Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/1/2005
Publication Date: 9/1/2005
Citation: Harmel, R.D., King, K.W. 2005. Uncertainty in measured sediment and nutrient flux in runoff from small agricultural watersheds. Transactions of the ASAE. 48(5):1713-1721.
Interpretive Summary: Nonpoint source pollution is generated when rainfall runs off from areas such as cities, farms, and forest operations. When excessive nonpoint source pollutants enter rivers and lakes, they can degrade fish and wildlife habitat, increase water treatment costs, and diminish recreational value. Because of these impacts on water quality, many efforts are underway to understand and control nonpoint source pollution. Automated sampling equipment is typically used to measure storm water runoff because manual sampling is especially difficult due to short storm durations, remote sampling sites, adverse weather conditions, and little advance warning. Although many agencies are using automated sampling equipment, little research on the errors associated with various sampling techniques is available. Therefore, our goal was to collect field data on the uncertainty of several automated sampling techniques. Results showed that errors were greater for sediment than for nitrate probably because sediment concentrations varied more than nitrate within runoff events. Also, increasing the sampling interval resulted in greater error than did increasing the number of samples composited into a sample bottle.
Technical Abstract: In an effort to quantify uncertainty in pollutant flux measurement for flow-interval sampling techniques, water quality data were collected from two watersheds in Central Texas. Each watershed was instrumented with two automated samplers. One sampler was programmed to take high frequency composite samples in order to determine the actual load for each storm. The other sampler collected discrete samples, from which strategies with 1.32 to 5.28 mm sampling intervals with discrete and composite sampling were produced. The measured or actual load was compared to load estimates from fifteen strategies to quantify error in the estimates. Errors were greater for sediment than for nitrate in both individual event and accumulated loads. This result supported the expectation that sampling errors are greater for constituents that experience greater within-event concentration variability, sediment in this case. Sediment loads were overestimated in high peak flow events, and the magnitude of overestimation increased as sampling interval increased. Cumulative nitrate loss was overestimated at the 1.32 mm sampling interval, but cumulative loss was underestimated as the sampling interval increased. Increasing the sampling interval by increasing the flow between samples resulted in greater sampling error than did increasing the number of composited samples.