluation criteria and perform a case study on a watershed in Uganda. The case study results and the evaluation criteria were then compared to an existing model in WEAP. The two selected hydrological modelling tools were MIKE SHE and SWAT. These two modelling tools were evaluated based on specified evaluation criteria. In the case study they modelled the same area in Kamwenge, Uganda. The output of the models was compared and calibrated against observed flow in the river at the outlet of the watershed. The results of the case study were incomplete since the MIKE SHE model was not calibrated successfully. The different models have different strengths. The format of output data and flexibility of the program is superior in MIKE SHE, while SWAT is more intuitive and demands less computer power. Over all SWAT is easier to use and model in for a novice user, while MIKE SHE requires more expertise in order to run a successful model. SWAT is able to model more of the Nexus perspectives and are therefore the preferred model. / Många faktorer påverkar tillgängligheten av sötvatten som är väsentligt för många mänskliga aktiviteter. Tre av FN:s globala hållbarhets mål, nummer 2, 6 och 7 relaterar direkt eller indirekt till tillgång till sötvatten. Mål nummer 2, ingen hunger; en del av att uppfylla detta mål är att det måste finnas vatten för att bevattna grödor. Mål nummer 6, tillgång till rent vatten och sanitet; det är möjligt att rena vatten eller att transportera det längre distanser, men det är ofta energikrävande och kostsamt. Därför är det viktigt att förvalta tillgängliga sötvatten resurser klokt. Hållbar energi för alla är mål nummer 7. En källa till hållbar och ren energi är vattenkraft, för att producera vattenkraft behövs en skillnad i potentiell energi och mycket vatten. För att kunna uppfylla alla dessa mål är det viktigt att inkludera alla perspektiv när man bestämmer policys för jordbruk, bygger vattenkraftverk eller gör andra stora ändringar i infrastrukturen. De komplicerade förhållandena mellan dessa olika intresseområden för vatten kan göra det svårt att förutse hur en förändring i en av dem påverkar de andra. Konceptet att koppla ihop olika perspektiv kallas Nexus. Målet med denna uppsats var att utvärdera olika hydrologiska modelleringsverktyg och hur de kan användas för Nexus bedömning. Det gjordes genom att en marknadsundersökning genomfördes och två modelleringsverktyg valdes ut. De två utvalda modelleringsverktygen var MIKE SHE och SWAT. Dessa två verktyg utvärderades baserat på specificerade kriterier. I fallstudien modellerade båda modelleringsverktygen samma område i Kamwenge, Uganda. De båda modellerna kalibrerades mot observerade utflöden från avrinningsområdet. Resultaten jämfördes med en existerande modell i WEAP. Resultaten från fallstudien är ofullständiga då kalibreringen för MIKE SHE modellen inte lyckades.

Variation of dissolved of organic carbon concentration in stream water is a consequence of process changes in the surrounding terrestrial environment. This study will focus on 1) Identify significant environmental factors controlling the spatial and temporal variation of DOC in terrestrial ecosystems of a watershed southeast of Boston, Massachusetts; 2) Model the DOC leaching from different land cover and examine the relationship between leaching flux and in-stream DOC. Our hypothesis is variations of in stream DOC is closely related to watershed properties and environmental factors at annual, seasonal, and daily scales, especially land cover type, watershed size and hydrology. To explore the relationship of hydrology and DOC variation at ungauged sub-basin, we examined the effectiveness of using simulated stream flow from Soil Water Assessment Tool (SWAT) to study terrestrial DOC dynamics. Our results demonstrated that streamflow, drainage area, and percent of wetland and forest were particularly strong predictors in watersheds with a large proportion of developed area. The resulting linear model is able to explain about 70.2% (R2=0.702) and 65.1% (R2=0.651) of the variance of in-stream DOC concentrations at seasonal and annual scales respectively. Results also suggest that more frequent DOC sampling is necessary to establish the quantitative relationship between simulated stream flows from the SWAT and in-stream DOC concentrations at daily scale. The physically based ecosystem model developed in this study shows that DOC leaching from various land cover are highly correlated (up to 80%) with in-stream DOC by using ecological process with incorporated different hydrological pathways. It shows that leaching of DOC from soil is a significant contributor to the in-stream DOC. The production of DOC is largely controlled by the vegetation type and soil texture. Considering the hydrologic control on DOC transport with different pathways of water at finer spatial and temporal scale highlights the need to identify the quantitative relationships between water and carbon flux.

