Study of water dynamics in the soil-plant-atmospheric continuum in a water-controlled ecosystem

The study of water exchange between soil, plants, and the atmosphere in response to seasonal or periodic droughts is critical to modeling the hydrologic cycle and biogeochemical processes in water-controlled ecosystems. This dissertation consists of four essential parts of water dynamics studies in an oak Savanna ecosystem.;The first study characterizes changes in evaporation and transpiration under water stress. The influence of soil moisture on evapotranspiration at the stand scale is studied using correlations between tower-based evapotranspiration measurements and representative soil moisture obtained by aggregating point measurements. The observed pattern of this effect is found in agreement with an existing model that features a linear reduction of the evapotranspiration when soil moisture falls below a critical value. The model parameters are inferred using a Bayesian framework and they are found to vary from year to year due to climate variability. The comparison between various aggregations of soil moisture at the stand scale from point measurements demonstrates that the spatial variability of the soil moisture as well as the water uptake capacity limited by the root biomass need be taken into account to produce a model that is most resistant to inter-annual variability.;The second study re-evaluated the theoretical basis of sap flow measurements using heat ratio method, whose fundamental basis was built on an idealized solution to heat transport process in sapwood. An improved solution is developed to model the same process with more realistic assumptions. Extensive comparisons on the difference of calculated temperature fields by idealized solution and improved solution reveal that most significant discrepancy occurs around the early times, whereas the difference diminishes over late time window. This study also present changes in the fundamental equation of heat ratio method to account for asymmetric probe alignments in practice.;The third study is on determining probe geometry and wood thermal diffusivity for sap flow measurements using heat ratio method. A statistical framework is presented to simultaneously estimate wood thermal diffusivity and probe geometry from in-situ heat response curves collected by the implanted probes of heat ratio apparatus. Conditioned on the heat response data, the parameters are inferred using a Bayesian inversion technique with Markov chain Monte Carlo (MCMC) sampling method. This procedure not only provides a systematic yet non-destructive to estimate the crucial parameters for sap flow calculation, it also enables direct quantification of uncertainty in estimated sap flow velocity. Experiments using synthetic data show that multiple tests on the same apparatus are essential to obtain reliable and accurate solutions, and the uncertainty in posterior distributions of the parameters is influenced by the prior knowledge on the probe geometry or heating power. When applied to field conditions, multiple tests are conducted during different seasons and automated using the existing data logging system. The seasonality of wood thermal diffusivity is obtained as a by-product of the parameter estimation process, and it is affected by both moisture content and temperature. Empirical factors are introduced to account for the influence of non-ideal probe geometry on the estimation of heat pulse velocity, and they are estimated in this study as well. The proposed methodology is ready to be applied to calibrate existing heat ratio sap flow systems at other sites. It is especially useful when alternative transpiration calibration device such as lysimeter is not available.;The fourth study investigated how an individual plant adjusts transpiration under the stressed conditions using continuous transpiration and soil moisture measurements. The objective is to appropriately calculate the potential transpiration, which is often needed in soil water dynamics models. The alternating conditional expectation (ACE) method is implemented to identify the optimal functional dependence of bulk canopy conductance on various environmental stresses including vapor pressure deficit, net radiation, and soil moisture. A multiplicative form of stress functions is found to be appropriate for the tree studied. The functional form of each individual stress is determined based on the optimal transformations identified by the ACE method, and MCMC is then implemented to estimate the model parameters. The continuous transpiration and soil moisture data are also used to investigate the water budget on the tree over the growing season of 2007. It is found that tree transpires much more water than what is provided by the root zone soil water during dry season, which is a strong evidence of tree tapping water from deeper soil and groundwater in dry seasons. This factor need be accounted for in further soil dynamics modeling by specifying appropriate boundary conditions.