Grassed waterways

Grassed waterways (GWWs) are a widely used best management practice (BMP) in countries with large-area farming. They are broad, shallow, grass-lined channels often located within large fields and have the primary function of draining surface runoff from farmland and preventing gullying along the natural drainage ways. They are rarely found in areas with relatively small fields, for example, in many European countries, where a single GWW would drain a number of fields. Presently, GWWs seem to attract much less attention as BMPs than grassed or vegetated filter strips at the down-slope end of fields or along the surface of bodies of water, even though the benefits of GWWs are obvious. Studies clearly show that well-established GWWs effectively prevent gully erosion, reduce sediment and agrochemical delivery, and dampen peak discharge rates. The effects of GWWs on plant and faunal diversity are insufficiently studied, and our knowledge of them is based mostly on analogies from grass strips or set-aside areas. Use of such analogies might be especially misleading with regard to the effects of GWWs on ecological connectivity within arable landscapes. From an agricultural viewpoint, the benefits of GWWs clearly seem to overcompensate for the main disadvantage: loss of land for cultivation. But arriving at general conclusions is difficult due to the huge variation in land prices and establishment costs. In economic terms, GWWs are thought to have a life span of ten years, a misjudgment that may be responsible for less relevance of GWWs since the 1970s. From political and societal viewpoints, outreach programs and financial incentives appear to be good investments. Apart from effectively preventing erosion damage (e.g., muddy floods), society would benefit from other ecological effects that are not included in the economic considerations of farmers. But scientific knowledge regarding the ecological and economic benefits and economic costs of GWWs is incomplete, impeding a holistic assessment of them. This is the case regarding the obviously intended effects on sediment delivery and discharge from agricultural land, because watershed-scale studies are rare and available modeling approaches therefore lack rigid testing.