Effect of the structural organization of stagnic luvisols on drainage outflow

Abstract

Drainflow and rainfall were recorded during a 10-yr period in 12 drainage plots, each 0.54 ha in area, that had been established on a stagnic luvisol (FAO soil classification) on the second terrace of the Garonne River in a 16-ha field cultivated with maize. Considerable variation in drainage efficiency and peak drainflow was noted between plots, unrelated to drain spacing (15, 20 and 30 m). This variability can be explained by the existence of two types of structure, prismatic and vertic, in the argic Btg horizon, with a gradual transition from one to the other as a function of the topographical undulations. Outflow is on average eight times greater in the vertic than in the prismatic facies. The drainflow data are in agreement with morphological observation of the soil structure and with the results of hydraulic conductivity measurements and piezometric and tensiometric monitoring. In practical terms, these results suggest that drain spacing should be closer in vertic than in prismatic facies.