Evaporation from soils of different texture covered by layers of water repellent and wettable soils

Water repellent soils are able to channel water deep into the soil profile by fingered flow, minimising water storage in the water repellent top layer where water is most susceptible to evaporation. To date, the effect of water repellent or wettable surface layer on evaporation from wet sublayer has only been reported for coarse materials, and an increase in water repellency led to a greater delay in water evaporation. The objective of this study was to assess the effect of water repellent vs. wettable top layers with different thickness on water evaporation from coarse and fine texture subsoils that were pre-moistened. Clay loam soil samples were taken from Pinus pinaster woodland of Ciavolo, Italy, and sandy soil samples from Pinus sylvestris woodland of Sekule, Slovakia. Evaporation from soil samples was determined from the loss of weight in laboratory conditions. Water in the clay loam soil from Ciavolo was held for a longer period due to slower evaporative loss than in the sandy soil from Sekule, and the impact of the water repellent layer on the loss rate over time is related to its thickness. Over 550 h, about 90% of the initial stored water was evaporated from the uncovered clay-loam soil sample from Ciavolo. In the same time, the 0.3, 1, and 2 cm-thick duff layers, respectively, saved about 23, 34, and 58% of water from evaporation, and evaporation of 90% of water took over 780, 1100, and 1450 h. It means that the clay loam soil cover with the 0.3, 1, and 2 cm-thick duff layers resulted in prolonging the evaporation by 10, 23, and 37.5 days, respectively. As to the sandy soil from Sekule, 98% of water was evaporated from the uncovered soil sample over 240 h. In the same time, the 0.3, 1, and 2 cm-thick water repellent soil layers, respectively, saved about 7, 45, and 59% of water from evaporation, and evaporation of 98% of water took over 330, 606, and 774 h. It means that the sandy soil cover with the 0.3, 1, and 2 cm-thick water repellent soil layers resulted in prolonging the evaporation by about 4, 15, and 22 days, respectively. It can be concluded that water repellent surface layers, created by pine trees, are able to delay evaporation significantly for both coarse and fine textured soils, which may be particularly beneficial for plants during hot and dry periods in summer.