Simulated Study On Hydrological Process Of Forest Litter Layer In Beijing Mountainous Area

Forest litter layer is a vital component of forest system,which has many hydrological functions,such as water conservation and runoff regulation.Most previous studies focused on the water-holding capacity of forest litter layer,seldom took terrain factor,such as gradient into consideration.It is difficult to simulate the real hydrological process of forest litter layer in forested hillslope.Forest litter from typical forests in Beijing mountainous area was selected as study objectives,and tested by two methods: immersion test and artificial rainfall simulation.Different gradients and rainfall conditions were set in order to investigate the hydrological process of forest litter of different litter types,and then model the corresponding process.Based on the above,we can explore the mechanism of hydrological portions,such as interception,infiltration and runoff,responding to gradient and rainfall.The test method may give a reference to future research,and the results may provide theory and data support for accurately evaluating hydrological function of forest and profoundly understand the mechanism of runoff generation in forested hillslopes.The main results were as follows:(1)In soaking test,the maximum water-holding capacity of forest litter were at the range of 2.27 to 25.94 t·hm-2.Several indexes including the maximum water-holding capacity,the maximum water-holding rate and the available water-retaining amount/rate/depth were shown in the same trend,following the order of Quercus variabilis forest > mixed forest with Quercus variabilis and Pinus tabuliformis > Pinus tabuliformis forest > Platycladus orientalis forest.Except for P.tabuliformis and P.orientalis,the maximum water-holding capacity and the maximum water-holding rate among four forest types showed significant difference.The dynamic water-holding amount increased with soaking time logarithmically,while the relationship between water-holding rate and soaking time followed power function.(2)In simulated rainfall experiments,the interception process of forest litter layer in both constant-intensity rainfall and varying-intensity rainfall have three stages: a rapid-intercepting stage,a slow water-absorption stage and a drainage stage after rainfall.In rainfall tests with changing intensity,the instantaneous interception rate of Q.variabilis showed greater fluctuation than P.tabuliformis,which means broad-leaved litter have better hydrological function of filling and drainage.(3)The average maximum interception storage capacity(Cmax)and the average minimum interception storage capacity(Cmin)of Q.variabilis were range from 1.56 to 3.22 mm and 1.08 to 1.68 mm,respectively.The Cmax and Cmin of P.tabuliformis were range from 1.3 to 2.73 mm and 0.72 to 1.14 mm,respectively.Except that rainfall intensity had no significant influence on Cmin of P.tabuliformis,gradients and rainfall intensities showed significant influence on Cmax and Cmin of both two kinds of forest litter.Additionally,variable-intensity rainfall had no effect on Cmax and Cmin.(4)According to former study and existing data,a dynamic interception model was established based on slope factor and rainfall factor.The model apply to individual rainfall and had a good precision,with RRMSE generally < 20%.Besides,this paper put forward a process model for drainage after rainfall,which had a high accuracy,with RRMSE <10% and NSE >0.9.Compared with immersion tests,simulated rainfall experiments can make a better description of the physical process of water storage or interception of forest litter layer.(5)In a rainfall event,the percentage of rainfall interception was the lowest,which less than 5%,while the vertical flow accounted for the biggest part,ranging from 53.90% to 99.17,and the rest of water was lateral flow along slope,which share 3.59% to 44.58% of rainfall.Since the difference of litter structure,the proportion of lateral flow of Q.variabilis was higher than that of P.tabuliformis.Gradients and rainfall intensities had great influence on the rainfall partitioning,while varying intensity rainfall event did not show significant effect on rain partitioning.In variable-intensity experiments,forest litter of Q.variabilis was found a better function of adjusting runoff than P.tabuliformis.