LDCP Framework for Setting Load Limits in Urban Catchments

Setting enforceable quality and quantity limits is a key purpose of the National Policy Statement for Freshwater Management (NPS FM). Setting limits in terms of catchment loads is supported by NPS FM. Therefore, it is important that stakeholders have a reliable means to determine the catchment load levels for various pollutants and accordingly set the limits. In urban catchments, the fate and transport of different pollutants depends on a number of conditions including catchment characteristics, weather conditions, stormwater management practices, and chemical properties of pollutants. The observed water quality displays variability depending on temporal and spatial scales of the data, which are controlled by sampling frequency and range. This makes it difficult to determine load limits at daily or weekly or monthly frequency.

In this paper, we present a “Load Duration and Catchment Prioritization” (LDCP) framework approach to characterize water quality and enable limit settings. This framework determines loading capacities for each sub-catchment. It accounts for how stream flow patterns affect changes in water quality over the course of a year. Using this framework, the frequency and magnitude of water quality standard exceedances, allowable loadings, size of load reductions, and catchment (or sub-catchment) prioritization are determined.

The underlying premise of the LDCP framework is correlation of water quality impairments to flow conditions. It helps identify maximum loads for a given time unit (day or week or month) which account for the variable nature of water quality associated with different stream flow rates. This framework provides a reasonable way to define allocations which reflect differences in the types of sources that may be dominant under various flow conditions. Allocations represent those portions of a receiving water’s loading capacity attributed to point sources or to nonpoint sources and natural background. Hence, it is useful to understand the load seasonality as well as the effect of temporal scale on load variability and water quality violations.

In the LDCP framework, each sub-catchment can be ranked according to the relative water quality improvement needed based on pollutants of concern, pollutant loadings, impairments in receiving waters, and regulatory requirements. This helps to develop a Catchment Prioritization Index (CPI) for each sub-catchment area. The CPI represents the relative water quality improvement needed for each sub-catchment on a scale from 1‐5 with 5 indicating the highest need for improvement.

A major advantage of LDCP framework in loading capacity development is the ability to provide meaningful connections between allocations and implementation efforts. Because the flow duration interval serves as a general indicator of hydrologic condition (i.e., wet versus dry and to what degree), allocations and reduction targets can be linked to source areas, delivery mechanisms, and the appropriate set of management practices.