Effects Of Contour Reverse-slope Terraceon Soil Moisture Movement In Red Soil Sloping Farmland In Central Yunnan Province

Soil moisture is a key ecological factor exerting significant influences on matter and energy migration in soil as well as the survival and growth of crops.Sloping farmland is a major type of cultivated land in Yunnan Province where the red-soil sloping farmland plays a predominant role.Furthermore,rainfall if the major source of soil moisture in the sloping farmland In terms of the red-soil sloping farmland,rainfall infiltration lessens due to the existence of the slope,which results in severe water and soil loss there and prominent eco-hydrological drought.For this reason,effective measures should be taken to reduce surface runoff,intensify moisture infiltration and improve water use efficiency.This is essential for preventing water and soil loss in the red-soil sloping farmland and elevating its productivity.Contour reverse-slope terrace is deemed as a feasible and effective soil and water conservation measure.In addition to significant reduction in sediment yield,such an approach is also used to trap surface runoff and transform the runoff into soil moisture urgent needed by crop growth so as to improve soil water regime in the sloping farmland and increase its water use efficiency accordingly.In this paper,the red-soil sloping farmland in the central part of Yunnan province served as the research object to perform comparative research on the sloping farmland of contour reverse-slope terrace and sloping farmland in original state as a contrast group.By virtue of TDR,soil moisture in diverse positions and depths was measured for the sloping farlland.In addition,runoff yield conditions of the sloping farmland soil were also measured by virtue of standard runoff plots.Field observation in fixed location was combined together with laboratory tests to not only explore the influence of contour reverse-slope terrace on surface runoff of the sloping farmland,spatial and temporal variations in soil moisture,soil water storage,water balance,and crop growth and yield,etc.,but also probe into crop growth promotion actions and nutrition-preserving capability of the contour reverse-slope terrace.Additionally,a rainfall runoff model and a soil water movement model were established for the sloping farmland of contour reverse-slope terrace and the sloping farmland in original state,so as to facilitate the studies on conservation of water and soil based on the contour reverse-slope terrace and the mechanism of improving soil moistxire conditions in the sloping farmland,and guide agricultural production of the cropland in the central part of Yunnan Province.Major conclusions of this study are described as follows:(1)The contour reverse-slope terrace has a significant influence on the surface natural rainfall runoff of the sloping farmland.However,under the circumstances of different rainfall patterns and water contents at the early phase,runoff reduction effects can be enormously different,which are mainly embodied in that runoff reduction effects are most significant in a condition of a low to medium amount of precipitation and low to medium rainfall intensity as far as the contour reverse-slope terrace is concerned,and the runoff reduction effect drops as the soil moisture increases.(2)In the case that rainfall conditions were simulated in a laboratory,contour reverse-slope terrace laid for the sloping farmland played a role in postponing the time of generating the initial runoff,extending the runoff yielding time after the rain,tremendously lowering the volume of runoff generated in the whole course of rainfall runoff yielding,and remarkably increasing moisture infiltration capacity in the sloping farmland soil.In general cases,the higher the rainfall intensity is,the greater the water content at the early phase will be.On this basis,runoff is yielded in the sloping farmland earlier,the amount of runoff yielded in unit time increases and the time of rainfall runoff yielding can be further extended.Earlier stage water content exerts diverse impacts on the rainfall runoff yielding process subjected to various rainfall intensities.To be specific,the lower the rainfall intensity is,the greater the influence of earlier stage water content on runoff yielding process will be.The arrangement of the contoizr reverse-slope terrace intensifies the effect of earlier stage water content on the precipitation process to a certain degree.