Civil and Environmental Engineering

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Biogeochemical Hotspots: Role of Small Water Bodies in Landscape Nutrient Processing
(University of Waterloo, 2017-05-09) Cheng, Frederick; Basu, Nandita
Increased loading of nitrogen and phosphorus from agricultural and urban intensification has led to severe degradation of inland and coastal waters. Lakes, reservoirs, wetlands, and streams retain and transform these nutrients, thus regulating their delivery to downstream waters. While the processes controlling nitrogen and phosphorus removal from the water column are relatively well-known, there is a lack of quantitative understanding of how these processes manifest across spatial scales. This thesis explores the relationship between hydrologic and biogeochemical controls on nutrient processing in a lentic water body (lakes, reservoirs, and wetlands). Here, our work revolves around three research questions: 1) What are the emergent patterns between nutrient processing rates and residence times in lentic systems? 2) What are the underlying mechanisms contributing to the observed patterns? 3) What is the relative magnitude of nutrient retention as a function of wetland size? These questions are addressed through a meta-analysis of existing literature, the development of a modelling framework, and an analysis through upscaling of the results. Within the meta-analysis, we synthesized data from 600 sites across the world and various lentic systems (wetlands, lakes, reservoirs) to gain insight into the relationship between hydrologic and biogeochemical controls on nutrient retention. Our results indicate that the firstorder reaction rate constant, k [T-1], is inversely proportional to the hydraulic residence time, τ [T], across six orders of magnitude in residence time for total nitrogen, total phosphorus, nitrate, and phosphate. This behavior prompted the hypothesis that the consistency of the relationship points to a strong hydrologic control on biogeochemical processing. Specifically, we hypothesized that small systems have a higher sediment surface area to water volume ratio that would facilitate the biogeochemical processes of the system. To validate the hypothesis, we developed a two-compartment model that links the major nutrient processes with system size: the water column and the reactive sediment zone are coupled through a mass exchange process, with nitrogen being removed through denitrification in the sediments and phosphorus transferring to long term storage via particle settling. The model analyses validated our hypothesis by replicating the empirical inverse k-τ relationship through deterministic modelling. Additionally, we demonstrated the inverse relationship between the sediment surface area to water volume ratio and size through an analysis of the bathymetric relationships. Finally, we focused on wetland systems that have been relatively less studied, and upscaled the k-τ relationships to the landscape scale using a wetland size-frequency distribution. Results highlight the disproportionately large role of small wetlands in landscape scale nutrient processing, such that for the same wetland area removed, the nutrient removal potential lost is larger when smaller wetlands are lost. The disproportionately larger role of small wetlands in landscape scale nutrient processing is important given previous research on the preferential loss of smaller wetlands from the landscape. Through the use of a cross-system meta-analysis that spanned multiple orders of magnitude of system size, we were able to quantify multi-scale behavior that is less apparent when studying individual systems. Our study highlights the need for a stronger focus on small lentic systems as potential nutrient sinks in the landscape due to their high reactivity rates in comparison to larger water bodies. With a growing recognition that wetlands play a critical role in landscape nutrient cycling, our work will help policy makers and water managers to better understand the suite of functions that is associated with the different size classes and types of wetlands.
Biogeochemical Signatures of the Great Lakes Watersheds
(University of Waterloo, 2018-08-21) Chowdhury, Qualbe Shadman; Basu, Nandita
Water quality in many regions of the Great Lakes Basin (GLB) has deteriorated due to numerous anthropogenic drivers, including increases in agricultural area, increased fertilizer use, intensive livestock production, and increases in human population densities. Excessive nutrient inputs from both point and non-point sources have accelerated eutrophication in inland watersheds and in receiving water bodies, and policy goals have recently been set to reduce phosphorus loading to Lake Erie by as much as 40%. Under such pressures, it is crucial to better our understanding of nutrient transport across the GLB and to identify key watershed drivers of both seasonal and annual nutrient loading from watersheds to the lakes. In this research study, I have utilized numerous metrics to characterize nutrient dynamics in Great Lakes Watersheds across a gradient of human impacts and have attempted to identify key controls on biogeochemical signatures. As a part of this work, I paired water quality data from over 200 Great Lakes watersheds with land use and climate data to identify dominant controls on stream nutrient concentrations at the annual, seasonal, and event scales. At the annual scale, standardized regression analysis identified significant relationships between flow-weighted concentration (FWCs) and selected catchment characteristics. FWCs were found to be strongly linked to land-use variables such as combined agricultural and urban land, wetlands and tile drainage. Our quantification of these relationships was used to create spatial maps of annual nutrient concentrations and loads and to identify nutrient hotspots across the GLB. Specifically, high nutrient concentrations and export were observed in the Maumee and Sydenham River catchments, whereas lower concentrations and loads were found in Lake Superior catchments. At the seasonal scale, three primary seasonal nutrient regimes were identified: (1) ‘in-phase’ (positive correlation between monthly concentrations and discharge), (2) ‘out-of-phase’ (negative correlation), and (3) ‘stationary’ (no significant relationship). While in-phase seasonality was found to be the most common concentration regime for watersheds with higher levels of agricultural land use, nitrate seasonality in particular was found to be muted in watersheds with the highest agricultural land use, but to be more extreme in watersheds with less agriculture but higher amounts of forested area and higher wetland densities. Out-of-phase seasonality was found to be significantly associated with higher population densities and higher percent urban areas. At the event-scale, concentrations were found to be more variable with discharge for phosphorus than for nitrate. Additionally, Lake Erie showed significantly lower concentration variability in relation to discharge compared to all the other Lakes. As the Lake Erie basin also has higher agricultural land use than the other lakes, the more chemostatic concentration dynamics in these watersheds appears to be linked to agricultural nutrient use and suggests that agricultural nutrient legacies may be an important driver of current patterns in nutrient delivery to the lakes.
