Comparisons of soil suction induced by evapotranspiration and transpiration of S. heptaphylla

For a given evapotranspiration (ETr), both soil evaporation and plant transpiration (Tr) would induce soil suction. However, the relative contribution of these two processes to the amount of suction induced is not clear. The objective of this study is to quantify ETr- and transpiration-induced suction by a selected tree species, Scheffllera heptaphylla, in silty sand. The relative contribution of transpiration and evaporation to the responses of suction is then explored based on observed differences in transpiration- and ETr-induced suction. In total, 12 test boxes were used for testing, 10 for vegetated soil with different values of Leaf area index (LAI) and Root Area Index (RAI), while two were for bare soil as references. Each box was exposed under an identical atmospheric condition controlled in a plant room for monitoring suction responses over a week. Due to the additional effects of soil evaporation, ET-induced suction could be 3% – 47% higher than transpiration-induced suction, depending on LAI. The significance of evaporation reduced substantially when LAI was higher, as relatively less radiant energy fell on the soil surface for evaporation. For a given LAI, the effects of evaporation were less significant at deeper depths within the root zone. The effects of RAI associated with root-water uptake upon transpiration were the dominant process of ETr affecting the suction responses.