The Morro Bay Watershed, which is located inSan Luis Obispo County,California, covers more than 48,000 acres of land and discharges intoMorroBaythrough the Morro Bay National Estuary (MBNE). The Chorro Creek Subwatershed consists of approximately 30,000 acres of the overall watershed. The MBNE provides an ecosystem that supports a variety of wildlife from the common sea gull to the endangered sea otter. The estuary is also home to over 200 species of birds. The operational waterfront of theMorroBayHarborwas and continues to be a strong supporter to the local economy of the City of Morro Bay. Numerous studies were conducted since the 1990s throughout the watershed to study the sedimentation of the estuary and bay and identified accelerated erosion and subsequent sedimentation as a major threat to sustainability of the bay. As a result, various Best Management Practices (BMPs) were implemented in the watershed to reduce sediment loading and transport to the bay. Localized evaluations of various BMPs have been performed to investigate effectiveness of individual BMPs. This paper consolidates this information and develops a comprehensive spatially distributed watershed simulation model (1) for detailed understanding of the erosion and sedimentation processes in the watershed; (2) to evaluate a watershed scale effectiveness of the conservation practices that were installed in the watershed; (3) to identify optimal BMP types and sites that may be used in the future to further reduce sedimentation of the bay at minimal cost; (4) to organize and document the various sources of data and studies that have been performed to date in the Chorro Creek subwatershed. Soil and Water Assessment Tool (SWAT) was used to develop the model and to evaluate the pre- and post-BMP implementation characteristics in the subwatershed. Combining the data and efforts of past BMP evaluations, land use, soil type, climate data, and streamflow data, statistical evaluations, and model sensitivity analysis will help build and calibrate a robust SWAT model that can be used to track BMP evaluation efforts, as well as other watershed management tasks. Through the evaluation of BMPs in the watershed, efforts can be made to implement the more successful BMPs in the watershed or in other similar watersheds. Sensitivity analysis was performed using a global sensitivity analysis method and streamflow and sediment yield was calibrated using the Shuffled Complex Evolution-University ofArizona.

As land cover throughout the Chesapeake Bay watershed is modified and climate patterns alter, changes in runoff occur which can impact water quality. A study was conducted using the Soil and Water Assessment Tool (SWAT) for the James River watershed in Virginia, the southernmost tributary of the Chesapeake Bay, for 1986 to 2018. A comparison of land cover changes throughout that time frame was included in the analysis. Land cover data was derived from satellite imagery with values extracted using Geographic Information System (GIS) analysis. The SWAT model was run within the web-based Hydrologic and Water Quality System (HAWQS). The research focuses on land use, precipitation, and water quality indicators. Findings suggest links between land cover modification, such as residential development, to degraded water quality indicators. Also denoted are direct improvements in water quality when forest land areas are preserved throughout the watershed terrain.

Watershed systems are experiencing rapid changes to water quality and hydrologic regimes due in part to climate-induced changes in temperature and precipitation, urbanization, and increases in aquatic invasive species. Aquatic invasive species are one of the primary threats to ecosystems, contributing to loss of biodiversity, altered hydrologic regimes, and stream degradation. Urban land use and climatic factors influence the spread of invasive species, presenting greater challenges for future invasive species management. There is a need for more research that evaluates the watershed process in connection with urban land use and climate change factors in relation to invasive species spread. This study will examine factors of climate change and land use that may be influencing the spread and occurrence of aquatic invasive plants within the Connecticut River watershed. There will be four species involved in this study: Eurasian milfoil (Myriophyllum spicatum), Variable milfoil (Myriophyllum heterophyllum), Hydrilla (Hydrilla verticillate), and the European water chestnut (Trapa natans). Hydrological conditions within the watershed will be analyzed using the SWAT model through the HAWQS interface. ArcGIS Pro will be used to combine and prepare data so that it may be utilized through MaxEnt. MaxEnt will be used to create species distribution models to estimate the probability of the presence of invasive aquatic plant species in the Connecticut river watershed.

The objectives of this research were to examine some representative surface waters in the Yazoo River Basin (YRB) during the growing season for a few selected herbicides; to compare the type and abundance of pesticides in the atmosphere at a agricultural and an urban site; to determine the load of nitrogen and phosphorus being discharged from the YRB to the Mississippi River and ultimately the Gulf of Mexico; to determine how the adsorptive properties of a common Delta soil differ under no-tillage (NT) versus conventional tillage (CT) for two commonly used herbicides; and to validate the Soil and Water Assessment Tool (SWAT) model for herbicide runoff in the YRB. Herbicides, in low concentrations (generally less than 10 parts per billion) were frequently detected in 3 streams in the YRB throughout the growing season. Pesticides were detected in air and rain samples from both urban and agricultural areas. The concentrations in the agricultural area generally were an order of magnitude higher and types of pesticides detected were different: more insecticides in the urban area and more herbicides in the agricultural area. The annual load of nitrogen being contributed to the Mississippi River from the YRB was less than what might be expected based on discharge, and the load of phosphorus was slightly higher than what might be expected. The amount of atrazine and fluometuron adsorbed was similar for a soil under CT and NT, but much more herbicide was adsorbed by the NT soil. At the scale of the Bogue Phalia Basin (too large for specific information to be available and too small for averaging to eliminate the need for site specific data) there are considerable uncertainties associated with input data and these, together with the simplifying assumptions within the model, mean that SWAT should not be used to predict the exact date, time, and concentration of a pesticide in a stream. However, the model does offer the potential to assess the likelihood of contamination of surface waters by a given compound in a given situation and as such could provide a useful tool for planning, management and regulatory purposes.