Biogeochemistry Across Spatiotemporal Scales: The Role of Reactive Interfaces in Modulating Landscape Scale Contaminant Fluxes
(University of Waterloo, 2022-09-09) Cheng, Frederick; Basu, Nandita
The intensification of agricultural and urban activities has been accompanied by increased fluxes of nutrients to inland and coastal waters, leading to concerns of eutrophication and drinking water contamination. Certain areas in the landscape can intercept, remove or retain nutrients and other contaminants at much higher rates than the overall landscape. While there has been extensive work documenting the extremely dynamic behaviour of these reactive interfaces, watershed-scale models are unable to explicitly capture the spatiotemporal heterogeneity of reactive interfaces. In this thesis, I developed methodologies to explicitly represent different reactive interfaces across the upland-stream continuum using a combination of data synthesis and deterministic modelling. In Chapter 2, I quantify the role of wetlands in retaining nitrogen (N) at the watershed across the US at the annual time scale. Here, I combined spatially explicit datasets of nitrogen surplus and wetland location to demonstrate that current N removal by US wetlands is limited by a spatial disconnect between high-density wetland areas and N hotspots. Simulations of wetland restoration scenarios also showed that spatially targeting areas of high N surplus can greatly increase N retention compared to current practices. I further explore the behaviour of wetland N retention with a focus on sub-annual temporal dynamics in Chapter 3. Using a reduced complexity model with thirty years of remotely sensed wetland inundation levels across eight wetlandscapes in the US, I show that the consideration of transient hydrologic dynamics can increase N retention estimates compared to steady-state models. This effect was more apparent in smaller, isolated wetlands, where the limited outflows of water limited the loss of N and increased the time for denitrification in the wetland. Next, I explore another common reactive interface found in agroecosystems: the hyporheic zone (HZ). While the HZ is often considered a sink for agricultural contaminants such as nitrogen, its role in modulating other contaminants is less clear. In Chapter 4, I developed a mechanistic HZ model to quantify the fate and transport of estrogen, an emerging contaminant in agroecosystems. Using the model, I show that the HZ actually functions as a source and increases the temporal persistence of estrogens in the stream network. Finally, I developed a coupled CN biogeochemical model to a humid water balance model to further our understanding of water fluctuations on nitrogen fluxes in Chapter 5. While the water table is commonly ignored in CN modeling, I show that consideration of water table fluctuations modify the CN export by modulating the redox conditions and altering the transport mechanisms and should not be ignored. My results show that reactive interfaces have complex spatiotemporal behaviours driven by the interactions between the hydrological and biogeochemical processes. Through the use of data synthesis and deterministic modelling, I quantify various environmental factors that contribute to the functionality of reactive interfaces and developed methodologies that can help decision makers predict and mitigate the contaminant fluxes from agroecosystems to protect our water resources.
Checkered Landscapes: Quantifying Dominant Control on Nitrogen Legacies and Time Lags along the River Continuum
(University of Waterloo, 2020-08-20) Liu, Yuhe (Joy); Basu, Nandita
In agricultural watersheds across the world, decades of commercial fertilizer application and intensive livestock production have led to elevated stream nutrient levels and problems of eutrophication in both inland and coastal waters. Despite widespread implementation of a range of strategies to reduce nutrient export to receiving water bodies, expected improvements in water quality have often not been observed. It is increasingly understood that long time lags to seeing reductions in stream nutrient concentrations can result from the existence of legacy nutrient stores within the landscape. However, it is less understood how spatial heterogeneity in legacy nutrient dynamics might allow us to target implementation of appropriate management practices. In this thesis, we have explored the dominant controls of legacy nitrogen accumulation in a predominantly agricultural 6000-km2 mixed-landuse watershed. First, we synthesized a 216 year (1800 – 2016) nitrogen (N) mass balance trajectory at the subbasin scale accounting for inputs from population, agriculture, and atmospheric data, and output from crop production using a combination of census data, satellite imagery data, and existing model estimates. Using these data, we calculated the N surplus, defined as the difference between inputs to the soil surface from manure application, atmospheric deposition, fertilizer application, and biological N fixation, and outputs primarily from crop production. We then used the ELEMeNT-N model, with the estimates of the N mass balance components as the model inputs, to quantify legacy accumulation in the groundwater and soil in the study basin and 13 of its subbasins. Our results showed that from 1950, N surplus across the study site rose dramatically and plateaued in 1980. Agricultural inputs from fertilizer and biological nitrogen fixation were the dominant drivers of N surplus magnitude in all areas of the watershed. Model results revealed that 40% of the N surplus to the watershed since 1940 is stored as legacy N, and that the proportion of N surplus that is stored as legacy vary across the watershed, ranging from 33% to 69%. Where legacy tends to accumulate also varies across the watershed, ranging from 49% - 72% stored in soil, and 28% - 51% stored in groundwater. Through correlation analysis, we found that soil N accumulation tends to occur where there is high agricultural N surplus, and groundwater N accumulation tends to occur where mean groundwater travel times are long. We also found that using the model calibrated mean groundwater travel times as an indication of lag times, we can identify the length of lag time in various regions in the watershed to help inform long-term management plans. Our modeling framework provides a way forward for the design of more targeted approaches to water quality management.
