HYDRUS simulation and verification of movement characteristics of upward capillary water flow in soil as affected by multi-factor

Abstract: The important mechanism of soil salinization is that soluble salt in groundwater accumulates continuously to the topsoil under the action of capillary force. In order to study the effect of various factors on the characteristics of soil upward capillary water movement under shadow ground water table, we used HYDRUS-1D software based on the theory of saturated-unsaturated soil water movement to simulate the upward capillary water movement under the action of multiple factors. A total of 5 factors were considered including soil bulk density, clay content, initial soil moisture, groundwater depth and infiltration time. The simulation experiment was carried out by orthogonal design method including 9 treatments and 1 controlled trial. In the simulation experiment, the soil bulk density was designed with different levels of 1.30, 1.35 and 1.40 g/cm3, the clay particles content was 1.25%, 5.00% and 15.00%, the initial soil moisture was 8%, 11% and 14% and the groundwater depth was 1.5, 2.5 and 3.5 m. The controlled trial, which adopted clay loam in arid area, was carried out in State Key Laboratory Base of Eco-hydraulic Engineering. During the experiment, the capillary water recharge was measured by difference of water levels in Markov bottle. The water levels were recorded every 1, 2 and 3 hours in the 1st day, every 5 hours on the 2nd day, and every 9 hours on the 3rd day, and every 12 hours from the 4th day to the 5th day. The capillary water rising height was obtained by plotting the position of the wetting front corresponding to the different infiltration times on the outer wall of the plexiglass column. Based on the analysis of simulation results, the empirical models between the upward capillary water recharge and the capillary water rising height and various influencing factors were established, respectively. The root mean square error of the model was 0.003 cm, the correlation coefficients were all greater than 0.99, and the coefficient of determination were all greater than 0.98(P<0.01), and the relative error between the measured and calculated values were less than 11.25%. The results showed that the models could better illustrate the quantitative relationship between the upward capillary water recharge and the capillary water rising height and various influencing factors. The influence degree of each influencing factor on the upward capillary water recharge was the highest for time, followed by soil bulk density, initial moisture, clay particles content, groundwater depth, and the influence degree on the upward capillary water rising height was the highest for time, followed by initial moisture, clay content, soil bulk density and groundwater depth. The capillary water recharge had a positive correlation with clay content and time, however, had a negative correlation with soil bulk density, initial moisture and groundwater depth. The capillary water rising height had a positive correlation with initial moisture, groundwater depth and time, however, had a negative correlation with soil bulk density and clay content. Moreover, the relationship of the capillary water recharge and the rising height were fitted by the means of (intercept=0) linear function, and the determination coefficients were larger than 0.96, which indicated a significantly positive linear relationship between the upward capillary water recharge and the rising height. The results would provide a basis for making measures such as irrigation and drainage as well as saline-alkali land improvement.