A comparative evaluation of the continuous and event-based modelling approaches for identifying critical source areas for sediment and phosphorus losses.

Proper identification of critical source areas (CSAs) is important for economic viability of any best management practices (BMPs) aimed at reducing sediment and phosphorus loads to receiving water bodies. Both continuous and event-based hydrologic and water quality models are widely used to identify and assess CSAs, however, their comparative assessment is lacking. In this study, we have used continuous Soil and Water Assessment Tool (SWAT) and event-based Agriculture Non-Point Source (AGNPS) pollution models to identify CSAs for sediment and phosphorus in a watershed in Ontario, Canada. Along with their original version, both models were re-conceptualized to incorporate saturation excess mechanism of runoff generation, which is also refereed as variable source area (VSA) integration. The models were set-up using high resolution spatial, crop- and land-management, and meteorological dataset; and calibrated with reasonable accuracy against streamflow, sediment and phosphorus concentration data at multiple locations. Threshold value (t-value) approach was used to identify CSA areas in the watershed. Results showed that both models were in agreement (up to 96% of fields) that summer season did not constitute hot-moments (<6% of the watershed area as CSAs) for both sediment and phosphorus. SWAT models identified winter (~50% of watershed area as CSA) and AGNPS models identified early spring (~50% of watershed areas as CSAs) season as the hot-moment for both sediment and phosphorus. Contrasting result, as indicated by low (1%) matching in field CSA potential, was observed in autumn season. In the same season, VSA integrated SWAT and AGNPS models showed better matching (43% for sediment and 31% for phosphorus), highlighting the importance of VSA integration in the models. Qualitative validation of model-based CSA potential with oblique aerial-photograph-based CSA potential in two soil moisture conditions (wetter and drier) indicated slightly better performance of the SWAT models, and over-prediction of the AGNPS models. However, a more comprehensive analysis based on more detailed field observations is needed to further confirm the results.

Predicting nitrous oxide emissions after the application of solid manure to grassland in the United Kingdom.

Nitrous oxide (N2 O) emission from agricultural soils represents a significant source of greenhouse gas to the atmosphere. We evaluated the suitability of a modified Soil and Water Assessment Tool (SWAT) model to estimate the N2 O flux from the application of solid manure at two grassland sites (North Wyke [NW] and Pwllpeiran [PW]) in the United Kingdom. The simulated N2 O emissions were validated against field observations measured in 2011 and 2012 for model calibration and validation, respectively. The SWAT model predicts water-filled pore space (WFPS) very well with Nash-Sutcliffe efficiency (NSE), R2 , RMSE, and percentage bias (PBIAS) values of 0.67, .72, 0.06, and 3.64, respectively, during the calibration period for NW site, whereas it gives 0.68, .69, 0.07, and 3.04, respectively during the validation period. At PW, the model predicted the NSE, R2 , RMSE, and PBIAS of 0.55, .69, 0.04, and -4.5, respectively, during calibration and 0.63, .71, 0.05, and -2.6, respectively, during the validation period. Compared with WFPS, the model resulted in a slightly lower fit for N2 O emissions for NW (NSE = 0.47, R2  = .63 during calibration, and NSE = 0.55, R2  = .58 during validation) and for PW (NSE = 0.54, R2  = .71 for calibration, and NSE = 0.47, R2  = .69 for validation). Results revealed that the SWAT model performed reasonably well in representing the dynamics of N2 O emissions after solid manure application to grassland.

Incorporating a non-reactive heavy metal simulation module into SWAT model and its application in the Athabasca oil sands region.

