Results
1 The change of cumulative infiltration depths with time in red plum apricot forest
Infiltration is a process in which water enter soil. The water infiltrated into soil have two function. One is to increase soil water content in a soil layer, and another is to increase cumulative infiltration depth. The two curve was used to estimate the depth of infiltration before and after a rain event or an infiltration process or several days. The infiltration depth for a rain event is equal to the distance from the surface to the crossover point between two soil water distribution curves with depth measured in the soil profile before and after a rain event or several days. A lot of the crossover points at the same height in the soil profile makes up the wetting peak. The annual precipitation is 536.2 mm, which is 120.6 mm more than the mean precipitation 415.6 mm and close to the maximum rainfall record of 634.7 mm in the National high quality red plum apricot demonstration area, see fig.1. After two effective rain events, 9 mm in May 20 and 19.7 mm in April 13, infiltration depth reach to 70 cm on April 28, 2018, see fig.2.
As time goes on, the cumulative infiltration depth increased with time in 2018 because the infiltration includes two stages: rainfall infiltration generally happening during a rain event and the cumulative infiltration (Chowdary and others 2006) or reinfiltration (Corradini and others 2000), which occurs in the period between two
(Fig.1 Here)
(Figures 2 Here)
rain events or a long term period in which there are more than two rain events happens because there is a cumulative effect on the infiltration process. After a heavy rain event, a high water-bearing soil layer formed under land surface. With time going by, the soil water content in the high water-bearing soil layer reduced because soil evaporate, plant root water absorption, or infiltrate into deeper soil layer and form another high water-bearing soil layer at deeper soil layer, and cumulative infiltration depth increases in the soil profile (Guo and Shao,2009; Guo 2017).When the soil water content in the upper layer of wetting peak is equal to the lower layer of wetting peak, the cumulative infiltration process stopped because there is not infiltration force, and water potential difference between the upper layer of wetting peak and the lower layer of wetting peak approaches to zero. At this time, the cumulative infiltration depth is the maximum cumulative infiltration depth (Guo 2020).That is to say, the maximum cumulative infiltration depth is the maximum infiltration depth. The cumulative infiltration depth arrives in 130 cm on May 28, up to 150 cm on June 16, get to 190 cm on July, 16 , and finally to 290 cm. So, the maximum infiltration depth is 290 cm in red plum apricot forest, see Figure 3.
(Fig3 Here)