Increasing Nutrient Circularity and Reducing Water Pollution Through Anaerobic Digesters
(University of Waterloo, 2024-12-11) Wallace, Nettie; Basu, Nandita; Mai, Juliane
While the intensification of agricultural practices over the last few decades has increased livestock and crop production, it has also led to unintended environmental consequences such as harmful algal blooms, drinking water contamination, and increased emissions of greenhouse gasses. Much of the increase in crop and livestock production can be attributed to a shift towards specialized agriculture which has resulted in the decoupling and spatial separation of livestock and crop systems. The spatial separation of the two systems has disrupted the circular flow of nutrients in agricultural systems. Relinking the livestock-nutrient economy has been identified as a strategy to reduce the overall environmental burden of the sector. The use of anaerobic digesters to manage livestock manure presents a promising pathway towards the recoupling of crop and livestock systems. Anaerobic digesters, or also referred to as biodigesters, utilize anaerobic decomposition to transform organic wastes into valuable by-products. During the digestion process, methane – a potent greenhouse gas emitted in traditional manure management – is captured to produce biogas which is a source of renewable energy. The process also produces digestate which is a nutrient rich effluent that can be applied to cropland as a fertilizer source. The nutrient dense nature of digestate, and the potential revenue from biogas production enable it to be economically transported over a greater distance than untreated manure – thereby providing a pathway to enhance the nutrient circularity in spatially separated livestock and crop systems. However, there is concern that digestate use can result in greater nitrogen leaching losses than manure. The work presented in this thesis estimates the nitrogen leaching losses from the corn and soybean cropland across 263 regions in Ontario and assesses the water quantity implications of manure and digestate land-application. To do this, a DeNitrification-DeComposition (DNDC) model was developed for each region. The models were calibrated individually to observed crop yields from 2011 and 2021. The calibrated models were able to capture the general magnitude and annual variation in reported corn and soybean yields across the study region. The median error between simulated and observed crop yields across all regions was 5.8% (mean absolute percent error). Corn crops were provided with synthetic fertilizer at an optimal rate, as determined by calibration. The results of the calibration showed that observed crop yields across the study region could be met through the application of 69% of the nitrogen fertilizer purchased in Ontario in 2021. This finding suggests corn nitrogen requirements are met through the application of purchased synthetic fertilizer while manure is applied to cropland in addition of crop needs. Next, I used livestock population data to estimate the quantity of manure nitrogen produced in each region. Using the calibrated DNDC models, I simulated a number of scenarios which explored various manure and digestate distribution configurations across the landscape. The results of this work show that when digestate was substituted for manure and subject to the same transportation constraints, the amount of nitrogen lost by leaching across the study region increased by 6% (from 46.77 to 49.42 kt N/yr). However, when the digestate distribution configuration was altered to reflect re-distribution from a centralized biodigester and its ability to be transported over a greater distance, the amount of nitrogen lost through leaching across the study region was reduced by 7% (43.42 kt N/yr). These findings show that when digestate was used as a direct substitute for manure and applied at equal rates based on total nitrogen content, it contributed to increased nitrogen leaching losses. However, when the distribution of the digestate was considered at a regional scale and the system dynamics of the biodigester were accounted for, the use of digestate reduced the total nitrogen leaching losses across the study region. This research shows that biodigesters can provide benefit to water quality when considered at a regional scale.
Land-to-Water Linkages: Nutrients Legacies and Water Quality Across Anthropogenic Landscapes
(University of Waterloo, 2025-01-06) Byrnes, Danyka; Basu, Nandita
An increasing population and the intensification of agriculture has driven rapid changes in land use and increases in excess nutrients in the environment. Globally, excess nutrients in inland and coastal waters have led to persistent issues of eutrophication, ecosystem degradation, hypoxia, and drinking water toxicity. Over the past few decades, we have seen policies set to mitigate the degradation in water quality. The existing paradigm of water quality management is based on decades of research finding a linear relationship between the net nitrogen inputs to the landscape and stream nitrogen exports. For instance, in the U.S., in response to these nutrient problems, working groups have spent approximately a trillion dollars to improve water quality by upgrading wastewater treatment plants and implementing nutrient management plans to decrease watershed nitrogen and phosphorus inputs. Despite concerted efforts, in many cases we have not seen marked improvements in water quality. In cases where water quality has improved, it is frequently after decades of nutrient management. The lack of or delayed water quality improvement suggests the importance of other drivers in modulating the relationship between nutrient inputs and watershed exports. Indeed, watershed nutrient loads are not just a function of current-year nitrogen inputs but can also depend on the history of inputs to the watershed. However, we still have little understanding of the relationship between nutrient inputs related to exports and the extent that accumulated stores of nitrogen and phosphorus influence this relationship. The central theme of my research has been an exploration of the history of anthropogenic nutrient use and the relationship between nutrient inputs and the response in water quality. Specifically, I have focused on the role of current nutrient inputs versus historical nutrient use in impacting water quality at the watershed scale, as well as the various landscape and climate controls that can mediate responses to changes in management. My research objectives will be to (1) develop a multi-decadal mass balance of nitrogen and phosphorus at the sub-watershed scale across the contiguous U.S. in order to investigate (2) the relationship between watershed nitrogen inputs and export and the drivers of changes in watershed nitrogen export, (3) the magnitude, spatial distribution, and drivers of nitrogen retention and legacy stores, and (4) the use and management of phosphorus in agricultural landscapes in the context of both food security and environmental health. I began by developing county-scale nitrogen and phosphorus surplus datasets, TREND-N and TREND-P, for the contiguous U.S.