Dale Hebb

Modeling Sediment and Nitrogen Export from a Rural Watershed in Eastern Canada Using the Soil and Water Assessment Tool

Watershed simulation models can be used to assess agricultural nonpoint-source pollution and for environmental planning and improvement projects. However, before application of any process-based watershed model, the model performance and reliability must be tested with measured data. The Soil and Water Assessment Tool version 2005 (SWAT2005) was used to model sediment and nitrogen loads from the Thomas Brook Watershed, which drains a 7.84 km rural landscape in the Annapolis Valley of Nova Scotia, Canada. The Thomas Brook SWAT model was comprised of 28 subbasins and 265 hydrologic response units, most of them containing agricultural land use, which is the main nonpoint nitrogen source in the watershed. Crop rotation schedules were incorporated into the model using field data collected within Agriculture and Agri-Food Canadas Watershed Evaluation of Beneficial Management Practices program. Model calibration (2004-2006) and validation (2007-2008) were performed on a monthly basis using continuous stream flow, sediment, and nitrogen export measurements. Model performance was evaluated using the coefficient of determination, Nash-Sutcliff efficiency (NSE), and percent bias (PBIAS) statistics. Study results show that the model performance was satisfactory (NSE > 0.4; > 0.5) for stream flow, sediment, nitrate-nitrogen, and total nitrogen simulations. Annual corn, barley, and wheat yields were also simulated well, with PBIAS values ranging from 0.3 to 7.2%. This evaluation of SWAT demonstrated that the model has the potential to be used as a decision support tool for agricultural watershed management in Nova Scotia.

Comparison of Crop Yield and Pollution Production Response to Nitrogen Fertilization Models, Accounting for Crop Rotation Effect

Alternative mathematical functional forms commonly applied in modelling crop and pollution production response to nitrogen (N) fertilizer use were investigated. Data were generated using soil and water assessment tool modeling, and explicitly accounted for rotation effects on regression parameters. The Mitscherlich–Baule model best represented potato, carrot, and alfalfa yield response, while the quadratic model best described corn, winter wheat, and barley yield response to N fertilization. The quadratic functional form also best represented nitrate-N leaching response to N fertilization for most crops. Maximum economic rates of N fertilization for crops were sensitive to residual N effects of previous crops.

Modifying SWAT with an energy balance module to simulate snowmelt for maritime regions

Rain-on-snow events are typical in maritime climates, and they can cause serious floods and excessive losses of soils and nutrients. It is assumed that energy balance snowmelt models (EBMs) perform better in simulating these events than temperature index models (TIMs), due to their consideration of physical conditions. In this study, an energy balance snowmelt model was modified and integrated with the Soil and Water Assessment Tool (SWAT) to predict snowmelt for maritime regions. The modified EBM was tested against field measurements and simulations of the currently used TIM in SWAT for eight watersheds across Atlantic Canada. Results indicated that the EBM improved the accuracy of predicting snowmelt compared with the TIM, especially for watersheds with low forest cover, mainly due to improved simulations of rain-on-snow events. In addition, the EBM was able to provide reliable estimates of snow depths important for simulating soil temperatures during winter in Atlantic Canada. SWAT2009 source code is modified to integrate an energy balance snowmelt module.The new module is tested in eight watersheds at an Atlantic maritime region.Modeling of rain-on-snow events is improved by the new module.The new module performs better than the original module in ungauged conditions.The modified SWAT is a better option for application in maritime regions.

Integrated surface water quality assessment of a rural watershed in Nova Scotia, Canada

Integrated water quality assessments that pertain not only to crop production systems but also public health at the watershed level are growing in popularity. The Thomas brook is a small (760 ha) tributary of the Cornwallis river located in the Annapolis Valley of Nova Scotia, Canada. The Annapolis valley is the most intensively managed agroecosystem in the province, characterized by both agricultural and residential land uses. The objective of this research was to: (i) quantify concentration and loading levels of nutrients (nitrogen and phosphorus) and total coliform bacteria in the surface waters and (ii) attempt to identify the primary sources of pollution within the Thomas brook. Five sampling stations were strategically stationed in the watershed based on the different land uses within the watershed. Water quality and stream flow were monitored at these monitoring stations during a three year period (May to November from 2001 to 2003). Water quality indicators monitored included total phosphorus (TP; 0.09 mg L-1), soluble reactive phosphorus (SRP; 0.09 mg L-1), nitrate-nitrogen (NO3 --N; 2.28 mg L-1), ammonium-nitrogen (NH4+ -N; 0.24 mg L-1) and E. coli (403 cfu 100ml-1). Results show that P concentrations were greatest in a section close to an adjacent dairy farm. The largest mass loadings of nitrogen to the stream were observed in the lower reaches of the watershed, which is surrounded by agricultural cropping systems. Whereas fecal coliform loading along stream reaches were affected by both livestock operations and residential dwellings. These findings support the suggestion that integrated water quality assessment is a key to develop management strategies that minimize health and environmental risks through the contamination of water.

