Soil infiltration is the basic hydrological process for water penetrating into the soil, and the amount of water infiltrating the soil surface directly affects the quantity of surface runoff and soil erosion, even the recharge of both soil and underground water. The initial soil water content is an important factor to soil infiltration, and its role in runoff controlling and soil erosion prevention has been taken into account by many researches. In this study, the double-ring infiltration method was used to quantitatively measure the infiltrability of surface soil (0-5 cm depth) covered with forestland and grassland, which were under two antecedent soil water contents of 12% and 40%, respectively. In addition, the changes of soil infiltration process over time were estimated with different infiltration models.
The results indicated that the initial soil infiltration rate of forestland and grassland tended to decrease with increasing initial soil water content. And an increase in initial soil water content results in an increase in steady infiltration rate and cumulative infiltration rate. Whether forestland or grassland, soil infiltration process with higher initial soil water content reaches the steady infiltration stage earlier than that with lower initial soil water content. For forestland with initial soil gravimetric water content rate of 12%, the initial soil infiltration rate was 8.95 mm/min, which was four times to the initial infiltration rate for forestland soil with gravimetric water content rate of 40%. However, its steady infiltration rate 0.24 mm/min was only one eighth of the forestland with initial soil gravimetric water content rate of 40%, and the cumulative infiltration in an hour was only two thirds of that. This phenomena may attribute to the faster wetting rate of drier soil in infiltration process, which cause stronger slaking forces of the soil and severe aggregates breakdown, enhance surface sealing or promote the destruction of soil structure, and then significantly decrease the soil infiltrability.
For grassland surface soil with ample organic matter, soil infiltrability and infiltration process suggested lower sensitivity to initial soil water content. Initial infiltration rate, steady infiltration rate, and one hour cumulative infiltration of grassland with 12% soil water content were 2.21 mm/min, 0.62 mm/min, 59.16 mm, respectively. And these parameters of grassland with 41% soil water content were 2.07 mm/min, 1.7 mm/min, 110.27 mm, which suggested less significant difference than that of forestland. Whether covered with forestland or grassland, soil instant infiltration rate decreased rapidly at the beginning of the infiltration process, and then decreased approximately linearly with time until approached to constant, especially under the condition of the initial soil water content to be 12%. The least square method was used to simulate the infiltration rate changes with time, and the results indicated that soil infiltration process of forestland and grassland were all fitted better by Horton and Kostiakov infiltration models than by Philip infiltration model. Besides, Horton infiltration model was the best model to describe soil infiltration process, which made it suitable to describe the soil infiltration process of forestland and grassland in the Three Gorges Reservoir area.