Influence of scale and land use pattern on the efficacy of grassed waterways to control runoff

Abstract

Grassed waterways (GWWs) are established where runoff from arable land concentrates. They provide travel distances of some hundreds meters over hydraulically rough, flat-bottomed surfaces. Studies in small watersheds (<100 ha) have demonstrated a large reduction in runoff volume and peak discharge but it is unknown to who extent large watersheds (>1000 ha) also benefit from these effects, when land uses other than arable land also contribute and when travel time increases due to the increasing flow path length. We analyzed this by a modeling approach because controlled experiments can hardly be applied for large watersheds. Two summers, one prior to and one after small grain harvest, and one winter condition and recurrence times of 2, 10, 20 and 50 yr were taken into account. Land use was assumed to be either dominated by arable land (80%) or varying between sub-watersheds with arable land contributing only 45% on average. Under predominantly arable land use 2.3% of the total land was found suitable to be converted to GWWs, while for a diversified land use only 0.8% of the total land called for a GWW. For all conditions the efficacy of GWWs to reduce runoff volume and peak discharge decreased only slightly with increasing watershed size. Under arable land use and summer conditions-runoff volume was reduced by about 30% and peak discharge by about 40% with somewhat higher values for more frequent storms and lower values for rare storms. The efficacy was considerably lower under winter conditions and for a diversified land use where only a small proportion of GWWs was assumed. Runoff reduction was affected more and may drop below 5% under unfavorable conditions (low GWW percentage, winter, large events) while still a reduction in peak discharge of at least 15% was observed even under most of the unfavorable conditions despite a loss of land of only 0.8%. GWWs hence contribute considerably to flood control even in watersheds larger than 1000 ha and especially when summer floods are the main problem.