Runoff mitigation on agricultural fields via micro-dams – Literature review and derivation of runoff curve number reductions

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Water runoff and soil erosion by water from agricultural fields lead to a loss of water available for plant growth, loss of topsoil and transport of plant protection products (PPPs), hence constituting contaminant pathways to adjacent surface water. Several mitigation measures are used, including small earthen dams or depressions, referred to as micro-dams. This practice has been applied worldwide since the 1930s. In the regulatory exposure assessment for the registration of PPPs, runoff curve numbers (CNs) are used in the PRZM model to quantify runoff and the effect of mitigation measures. An overview of the pertinent literature is presented to enlarge the knowledge base to give robust recommendations for the inclusion of the practice in the regulatory exposure assessment of PPPs. Literature on micro-dams was collected here in a review of the reported results from field trials in terms of runoff, erosion, and PPP transport mitigation. Selected data were further evaluated to derive runoff CNs. From 21 studies with 252 individual setups and trials, we derived a geometric mean reduction of runoff of 62% (42%–90%); results for maize and potatoes only were 62% (53%–73%) and 81% (71%–93%), respectively. Erosion was reduced by 73% (60%–90%; maize: 75% [63%–88%]; potatoes 89% [83%–97%]). From 19 studies with 246 individual setups and trials, the geometric mean of the CN reduction is 11% (4%–25%; maize: 7% [3%–17%], potatoes: 14% [6%–36%]). For PPP transport (5 studies; 10 different PPP), a geometric mean reduction of 67% (48%–94%) was derived (maize: 56% [38%–81%; 3 studies; 10 PPP]; potatoes: 91% [1 study; 4 PPP]). The application of micro-dams provides considerable reductions and can therefore be factored into the environmental exposure assessment by using percent reductions of runoff, erosion, and PPP transport or lowering the runoff CN in numerical modelling.