Study On The Purple Soil Water Properties Under Contour Hedgerow-Intercroping System In The Three Gorges Reservoir Area

The coexistence of seasonal drought and soil and water losses is one major ecological problem in the three gorges reservoir area now.Contour hedgerow is a very common and effective technique around the world,which is often employed to revegetate on cultivated slope land and conserve water and soil resources.And it is also considered as an available way of sustainable development of agroecosystem in the three gorges reservoir area.Soil water properties under contour hedgerow-intercroping system were studied thoroughly in two respects of soil water morphology and soil water dynamics.The purpose of this research was to discover soil water availability,soil permeability,and theirs spatial variability,as well as to determine hydraulic parameters of different textural purple soils and to discuss methods of estimating hydraulic parameters indirectly in term of applicability on purple soils.On the basis of former research,process of water infiltration in special soil profile was simulated according to alluvial soils in front of hedges through physical models and mathematical methods.The main results were as follows:(1) Three hedges of Leucaena leucocephala,Vitex negundo and Coriaria sinica were selected to discuss the benefits of soil and water conservation and drought-resistance.The results showed that because of trapping and depositing by contour hedgerows,the height of silt had increased to 80cm and the gradient had been eliminated 20°after 10 years, which reflected that the effect of sediment trapping and depositing was significant Compared with the control,within the contour hedgerow treatments,soil organic matter increased from 52%to 65%,soil bulk density decreased from 14%to 17%,and soil non-capillary porosity increased from 66%to 124%especially.The effects of fertilizer maintenance and improvement under the contour hedgerow system were significant and soil structure was improved obviously.According to the effective thickness of soil layer of 60cm,soil water total volumetric capacity in the treatments of Leucaena leucocephala, Vitex negundo and Coriaria sinica increases 271mm,257mm and 247mm respectively, which then improves the soil ability of drought-resistance greatly.(2) Alluvial soils in front of hedges(Leucaena leucocephala and Vitex negundo) were taken as the objects,to evaluate the effects on soil water and physical properties from different distances to hedgerows at different soil depths by contour hedgerow-intercroping system.The results showed that soil non-capillary porosity, saturated hydraulic conductivity,saturated water content and natural water content in the hedgerows treatments were higher than in the rock ridge terrace treatment,while the soil bulk density in the hedgerows treatments was lower than in the rock ridge terrace treatment,which reflected that the effects on water and soil conservation by hedgerows were significant,especially in the top and second layer.Based on the same depth,with the extending of the distance to hedgerows(ridge),in the hedgerows treatments soil bulk density and natural water content increased gradually,and soil non-capillary porosity, saturated hydraulic conductivity and saturated water content decreased gradually,while no apparent change in the rock ridge terrace treatment.With the deepening of the soil layer,in each treatment soil bulk density and natural water content increased gradually, and soil non-capillary porosity,saturated hydraulic conductivity and saturated water content decreased gradually accordingly,and their differences among treatments were also lessened gradually.The distance to hedgerow reflected the degree of effects on soil water and physical properties by hedgerow indirectly.The closer to hedgerow,the greater the effects were.Comparing treatments of Leucaena leucocephala and Vitex negundo, their effects were equivalent.(3) Alluvial soils in front of hedges(Leucaena leucocephala and Vitex negundo) and rock ridge terrace were taken as the objects,to study the soil water infiltration properties and theirs factors from different distances to hedges/ridge at different soil depths,as well as fit the model of the soil infiltration process.The results showed that,based on the same depth,with the extending of the distance to hedgerows(ridge),soil infiltration properties represented the trend of decreasing gradually for all treatments,and there was big difference among different distances to hedges for hedgerow treatments but no difference among different distances to ridge for rock ridge terrace treatment.With the deepening of the soil layer,soil infiltration properties decreased gradually for hedgerow treatments, while decreased rapidly for rock ridge terrace treatment.In the top layer the average soil infiltration properties for rock ridge terrace treatment was better than those for hedgerow treatments,but in the second layer the average soil infiltration properties