To prevent geotechnical hazards caused by rainfall, vegetation is increasily adopted to induce soil suction by root-soil hydraulic interactions. However, the typical Feddes model is unable to consider the significant effects of plant morphology. This study aims to model and evaluate the effects of plant leaf, root characteristics and height on (i) root water uptake ability, (ii) induced soil suction and (iii) resulted suction response and performance of vegetated soil during drying and wetting.

The objectives are achieved by a series of theoretical and laboratory studies. A new theoretical root water uptake model is built based on previously plant physiological studies and flow equations in soil-water-atmosphere continuum. A back-analysing technique is developed, enabling the c...[ Read more ]

To prevent geotechnical hazards caused by rainfall, vegetation is increasily adopted to induce soil suction by root-soil hydraulic interactions. However, the typical Feddes model is unable to consider the significant effects of plant morphology. This study aims to model and evaluate the effects of plant leaf, root characteristics and height on (i) root water uptake ability, (ii) induced soil suction and (iii) resulted suction response and performance of vegetated soil during drying and wetting.

The objectives are achieved by a series of theoretical and laboratory studies. A new theoretical root water uptake model is built based on previously plant physiological studies and flow equations in soil-water-atmosphere continuum. A back-analysing technique is developed, enabling the calculation of local root water uptake through measured soil suction distribution. Laboratory tests are performed by planting a single tree species, Schefflera heptaphylla, in cylindrical drums with suction distribution monitored. A total of 26 individuals, in 300, 500, 800, 1000 and 1200 mm plant height groups, are planted in these drums, subjected to drying and simulated rainfall. The theoretical model is further evaluated by measured and back-calculated results.

For the first time, the new theoretical model reasonably explains both the remained maximum and dramatic drop of root water uptake with soil total suction being lower and higher than a threshold, respectively. The model also pioneers the solutions to the threshold soil suction values, either starting water uptake reduction or entirely prohibiting water uptake, with input parameters all physically measurable through experiments. Some quantitative relationships of plant morphology and root water uptake ability are further predicted by the model, unnoticed by previous studies.

According to laboratory test results, the induced and retained soil suction is observed to be highly relevant to plant morphology. Plants with greater leaf area tend to induce higher suction when drying, while larger root zone leads to larger influence zone but a reduction local induced suction. Shorter individuals tend to induce shallow local excessive suction, while suction is more evenly distributed within root zone for taller ones. As suction at deep depth is more difficult to be destroyed during rainfall, less infiltration and percolation occur for soil vegetated with taller plants.

The back-calculated results successfully verify the assumptions and predictions of the theoretical model. water uptake is similar for root with unit length for specific individual, which is defined as water uptake length ability (WULA). Threshold suction for water uptake reduction tends to decrease linearly with LR ratio at a rate positively related to plant height. The limiting soil suction, at which plant stops water uptake, appears to be about all about 130 kPa, dominated by environmental parameters instead of plant morphology. The reliability of theoretical model is confirmed with less than 15% difference with back-calculated WULA from tests.