—with surplus defined as the difference between anthropogenic inputs (fertilizer, manure, domestic inputs, biological nitrogen fixation, and atmospheric deposition) and non-hydrological export (crop and pasture uptake). In Chapter 2, I present the updates to a previously published TREND-N county-scale nitrogen mass balance dataset, improving crop and pasture uptake and livestock excretion methods. In Chapter 3, I develope new county-scale phosphorus surplus dataset, using similar methods. These datasets were then downscaled to a 250 m gridded-scale dataset, known as gTREND-Nitrogen and gTREND-Phosphorus, a step led by my collaborator Shuyu Chang. These novel datasets serve as the foundational data for the subsequent chapters. Next, in Chapter 4, I explored the relationship between net nitrogen inputs and nitrogen export for over 400 watersheds across the U.S. I used the newly developed nitrogen surplus dataset to understand how watershed-scale nitrogen surplus magnitudes and exports change over time and examine how the relationships are influenced by both natural and anthropogenic controls within watersheds. To achieve this, I used a set of 492 watersheds with nitrogen input and export data spanning from 1990 to 2017. We found that 284 watersheds had a significant (p<0.1) increasing or decreasing trend in both nitrogen surplus and nitrogen load. Of these watersheds, we identified 62 where both nitrogen surplus and export have been significantly increasing over the last two decades. These input-driven watersheds are characterized by high livestock density, agricultural area, and tile drainage. In contrast, nitrogen surplus and export have been decreasing in 127 "bright spot" watersheds, characterized by high population density and urban land use. Nitrogen surplus is also decreasing in 60 "transitioning" watersheds, but export is increasing as nitrogen surplus decreases. We argue that these watersheds are transitioning from agriculture to more urban areas, such that fertilizer inputs have decreased, but the higher nitrogen export is driven by legacy nitrogen stores. Finally, we found 35 watersheds demonstrating a delayed response, with nitrogen export decreasing despite an increase in nitrogen surplus. Climate appears to be the driver of response in these watersheds, with aridity likely driving lower nitrogen export, despite increasing inputs. The four typologies of nitrogen inputs and export relationships suggest that watersheds can act as filters and modulate the movement of nitrogen. Our results provide insights into the complex dynamics of nitrogen surplus and export relationships, as well as how the landscape, climate, and legacy nitrogen can influence these relationships. In Chapter 4, I analyzed relationships between changes in nitrogen inputs and export, to understand what drives changes in watershed export, finding that legacy stores may be modulating the watershed response to changing net nitrogen inputs. However, we have limited knowledge of the magnitude and spatial distribution of legacy stores across North America. Therefore, in Chapter 5, we quantified how much nitrogen retention, which is the mass of nitrogen stored in legacy pools and nitrogen lost to denitrification, has accumulated in watersheds, and where it can be accumulating. To achieve this, we used existing datasets and machine learning algorithms to calculate the mass of ‘retained’ nitrogen in the landscape—defined as the nitrogen stored in the soil organic nitrogen pool, the groundwater pool, or lost through denitrification. Specifically, we built a random forest modeling framework trained on the watersheds’ nitrogen surplus and components, loads, and characteristics to predict nitrogen loads at the HUC8 scale across the U.S. We calculated retention for HUC8, which is the difference between nitrogen surplus and predicted loads, and found that nitrogen retention is highest in the Midwestern and Eastern U.S. because of low exports in regions with high agricultural inputs or high population density. Next, we used a data-driven approach to estimate legacy stores by allocating retained nitrogen mass into their legacy pools. We partition nitrogen retention in the Upper Mississippi region HUC8 watersheds into the mass stored in the groundwater pool, soil organic nitrogen pools, and mass lost to denitrification. We found that, on average, 42% of the mass is stored in the soil organic nitrogen pool, 16.5% is stored in the groundwater pool, and 40% is lost to denitrification. While these two chapters focused on nitrogen, in my final chapter we shifted to explore phosphorus use in agricultural systems. In my final chapter, we used the new gridded phosphorus surplus and components dataset to explore current and historical agricultural phosphorus use and management in landscapes within the context of both food security and environmental health. To characterize the extent of phosphorus depletion and excess, we employed indicators such as annual and cumulative phosphorus surplus and phosphorus use efficiency (PUE). We found that the evolution of agricultural phosphorus management is shaped by changing fertilizer management, the proliferation of concentrated animal operations, climate, and the landscape's memory of past phosphorus use. We further integrated both cumulative phosphorus surplus and PUE into a framework to quantify phosphorus sustainability in intensively managed landscapes. We found that in the 1980s, much of the agricultural land was undergoing ‘intensification,’ with positive and increasing cumulative stores because phosphorus inputs exceeded crop uptake (PUE < 1). By 2017, 29.5% of the agricultural land was undergoing ‘recovery’ and had positive cumulative phosphorus stores that were being depleted through improved phosphorus management (PUE > 1). However, 70% of the agricultural area in the U.S. is still undergoing ‘intensification,’ particularly in areas with more of their inputs from livestock manure, pointing to the need to treat manure as a resource instead of the current approach of treating it as a waste product. By using novel datasets, we have been able to explore nutrient use across space and time and its impact on food security and environmental outcomes. I have made significant contributions towards expanding the discussion of nutrient us and fate, understanding the magnitude and distribution of cumulative net nutrient inputs stores in the landscape, as well as the ways in which intrinsic watershed properties, climate, land management, and historical nutrient use can modulate the relationship between inputs and export. Overall, my findings underscore the importance of nuanced, place-based, and context-dependent nutrient management strategies, with a focus on manure management, to address the diverse challenges of different agricultural systems and prevent unintended environmental consequences.
