Responses Of Runoff Generation And Vertical Hydrological Connectivity To Different Terraces In The Three Gorgrs Reservoir

Terraces are of great significance to the reduce of soil and water loss and the development of agricultural activities in the Three Gorges Reservoir（TGR）.They are a guarantee guarantee of grain security in the TGR and support the national key projects for soil and water conservation in upper Yangtze River basin.It is of great significance for the response of runoff generation and hydrological connectivity to different terraces in the TGR.In this study,the terraces of different ladder wall in the TGR were studied,the historical data of 6 types of terraced plots（farming:Rock Ridge Terrace（RRT）,Hedgerow terrace（HT）and Sloping Land（SL）;citrus:Soil Ridge Terrace（SRT）,HT and SL）were analyzed.The natural rainfall and soil moisture of 3 farming terraces were monitored,and the simulated flooding test of two consecutive stepped fields was also carried out.By analyzing the response of runoff and sediment yield to rainfall factors and soil moisture,and exploring the path of soil moisture movement in different terraces,the research conclusions can reveal the impact of different terraces on rainfall runoff yield and vertical hydrological connectivity in the TGR.The finding are as follows:（1）According to the rainfall duration（T）,precipitation（P）and maximum 30minutes rainfall intensity(I₃₀),153 erosive rains are divided into 4 types.R1 are characterized by long duration,large precipitation and medium intensity;R2 are characterized by the longest duration,the largest precipitation and medium intensity;R3 are characterized by the shortest duration,small precipitation and the highest intensity;R4 are characterized by medium duration,medium precipitation and low intensity.The results showed,the runoff and sediment reduction benefits of different types of terraces are generally good.The average runoff reduction benefits are as follows:HT+citrus>HT+farming>RRT+farming>SL+citrus>SRT+citrus,and the sediment reduction benefits are as follows:HT+citrus>RRT+farming>SL+citrus>HT+farming>SRT+citrus.The runoff coefficient and soil erosion amount under different rainfall types were significantly different.Terraces are generally easier to produce runoff sediment under R3 and R4,while the runoff coefficient and soil erosion are generally small under R1 and R2.Additionally,the RRT under R2 and the SRT under R1,R3 and R4 can not reduce soil erosion.（2）The dynamic change of soil moisture generally shows that the response time advances with the increase of rainfall,lags behind or even has no response with the deepening of soil layer;The influence of slope position and terrace type on soil water content change could not be reflected under the condition of light to moderate rain;Under heavy rain conditions,the increase and increment of soil moisture in the downslope are higher than those in the upslope.The increment of soil moisture in the rock ridge terrace is slightly higher than that in the hedgerow terrace and the slope.The runoff thresholds of P,I₃₀ and P+SWC（precipitation+antecedent soil water content）in RRTwere 42.40mm,30.80mm/h and 56.77mm respectively;The runoff thresholds of P,I₃₀ and P+SWC in HT were 42.40mm,13.70 mm/h and 58.00mm respectively;The runoff thresholds of P,I₃₀ and P+SWC in SL are 40.20mm,2.40mm/h and 51.51mm respectively.Multiple regression analysis showed that runoff and sediment yield in terraces were affected by rainfall factors and SWC,but rainfall factors were the main driving factors.The analysis of rainfall process shows that the water under low intensity rainfall is mainly infiltration,no runoff and sediment are produced,and there is no difference between different terraces;Under moderate rainfall intensity,the water content of terrace is mainly infiltration,and the runoff coefficient of slope is large;Under high-intensity rainfall,the runoff coefficient is shown as RRT>SL>HT,but the sediment content is shown as HT>RRT>SL.（3）The study on the soil physical properties on the the center of the terrace and behind the ladder wall（or fence）found that the construction of the terrace would lead to more uneven soil properties in different spatial positions within the terrace,that is,better soil water holding capacity in the center of the field surface,and the worst soil water holding capacity behind the ladder wall of the RRT.The results of simulated flooding test show that,when the water supply is small,all plots are dominated by vertical infiltration,and there is no water connection between the upper and lower stepped fields or the upper and lower slopes.In the case of large water supply,preferential flow is formed through loose soil behind the ladder wall,as well as large porosities and piping systems in RRT,which in turn causes an increase in soil moisture on the surface of the next terrace.Subsurface flow of the soil is generated through the soil-stone interface,causing an increase in soil moisture in the deeper layers of the next terrace,so that the above two processes connect the soil moisture of the upper and lower terraces;The soil moisture of two successive HT is mainly connected through surface runoff,and there is no soil moisture connection within the terraces;The soil moisture of SL is mainly affected by the subsurface flow in the deep layer along the slope direction and the surface runoff,which is quite different from the terrace.