Quantifying Preferential Flow For The Soil Water Infiltration

The Hebei Plan is one of the largest bases for agriculture in China, which is suffered shortage of water resource. The groundwater is the mainly source for drinking, irrigation and industry. The percentage of water amount for irrigation accounts for72%of the total water resource. At present, over-exploitation caused a series of environmental problems, e.g., continued declining of the groundwater level, increasing of cones of depression, moving down of interface between fresh and saltine water, and superficial deposit, which were attributed to unreasonable management of groundwater. The soil water infiltration is vital for estimating the groundwater resources. However, the piston model was applied to estimate the groundwater recharge, which doesn’t agree with the case of the field, because the preferential flow was common in the filed, the main way for soil water infiltration.This paper discussed the mechanism of preferential flow on the condition of different time and space scales. The percentage of preferential flow was also estimated when the irrigation and precipitation infiltrated into soil, and even the groundwater. In Luancheng site of Chinese Academy of Science, the soil water content and potential were monitored from2003, April to2013, April, and the Bromine and Tritium tracers’test, and Brilliant Blue dyeing test were conducted. The experimental soil columns and simulated method were also applied to study the preferential flow. The results showed that:1. The phenomenon of preferential flow was common in the filed. The preferential flow paths, e.g., the plant roots and earthworm holes develop, decreasing exponentially with the depth of soil profile. The preferential flow occurred during the irrigation and rainfall, and percentages of infiltrated water for preferential flow and matrix flow were54.5and45.5%, respectively. In generally, the percentages of preferential flow were larger for more amounts of irrigation or rainfall and higher soil water content, however, the increased extent was limited.2. The multi-region model considers that the distributions of the tracers’concentrations in the soil profile were attributed to the effect of soil structure on the soil water movement. The annual recharges of Luancheng and Hengshui sites were124.3and13.7mmyr-1for the multi-region model, respectively, however,103.3and0.0mmyr-1for the traditional method, which improved the precision of groundwater recharge, especially for the areas of much lower annual recharges without peaks of tracers’concentration in the soil profile. The annual recharges of preferential and matrix flows were36.05and0.32mmyr-1, respectively, and accounted for28.7and2.3%. The straw mulching restricted preferential flow, and increased the storage of soil water in the plant root zone. Plants improved the infiltration of irrigation and rainfall, increasing preferential flow. The extent of preferential flow was controlled by the soil structure rather than the total amount of infiltration.3. The results of dyeing test showed that the model of preferential flow varied in three dimensions. The percentages of dyeing area changed from28.2to41.7%, with the average value of34.1%, which decreased with depth in the soil profiles. The earthworm holes were the main preferential flow paths rather than the plant roots. The distributed area of Bromine was larger than Brilliant Blue, which resulted in less2.69to24.74%of infiltration for the Brilliant Blue concentration profile than the Bromine. 4. The results of saturated experiment showed that the saturated hydraulic conductivities of undistributed soil columns were1to3orders of magnitude higher than the distributed. Better growth of macropores or fracture between soil aggregates results in larger difference of saturated hydraulic conductivities between undistributed and distributed. The average percentage of preferential flow for the undistributed soil column was87.6%.5. Contrastive experiment of undistributed and distributed soil columns showed that the preferential flow of the undistributed soil columns developed well, and the percentages of preferential flow were from73.00to99.99%. In total, the effluent velocity at the bottom of soil columns increased with higher intensity. The percentages of dyeing area in the whole soil profile were10to20%for the distributed, while40.37to74.90%for the undistributed. The extents of preferential flow of undistributed columns were increased with depth. The coefficients of variation for percentages of dyeing area with depth in the soil profile could indicate the extent of preferential flow, i.e., larger coefficients of variation for lower extents of preferential flow, which wasn’t suitable for the distributed.6. The results of modeling for soil water and potential test in Luancheng site showed that the groundwater recharge was largest in the rainy seasons, and then decreased to the dry seasons. The annual recharge was220mmyr-1. The recharge of preferential flow was211mmyr-1, accounting for96.3%of the total recharge.7. The experiments of undistributed columns showed that the percentages of preferential flow were large, average value of98.7%. The soil hydraulic parameters of the dual-permeability model varied on the different condition at the upper boundary, owing to the stochastic characters of preferential flow. Therefore, the optimizational model could be applied in field with the same or similar irrigation pattern, but couldn’t be suitable for the cases which were different from irrigation of the experiment.8. The results of this thesis could be useful for estimation of groundwater resource, management of agriculture water-saving, and risk evaluation of groundwater suffering contamination, which improved the understanding of soil water infiltration, and modified the traditional method basing on the piston flow model. Water-saving technique, e.g., straw mulch, intermittent irrigation pattern, less per-irrigation (i.e., increasing the number with the same total irrigation), increasing the plowing depth and frequency, could restrain the preferential flow, improve the ability of retaining soil water during the root zone and utilization efficiency of irrigation or rainfall, and cut down the risk of groundwater suffering contamination.