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Submitted to: Meeting Abstract
Publication Type: Abstract Only Publication Acceptance Date: 1/23/2015 Publication Date: 1/30/2015 Citation: Buda, A.R. 2015. The role of hydrology in connecting agricultural phosphorus sources to surface water. Phosphorus Symposium. p. 1. Interpretive Summary: Technical Abstract: Minimizing the risk of phosphorus (P) loss from land to water represents one of the most important priorities of nutrient management in the Chesapeake Bay watershed. Simply put, for P to pose a water quality problem, there must be a source of P that can readily be connected to surface water by hydrologic transport processes. Areas with high P sources that lack hydrological connectivity do not typically constitute a water quality risk. By the same token, areas with high hydrological connectivity that do not link to high P sources also pose little threat. While the role of hydrological connectivity in P loss is simple to articulate, uncertainty arises when we try to observe the hydrological processes and landscape features linking P sources to receiving waters, to model and represent these processes in risk assessment tools, and to control them with targeted management measures. Indeed, recognizing and addressing these uncertainties is central to advancing the science of P management across the Chesapeake Bay watershed. The critical source area concept is perhaps the most recognizable illustration of how hydrological connectivity has been incorporated into P management, most notably the P Index. The critical source area concept posits that areas of the landscape posing the greatest risk to P loss are those where high P sources and high transport potential coincide. In upland areas of the Bay watershed, variable source area hydrology dominates such that small areas of watersheds are responsible for the majority of surface runoff. These runoff generating zones, typically found near streams and in lower landscape positions, expand and contract during storms and over seasons as a result of interactions between ground and surface water, storm characteristics, and soil properties. It is from this perspective that distance to receiving water has oft been viewed as one of the best proxies for hydrological connectivity in nutrient management. While the greatest risk of P loss in upland regions is usually confined to variable source areas near streams, a host of hydrological processes can modify and enhance hydrological connectivity, thus activating distant P source areas that otherwise would not normally contribute to P loss in agricultural watersheds. Chief among these are preferential flow pathways, including macropores, soil pipes, and fractures, as well as shallow lateral flows induced by the presence of soil or bedrock confining layers. While digital soil mapping, near-surface geophysics, and tracer studies can offer insight into the spatial arrangement and extent of preferential flow paths, the activation of these pathways in time and space remains dynamic and threshold-dependent (e.g., affected by variable rainfall characteristics and antecedent conditions), which complicates our ability to adequately represent these flow paths in P risk assessment tools. Even when we have good knowledge of preferential flow networks and their hydrological behavior, determining whether they are linked to P source areas is often hindered by fact that P sources (edaphic and applied) are rarely, if ever, mapped in detail across agricultural landscapes. Farm infrastructure and daily farming activities also play a role in shaping the hydrological connectivity of agricultural landscapes. For example, impervious surfaces such as barnyards, roads, and roofs can rapidly generate stormwater runoff and connect P sources on the farm with nearby receiving waters. This problem may be especially acute in areas where farms are intensifying operations by adding significant infrastructure. Soil compaction by heavy farm machinery and animal activity, as well as practices that orient soil roughness features parallel with topographic slope (e.g., the direction of plowing and cropping patterns) also create concentrated flow pathways that increase the risk |