Modeling Nutrient Legacies and Time Lags in Agricultural Landscapes: A Midwestern Case Study
(University of Waterloo, 2019-05-21) Ilampooranan, Idhayachandhiran; Basu, Nandita
Land-use change and agricultural intensification have increased food production but at the cost of polluting surface and groundwater. Best management practices implemented to improve water quality have met with limited success. Such lack of success is increasingly attributed to legacy nutrient stores in the subsurface that may act as sources after reduction of external inputs. These legacy stores have built up over decades of fertilizer application and contribute to time lags between the implementation of best management practices and water quality improvement. However, current water quality models lack a framework to capture these legacy effects and corresponding lag times. The overall goal of this thesis is to use a combination of data synthesis and modeling to quantify legacy stores and time lags in intensively managed agricultural landscapes in the Midwestern US. The specific goals are to (1) quantify legacy nitrogen accumulation using a mass balance approach from 1949 - 2012 (2) develop a SWAT model for the basin and demonstrate the value of using crop yield information to increase model robustness (3) modify the SWAT (Soil Water Assessment Tool) model to capture the effect of nitrogen (N) legacies on water quality under multiple land-management scenarios, and (4) use a field-scale carbon-nitrogen cycling model (CENTURY) to quantify the role of climate and soil type on legacy accumulation and water quality. For objectives 1 and 2, the analysis was performed in the Iowa Cedar Basin (ICB), a 32,660 km2 watershed in Eastern Iowa, while for objective 3, the focus has been on the South Fork Iowa River Watershed (SFIRW), a 502 km2 sub-watershed of the ICB, and for objective 4 the focus was at the field scale. For the first objective, a nitrogen mass balance analysis was performed across the ICB to understand whether legacy N was accumulating in this watershed and if so, the magnitude of accumulation. The magnitude of N inputs, outputs, and storage in the watershed was quantified over 64 years (1949 – 2012) using the Net Anthropogenic Nitrogen Inputs (NANI) framework. The primary inputs to the system were atmospheric N deposition (9.2 ± 0.35 kg/ha/yr), fertilizer N application (48 ± 2 kg/ha/yr) and biological N fixation (49 ± 3 kg/ha/yr) and while the primary outputs from the system was net food and feed that was estimated as 42 ± 4.5 kg/ha/yr. The Net Anthropogenic Nitrogen Input (NANI) to the system was estimated to be 64 ± 6 kg/ha/yr. Finally, an estimated denitrification rate constant of 12.7 kg/ha/yr was used to estimate the subsurface legacy nitrogen storage as 33.3 kg/ha/yr. This is a significant component of the overall mass budget and represents 48% of the NANI and 31% of the fertilizer added to the watershed every year. For the second objective, the effect of crop yield calibration in increasing the robustness of the hydrologic model was analyzed. Using a 32,660 km2 agricultural watershed in Iowa as a case study, a stepwise model refinement was performed to show how the consideration of additional data sources can increase model consistency. As a first step, a hydrologic model was developed using the Soil and Water Assessment Tool (SWAT) that provided excellent monthly streamflow statistics at eight stations within the watershed. However, comparing spatially distributed crop yield measurements with modeled results revealed a strong underestimation in model estimates (PBIAS Corn = 26%, PBIAS soybean = 61%). To address this, the model was refined by first adding crop yield as an additional calibration target and then changing the potential evapotranspiration estimation method -- this significantly improved model predictions of crop yield (PBIAS Corn = 3%, PBIAS soybean = 4%), while only slightly improving streamflow statistics. As a final step, for better representation of tile flow, the flow partitioning method was modified. The final model was also able to (i) better capture variations in nitrate loads at the catchment outlet with no calibration and (ii) reduce parameter uncertainty, model prediction uncertainty, and equifinality. The findings highlight that using additional data sources to improve hydrological consistency of distributed models increases their robustness and predictive ability. For the third objective, the SWAT model was modified to capture the effects of nitrogen (N) legacies on water quality under multiple land-management scenarios. My new SWAT-LAG model includes (1) a modified carbon-nitrogen cycling module to capture the dynamics of soil N accumulation, and (2) a groundwater travel time distribution module to capture a range of subsurface travel times. Using a 502 km2 SFIR watershed as a case study, it was estimated that, between 1950 and 2016, 25% of the total watershed N surplus (N Deposition + Fertilizer + Manure + N Fixation – Crop N uptake) had accumulated within the root zone, 14% had accumulated in groundwater, while 27% was lost as riverine output, and 34% was denitrified. In future scenarios, a 100% reduction in fertilizer application led to a 79% reduction in stream N load, but the SWAT-LAG results suggest that it would take 84 years to achieve this reduction, in contrast to the two years predicted in the original SWAT model. The framework proposed here constitutes a first step towards modifying a widely used modeling approach to assess the effects of legacy N on time required to achieve water quality goals. The above research highlighted significant uncertainty in the prediction of biogeochemical legacies -- to address this uncertainty in the last objective the field scale CENTURY model was used to quantify SON accumulation and depletion trends using climate and soil type gradients characteristic of the Mississippi River Basin. The model was validated using field-scale data, from field sites in north-central Illinois that had SON data over 140 years (1875-2014). The study revealed that across the climate gradient typical of the Mississippi River Basin, SON accumulation was greater in warmer areas due to greater crop yield with an increase in temperature. The accumulation was also higher in drier areas due to less N lost by leaching. Finally, the analysis revealed an interesting hysteretic pattern, where the same levels of SON in the 1930s contributed to a lower mineralization flux compared to current.
