Plant age effects on soil infiltration rate during early plant establishment

Infiltration rate affects slope stability by determining the rate of water transport to potential failure planes. This note considers the influences of vegetation (grass and willow) establishment and root growth dynamics on infiltration rate, as related to establishing vegetation on bioengineered slopes. Soil columns of silty sand with and without vegetation were tested by constant-head infiltration tests at 2, 4, 6 and 8 weeks after planting. Infiltration rate increased linearly with plant age and below-ground traits including root biomass and root length density. Infiltration rate for willow-rooted soil was an order of magnitude higher than for fallow soil. The plant age effect was more prominent for willow, which grew faster and with thicker roots than the grass. Illustrative seepage analysis suggests that ignoring the plant age effects could underestimate wetting front advancement to greater depths during rainfall, and underestimate suction recovery at shallow depths during internal drainage.