The effect of water repellent soil surface layers on preferential flow and bare soil evaporation

Wetting patterns produced by water repellent soils are able to preferentially channel moisture deep into the soil profile, minimising storage in surface layers where it is most susceptible to evaporative loss. Although this effect has been repeatedly described in the literature, the significance of such effects under field conditions remains unclear. In order to quantify the impact of water repellency, preferential flow and evaporation rates were monitored in a series of portable soil tanks packed with soil sourced from a water repellent field site. Tanks were placed outside to expose them to environmental forcing factors and their weights after rainfall and subsequent periods of drying were recorded daily. Increased water repellency across the wettable, low and medium water repellency classes led to increased maximum pathway depths and decreased cumulative evaporation. However, the high repellency class exhibited no difference to the medium repellency class. Soils layered to generate decreasing water repellency over 10–30 cm depth in distributions similar to that seen in the field recorded evaporative losses 70–80% lower than that in wettable control soils over 4 days of drying in autumn. Shallower layers of 5–15 cm examined during winter had evaporation reduced by 40–80% over a 4 day period even in a period of much reduced potential evaporation. It is concluded that water repellent surface layers are able to effect significant reductions in net evaporative moisture loss, in patterns which may be particularly beneficial during periods of high moisture stress in summer or during low-rainfall years. Though water repellency substantially breaks down in the field during winter, our results suggest it may continue to aid moisture conservation well into the winter season.