Development of Laboratory Experiments and Numerical Modeling Techniques to Quantify Vadose Zone Water Fluxes in Arid Regions

The quantification of vadose zone water fluxes in arid regions poses many difficulties due to low water input, coupled thermal and isothermal processes, spatial and temporal highly variable meteorological conditions and measurement deficits regarding spatial and temporal resolutions as well as measurement errors. Nevertheless, water flux processes in the vadose zone have to be understood and quantified as they govern rates of direct groundwater recharge. To quantify vadose zone water fluxes under controlled conditions, laboratory soil column experiments were developed that mimic atmospheric and soil water conditions as they can be expected in arid regions. The experimental setup allowed to measure water content and temperature distribution within a 92 cm deep soil profile in high temporal and spatial resolution. At the top of the column, a head space with controllable air stream, water input and applied temperature accounted for the simulation of changing atmospheric conditions. At the column bottom, temperature and outflow pressure were applied to obtain a temperature gradient within the column and water discharge under controlled pressure conditions. By applying different initial and boundary conditions, soil water dynamics and temperature distributions were studied for two different sands. Water content profiles, that were measured with a TDR “Taupe” cable, showed almost uniform infiltration fronts for steady-state experiments. Subsequent experimental runs indicated the high impact of irrigation amount and intensity on water infiltration, evaporation and redistribution within the sands. Obviously, only single irrigations exceeding potential evaporation and lasting long enough to infiltrate deeper than 20 cm, could account for discharge at the bottom of the column, depending on successively applied irrigations. According to the experiments, a numerical model was set up in Hydrus-1D, simulating coupled water, vapor and temperature fluxes in variably-saturated media (Šimunek et al., 2009). Hydraulic and thermal soil parameters, which are implemented into the model, were calibrated with experimental data of water content and temperature profiles at different times as well as transient water discharge and evaporation. Amongst the calibrated parameters, those controlling high saturated flow were less sensitive than those controlling evaporation and drainage, whereas highest sensitivities were obtained for the air entry pressure of the retention function of Brooks and Corey (1964). With the calibrated model, predictive scenario modeling was performed representing annual changing soil moisture conditions to identify parameters of primary importance for possible groundwater recharge in arid regions. The predictive modeling emphasized the high importance of single precipitation amounts on deep infiltration and percolation which can induce groundwater recharge. For annually low precipitation amounts, the residual water content of the ambient soil mainly determined percolation processes. Vapor fluxes, induced by temperature gradients, played a major role in total water fluxes under low saturated conditions. The laboratory experiments were a good tool for first estimates of vadose zone water fluxes under arid conditions and were essential for the model setup and calibration. Based on the calibrated model further predictions upon vertical water fluxes and deep percolation for critical meteorological conditions could be made. By this, the model offers a valuable tool for groundwater management issues, especially regarding smart field observation and measurement schemes and initial predictions on soil water states for expected future hydrological and microclimatological changes

Soil column experiments to quantify vadose zone water fluxes in arid settings

For the determination of groundwater recharge processes in arid environments, vadose zone water fluxes and water storage should be considered. To better understand and quantify vadose zone processes influencing groundwater recharge, a soil column experimental setup has been developed that mimics arid atmospheric conditions and measures water and temperature fluxes in high temporal and spatial resolution. The focus of the experiment was on the determination of water infiltration, redistribution, evaporation and percolation under non-isothermal conditions. TDR rod sensors and a specific TDR "Taupe" cable sensor were used for water content measurements and allowed the infiltration fronts to be traced over the whole column length. Applying single irrigations of different amount and intensity showed the applicability of the experimental setup for the measurement of water movement in the unsaturated soil column