A Geotechnical Engineering Approach to Plant Transpiration and Root Water Uptake
Tratch, David Jonathon
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In recent years, the field of geotechnical engineering has identified the significance of the surface mass flux boundary condition on the soil water flow conditions through the vadose zone of the soil profile. This thesis investigates the soil surface flux which is directly a result of the vegetative cover in the form of transpiration. Application of transpiration theory may be extended to any geotechnical engineering problems which involves vegetated surfaces. Typical applications include moisture redistribution evaluation for volume change and stress analysis, contaminant transport and evaluation of the performance of soil covers over hazardous waste sites. Historical literature on the available methods to predict transpiration and root water uptake were consulted. Methods available varied between highly empirical methods, which lack accuracy, to physically based methods which are too complicated for practical use. A semi-empirical method of evaluating the transpiration rate by the potential evaporation rate, degree of vegetative cover and matric suction profiles through the root zone was determined to be reasonable for use by geotechnical engineers. The method was incorporated into an existing l-dimensional heat and mass transfer finite element computer program by the name of ' SoilCover'. A laboratory program was conducted to measure the evapotranspiration flux rate from a vegetated soil surface and the moisture redistribution patterns through the soil profile. The experiment consisted of a vegetated soil column system placed in an environmental chamber. The surface of the soil column was allowed to evapotranspire at the prevailing rates and the base of the soil column was subject to a hydraulic head boundary condition for a period of 47 days followed by a zero flux boundary condition. In addition to the base and surface flux rates of the soil column, the soil column profile was instrumented for matric suction, temperature and manual water content determinations. Climatic variables measured in the environmental chamber included pan evaporation rates, evaporating pan and ambient temperature, and ambient relative humidity. Data from the laboratory program were used to verify the proposed methodology to predict transpiration. As well, a field data set from the Matador Site program, was used to verify the predictive methodology using typical field data. In both cases, the analytical solution provided by SoilCover was in agreement with the measured data trends. The predictive methodology appears to be a simple method with variables which are relatively easy to define and measure by geotechnical engineers. The most difficult factor in the predictive methodology involves the definition of the boundaries of the active root zone. In the laboratory program, the upper limit of the active root zone did not perform in a reproducible fashion. Alternatively, the field data simulation results show the lower root boundary to behave in a relatively unpredictable manner. However, the active root boundaries could be calibrated using measured surface flux and water content profiles for both the laboratory and field cases. Also, relatively minor changes to the definition of the active root zone result in significantly modified results for both data sets. The unpredictable behavior of the active root zone is of particular concern to predictive modelling attempts, if a detailed data set is not available. In summary, this thesis demonstrates the ability to incorporate knowledge and theories which were developed by soil scientists. With this knowledge, geotechnical engineers can be successful in providing analytical solutions to the problem of the surface flux transpiration boundary condition.