Phosphorus Legacies and Water Quality Trajectories Across Canada
(University of Waterloo, 2024-10-15) Malik, Lamisa; Basu, Nandita
Phosphorus (P) pollution in freshwater is a critical environmental issue, primarily driven by agricultural runoff, wastewater discharge, and industrial effluents. Across Canada, lakes such as Lake Erie and Lake Winnipeg experience severe and persistent algal blooms driven mainly due to excess phosphorus loading. Excessive phosphorus loading leads to eutrophication, which causes harmful algal blooms and hypoxia which disrupt aquatic life, reduce biodiversity, and impair water quality, making human consumption and recreational activities unsafe. Despite policies aimed at reducing phosphorus loading, such as improved farming practices and wastewater treatment upgrades, we have not seen a marked decrease in riverine loads. Phosphorus management goals often fall short due to the persistence of legacies – phosphorus that has accumulated in soils and sediments over decades of agricultural applications – which continue to release phosphorus into water bodies for decades after its initial application. Despite recognizing the existence and significant regional and global impact of legacy P on water quality and aquatic ecosystems, our understanding of the magnitude and spatial distribution of these P stores remains limited. Understanding the legacy P stores and their contributors is crucial for efficiently managing water quality, highlighting the importance of studying these factors to develop more effective and sustainable management strategies. The central theme of this thesis is the exploration of the phosphorus legacy across various landscapes. My work has three objectives. First, I explore phosphorus legacies and water quality trajectories across the Lake Erie basin. Second, I quantify various legacy P stores and evaluate their current and future impacts on water quality. Third, I quantified phosphorus accumulation for Pan Canada. In the first objective, I develop a comprehensive phosphorus budget for the Lake Erie Basin, a 60,000 km² transboundary region between the U.S. and Canada, by collecting, harmonizing, and synthesizing agricultural, climate, and population data. The phosphorus inputs included fertilizer, livestock manure, human waste, detergents, and atmospheric deposition, while outputs focused on crop and pasture uptake, covering a historical period from 1930 to 2016. The budget allowed us to calculate excess phosphorus as phosphorus surplus– surplus defined as the difference between P inputs and non-hydrological exports. A random forest model was then employed to describe in-stream phosphorus export as a function of cumulative P surplus and streamflow. The results indicated a significant accumulation of legacy P in the watersheds of the Lake Erie Basin. Notably, higher legacy P accumulation corresponded strongly with greater manure inputs (R²=0.46, p < 0.05), whereas fertilizer inputs showed a weaker relationship. For the second objective, I model the long-term nutrient dynamics of phosphorus across 45 watersheds in the Lake Erie basin using the ELEMeNT-P model. This aimed to quantify legacy phosphorus accumulation and depletion across different landscape compartments, including soils, landfills, reservoirs, and riparian zones, and to assess the potential for phosphorus load reductions under future management scenarios. The model sought to identify key legacy phosphorus pools and explore the feasibility of achieving significant reductions in phosphorus loading, with results indicating that 40% reductions are attainable only through aggressive management efforts. For the last objective, I develop a high-resolution phosphorus budget dataset for Canada, spanning the years 1961 to 2021, at both county and 250-meter spatial scales. This dataset aimed to capture phosphorus inputs from fertilizers, manure, and domestic waste, along with phosphorus uptake by crops and pastureland, across all ten provinces. With this dataset, I aim to better understand the state and progress of phosphorus management across space and time. The results reveal significant variation in P surplus attributable to differences in land use and management practices. The highest surpluses were observed in southern Ontario and Quebec, with approximately 50 kilotons in 2021, contributing to an accumulation of over 2 tera tons of phosphorus over the past 60 years.
A Random Forest in the Great Lakes: Exploring Nutrient Water Quality in the Laurentian Great Lakes Watersheds
(University of Waterloo, 2020-09-22) Dony, John; Basu, Nandita
A data driven approach was used in this study to investigate the drivers of nutrient water quality across the Laurentian Great Lakes drainage basin. Monitored time series of nutrient water quality and discharge were modelled using a dynamic regression-based model. Random forest machine learning was used as a framework to assess drivers of nutrient water quality, using mean annual flow-weighted concentrations (FWCs) and ratios calculated from modelled water quality, combined with spatial factors from monitored watersheds. Analysis revealed that landscape variables of developed land use, tile drained land, and wetland area played important roles in controlling nitrate and nitrite (DIN) and soluble reactive phosphorus (SRP) FWCs, while soil type and wetland area was important for controlling particulate phosphorus (PP) FWCs. Fertilizer and manure practices were important controls in nutrient ratios of SRP:Total Phosphorus (TP), and DIN:TP, with developed land use, manure application, and tile drained land important for the former, and developed land use and manure application (vs synthetic fertilizer application) important for the latter. Plots of feature contribution were generated to isolate the effect that spatial variables had in machine learning models and revealed underlying behaviour of important controls in driving nutrient water quality across the basin. Random forest models were further developed to predict FWCs and ratios of nutrients across all watersheds within the Great Lakes drainage basin. Modelled results revealed hot spots of high DIN, SRP and PP in the watersheds along the southeastern shores of Lake Huron, on the eastern watersheds of the Huron-Erie corridor, and in the southwestern watersheds of Lake Erie. High SRP:TP ratio hot spots were seen in watersheds along the southeastern shores of Lake Huron and along the eastern side of the Huron-Erie corridor. Hot spots of low DIN:TP ratios with high nutrient export were seen in the southwestern watersheds of Lake Erie, which has implications for harmful algal growth. Nutrient ratios across the Great Lakes watersheds compared similarly to other heavily human impacted catchments of the Baltic Sea and western Europe. Annual basin loads of DIN, SRP, and TP were estimated from random forest models for each year from 2000-2016. Calculated annual nutrient loadings of SRP and TP were consistent with other published values of Great Lakes watershed estimates and revealed highest loadings during 2011 when the largest recorded algal bloom in Lake Erie occurred to date. Overall, this data-driven analysis of nutrient water quality reinforces and refines our process understanding of nutrient pollution dynamics across the Great Lakes drainage basin.
