Study On Regularities Of Water Infiltration And Migration In Woodland Drying Soil In The Loess Hilly Region

The phenomenon of woodland drying soil was existed on a large-scale in the northern of Shaanxi province.With the rapid development of the vegetation construction,the problem of drying soil was becoming more and more serious due to the unreasonable selection of vegetation types and planting densities.Then the prevention and restoration of the soil drying problem was becoming one of the focuses of the ecological and soil disciplines.Because of the landform was special in this region,and the buried depth of groundwater was relatively great,irrigation conditions were not possessed.Natural rainfall became the only source of soil moisture to the drying soil.In order to explore the regularities of rainfall infiltration and water migration,a field soil column up to 10 m was established to simulate the jujube forests drying soil in Mizhi experimental station which is located in the North of Shaanxi.Through long-term monitored,this study analyzed the regularities of soil water infiltration and migration during different growth periods and under different rainfall types.Further simulated prediction using Hydrus-1D model was put forward on the basis of the observation.The study could improve the scientific management of the soil moisture and made a great theoretical and practical sense to the woodland drying soil.The main conclusions are as follows:(1)Regularities of soil water infiltration and migration under independent rainfall conditions presented different from that under intermittent rainfall conditions.Under the condition of independent rainfall,the depths of infiltration and migration were affected clearly by precipitation and meteorological factors.What's more,other factors such as rainfall intensity and initial soil moisture content also influenced it.The greater the rainfall intensity and the higher the initial soil water content was,the deeper the depths were.Contrast with the regularities under independent rainfall conditions,it showed a stronger law in the condition of intermittent rainfall.The former rains improved the soil moisture for the latter,and the latter ones occurred on the basis of the former.Under intermittent rainfall conditions,several rains promoted infiltration and migration interactively.Compared with independent rainfall,the intermittent rainfall could significantly increase the infiltration depth by 100%~160%,and the migration depth by 91%~197% with the same precipitation.(2)Soil water transported in the vertical profile tending to be layered.They were evapotranspiration layer,transition layer,and infiltration layer in turn.During the growth period of the jujube,the depths of the layers were 0~60cm,60~120cm and 120~220cm respectively.In the dormant period,they were 0~100cm,100~140cm and 140~240cm.The evapotranspiration layer was greatly affected by the precipitation and evapotranspiration.The soil water storage in this layer presented linear change with the precipitation during the two periods.The linear functions were Wgrowth=0.2332P+81.25(R2=0.41)Wdormacy=0.7011P+133.24(R2=0.66).The transition layer was influenced by the evapotranspiration weakly.The soil water storage in this layer just fluctuated gently.In the growth period,infiltration of soil water was greater than the evaporation,and in the dormant period on the contrary.The infiltration layer was not affected by the evaporation.In this layer,soil water transported to the deep layers continuously and the soil water storage increased with time during the both periods.(3)During the growth period,monthly evaporation was 40.1 mm on average.The evaporation coefficient remained at about 0.87.It meant that nearly 87.0% of the precipitation was evaporated,and the remaining 13.0% was transported into the deep soil.While during the dormant period,monthly evaporation was 24.7 mm averagely.The evaporation coefficient was as high as 5.1.In this period,79.4% of the evaporation was from the precipitation,and 20.6% was from the soil water storage approximately.Therefore the period of dormancy was an important stage for the soil water loss.(4)In the Loess Region,not all the rainfall could influence the soil.During the period from 2014.8.1 to 2016.12.31,the effectiveness of the rainfall frequency and amount were 21.3%,62.6% respectively.Soil water movements presented a hysteresis and the vertical profile was layered.In the period of the observation,the effective rainfall could repair the drying soil to depths of 350~400 cm on soil moisture.(5)The model of Hydrus-1D could better simulate the soil moisture dynamic changes of the soil column.A simulation was established using by the Hydrus-1D model under the typical normal flow year.The results showed that the depth of 600 cm in the drying soil need about 8 or 9 years to repair.The equation to simulate the depths changing with time was also concluded.It confirmed that the repair of the soil moisture to the drying soil was a long-term and slow process.