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 Canada's 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.

Construction of large on-site detention pond to manage the stormwater runoff in the project area is not only expensive, but also a waste of developable land. To minimize the pond size, a systematic design of storm fl ow routing followed by model verifi cation is necessary. This study presents a challenging stormwater management design for a site located in a complex urban setting at Tallahassee, Florida, USA. The site, a 4.6-hectare (11.4 acre) wooded area, was developed into a swimming pool complex resulting in increased post-development runoff. This increased runoff was managed by designing an on-site pond, minimized by placing it in series with an existing downstream off-site pond of a closed basin. The available storage of the downstream pond was effi ciently used to reduce the upstream pond size. To minimize the on-site pond, the design considered rearrangement and re-sizing of pre-development basins that allowed releasing some portion of post-development runoff below its pre-development level in the directions where it was allowed to drain. The excess runoff generated from the area was routed through the on-site pond and discharged into the existing off-site pond, where all runoff was retained to meet the guidelines of a closed basin. The short duration simulation results (8-hr and 24-hr design storms) confi rmed signifi cant off-site runoff reduction for the postdevelopment condition. Besides short duration simulations, the extended simulation results (for the entire 1-yr period) also revealed that the on-site and off-site ponds can jointly manage all extreme runoff including the runoff of a historical extreme wet year.

In a closed basin, where runoff is not allowed to discharge outside the watershed, a systematic design of storm water routing with retention facility is necessary to manage the runoff. This article presents a successful design to route the excess runoff generated due to the change in land-cover from an 11.4 acre site in Tallahassee, Florida, USA. The site development involved conversion of a wooded area to an aquatics complex with pools, buildings, parking lots, and driveway access etc. creating 2.4 acre impervious area. A pond with adequate storage capacity was not feasible to construct within the site due to site constraints and high-cost. The runoff generated from the new impervious area was designed to route through the newly designed small onsite dry-detention pond and discharged to an existing offsite pond, located within a closed basin. The analysis showed successful design of an onsite pond that retains as much water as possible within the site and safely releasing excess volume downstream to the offsite pond. The results confirmed that the offsite pond in the closed basin can retain all runoff without any discharge and the two ponds in series can handle any extreme storm in an integrated manner.