Evaluating connectivity risk of farm roadway runoff with waters - Development and sensitivity analysis of a semi quantitative risk model

Farm roadways are an important sub-component of the nutrient transfer continuum (NTC) and roadway runoff (RR), leading to nutrient pressures in receiving waters at different times of the year at catchment scale. This study developed a semi-quantitative risk assessment model for dairy farms that once populated with data identifies roadway sections where RR enters waters. The model contains parameters that represent source, mobilisation and transport-connectivity stages of the NTC defined as continuous or categorical variables. Each parameter has a corresponding scoring system in terms of connectivity likelihood to waters (L) and the associated impact on water quality (I) from which field data can be converted to a risk score (RS). The connectivity or impact risk of any roadway section is a sum of all parameter scores, i.e. ‘Total Risk Score’ (TRS). The risk scores were classified into 5 categories (very low, low, moderate, high and very high). Field data from seven farms enabled five equal interval risk score classifications to be developed (very low (110–134), low (135–158), moderate (159–182), high (183–206), very high (207–230)). Fieldwork data showed differences between the number of mapped roadway sections ranging from 35 to 76, with the lowest and highest risk scores being 110 and 230, respectively. Out of all sections scored 25.9 %, 45.6 %, 20.4 %, 6.4 %, and 2 % were in very low, low, moderate, high and very high categories, respectively. In terms of management, only 8.4 % (i.e. high or very high scores) had all components of the NTC and required RR mitigation. An examination of the mobilisation parameter showed that the % of roadway sections needing mitigation is likely to increase if rainfall increases on these farms. An uncertainty assessment limiting the model to different levels of connectivity confirmed that all components of the NTC and those with greater than moderate risk should only be considered in future mitigation plans. Future work should concentrate on adapting this methodology to a wide range of farm enterprises.