Characteristics Of Soil Moisture And Temperature And Their Coupling Effects On Sloping Land In Loess Hilly Region

Sloping land is one of the main landform types in the loess hilly region.The area of sloping land above 7°accounts for 75.7%of the area of the loess hilly region.Sloping land is also the main source of soil erosion in the region.Therefore,the rational development and utilization of sloping land is of great significance for the ecological construction and the sustainable development of economy and society in the loess hilly region.In the arid and semi-arid loess hilly region with scarce water resources,understanding the soil moisture and temperature conditions of sloping land is an important prerequisite for rational development and utilization of sloping land.At the same time,the sloping land use pattern affects the soil moisture and temperature.Therefore,it is of great theoretical and practical significance to reveal the characteristics of soil moisture and temperature changes and their coupling effects under typical sloping land use patterns in the loess hilly region for efficient and sustainable utilization of sloping land resources.In this paper,four typical sloping land use patterns(i.e.,soybean sloping field,maize terraced field,jujube orchard,and grassland)in the loess hilly region were selected as the research objects.The variation characteristics of soil moisture and temperature,their responses to major environmental factors,and the complexity of soil moisture and temperature changes under different sloping land use patterns were studied by field positioning monitoring,laboratory experiments,and statistical analysis.A numerical model of one-dimensional vertical soil water-heat coupled transport was established.As an example,the temporal variation and spatial vertical distribution of soil moisture and temperature in jujube orchard were simulated and predicted.The effects of soil temperature gradient and root water uptake on soil water-heat coupled transport in jujube orchard were further studied.The main conclusions were summarized as follows:(1)The dynamic changes of soil moisture and temperature under different sloping land use patterns were elucidated.The consistent results were presented in four sloping land use patterns,with the soil moisture seasonal fluctuation in 0-60 cm layer and relatively flat change in 60-160 cm layer.The soil moisture of jujube orchard in 0-60 cm layer was significantly lower than that of other three sloping land use patterns(p<0.05)during the growing season both in the normal precipitation year(2014)and in the drought year(2015).The total water storage of 0-160 cm soil layer in jujube orchard in the normal precipitation year was 5.9%,6.6%,and 17.7%lower than that in soybean sloping field,maize terraced field,and grassland,respectively.The maize terraced field showed adequate soil water conservation capacity,with a low soil water deficit and the soil moisture in 0-160 cm layer being significantly higher than that of other three sloping land use patterns(p<0.05)during the growing season in the drought year.After typical rainfall,with the duration of drought,the soil moisture loss rate of jujube orchard increased significantly,while only moderate increase was observed in maize terraced field.For all sloping land use patterns,with the deepening of soil layer,soil temperature gradually decreased and the seasonal variation was also reduced.The jujube orchard showed obvious warming effect,which increased the soil temperature daily amplitude and daily range during the growing season.Contrarily,maize terraced field could decrease soil temperature daily and seasonal amplitude,and also reduced soil temperature daily range.In summary,as a week performance in conserving soil moisture and regulating soil temperature for jujube orchard,corresponding field management measures are urgently needed to effectively improve soil hydrothermal conditions and achieve sustainable development.Maize terraced field had good effect of soil moisture conservation and temperature regulation,it is recommended to transform sloping fields into terraced fields to achieve the dual goals of efficient use of rainwater and regulation of soil temperature.(2)The complexity of soil moisture and temperature changes under different sloping land use patterns were revealed.During the growing season both in the normal precipitation year(2014)and in the drought year(2015),the jujube orchard increased the variation amplitude,degree,and active layer of soil moisture in 0-160 cm layer,and also improved the variation amplitude and degree of soil temperature in 0-100 cm layer;the inconsistent results were observed in maize terraced field that the variation amplitude,degree,and active layer of soil moisture,and the variation amplitude and degree of soil temperature were both reduced.Under different rainfall years,the fractal dimension of soil moisture and temperature in different soil layers of jujube orchard was higher than that of other three sloping land use patterns,while the fractal dimension of maize terraced field was the lowest.During the growing season both in the normal precipitation year(2014)and in the drought year(2015),the order of approximate entropy of soil moisture in 0-20 cm layer under four sloping land use patterns was as follows:the approximate