Development of a Decision-Making Framework for BMP Design to Reduce Loads During “Hot Moments”

Abstract. Inequality is an emergent property of many complex systems. For a given series of stochastic events, some events generate a disproportionately large contribution to system responses compared to other events. In catchments, such responses cause streamflow and solute loads to exhibit strong temporal inequality, with the majority of loads exported during short periods of time, generally corresponding to high-flow events. These time periods are commonly referred to as “hot moments” or “hot events”. Although this temporal inequality is widely recognized, there is currently no uniform metric for assessing it. We used a novel application of Lorenz Inequality, a method commonly used in economics to quantify income inequality, to analyze the transport of nutrient and sediment loads in the Chesapeake Bay Watershed. The analysis was used to identify “hot moments” and corresponding flowrates associated with large solute loads. These flow conditions can ultimately be used to design structural best management practices (BMPs) such as vegetated filter strips, riparian buffers, and constructed wetlands. Based on the results of the temporal inequality analysis, we developed a decision-making framework for 14 subwatersheds in the Chesapeake Bay Watershed. The goal of this framework is to identify the “windows of opportunity” for reducing nutrient loads and informing BMP design and implementation. This approach allows users to identify the fraction of time and corresponding flowrates during which a given percentage of load is exported, thereby enabling the development of a site-specific tool that can be used to reduce loads and achieve water quality goals.