Heavy metal contaminations in an aquatic environment is a serious issue since the exposure to toxic metals can cause a variety of public health problems. A watershed-scale model is a useful tool for predicting and assessing heavy metal fate and transport in both terrestrial and aquatic environments. In this study, we developed a simulation module for non-reactive heavy metals and incorporated it into the widely used Soil Water Assessment Tool (SWAT) model. The simulated processes in the developed model include heavy metal deposition, partitioning in soil and water, and transport by different pathways in both terrestrial and aquatic environments. Three-phase partitioning processes were considered in the module by simulating heavy metals portioning to dissolved organic carbon in the soil and stream. This developed module was used for watershed-scale simulation of heavy metal processes in the Muskeg River watershed (MRW) of the Athabasca oil sands region in western Canada for the first time. The daily streamflow and sediment load from 2015 to 2017 were first calibrated and validated. Subsequently, the daily Lead and Copper loads at the outlet station were used for heavy metal calibration and validation. The performances for the daily heavy metal loads simulation during the whole simulation period can be considered as "satisfactory" based on the recommended model performance criteria with the Nash-Sutcliffe efficiency as 0.41 and 0.71 for Pb and Cu loads, respectively. The simulation results indicate that the spring and summer are hot moments for heavy metal transport and the snowmelt in spring and rainfall runoff events in summer are the main driving forces for the metal transport in the MRW. We believe the developed model can be a useful tool for simulating the fate and transport of non-reative heavy metals at watershed scale and further used to assess management scenarios for mitigating heavy metal pollution in the Athabasca oil sands region.

Assessing climate change impacts on stream temperature in the Athabasca River Basin using SWAT equilibrium temperature model and its potential impacts on stream ecosystem.

Stream temperatures, which influence dynamics and distributions of the aquatic species and kinetics of biochemical reactions, are expected to be altered by the climate change. Therefore, predicting the impacts of climate change on stream temperature is helpful for integrated water resources management. In this study, our previously developed Soil and Water Assessment Tool (SWAT) equilibrium temperature model, which considers both the impacts of meteorological condition and hydrological processes, was used to assess the climate change impact on the stream temperature regimes in the Athabasca River Basin (ARB), a cold climate region watershed of western Canada. The streamflow and stream temperatures were calibrated and validated first in the baseline period, using multi-site observed data in the ARB. Then, climate change impact assessments were conducted based on three climate models under the Representative Concentration Pathways 4.6 and 8.5 scenarios. Results showed that warmer and wetter future condition would prevail in the ARB. As a result, streamflow in the basin would increase despite the projected increases in evapotranspiration due to warmer condition. On the basin scale, annual stream temperatures are expected to increase by 0.8 to 1.1 °C in mid-century and by 1.6 to 3.1 °C in late century. Moreover, the stream temperature changes showed a marked temporal pattern with the highest increases (2.0 to 7.4 °C) in summer. The increasing stream temperatures would affect water quality dynamics in the ARB by decreasing dissolved oxygen concentrations and increasing biochemical reaction rates in the streams. Such spatial-temporal changes in stream temperature regimes in future period would also affect aquatic species, thus require appropriate management measures to attenuate the impacts.

Current and future hot-spots and hot-moments of nitrous oxide emission in a cold climate river basin.

An ecosystem in a cold climate river basin is vulnerable to the effects of climate change affecting permafrost thaw and glacier retreat. We currently lack sufficient data and information if and how hydrological processes such as glacier retreat, snowmelt and freezing-thawing affect sediment and nutrient runoff and transport, as well as N2O emissions in cold climate river basins. As such, we have implemented well-established, semi-empirical equations of nitrification and denitrification within the Soil and Water Assessment Tool (SWAT), which correlate the emissions with water, sediment and nutrients. We have tested this implementation to simulate emission dynamics at three sites on the Canadian prairies. We then regionalized the optimized parameters to a SWAT model of the Athabasca River Basin (ARB), Canada, calibrated and validated for streamflow, sediment and water quality. In the base period (1990-2005), agricultural areas (2662 gN/ha/yr) constituted emission hot-spots. The spring season in agricultural areas and summer season in forest areas, constituted emission hot-moments. We found that warmer conditions (+13% to +106%) would have a greater influence on emissions than wetter conditions (-19% to +13%), and that the combined effect of wetter and warmer conditions would be more offsetting than synergetic. Our results imply that the spatiotemporal variability of N2O emissions will depend strongly on soil water changes caused by permafrost thaw. Early snow freshet leads to spatial variability of soil erosion and nutrient runoff, as well as increases of emissions in winter and decreases in spring. Our simulations suggest crop residue management may reduce emissions by 34%, but with the mixed results reported in the literature and the soil and hydrology problems associated with stover removal more research is necessary. This modelling tool can be used to refine bottom-up emission estimations at river basin scale, test plausible management scenarios, and assess climate change impacts including climate feedback.

