Seasonal variation of grassed waterway effectiveness in reducing runoff and sediment delivery from agricultural watersheds in temperate Europe

Grassed waterways (GWWs) with large hydraulic roughness exhibit a great potential to reduce runoff, sediments and pollutants coming from agricultural watersheds. For conservation planning the knowledge of overall effectiveness and its seasonal variation is highly relevant. Our objectives were to (i) evaluate the seasonal variation in runoff reduction and sediment trapping in a GWW, (ii) identify the driving parameters and (iii) measure and analyse the seasonal variation of each of these parameters. and sediment delivery were measured between 1994 and 2001 in two paired subwatersheds, both optimised to reduce runoff and sediment delivery by an intensive soil conservation system within the fields. In one of the subwatersheds additionally a GWW (290 m long, 37 m wide) was established to further improve soil and water conservation. During the observation period it reduced runoff and sediment delivery by 87 and 93%, respectively. 70% of total outflow and 68% of total sediment output occurred between February and April, mainly controlled by watershed hydrology. Seasonal changes in GWW properties, namely soil water content and hydraulic roughness, had a minor effect. It was most notably in May and June, when available field capacity averaged 59% while inflow was dominated by single heavy rain events (15% of total inflow). In general, the results indicate the high potential of GWWs for reducing runoff and sediment delivery, especially if combined with an intensive soil and water conservation system in the draining fields. For conservation planning, the least effectiveness at the end of winter should be taken into account.