Consequences on runoff of the inexistence of permanent landscape elements in an agricultural landscape.
Résumé
The loam belt of the Northern Europe is periodically affected by muddy floods. Runoff coming from arable land is one of the main causes of damage, and the amount of runoff produced in arable catchments is directly influenced by the organisation of fields and permanent landscape elements in the catchments. In order to analyse the influence on runoff production of the inexistence of permanent elements in agrarian landscape we simulated runoff in several arable catchment basins (about fifty to one thousand hectares) of the Pays de Caux, France. Runoff simulations have been done with the STREAM model with data about land use at the field level collected over six crop years (1996-2001) and with two rainfall events and soil surface states representative of winter conditions (29.6 mm – 4:57 h / 22 mm – 2:19 h). Results of simulations showed that the runoff amount simulated at the outlet of a small catchment basin with the same rainfall event can be very different from one year to another. Between the two extreme years, depending on the catchment, runoff varies from 15% to 56% with the more intense rainfall event, and from 25% to 77% with the less intense rainfall event. This is due to the crop rotation on fields and to the inexistence of permanent landscape elements. Indeed, even if the mean area of fields is rather moderate, as there is almost no grass margin or hedgerow between fields, very large fields can appear. It results from the presence of the same crop in different adjoining fields cultivated sometimes by one farmer, but most of the time by several farmers. One of the catchment has been analysed more precisely: the mean area crop patches is double than the mean area of fields. As a consequence, planning fields organisation in an agricultural catchment without permanent landscape elements should take into account these possibilities of fields aggregating.