Predicting sediment yield and transport dynamics of a cold climate region watershed in changing climate.

The effects of climate change on sediment yield and transport dynamics in cold climate regions are not well understood or reported. In this study, the Soil and Water Assessment Tool (SWAT) has been built-up, calibrated, and validated against streamflow and sediment load at several monitoring stations in a cold climate region watershed - the Athabasca River Basin (ARB) in Alberta, Canada. The model was then fed with bias-corrected spatial disaggregated high-resolution (~10km) future climate data from three climate models for two emission scenarios (RCP 4.5 and 8.5), and two periods (mid- and end-century). Results show that channel erosion and deposition are the dominant processes over hill slope erosion in the basin. On average, a predicted warmer and wetter future climate has both synergetic and offsetting effects on sediment yield. Changes are sub-region specific and land-use type dependent, thus reflecting a marked spatial and temporal heterogeneity within the basin. Increases on sediment yield in future periods in the agricultural areas are up to 0.94t/ha/yr, and are greater than reported soil formation rates in the region. Similarly, while substantial increases (by more than two fold) in the sediment load transport through the river reaches were obtained, the changes show both temporal and spatial variability, and are closely aligned with the trend of stream flows. We believe that availability of such models and knowledge of the effect of future climatic conditions would help water managers formulate appropriate scenarios to manage such basins in a holistic way. However, significant uncertainties in future sediment yield and transport, as a result of variations in climatic forcing of different climate models, need to be considered in any adaptation measures.

Modeling nitrous oxide emissions from rough fescue grassland soils subjected to long-term grazing of different intensities using the Soil and Water Assessment Tool (SWAT).

Given the rising nitrous oxide (N2O) concentration in the atmosphere, it has become increasingly important to identify hot spots and hot moments of N2O emissions. With field measurements often failing to capture the spatiotemporal dynamics of N2O emissions, estimating them with modeling tools has become an attractive alternative. Therefore, we incorporated several semi-empirical equations to estimate N2O emissions with the Soil and Water Assessment Tool from nitrification and denitrification processes in soil. We then used the model to simulate soil moisture and the N2O flux from grassland soils subjected to long-term grazing (> 60 years) at different intensities in Alberta, Canada. Sensitivity analysis showed that parameters controlling the N2O flux from nitrification were most sensitive. On average, the accuracy of N2O emission simulations were found to be satisfactory, as indicated by the selected goodness-of-fit statistics and predictive uncertainty band, while the model simulated the soil moisture with slightly higher accuracy. As expected, emissions were higher from the plots with greater grazing intensity. Scenario analysis showed that the N2O emissions with the recommended fertilizer rate would dominate the emissions from the projected wetter and warmer future. The combined effects of fertilization and wetter and warmer climate scenarios would increase the current N2O emission levels by more than sixfold, which would be comparable to current emission levels from agricultural soils in similar regions.

Assessing climate change impacts on fresh water resources of the Athabasca River Basin, Canada.

Proper management of blue and green water resources is important for the sustainability of ecosystems and for the socio-economic development of river basins such as the Athabasca River Basin (ARB) in Canada. For this reason, quantifying climate change impacts on these water resources at a finer temporal and spatial scale is often necessary. In this study, we used a Soil and Water Assessment Tool (SWAT) to assess climate change impacts on fresh water resources, focusing explicitly on the impacts to both blue and green water. We used future climate data generated by the Canadian Center for Climate Modelling and Analysis Regional Climate Model (CanRCM4) with a spatial resolution of 0.22°×0.22° (~25km) for two emission scenarios (RCP 4.5 and 8.5). Results projected the climate of the ARB to be wetter by 21-34% and warmer by 2-5.4°C on an annual time scale. Consequently, the annual average blue and green water flow was projected to increase by 16-54% and 11-34%, respectively, depending on the region, future period, and emission scenario. Furthermore, the annual average green water storage at the boreal region was expected to increase by 30%, while the storage was projected to remain fairly stable or decrease in other regions, especially during the summer season. On average, the fresh water resources in the ARB are likely to increase in the future. However, evidence of temporal and spatial heterogeneity could pose many future challenges to water resource planners and managers.