Root water uptake under non-uniform transient salinity and water stress

The study described in this thesis focuses on the quantitative understanding of water uptake by roots under separate and combined salinity and water stresses. The major difficulty in solving Richards' equation stems from the lack of a sink term function that adequately describes root water uptake. From the existing microscopic and macroscopic sink term functions, the empirical macroscopic approach was chosen because it requires the least number of parameters whose values can readily be determined. All existing reduction functions as well as those newly developed in this study are used in the macroscopic model and tested against experimental data. The experimentally obtained data are used to derive the parameter values needed for the simulation model HYSWASOR. The experiments cover root water uptake by alfalfa under salinity stress, water stress, and combined salinity and water stress. This order is followed with the analysis of the data and the simulation. Under salinity stress , both experimental and simulated results indicate that the well-known linear crop response function can be used as a reduction function. The parameter values available in the literature for different reduction functions cannot provide acceptable agreement with the experimental data. When experimentally derived parameters are used in the simulation model, the agreement becomes much closer, but calibration is still needed. The parameter values obtained by calibration differ slightly from the experiments, because the experimentally derived parameter values are based upon mean soil solution salinity. Both experimental and simulation results indicate that different salinity reduction functions can provide almost the same results if the parameter values are well specified. For practical use the linear reduction function with the least number of parameters appears to be adequate. Under water stress , all existing reduction functions as well as the one developed in this study are tested on the experimental data. Since the trend of the experimental relative transpiration versus mean soil water pressure head is nonlinear, the linear reduction function cannot fit the data. The existing nonlinear reduction functions can fit only half of the data range satisfactorily. The best agreement is obtained with the newly developednonlinear two-threshold reduction function . The parameter values obtained by calibration differ only slightly from those of the experiments. Soil water pressure head heterogeneity over the root zone does not play an important role in water uptake. The roots appear to take up water from the relatively wetter parts of the root zone to compensate for the water deficit in the drier parts. On the first day after irrigation both relative transpiration and relative leaf water head are almost the same for the stressed and non-stressed plants. While the simulated transpiration agrees closely with the experimental data, the main reason for the discrepancy between the simulated and actual water contents appears to be water uptake during the night. Under combined water and salinity stress , the additive and multiplicative reduction functions are first tested against the experimental data and then inserted in the simulation model. A new combination reduction function is introduced that differs conceptually from the additive and multiplicative functions. Both the experimental and simulated results show that the newly proposed model fits the data best, while the worst results are obtained with the simple additive model.