Recoupling the Livestock Nutrient Economy: A Path forward for Water Quality Improvement
(University of Waterloo, 2019-09-23) Werenka, Alexander; Basu, Nandita; Tolson, Bryan
Intensification of farming operations and increased nutrient application rates have led to higher crop yields and greater food security. At the same time, widespread use of commercial nitrogen (N) and phosphorus (P) fertilizers and large-scale livestock production have led to unintended environmental consequences, including eutrophication of both coastal and inland waters, threats to drinking water, and increased production of N2O, a potent greenhouse gas. In the past, crop and livestock production were typically more integrated, allowing most livestock to be fed by local crops, and most livestock manure to be applied directly to nearby cropland. Under current intensive agriculture practices, however, there is frequently a spatial decoupling of crops and livestock, leading to hot spots of manure production and a lack of opportunities for cost-efficient and environmentally sensitive disposal. In recent years, there has also been increased interest in the use of both farm and regional-scale bioreactors to convert excess manure to energy, thus exploiting a renewable energy source and increasing the potential to recycle animal waste. In the present work, I develop a spatially distributed optimization approach to identify hotspots of manure production, and, using both economic and environmental criteria, evaluate the economic feasibility of (1) transporting manure for spreading on cropland to meet established nutrient requirements, and (2) constructing biogas reactors to process excess manure in areas where long-range transport is found to be infeasible. This work is focused on manure redistribution, and potential for biogas construction at the continental US scale. In order to identify the spatial disconnect between livestock and crop production, I developed a gridded data set where each cell was 6 km x 6 km and calculated the crop requirements and manure production in each cell. After finding the P requirements in each cell, I found that 530,000 tonnes of phosphorus in manure was located in areas where, if applied, it would be in excess of the local crop requirements. I then examined the feasibility of transporting manure from excess locations (cells) to other locations to use as fertilizer by formulating an optimization problem to maximize the financial benefits of transporting the manure. Savings from transporting manure was calculated as the financial benefit from buying less mineral fertilizer minus the cost of transporting the manure. The solution to this optimization problem shows that transporting manure was able to reduce the excess phosphorus applied to fields by at least 88% with savings of up to $3 billion USD. Finally, I examined the costs and benefits of using the remaining excess manure (after transportation for fertilizer) as fuel to operate biogas plants. For this, I formulated an optimization model to site biogas plants across the continental US such that net profits from the biogas plants were maximized. Biogas net profits were defined as the money made from selling electricity minus the annualized costs for constructing and operating the biogas plants and transporting the manure to the biogas plants. The solution to this problem shows that constructing and operating 387 biogas plants yielded a net profit of $100 million USD and would utilize all of the manure remaining after transportation for fertilizer. This 100% utilization rate of excess manure would have great environmental benefits in terms of removing potential sources of non-point source pollution from farms that would otherwise be available to runoff into waterways.
The Socio-ecohydrology of Rainwater Harvesting in India: Understanding Water Storage and Release Dynamics at Tank and Catchment Scales
(University of Waterloo, 2016-05-02) Steiff, Michael; Basu, Nandita
Rainwater harvesting (RWH), the small-scale collection and storage of runoff for irrigated agriculture, is recognized as a sustainable strategy for ensuring food security, especially in monsoonal landscapes in the developing world. In south India, these strategies have been used for millennia to mitigate problems of water scarcity. However, in the past 100 years many traditional RWH systems have fallen into disrepair due to increasing dependence on groundwater. This dependence has contributed to accelerated decline in groundwater resources, which has in turn led to increased efforts at the state and national levels to revive older RWH systems. Critical to the success of such efforts is an improved understanding of how these ancient systems function in contemporary landscapes with extensive groundwater pumping and shifted climatic regimes. Knowledge is especially lacking regarding the water-exchange dynamics of these RWH “tanks” at tank and catchment scales, and how these exchanges regulate tank performance and catchment water balances. Further, the effects of imposing management controls on improving tank system sustainability and the ability to meet crop water requirements are not well understood. In this thesis, I have attempted to quantify the water exchange dynamics in a cascade of four RWH tanks, using a conjunction of field data and modeling, in the Gundar Basin watershed in Southern Tamil Nadu. Water level sensors were installed in the tanks over the NE monsoon season. Using fine-scale water-level variations, the White method was used to estimate daily fluxes of groundwater exchange (GE), and evapotranspiration (ET) in the four tanks over the 2013 northeast monsoon season. Groundwater recharge and irrigation outflows comprised the largest fractions of the tank water budget, with ET accounting for only 13-22% of the outflows. While water from the tanks directly satisfied ~ 40% of the crop water requirement across the northeast monsoon season via surface water irrigation, a large fraction of the tank water was not available for direct use in the tank’s irrigated area. This is because the sluices were not managed properly, and discharged continuously, instead of only supplying water when it was required for irrigation. For the cascade, a distinct spatial pattern in groundwater-exchange dynamics was observed, with the frequency and magnitude of groundwater inflows increasing down the cascade of tanks. The significant magnitude of return flows along the tank cascade leads to the most downgradient tank in the cascade having an outflow-to capacity ratio greater than 2. The presence of tanks in the landscape dramatically altered the catchment water balance, with runoff decreasing by nearly 75%, and recharge increasing by more than 40%. The second major output is a tank water balance model to evaluate the effect of climate versus management controls on tank water dynamics. The model was run with a 65-year long (1906- 1969) rainfall dataset to evaluate climatic controls, while the two primary management controls imposed were those of an alternate planting date, and the management of sluice outflows to discharge only the amount of water needed for the crops in the irrigated area. Following the imposition of management controls, these previously unutilized outflows were converted more effectively into groundwater recharge (24-54%) and sluice outflow to meet crop water requirements (9-54%) than ET (5-21%). For the long-term (65 year) simulation, catchment scale reductions in runoff (60-80%) and increases in recharge (17-53%) were largely dependent on variations in seasonal rainfall, with proportionally larger decreases in catchment runoff for years of higher seasonal rainfall. Additionally, three sustainability metrics were defined, namely reliability (probability of successfully meeting crop water requirements), resilience (likelihood of meeting crop requirements after a year of crop failure), and vulnerability (severity of crop water requirement shortfall during failure years) to explore the effects of management controls on tank system performance. Evaluation of the sustainability metrics revealed sluice management driven increases in reliability and resilience for tanks 1 and 4. In tanks 2 and 3, increased reliability and resilience was found as a result of changes in the planting date. Vulnerability remained largely unchanged except for tank 2 which became less vulnerable following the imposition of management controls.
