The Effect Of Rainfall And Vegetation On Dynamic Distribution Of Soil Moisture During Dry-Wet Season In A Red Soil Slope

The red soil slope,as important resources for agricultural and forestry production,is widely distributed in southern China.The uneven distribution of the water and heat in seasons causes obvious differences of the soil moisture between wet and dry seasons,which is one important reason for the formation of seasonal drought which in the red soil slope.However,people do not fully grasp the soil moisture dynamics in dry and wet seasons in the red soil slope.More work has to be carried out to study that how vegetation and precipitation drive the soil moisture changes during the dry/wet season.Our study site is located in southern Hubei,and this site belongs to a typical red soil sloping land.Tea tree field,photinia frondosa field,sapindus field,and rapeseed field on the sloping land were selected as sample plots.Positioning high frequency monitoring were conducted to get the data of rainfall amount,vegetations features and soil moisture contents in these four fields for 3 years.Then we analyzed the characteristics of rainfall and its effect on the soil water storage,explored the characteristics of aboveground and underground parts of vegetation and their effects on soil moisture dynamics and soil water profile,and addressed the coupling effects of rainfall and vegetation on soil moisture dynamics.The main results are as follows:(1)There is a significant positive correlation between soil water storage and rainfall in the study sloping land,and the heavy rain(daily rainfall amount>25 mm)can effectively increase the soil water storage capacity.Under the same rainfall amount,different rainfall intensities are the main factor for soil moisture changes.The soil water contents will increase greatest when rainfall intensities are between 8~16 mm/h.According to the characteristics of rainfall amount and rainfall intensity,the rainfall type was divided into four types including rain type I(heavy rainfall with high rain intensity),rain type II(moderate rainfall and medium rainfall intensity),rain type III(small-medium rainfall and heavy rain intensity)and rain type IV(small rainfall and small rain intensity).The rain type IV(reaching 74.5%of the total rainfall times)is the main type in the slope,but it has the least effect on increasing the soil water storage.The capacity of these four rainfall types for increasing the soil water storage is in the decreasing order:rain type I>rain type II>rain type III>rain type IV.The difference in soil water storage between different rainfall types is significant.(2)There are some differences in the soil hydraulic properties between different vegetation plots.In general,the tea tree plot has higher values of the rainfall penetration capacity(surface)and of the water holding capacity during drought,followed by the sapindus and rapeseed plots,and the photinia frondosa plot has the minimum values.The litter biomasses of tea tree,photinia frondosa,sapindus,and rapeseed were 23.3 g/m~2,127.8 g/m~2,22.6 g/m~2,7.5 g/m~2,and the maximum water holding capacity separately reached 1.94 times,2.50 times,2.21 times,and 2.41 times of the dry weight of the litters.Leaf area indexes of these four vegetation types are 1.13,3.00,0~1.50(Leafless period from November to March of the following year is 0),and 0~1.20(leisure period from June to October is 0),respectively.Soil hydraulic properties,vegetation canopy and litter holding capacity are different,which is one of the important reasons for the difference in soil water storage,soil water dynamics and soil water profile distribution among vegetation plots.(3)The difference in soil moisture profile among vegetation plots is high related to the root distribution of different vegetations and their root water absorption capacities(root water consumption).The tea trees roots are concentrated above the 40 cm soil,roots of the rapeseed and sapindus are mainly located above the 60 cm depth,and the photinia frondosa root can reach a depth of 90 cm.Photinia frondosa belongs to deep root vegetation,and its highest total root long density(RLD)appears at the depth of 40 cm.The photinia frondosa's RLD in different depths are significantly higher than those of other vegetation plots.The water consumption of roots at different depths has positive correlation with the total length of fine roots.The root water consumption in the wet season reduces as follows:photinia frondosa>sapindus>rapeseed>tea tree.In the drought process in the dry season,they decrease in the order:photinia frondosa>sapindus>tea Tree>rapeseed(leisure period,a small amount of weed growth).(4)Rain type(rainfall,rain intensity and rain duration),vegetation characteristics(canopy,litter,root system)and soil water content in the early stage affect the soil water responsing depth and hydrological process characteristics of the red soil slope(lateral flow and priority flow occurrence).Rain type I,rain type II and rain type III can cause obvious responses of soil moisture at different depths.But in most cases,rain type IV only causes water changes in the surface soil(such as 20 cm depth).Under the same rain type,continuous rainfall can increase more soil water storagethan the individual rainfall event do.In general,all kinds of rain patterns have soil lateral flow.Especially in the case of continuous rainfall or high water content in the early stage,the lateral flow in soil is more likely to occur;for the vegetation plots,no lateral flow occurs in the tea tree block in the highest slope location,while lateral flow appears in other plots in lower slop loction.The preferential flow mainly appears in the heavy rain intensity,including rain type I and rain type III.In addition,the root channel is also an important condition for the occurrence of preferential flow.For example,the root system of the photinia frondosa is the deepest,which can reach a depth of 90 cm.It is the only plot where preferential flow appeared at the depth of 90 cm.