A runoff risk model based on topographic wetness indices and probability distributions of rainfall and soil moisture for central New York agricultural fields

Nonpoint source (NPS) pollution continues to be the leading cause of US surface water degradation, especially in agricultural areas. In humid regions where variable source area (VSA) hydrology dominates storm runoff, NPS pollution is generated where VSAs coincide with polluting activities. Mapping storm runoff generating areas could allow for more precise and informed targeting of NPS pollution mitigation practices in agricultural landscapes. Previous efforts to do this have relied on simulation models or potentially over-simplifying assumptions about the interactions between rainfall and landscape features that generate storm runoff. We surveyed volumetric water content (VWC) across five agricultural field sites in central New York over two years (2012 to 2014) and converted these into VWC probability maps based on a soil topographic index (STI). We assumed that the threshold for runoff occurred when the combination of antecedent soil water and rainfall were sufficient to saturate the soil. We combined VWC probability distributions with rainfall frequency to determine saturation-excess runoff probability. This analysis indicated that the risk of NPS pollution in runoff can be reduced by 40% to 97% by taking 10% of the highest-runoff-risk agricultural land out of production or halting polluting activities in these areas. This approach will be most effective if applied to watersheds rather than field-by-field. This analysis can be used to determine the optimal placement of conservation easements or management practices for the protection of water quality.