A spatially explicit statistical model to quantify nutrient sources,pathways, and delivery at the regional scale

Nutrient loading has been linked to many issues including eutrophication, harmful algal blooms, and decreases in aquatic species diversity. In order to develop mitigation strategies to control nutrient sources, the relative contribution and spatial distribution of nutrient sources must be quantified. Though many watershed nutrient models exist in the literature, there is generally a tradeoff between the scale of the model and the level of detail regarding the individual sources of nutrients and basin transport and fate characteristics. To examine the link between watershed nutrient sources, landscape processes, and in-stream loads in the Lower Peninsula of Michigan, a spatially explicit nutrient loading model was developed. The model uses spatially explicit descriptions of nutrient sources and a novel statistical model describing spatially-explicit nutrient attenuation along transport pathways to predict total nitrogen and phosphorus loads. Observations collected during baseflow and melt conditions from 2010–2012 were used to calibrate and validate the model. The model predicts nutrient loads, provides information on the sources of nutrients within each watershed, and estimates the relative contribution of different sources to the overall nutrient load. The model results indicate that there is a high degree of variability in seasonal nutrient export rates, which can be significantly greater during snow melt conditions than during baseflow. In addition, the model highlights the considerable variability in seasonal pathways and processes that impact nutrient delivery. The model performance compares favorably to other regional scale nutrient models. This work has the potential to provide valuable information to environmental managers regarding how and where to target efforts to reduce nutrient loads in surface water.