| Soil hydraulic properties are affected by the composition of particles,pore structure and complex arrangement of aggregates,which may change with climate,ecological evolution and land management process.Soil hydraulic characteristic parameters are the basis and premise of numerical simulation of soil hydrodynamic process.Due to the influence of tropical monsoon climate,the vegetation coverage of tropical monsoon rain forest in Xishuangbanna is high.There are seasonal differences in rain and heat conditions,and there are deciduous broad-leaved forests.Deciduous leaves will also change the soil hydraulic properties.It is of great significance for the study of Hydrometeorological process and ecological evolution to analyze the process of soil water movement and the change of soil hydraulic characteristic parameters in different seasons.Based on the observation data of Xishuangbanna tropical monsoon rain forest flux station from 2003 to 2008,this paper analyzes the seasonal characteristics of Xishuangbanna tropical monsoon rain forest.By using HYDRUS-1D,the soil water content was simulated by different parameter setting strategies,and the seasonal variation of soil permeability was analyzed by parameter inversion method.The main conclusions are as follows:(1)The year in the study area can be divided into dry and hot season,rainy season and fog and cool season.The results showed that the rainfall and temperature were the largest in rainy season,followed by dry hot season,and the smallest in foggy cool season;the net radiation was larger in rainy season,but smaller in dry hot season and foggy cool season;the soil water content fluctuated with rainfall,and kept a high level in rainy season,but was lower and decreases slowly in dry hot season and foggy cool season;the soil temperature rose slowly in dry hot season,and kept a high level in rainy season,and decreased slowly in foggy cool season.(2)Penman-Monteith(P-M)and Bowen-ratio method were used to estimate the evapotranspiration from 2003 to 2008.The evapotranspiration estimated by Penman-Monteith(P-M)method and Bowen-ratio method was the largest in rainy season,followed by dry and hot season,and the smallest in foggy and cool season.There were differences between the two methods in different seasons: the evapotranspiration estimated by the two methods was the most similar in dry and hot seasons,and the P-M method was obviously overestimated compared with Bowen-ratio method in rainy season.The results of Bowen-ratio method could better reflect the effect of soil moisture on evapotranspiration,while P-M method could better reflect the effect of air temperature on evapotranspiration.Evapotranspiration in rainy season was significantly less than rainfall,and it was greater than rainfall in foggy cool season and dry hot season.Evapotranspiration in dry hot season and foggy cool season mainly consumed the water stored in the soil in the previous rainy season.(3)The P-M formula was used to calculate the stomatal conductance,the diurnal variations of albedo,Bowen's ratio and stomatal conductance in different seasons were different.The diurnal variation of albedo in different seasons was affected by rainfall.It was "U" shape in foggy and cool season with less rainfall,and it had a peak value in dry and hot season and rainy season due to the influence of rainfall and cloud.Bowen ratio was less than 1 as a whole.It was mainly latent heat in the daytime,mainly affected by air temperature in rainy season,and mainly affected by soil water content and air temperature in dry hot season and foggy cool season.The variation range of Bowen ratio in dry hot season and foggy cool season was larger than that in rainy season.Stomatal conductance was affected by soil water content and transpiration,which was lower in dry hot season and rainy season around noon(12:00).The variation range in rainy season was smaller than that in dry hot season,and the variation characteristics were not obvious in foggy cool season due to low temperature.(4)The simulation results of soil moisture under different rainfall intensity was different.When rainfall was scarce,the simulated regression curves of surface layer(5 cm)and subsurface layer(20 cm)were below the measured regression curves.After heavy rainfall,the simulated value was larger than the measured value when the rainfall interval was longer.When the rainfall was concentrated and the rainfall intensity was heavy,the simulated value of deep soil water(40 cm)content was overestimated and the deviation is large.(5)Soil water content was simulated by using the strategy of which was simulation with one set of parameters during the whole period and which adopt multiple sets of parameters for different periods.Soil water content was simulated by using diverse parameters in different periods could significantly improve the simulation effect of soil water content in different seasons and depths.Under different parameter setting schemes,the simulation effect of fog cool season was the worst,and the simulation effect of rainy season was the best.Compared with the whole simulation method with one set of parameters,the simulation accuracy of soil water content could be improved by using time-division parameters according to the seasonal law,which could better correct the problem of large deviation between the simulated value and the measured value of soil water content in the process of intermittent period of rainfall,and more accurately described the soil hydrodynamic process.(6)HYDRUS-1D was used to invert the soil hydraulic parameters from 2003 to 2008,the soil hydraulic parameters of van Genuchten model had certain seasonal characteristics: The residual water content was larger in rainy season and smaller in dry and hot season.The saturated water content was smaller and changes gently in the middle and late rainy season.The field water capacity decreased from July to August,and increased from August to November.The value of saturated hydraulic conductivity obtained by inversion was small and gentle in the rainy season and early foggy cool season,but large and violent in other seasons.The parameter? of surface soil was smaller in rainy season,and the multi-year error of ? increased with the increase of depth;for parameter n,the multi-year error and variation range of surface soil was larger than that of deep soil.The seasonal variation characteristics of saturated water content and saturated hydraulic conductivity were obvious,which were related to the seasonal variation of soil moisture,litter and soil respiration.In rainy season,soil porosity and organic matter content decreased,while saturated water content and saturated hydraulic conductivity decreased,and vice versa in dry hot season and fog cool season.In addition,the tropical monsoon rain forest ecosystem in Xishuangbanna was the carbon source in the late dry and hot season(April)and the middle and early rainy season(May to August),and the carbon sink in other periods,which provided the basis for the reduction of organic matter content in the rainy season. |
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