Development and sensitivity of a method to select time- and flow-paced storm event sampling intervals for headwater streams

Water quality research and monitoring programs often form the basis from which related legislation is derived. Yet, no standard, protocol, or method is available for guiding the selection of a water quality sampling strategy for runoff from fields and small watersheds. The objective of this study was to develop a methodology that provides guidance in the selection of a water quality sampling strategy for headwater streams (drainage areas less than 2500 ha; 6,177 ac). The developed method is based on the dimensionless unit hydrograph and relationships of measured pollutant concentrations to discharge hydrographs. The methodology was designed for storm events with a specific return interval and a selected acceptable level of error in pollutant load. Nine input parameters (hydraulic length, watershed slope, curve number, drainage area, runoff coefficient, 10-year, one hour precipitation amount, 100-year, one hour precipitation amount, 10 year, 24 hour precipitation amount, and recurrence interval) were used to develop the design hydrograph. Both time- and flow-paced sampling techniques were considered. A global sensitivity analysis of the method indicated that time-paced sampling was primarily sensitive to parameters included in the time of concentration calculation (hydraulic length, watershed slope, and curve number). Flow-paced sampling showed some sensitivity to all nine input parameters. An example application illustrates the utility of the method. Use of the method should facilitate the selection of water quality sampling strategies for field and small watershed scale studies and aid in budgetary planning for sample collection and analysis. The measurements taken based on the recommended sampling strategy will provide more confidence in the pollutant storm load estimates.