The presence of vegetation at the interface between the soil and the atmosphere is the situation most commonly found in nature. The hydrological regime in the vadose zone is therefore influenced by the presence of plants and by the mutual interaction between
the different components of the soil-plant-atmosphere continuum. A better understanding of the hydrological response of a vegetated ground is key in several fields, from agriculture and the study of ecosystems to the use of vegetation as a remedial measure for a number of geotechnical problems (e.g. slope instability). The study undertaken in this dissertation aimed at improving the understanding of the hydraulic process of water removal from a vegetated system with the long term objective of ‘engineering’ vegetation to stabilise natural and man-made slopes. The initial focus was on the development of an integrated system for the monitoring of the hydraulic behaviour of the coupled soil-plant system in a coherent and consistent way. The work has then investigated the different mechanisms of water extraction occurring
in a vegetated and a bare ground. A novel technique was developed to monitor the xylem water pressure. The High Capacity Tensiometer (HCT), developed by geotechnical researchers, was tested on plants to measure the xylem water pressure. The instrument showed it to be consistent with techniques routinely used in plant science, both in the field and laboratory conditions, for discontinuous and continuous monitoring of xylem water pressure. The novel procedure for the measurement of negative xylem water pressure is a step change in the study of continuous flow along the soil-plant system, especially in the geotechnical field. This allows the use of a single instrument to monitor the entire soil plant continuum. The effectiveness of vegetation in removing soil water by transpiration was then investigated. The overall methodology consisted in comparing the soil water regime generated by transpiration (from vegetated soil) with the one generated by evaporation (from bare soil), both in the laboratory and in the field. Two soil columns were developed, one vegetated and one left bare to compare the transpiration and evaporation respectively under the same laboratory atmospheric conditions. The experiments showed that differences in water extraction between bare and vegetated soils depend on whether transpiration/evaporation occurs in the water limited or energy limited regimes, with plants showing better efficiency in generating suction only in the case of water limited regime. For the case of bare ground, the concentration of water extraction at the soil-atmosphere interface generated high hydraulic gradients that, in turn, reduced the hydraulic conductivity of the soil at such an interface with the effect of reducing the outward flow. The outcome of the experiments was confirmed by numerical simulations. In these simulations, the water flow was modelled on the basis of the water pressure in the soil-plant system monitored
using High-Capacity Tensiometers. The interpretation of the results allowed a more robust experimentally-based approach to be developed for the estimation of the coefficients of the Feddes function. The experimental setup validated at the laboratory was ‘scaled up’ to the field by monitoring a plantation of Poplar trees in Southern France for four months. The water content profile was monitored throughout the dry season and the following rainy period in a poplar vegetated area as well as in the adjacent ploughed (virtually bare) field. The results were interpreted based on the conceptual framework developed during the experiments undertaken under controlled conditions in the laboratory. The
field test allowed the hydraulic behaviour of the SPAC to be monitored identifying potential and current limitations. In summary, this dissertation has developed a specific experimental procedure and tested the use of High-Capacity Tensiometers for the measurement of water pressure along the soil-plant system. Furthermore, it has configured an experimental setup to be implemented in the field for the study of the water extraction efficiency of vegetation considering the interplay between soil, plant, and atmosphere. -
| Date of Award | 13 Mar 2020 |
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| Original language | English |
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| Awarding Institution | - University Of Strathclyde
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| Sponsors | University of Strathclyde |
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| Supervisor | Alessandro Tarantino (Supervisor) & Charles Knapp (Supervisor) |
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