Soil Hydraulic Characteristics And Responds Of Soil Water And Runoff To Rainfall On Sloping Farmland Of Purple Soil

Sloping land is an important landform and it is the main part of China’s agricultural landresources. China is a mountainous country. As the main form of cultivated land, slopingfarmland accounts for about2/5of the arable land, and the grain yield accounts for1/5-1/3ofthe total output. Soil erosion, soil degradation and geological disasters restrict thedevelopment of mountain area. In addition, mountain ecosystems tend to be more sensitive tothe changing environment. The Purple Soil which concentrated in the Sichuan Basin is themost important farmland resources in the upper reaches of the Yangtze River (total area of160,000km~2). This soil is very shallow with only40-100cm depth. The main characteristicsof purple soil are high porosity, high saturated hydraulic conductivity and great heterogeneityof the soil hydraulic characteristics which are prone to soil erosion. On the other hand, theother characteristics of purple soil is juvenile. The dynamics of runoff and soil water contentis particular to others. Understanding of the spatial distribution characteristics and dynamicsof soil water content and runoff on the hillslope are useful to reveal the hydrological cyclecharacteristics of the mountains, agricultural chemical migration and the fate of pollutantsload.Field campaign was conducted to measure the soil hydraulic characteristics of the purplesoil in the Wan’an River basin. The aim is to establish the Soil pedo-transfer functions andcalculate other hydraulic parameters in this region. At Yanting Agro-ecological ExperimentalStation of Purple Soil, Chinese Academy of Sciences, an automatic monitoring system forsimultaneous observation of soil moisture, water potential and runoffs was built on a typicalsloping farmland of purple soil. For a typical rainfall event, the dynamics of soil water contentand runoff response were continuously recorded. In addition, the factors which affect thoseprocesses were also analyzed. The main results and conclusions were listed as follows:(1) Using multiple linear stepwise regression, point estimate models and parameterestimation models were able to well predict the soil hydraulic properties of purple soil by thesoil basic properties at southwest hilly area of purple soil (Upstream of Jialing River). Three different point estimation models which had different variables showed they are different onthe modeling performance. The model which adds the field capacity and wilting coefficientsas the variables had the highest correlation coefficient when to count soil moisture underdifferent matrix potentials. But the point model which was used to estimate the saturatedhydraulic conductivity had poor performance with the correlation coefficient R of0.514. Theparameter estimation model had lower predicted soil water content ability than the observeddata under different matrix potentials with the correlation coefficient R were0.5to0.87.Consequently, this model had a bad performance.(2) Field measured results showed that the topsoil in the slope farmland was influencedby cultivation activities. The tilled soil had large macropores spaces (nearly saturatedconditions on water filled pore space). The disc infiltrometer method was more accurate thanvariable water head method. The water characteristic curve of purple soil’s vG parameterestimation, the purple soil available water content was low and the soil water holding capacitywas poor. Field measured soil moisture characteristic curve shape was like a logarithmiccurve.(3)The result revealed the obvious response of soil moisture to the rainfall on the slopefarmland during the typical rainfall process. In October, the soil moisture trend showed thatthe soils on different sections of slope farmlands had differential water storage ability,following the order: upslope<middle slope<down slope. The rain characteristics was the mainfactor to affect the soil water content change. In different depths of soil, the soil moisturechange was differential to the same rain. With the depth increased, the water content showedless obvious change. In addition to the rainfall characteristics, great heterogeneity of the soilhydraulic characteristics in different slope positions and depth also affected the soil watermovement pattern.(4)The hydrological process of deep subsurface flow to a typical rainfall event onSeptember and October2012showed the obvious delay between rainfall and runoff. And thelagged time was about8hours. Different intensity rainfall events had different subsurfacerunoff coefficient (proportion of precipitation that becomes runoff). The large rainfall event,subsurface runoff coefficient was53.6%while the coefficient was only1.6%to the smallrainfall. In addition, the antecedent soil moisture content also significantly affected the runoffcoefficient in the vadose zone.(5) The analysis of water balance on the sloping farmland showed that soil water contentwas significantly higher than the measured soil moisture changes during a typical rainfallevent. The reason were listed:①The water holding capacity and water content change of themudstone bed was taken no account;②Sandstone below the mudstone had poor matrix permeability, but the fissures were often present. Thus, the water leakage amount which onlytook account of the underground runoff could be small.