Transactions of the ASAE
Vol. 47(5): 1457−1463
2004 American Society of Agricultural Engineers ISSN 0001−2351
COMPARISON OF TIME−BASED SAMPLING STRATEGIES TO
DETERMINE NITROGEN LOADING IN PLOT−SCALE RUNOFF
K. W. King, R. D. Harmel
ABSTRACT. 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 m2 Bermuda grass (Cynodon
dactylon L. Pers.) sod plots with 5% slope and analyzing for NO3+NO2−N and NH4−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 NO3+NO2−N and NH4−N was not significantly different ( = 0.05) from the total load when using
time−discrete sampling at 1, 2, or 3 min time intervals. No significant difference ( = 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.
Keywords. Bermuda grass, Concentration, Monitoring, Water quality.
he U.S. Environmental Protection Agency (U.S.
EPA, 1995, 2000) continues to identify surface wa-
ter quality as a prim