Prediction of concentrated flow width in ephemeral gully channels

Empirical prediction equations of the form W = aQ(h) have been reported for rills and rivers, but not for ephemeral gullies. In this study six experimental data sets are used to establish a relationship between channel width (W, m) and flow discharge (Q, m(3) s(-1)) for ephemeral gullies formed on cropland. The resulting regression equation (W = 2.51 Q(0.412); R-2 = 0.72; n = 67) predicts observed channel width reasonably well. Owing to logistic limitations related to the respective experimental set ups, only relatively small runoff discharges (i.e. Q < 0.02 m(3) s(-1)) were covered. Using field data, where measured ephemeral gully channel width was attributed to a calculated peak runoff discharge on sealed cropland, the application field of the regression equation was extended towards larger discharges (i.e. 5 x 10(-4) m(3) s(-1) < Q < 0.1 m(3) s(-1)). Comparing W-Q relationships for concentrated flow channels revealed that the discharge exponent (b) varies from 0.3 for rills over 0.4 for gullies to 0.5 for rivers. This shift in b may be the result of: (i) differences in flow shear stress distribution over the wetted perimeter between rills, gullies and rivers, (ii) a decrease in probability of a channel formed in soil material with uniform erosion resistance from rills over gullies to rivers and (iii) a decrease in average surface slope from rills over gullies to rivers. The proposed W-Q equation for ephemeral gullies is valid for (sealed) cropland with no significant change in erosion resistance with depth. Two example, illustrate limitations of the W-Q approach. In a first example, vertical erosion is hindered by a frozen subsoil. The second example relates to a typical summer Situation where the soil moisture profile of an agricultural field makes the top 0.02 in five times more erodible than the underlying soil material. For both case's observed W values are larger than those predicted by the established channel width equation for concentrated flow on cropland. For the frozen soils the equation W = 3.17 Q(0.368) (R-2 = 0.78; n = 617) was established, but for the summer soils no equation could be established. Copyright (C) 2002 John Wiley Sons, Ltd.