entropy of jujube orchard was the highest,followed by grassland,soybean sloping field was the third,and maize terraced field was the lowest.During the growing season in the normal precipitation year,the approximate entropy of soil moisture in 0-160 cm layer of maize terraced field was the minimum.During the two growing seasons,the approximate entropy of soil temperature in 0-100 cm layer of jujube orchard was the maximum,while the approximate entropy of maize terraced field was the minimum.In summary,the changes of soil moisture and temperature in jujube orchard were complex,while the maize terraced field could reduce the complexity of soil moisture and temperature changes,which was conducive to maintaining soil moisture stability and regulating soil temperature.It is feasible to evaluate the complexity of soil moisture and temperature under typical sloping land use patterns in loess hilly region based on wavelet fractal dimension estimation method and approximate entropy theory.(3)The main environmental factors affecting soil moisture and temperature in four sloping land use patterns were identified.During the growing season both in the normal precipitation year(2014)and in the drought year(2015),air temperature and air humidity were the main controlling factors affecting soil moisture variation in 0-60 cm layer of soybean sloping field and grassland;air humidity and wind speed were the main meteorological factors affecting soil moisture variation in maize terraced field;the extremely significant negative effect(p<0.01)was observed for air temperature on soil moisture variation in jujube orchard;a negative correlation(p<0.01)between soil moisture and temperature was presented in all sloping land use patterns.During the growing season in the normal precipitation year,the direct and comprehensive effects of air humidity on soil moisture of soybean sloping field,maize terrace field,and grassland were the most significant;soil temperature showed a higher effect on soil moisture in jujube orchard.During the growing season in the drought year,the direct and comprehensive effects of soil temperature on soil moisture of all sloping land use patterns were the greatest,and both of them were negative effects.The grey relational grade between air temperature and soil temperature in 0-60 cm layer of four sloping land use patterns was relatively high,and it decreased with soil depth.Under the similar meteorological conditions,the grey relational grade between soil temperature at the same depth and the same meteorological factors was different under different sloping land use patterns.The determination coefficient between air temperature and soil temperature in 0-20 cm layer of maize terraced field was relatively low,indicating that the influence of air temperature on maize terraced field was weak.(4)One-dimensional vertical soil water-heat coupled transport model was established on the basis of the influence of heat transfer on soil moisture transport under soil temperature gradient,the influence of soil moisture transport on heat transfer,and the influence of root water uptake on soil moisture transport and heat transfer.The finite difference method was used to solve the model.The model was validated by moisture content and temperature in 0-100 cm soil layer of jujube orchard in the drought year.The results showed that the determination coefficients(R~2)of simulated and measured soil moisture content during the simulation period were between 0.8581 and 0.9901,and the root-mean-square error(RMSE)and mean absolute error(MAE)were 0.7719%and 0.6856%,respectively.The average values of R~2,RMSE,and MAE of simulated and measured soil temperature at four growth stages in0-100 cm layer of jujube orchard were 0.8694,1.2884?,and 1.1894?,respectively.The above results show that the model has high accuracy for simulating the changes of soil moisture and temperature with time and depth in jujube orchard,and can accurately reflect the characteristics of soil moisture and heat transfer in jujube orchard at different growth stages.The effects of soil temperature gradient on soil moisture transport and root water uptake on heat transfer were studied by using the one-dimensional vertical soil water-heat coupled transport numerical model.The results show that when considering the effects of soil temperature gradient on soil moisture transport and root water uptake on heat transfer,the simulation results were obviously better than those of only considering one factor.Based on two-year continuous monitoring data,this study analyzed and discussed the soil moisture and temperature characteristics and their coupling effects under four typical sloping land use patterns in the loess hilly region both in the normal precipitation year and in the drought year.In this study,the dynamic changes of soil moisture and temperature,the complexity of soil moisture and temperature changes,the dominant environmental factors affecting soil moisture and temperature,and the construction and simulation of soil water-heat coupling model under different sloping land use patterns were thoroughly understood.The conclusions from this study can provide a scientific basis for the optimization of sloping land use structure and the efficient and sustainable utilization of sloping land resources in the loess hilly region.