Typologies of Nitrogen Surplus Across Continental US: Shifting Hotspots and Dominant Controls
(University of Waterloo, 2019-12-17) Byrnes, Danyka Kimberly; Basu, Nandita
Flows of reactive nitrogen (N) have significantly increased over the last century, corresponding to increases in the global population. The pressures on the N cycle include human waste, fossil fuel combustion as well as increasing food production (i.e., increasing fertilizer consumption, biological N fixation, and livestock manure production). The result is humans causing a 10-fold increase in the flow of reactive N globally. The influx of anthropogenic N into aquatic environments degrades water quality, alters fresh and saline ecosystem productivity, and poses an increasing threat to drinking water sources. In the U.S., decades of persistent hypoxic zones, created by elevated concentrations of nitrate from the landscape, have altered ecosystem trophic structure and productivity. Additionally, increasing N contamination of groundwater aquifers places over 20% of the U.S. population at increased risk of diseases and cancers. Despite billions of dollars of investment in watershed conservation measures, we have not seen proportional improvements in water quality. It has been argued that delayed improvements in water quality can be attributed to legacy stores of N, which has accumulated in the landscape over many decades. There is considerable uncertainty associated with the fate of N in the landscape; however, studies quantified increasing stores of N in the subsurface, suggesting increasing stores of N in groundwater aquifers, in soil organic nitrogen pools, and the unsaturated zone. Nevertheless, the spatial distribution of legacy N across the conterminous U.S. is poorly quantified. Here, we have synthesized population, agricultural, and atmospheric deposition data to develop a comprehensive, 88-year (1930 to 2017) dataset of county-scale N surplus trajectories for the U.S. N surplus, defined as the difference between N inputs and usable N outputs (crop harvest), provides insight into the trends and spatial distribution of excess N in the landscape and an upper bound on the magnitude of legacy N accumulation. Our results show that the spatial pattern of N surplus has changed drastically over the 88-year study period. In the 1930s, the N inputs were more or less uniformly distributed across the U.S., resulting in a few hotspots of N surplus. The following decades had sharp increases in N surplus, driven by the exponential use of fertilizer and combustion of fossil fuels. Contemporary N surplus distribution resembles a mosaic of varying degrees of excess, concentrated in the heavily cultivated areas. To understand dominant modes of behavior, we used a machine learning algorithm to characterize N surplus trajectories as a function of both surplus magnitudes and the dominant N inputs. We find ten primary clusters, three in crop dominated landscapes, four in livestock dominated landscapes, two in urban dominated landscapes, and one in areas minimally impacted by humans. Using the typologies generated can facilitate nutrient management decisions. For example, watersheds containing urban clusters would benefit from wastewater treatment plant upgrades. In contrast, those dominated by livestock clusters would have more success in managing nutrients by implementing manure management programs. The estimates of cumulative agricultural N surplus in the landscape highlights agronomic regions that are at risk of large stores of legacy N, possibly leading to groundwater and surface water contamination. In these agronomic regions, the average cumulative N surplus exceeds 1200 kg-N/ha by 2017. Despite having minimal agricultural activity in urban areas, urban fertilizer use has led to an average cumulative N surplus of over 900 kg-N/ha. While our estimates are an upper bound to legacy stores, significant uncertainty remains regarding the magnitude of the estimate of N accumulation. However, our results suggest that legacy N is at varying degrees, impacting most counties in the U.S. The significant investment and corresponding lack of returns can lead to disillusionment in farmers, watershed managers, and the general public. Developing such N surplus typologies helps improve understanding of long-term N dynamics. Beyond refining the supporting science, appropriately communicating uncertainties and limitations of water quality improvements to the stakeholders, authorities, and policymakers are essential to continuing efforts to improve national water quality.
The Urban Metabolism of the Greater Toronto Area: A Study of Nitrogen and Phosphorus Fluxes across the Urban, Suburban, and Rural Continuum
(University of Waterloo, 2019-05-21) Samson, Melani-Ivy; Basu, Nandita
It has been predicted that approximately 65% of the developing world and 85% of the developed world will be living in cities by 2050. Toronto, the largest city in Canada and the fourth largest in North America, is expected to double in population in the next 50 years. Although such rapid urbanization can lead to enormous social, economic, and environmental change, little is understood about how population growth in Toronto and the “Golden Horseshoe” region around Lake Ontario will impact the ecological systems of Southern Ontario. In our study, we are particularly interested in the ways in which increasing population densities in the Greater Toronto Area are impacting nutrient flows across Southern Ontario’s urban/rural continuum and how changing nutrient dynamics may lead to increasingly impaired water quality in Lake Ontario and beyond. In this work, we utilize a mass balance approach to quantify the flow of nutrients through urban, suburban, and agricultural areas of the Greater Toronto Area. A wide range of factors are considered, including human behaviour, domestic animals, stormwater management, and wastewater treatment processes. The present results suggest that any study of urban metabolism must take into account not only nutrient flows within urban boundaries, but must also identify externalities of urban development associated with a range of processes, from global trade to regional waste management.

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