for hedgerow treatments were better than those for rock ridge terrace treatment on the contrary,and in the third layer,they were equivalent.There was clear space variation in vertical and horizontal directions of soil infiltration properties for Vitex negundo treatment.In horizontal direction,the distance to hedgerow reflected the degree of effects on soil water and physical properties by Vitex negundo indirectly.Bulk density,sand/clay ratio and OM had significant influence on all of soil infiltration properties,but initial soil water content only had significant influence on initial infiltration rate.Sand/clay ratio had the most effect on initial infiltration rate and infiltration in 30min,and bulk density had the most effect on stable infiltration rate.Infiltration models of Green-Ampt and Philip were more suitable for describing and simulating the soil infiltration process in this research with higher accuracy.(4) Top purple soils of alluvial soil in front of Leucaena leucocephala,rock ridge terrace and natural slope were taken to measure hydraulic parameters in laboratory.The results were as follows,for purple soil water retention cuvre,there was a negative correlation between soil water suction and soil water content.Under the same soil water suction,water content of fine soil was the highest,and water content of coarse soil was the lowest.Both Gardner model and Van Genuchten model could be used to describe purple soil water retention curve.For purple soil water diffusivity,there was a positive correlation between soil water diffusivity and soil water content.With the increase of water content(θ＞0.25),the difference among different textural soils became more obvious(coarse soil＞fine soil).The existing models could be used to express purple soil water diffusivity.For purple soil conductivity,there was a positive correlation between soil conductivity and soil water content.Under the same water content,conductivity of coarse soil was higher than fine soil.Power function,Brooks-Corey model and Mualem-Van Genuchten model could be used to express purple soil conductivity.(5) Through conducting experiments of horizontal one-dimension infiltration,soil water redistribution and soil particle analysis,indirect approaches of determining soil unsaturated hydraulic conductivity and diffusivity were discussed,in term of applicability on purple soils.The results showed that,based on the linear function between the fractal dimension of soil particle size distribution and the fractal dimension of soil water retention curve,soil water retention curve was predicted through Tyler-Wheatcraft's model.It was quite obvious that the predicted soil water retention curve agreed well with the measured data.Hydraulic parameters were estimated by simple infiltration method and horizontal infiltration method.The estimated soil water retention curve was accurate,while the accuracy of estimated soil water diffusivity and conductivity was poor.Based on soil water redistributions,the estimated purple soil water diffusivity was in good agreement with the measured,but the estimated purple soil conductivity deviated greatly.While,the purple soil conductivity to be estimated agreed well with the calculated,by combining one-dimension redistributions of soil water and the measured soil water retention curve. In addition,this method was more suitable for the low soil moisture.(6) On the basis of research(4) and(5),models of water infiltration in special soil profile were derived according to alluvial soils in front of hedges for the first time,and verificated by laboratory simulation.The results showed that in homogeneous soil profile, wetting front rate for coarse soil was faster than free soil.But in heterogeneous soils profile,under the same boundary conditions,vertical wetting front rate of half part profile of coarse soil was faster than fine soil.Furthermore,vertical wetting front rate of half part profile of coarse soil in triangular profile with fine upper and coarse lower was faster than in triangular profile with coarse upper and fine lower.But there was no difference for half part profile of fine soil.There was no significant effect on incline wetting front rate by heterogeneous soils profile,whether coarse upper and fine lower,or fine upper and coarse lower.There was a very significant power relationship between wetting front,infiltration rate and time for different soil profiles.The established infiltration models for special profile of purple soils were verified by experimental data,and the results indicated that the estimated agreed well with the observed.It was suggested that these models be reliable and reasonable,so that these models could be used to express the infiltration process in special profile of purple soils.Meanwhile,there was well consistency between the established infiltration models and the existing empirical models,and they could be transformed each other.Moreover,parameters of established theoretical models had some certain physical meaning.