(3)The contour reverse-slope terrace is capable of significantly improving soil moisture in various time scales.On a yearly scale,the contour reverse-slope terrace applies a more significant effect of increasing the soil moisture in low flow years.On a monthly scale,as the contour reverse-slope terrace is laid for the sloping farmland,soil moisture content is significantly improved in different period and diverse soil layers,which can be more obvious in May as a soil water supplement period and November as a soil discharge period.Daily entropy increase rates of the annual contour reverse-slope terrace to soil 5cm,20cm,40cm,60cm,80cm and 100cm deep in the sloping farmland range from 4.95%to 24.66%,4.21%to 26.61,10.14%to 21.88%?10.53%to 20.49%,13.28%to 20.22%,and 13.07%to 20.18%respectively during the test.As a whole,the entropy increase rate of contour reverse-slope terrace to deep soil is slightly higher than that between that and the shallow soil.Moreover,the soil closer to the shallow layer has an entropy increase rate varying in a greater amplitude.It can be found by dividing the periods for plough layers of the sloping farmland to be dry and humid,the contour reverse-slope terrace has the capability to significantly increase soil moisture content in the sloping farmland in different periods,extend the humid period and shorten the drought period.Hopefully,soil moisture in the sloping farmland can be high enough to remain its soil humid for a long term.(4)The contour reverse-slope terrace places certain influence on soil moisture vertical distribution in sloping farmland soil profile.Based on a variation coefficient method,variations in sloping farmland soil moisture profile are divided into four levels.In terms of the sloping farmland of an original state,that 0～3 0cm deep is referred to as a fast changing layer,that 30～70cm as an active layer,that 70～90cm as a secondary active layer,and that above 90cm as a relative stable layer.As for the sloing farmland where the contour reverse-slope terrace has been laid,fast changing layer of the soil is 0～30cm deep,the active layer is 30～50cm deep,the secondary active layer is 50～90cm deep,and the relative stable layer is 90cm above in depth.After the contour reverse-slope terrace has been laid for the sloping farmland,soil moisture variations in the soil layer 0～90cm deep are alleviated,while those in the soil layer with at least 90cm in depth are intensified.Additionally,layout of the contour reverse-slope terrace accelerates a downward migration velocity of its wetting front to some extent.(5)In 2016 and 2017,soil water storage of the sloping farmland for which contour reverse-slope terrace has been laid was significantly higher than that of the sloping farnland of an original state.To be specific,the average soil water storage of the sloping farmland increased by 12.00%and 14.22%respectively in such two years;and the annual variation coefficient of soil water storage drops if compared with that of the sloping farmland of an original state.This signifies that contour reverse-slope terrace is capable of not only significantly improving the soil water storage of the sloping farmland,but also regulating the soil water storage.Through water balance calculations,soil water storage was supplemented by 10.55mm and 1.43mm respectively in years of 2016 and 2017 as far as the sloping farmlands of an original state were concerned.Nevertheless,since the contour reverse-slope terrace has been arranged,the soil water storage increases by 73.34mm and 42.78mm in these years,indicating that more moisture can be conserved in soil where the contour reverse-slope terrace is laid.(6)By combining the kinetic wave model and the Green-Ampt model together,an implicit differential method was adopted to solve the model,thus simulating the amount of runoff yielded through 32 typical rainfalls in the test period.According to the simulation results,relative error of the total runoff volume generated by the sloping farmland of an original state obtained by simulating 32 rainfalls to the actual amount of runoff yielded is merely-5.4%.By contrast,after the sloping farmland has been treated with the contour reverse-slope terrace,such a relative error turms into-0.54%,signifying a good imitative effect of the model.Additionally,Richards equation representing the movement of water in unsaturated soil was employed to simulate vertical movement of the soil moisture.The corresponding model was solved by a finite difference method.It can be found based on relevant model fitting results that the measured value preferably coincides with the simulated value;and the maximum error between them only reaches 4.56%.(7)In addition to significantly improving soil moisture conditions in the sloping farmland,contour reverse-slope terrace generates significant benefits in facilitating soybean growth and development,enhancing water use efficiency,effectively increasing crop yield and enhancing soil fertility.Specific to the phenomena that water and soil loss caused by agricultural production is severe in the sloping farmland in mountainous areas of China and the capacity of conserving water and maintaining nutrients is poor,it is necessary to extensively popularize the contour reverse-slope terrace targeted at the sloping farmland.