Approaches for Quantifying and Managing Diffuse Phosphorus Exports at the Farm/Small Catchment Scale

Abstract

Quantifying and managing diffuse P losses from small catchments or at the farm scale requires detailed knowledge of farming practices and their interaction with catchment processes. However, detailed knowledge may not be available and hence modeling is required. This paper demonstrates two approaches to developing tools that assist P losses from New Zealand or Australian dairy farms. The first is largely empirical and separates sources of P within a paddock into soil, fertilizer, dung, and treading impacts (including damage to grazed pasture). This information is combined with expert knowledge of hydrological processes and potential point sources (e.g., stream crossings) to create a deterministic model that can be used to evaluate the most cost and labor efficient method of mitigating P losses. For instance, in one example, 45% of annual P lost was attributed to the application of superphosphate just before a runoff event for which a mitigation strategy could be to use a less water soluble P fertilizer. The second approach uses a combination of interviews, expert knowledge and relationships to develop a Bayesian Network that describes P exports. The knowledge integration process helped stakeholders develop a comprehensive understanding of the problem. The Network, presented in the form of a "cause and effect", diagram provided a simple, visual representation of current knowledge that could be easily applied to individual circumstances and isolate factors having the greatest influence on P loss. Both approaches demonstrate that modeling P losses and mitigation strategies does not have to cover every process or permutation and that a degree of uncertainty can be handled to create a working model of P losses at a farm or small catchment scale.

The Potential of P Loss from Livestock Farms to Watercourses in Wensleydale; A Risk Mapping Approach

“…As stated above, P has been pointed out by numerous authors [10,[23][24][25], including the WFD, as a key nutrient for addressing water eutrophication derived from diffuse nutrient loss from agriculture. However, this is a hard task considering the complexity of the processes involved in the delivery of P to freshwaters [15].…”

Land Use: Catchment Management

“…Many of the mitigation methods implemented on farms have been recommended to or by policy makers a result of process studies that relate action to response for individual or small groups of measures (Kronvang et al, 2005;Carton et al, 2008;McDowell et al, 2009). Effectiveness of implementation of measures can also be informed by scenario analysis and modelling of the complex interactions of land management with environmental variables (Silgram et al, 2008;Oenema et al, 2009;Vigiak et al, 2011;van Grinsven et al, 2012).…”

Direct Exports of Phosphorus from Fertilizers Applied to Grazed Pastures

“…The decay curves were then used with 2 yr of field monitoring data to simulate P exports from fertilizer applications occurring on 1 June and 1 December. From this semi‐empirical approach, it was concluded that if applied in December, soluble P fertilizers would account for <10% of total P exports for the year (McDowell et al, 2009b; McDowell and Catto, 2005). While the methodology did not readily lend itself to detailed scenario testing and the outcomes of the study have been questioned (Quin et al, 2004), they are consistent with other modeling (Vadas et al, 2015).…”