COMPARISON OF TIME-BASED SAMPLING STRATEGIES TO DETERMINE NITROGEN LOADING IN PLOT-SCALE RUNOFF

摘要

Water quality loadings are generally calculated without knowledge of the relationship of the calculated loads to the total loads. A laboratory runoff study was designed and conducted to compare total loads with loads calculated from time-based sampling strategies. Total loads were measured by capturing all the runoff from 2.2 m 2 Bermuda grass ( Cynodon dactylon L. Pers.) sod plots with 5% slope and analyzing for NO ₃ +NO ₂ -N and NH ₄ -N. Runoff samples were also manually collected on 1 min intervals during 2 h overland flow events. Total loads were compared to time-discrete and time-composite sampling strategies. The strategies 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 aliquots per composite sample based on the same time-discrete intervals. In addition, loads were also calculated from a composite sample derived from aliquots collected at 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 min intervals. The calculated load of NO ₃ +NO ₂ -N and NH ₄ -N was not significantly different ( a = 0.05) from the total load when using time-discrete sampling at 1, 2, or 3 min time intervals. No significant difference ( a = 0.05) from the total load was found when using a composite approach with 2, 3, 4, or 5 aliquots collected at 1, 2, or 3 min time intervals or when using a composite sample with aliquots collected on a 1 min interval. To preserve the total 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 plot-scale studies