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Comparison of True Loads to Various Time-Based Sampling Schemes

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Paper number  032178,  2003 ASAE Annual Meeting . (doi: 10.13031/2013.13795) @2003
Authors:   Kevin King, Daren Harmel, Eric Zwierschke
Keywords:   quality, monitoring, runoff, concentration, nutrients, bermudagrass, plot scale

Estimates of water quality loadings are often taken without any knowledge of the relation of the estimates to the true loads. A laboratory runoff simulation study was designed and conducted to compare true loads with several time-based sampling schemes. True loads were measured for three constituents (NO3+NO2- N, NH4-N, and PO4-P) on three 2.2m2 plots topped with bermudagrass (Cynodon dactylon L. Pers.) sod and set at a 5% slope. Water samples were manually collected on 1-min intervals for a runoff duration of 2-hr. The true loads were compared to an array of time-discrete and time-composite sampling schemes. The schemes included time-discrete sampling at 1, 2, 3, 4, 5, 10, 15, and 30-min and composite sampling that included 2, 3, 4, and 5 sample composites based on the same time-discrete intervals as well as a single composite sample collected at 1-min intervals. Time-discrete sampling at 1-min intervals was the only discrete sampling scheme that produced load calculations not statistically different ( = 0.05) from the true loads. A single composite sample based on 1-min interval sampling and a 5 sample composite based on 1-min intervals were the only tested composite schemes that produced loads not significantly different from the true loads. To preserve the true load from plot-scale studies, more intensive sampling is required. The results from this study will facilitate the selection of time-based sampling strategies for small plot studies.

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