A Watershed Modeling Framework for Phosphorus Loading from Residential and Agricultural Sources

Phosphorus (P) loading from residential onsite wastewater systems (OWSs) into neighboring surface waters is a poorly understood process in rural watersheds; this can be further challenged when rural residential dwellings are intermixed with agricultural land use. The objectives of this research were (i) to design a P onsite wastewater simulator (POWSIM) to assess P loads from individual or clusters of residential OWSs typically used in Nova Scotia, Canada; and (ii) to simulate OWS P loads in a mixed agricultural watershed (Thomas Brook Watershed [TBW], NS) using the Soil and Water Assessment Tool (SWAT) model in conjunction with POWSIM, to predict and compare P loading from agricultural and residential sources. The POWSIM loading tool has three computational components: (i) disposal field selection and treatment media mass calculation, (ii) disposal field P treatment dynamics, and (iii) soil subsurface plume P treatment dynamics. The combination TBW POWSIM and SWAT modeling approach produced a better simulation of baseflow total P (TP) loads in both a predominantly residential subcatchment and one dominated by agriculture than the SWAT model without POWSIM. The residential subcatchment had 48% of its average annual land use TP load (simulated) contributed by OWSs, whereas the agricultural subcatchment had 39%. Watershed-scale sensitivity analyses of POWSIM input parameters for 18- and 50-yr OWS operation periods found the P loading rate into the disposal field, long-term P removal rates in the disposal field and soil systems, soil maximum P sorption capacity, and mass of native soil involved in P treatment to be most sensitive.

An Integrated Modeling Approach for Evaluation of On-Site Wastewater System Phosphorus Loading in Rural Nova Scotia Watersheds

Phosphorus (P) loading from poorly designed or hydraulically failed residential on-site wastewater treatment systems (OWS) into neighbouring surface water systems is typically an unquantified process in many rural and suburban watersheds. The transport of P from OWS to surface waters is typically related to the subsoil conditions surrounding the OWS disposal field (DF), which impact lateral movement of the wastewater plume and P sorption capacity, and the level of OWS management by the homeowner (Gold and Sims, 2000). In Nova Scotia (NS), Canada there is a wide abundance of low permeability soils, shallow bedrock and high water tables that can cause failure and improper functioning of OWS DFs (Havard et al., 2008). McCray et al. (2005) identified a need for quantitative approaches, such as watershed computer models, to assess OWS pollutant loads because of the increased importance of total maximum daily load (TMDL) planning and watershed management. Current updates to the Soil and Water Assessment Tool (SWAT) watershed scale model (version 2009) include algorithms for simulating OWS using a biozone based treatment process (Jeong et al., 2011). However, there are no specific algorithms within SWAT (version 2009) to simulate P transport via lateral subsurface flow to surface water systems, demonstrating a need for an integrated approach to simulate P fate and transport from OWS at the watershed scale.

Growing Season Surface Water Loading of Fecal Bacteria within a Rural Watershed

Water quality within the Thomas Brook watershed, a 750 ha mixed land-use catchment located in the headwaters of the Cornwallis River drainage basin, was assessed using an integrated monitoring program. The research objective was to examine spatial and temporal characteristics of fecal bacteria loading from a watershed during the growing season. Fecal coliform, Escherichia coli, total suspended solids (TSS) concentrations and stream flow were monitored at five points in the watershed during a six growing season period (May to Oct, 2001-2006). A nested watershed monitoring approach was used to determine bacterial loading from distinct source types (residential vs. agricultural). Daily loading was further differentiated into stormflow and baseflow. Bacterial loading per hectare increased in each nested watershed moving downstream through the watershed and were highest in the three subcatchments dominated by agricultural activities. Upper watershed bacterial loading from an agricultural subcatchment (Annual Avg 8.92x1010 CFU/ha) was consistently higher than a residential subcatchment (Annual Avg 8.43x109 CFU/ha). Annual stormflow bacterial loads were higher than baseflow loads. The highest daily fecal bacteria load per annum ranged from 8.1 to 20.1% of the total annual growing season load and were all preceded by at least 38 mm of precipitation. Annual fecal bacteria loads were found to be greater during growing seasons with greater annual precipitation. A positive linear relationship was observed between E. coli and TSS loading during the 2005 and 2006 growing seasons when both parameters were monitored. The E. coli and TSS loading relationship was weakest during baseflow periods (R2=0.40), higher for stormflow periods (R2=0.50), and strongest (R2=0.60) when all flow conditions were included in the regression. Further year round watershed and intensive stormflow monitoring are required to expand the understanding of fecal bacteria loading in the Thomas Brook watershed.