The Study Of The Infiltration Factors Of Purple Soil Under Different Water Supply Conditions

Infiltration process is an important part of hydrological cycle, which closely relates to soil moisture redistribution, soil erosion, nutrient migration with moisture, and agricultural non-point source pollution and so on. As the major plantation resource in hilly area of Sichuan, purple soil is developed by purple shale, which characters includes large porosity, strong infiltration capacity, shallow soil, and underlying shale with extremely weak permeability. What's more, the unique structural features of purple soil and the heterogeneity of rainfall in spatial and temporal distribution caused that slope cropland of purple soil becomes the most serious soil erosion in the upstream of Yangtze River. The non-point source pollution which is caused by soil erosion is the difficulty and emphasis to control water pollution. With the laboratory physical and chemical tests, the study carried out research and analysis of the infiltration characteristics and factors under different conditions focused on sticky soil type purple soil. The study discussed the influence of bulk density, initial moisture content and hydrocephalus depth on the purple soil infiltration properties under the conditions of hydrocephalus. The influence of different rainfall intensity, ground slope and amount of Polyacrylamide (PAM) on the purple soil infiltration properties under the conditions of artificial simulated rainfall was also discussed. Finally, the effects of different cycle time and circulation rate on the purple soil infiltration properties under the conditions of intermittent water supplies were also studied. The results were shown as following:1) The purple soil infiltration capacity under the conditions of hydrocephalus can be affected by the hydrocephalus depth, initial moisture content and soil bulk density. It was found that under the conditions of 1.20 g.cm^-3 for the purple soil density, 10% for the initial moisture content, 1cm, 3cm, 6cm and 10cm for the water depth, both purple soil infiltration capacity and infiltration rate changed little at the same infiltration time, which can be neglected under normal circumstances. Under the conditions of 1.36g.cm^-3 for the purple soil density, 6cm for the water depth, with the initial moisture content from 5% to 20%, the initial infiltration rate can be reduced from 258mm/h to 207mm/h, the infiltration decay rate can be accelerated, the time to steady infiltration rate can be shorten. The steady infiltration rate can be reduced from 15.6mm/h to 8.4mm/h; the cumulative infiltration volume can reduced respectively. The study also showed that soil bulk density affected on infiltration capacity of purple soil significantly. The purple soil bulk density was negatively related to the cumulative infiltration and infiltration rate. In another word, with the increase of bulk density, both purple cumulative infiltration and infiltration rate showed a decreasing trend. Under the experimental conditions, as the increase of the purple soil bulk density from 1.20g.cm^-3 to 1.60g.cm^-3, stable infiltration rate was reduced from 21.6mm/h to 3.6mm/h.2) The infiltration capacity of purple soil under the conditions of artificial simulated rainfall can be affected by the rainfall intensity, ground slope and the quantity of Polyacrylamide (PAM). It was found that, with the increase of rainfall intensity, the initial runoff time can be ahead, the cumulative infiltration can be increased, and the steady infiltration rate had an increasing trend. The steady infiltration rates were 26mm/h, 40mm/h and 48mm/h when the rainfall intensities were 60 mm/h, 96mm/h and 180 mm/h. With the increase of slope, purple soil stable infiltration rate can be increased because of shallow soil and the weak permeable bed. Under the conditions of 1.0mm/min for the rainfall intensity, without PAM, with the increase of the ground slope from 0°to 10°, the steady infiltration rate can be increased from 13mm/h to 45mm/h. PAM can improve the purple soil's infiltration capacity by influencing the purple soil's basic physical properties, such as soil porosity, bulk density, the content of water stable aggregates and so on. However, it was not always excellent with the increasing quantity; the infiltration rate was the highest for the 5g/m2 in this test.3) Different from the condition of continuous water application, the law of purple soil infiltration under the condition of intermittent water application was more complex. The infiltration curve of purple soil under the condition of intermittent water application was intermittent non-continuous curve. Water infiltration stage of the first cycle was consistent with the infiltration process of continuous water application. Starting from the second cycle, both cumulative infiltration and infiltration rate under the condition of intermittent water application was larger than continuous infiltration under the same conditions at the same infiltration time. The intermittent infiltration performance was closely related with cycle time and circulation rate. Setting the circulation rate of 0.5, with the increase of cycle time, both the infiltration rate and cumulative infiltration have an increasing trend. Setting the cycle time of 20min, with the increase of circulation rate, both the infiltration rate and cumulative infiltration can be increased.