Variable-width buffers to reduce sediment pollution from potato production on steep slopes: analysis of Black Brook Watershed using AgBufferBuilder

Abstract

Intensive agricultural production employs tillage, fertilizers and pesticides to obtain profitable yields. Intensive row crop production leaves soil vulnerable to erosion by water, carrying sediment, nutrients and pesticides into surface waters. Riparian buffer zones mitigate water pollution by trapping sediments, absorbing nutrients, and degrading pesticides. Many provinces in Canada have mandatory setbacks of fixed-width to be maintained between cropped land and waterways. Fixed-width buffers are easy to apply and regulate and are most effective where runoff is uniformly distributed through the entire buffer area. However, runoff flow is often not uniform, diverging from subtle ridges and concentrating into swales, reducing the effectiveness of fixed-width buffers. Effectiveness can be boosted by widening the buffer where loads are greater and narrowing it where loads are smaller, creating variable-width buffer. AgBufferBuilder is a GIS-based tool that evaluates terrain, accounts for non-uniform runoff, and assists decision-making about where to place buffers to provide the greatest environmental mitigation at less cost. AgBufferBuilder was used to examine crop fields in Black Brook Watershed, a 1450-ha model watershed in New Brunswick’s steep-sloped “Potato Belt”. The software was used to estimate the sediment trapping efficiency of existing buffers and to propose alternative placements that would trap 75% of sediments leaving these fields. Field-by-field analysis revealed examples where existing buffers were ineffective. In one case, buffers were just 9% effective; AgBufferBuilder suggested a design that used less area to obtain 47% mitigation. These numbers point to possible economic advantage for farmers who could plant for greater efficacy at less cost. Analysis of the watershed’s five sub-basins revealed advantages to using AgBufferBuilder in planning buffers at a local landscape level rather than field by field. Use of the AgBufferBuilder tool can potentially increase the cost-effectiveness of buffer installations and programs for improving water quality in New Brunswick.