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Estimating Nitrogen, Phosphorus, and Carbon Fluxes in Forested and Mixed-Use Watersheds of the Lower Coastal Plain of North Carolina: Uncertainties Associated with Infrequent Sampling

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

Citation:  Transactions of the ASABE. 54(6): 2099-2110. (doi: 10.13031/2013.40668) @2011
Authors:   F. Birgand, T. W. Appelboom, G. M. Chescheir, R. W. Skaggs
Keywords:   Annual nutrient loads, Coastal plain watershed, Dissolved carbon, Measurement uncertainty, Nitrogen, Phosphorus, Sampling strategies, Water quality

Assessing the impact of a land use change or the water quality improvement provided by a treatment system almost always involves computation of the difference in nutrient loads before and after implementation, or upstream and downstream of the system studied. Reporting meaningful values on mass balance or differences in nutrient loads implies that the uncertainty in the computed loads is several times smaller than the difference itself. This may imply very small uncertainties for the nutrient load measurements. The level of uncertainty induced by infrequent sampling on annual loads was investigated for a suite of nutrients in runoff from a forested watershed and a mixed land use watershed in the lower coastal plain of North Carolina. Reference data were used to simulate discrete sampling and to calculate new annual load estimators, which were then compared to the reference loads to calculate the level of uncertainty. Uncertainties depended on the watershed and the nutrients and other constituents, but their level was generally found to be high, around 20% and 40% or more for weekly and monthly sampling for most nutrients. This was generally attributed to the short periods of active flow in these watersheds and the flashiness of flow associated with subsurface drainage. The results suggest that to obtain uncertainties of 2% or 5% for nitrogen forms, 100 or more than 200 samples over six months of the year might be necessary in the forested and mixed-use watersheds of the lower coastal plain.

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