Runoff on drained soils: identification of genesis mechanisms

Abstract

Agricultural subsurface drainage increases the evacuation of infiltrated water and reduces surface runoff generated by soil profile saturation. However, other factors may contribute to surface runoff generation: e.g. limitation of drainage network capacity, soil surface sealing or compaction due to wheel tracks. The objective of this study is to identify the processes involved in the surface runoff occurrences observed on a tile drained field. Such knowledge aims at improving the efficiency of solutions implemented to control the surface runoff.This study is based on results from two experimental sites: La Jaillière (Loire-Atlantique, France) and l'Orgeval (Seine-et-Marne, France). Contrary to La Jaillière, l'Orgeval soils are prone to crusting.It is shown that artificial drainage decreases significantly surface runoff in winter: the ratio of surface runoff amount on rainfall amount is 70%-80% from a non drained field and 6-12% from a drained field. Nevertheless, the drainage influence is lower during intense rainfall events and events that don't occur during the drainage season (when the shallow water table is not established). For both experimental sites, the profile saturation at midpoint between two drains explains the main surface runoff occurrences during winter. A hydraulic diagnostic of a part of the Orgeval tile drainage network allows to specify the origin of the profile saturation: the network is under dimensioned (depth, diameter and distance between drain, diameter of collector...) or the rainfall exceeds the design rainfall event. Moreover, we observed that surface runoff events induced by high water table level leads to sedimentary crust, for the soils prone to crusting. Therefore, this limits infiltration capacity at the end of winter.