| Preferential flow is a common type of rapid and non-equilibrium soil water movement process,which can promote the rapid migration of water and solute to the deep layer of soil,resulting in a significant decrease in nutrient and water utilization rate,and affecting the quality of groundwater.Expanding preferential flow research scale from slope and to catchment will help fully understand the impact of preferential flow on the regional water cycle,thus providing certain theoretical basis for effective management and protection of water resources.This study was conducted in a typical catchment of Simian Mountain,Chongqing.Multiple-tracer experiments,ponding infiltration methods,the morphological image processing technology,variance analysis,pared-samples Ttest,and variability analysis were used in this research.Staining morphological characteristics,the spatial distribution of infiltration depths,variability characteristics and quantitative evaluation of dyeing morphological characteristics were systematically analyzed in the 16 plots(S1-S16)of different position in the catchment under 2 different infiltration amouts of 25 mm and 60 mm.The effect of preferential flow in the catchment on nutrients migration was also investigated.Although the preferential flow infiltration depth was smallest at the bottom of the catchment,obvious lateral movement of water was found in the same area.The main results were summarized as follows:(1)Obvious spatial heterogeneity of staining patterns was found both in the interior of each plot and the entire catchment scale.In general,matrix flow and preferential flow occurred simultaneously,while the former was mainly distributed in surface layer of the soil,while the latter occurred below the surface layer.With the increase of soil depth,staining ratios of horizontal and vertical slices basically decreased and the staining area distribution gradually changed from flake distribution to block or point distribution.Preferential flow in S7-S10 located in the middle and lower part of the catchment was relatively higher,which can infiltrate to a maximum depth of 60 cm,while S 14-S16 located in the bottom of the catchment was usually lower.Higher variations of staining area were showed in the central part of the catchment under infiltration amount of 25 mm,compared with lower ones at the bottom.Matrix flow area,staining ratio of each layer,and flake distribution ratio of the staining area increased with the increase of infiltration amount from 25 cm to 60 mm,while the degree of staining ratios variation in the catchment scale decreased.(2)Spatial heterogeneity of actual and maximum infiltration depths in the interior of each plot and the entire catchment scale indicated that the preferential flow movement has obvious non-uniformity characteristics.Actual and maximum infiltration depths were both significantly improved with the increase of infiltration amount from 25 cm to 60 mm,and achevied the maximum infiltration depths of 55 cm and 60 cm,respectively.Although greater differences of actual infiltration depths were found in the interior of each plot,its variation range was generally less than of maximum infiltration depths.The variations of actual and maximum infiltration depths could be reduced as infiltration amount increased Similar to the change trend of staining ratio,variation coefficients of actual and maximum infiltration depths above the central part of the catchment were significantly greater than that of the bottom,showing a greater variability.(3)Significant spatial differences in total staining ratio(Dc),matrix flow depth(UF),preferential flow ratio(PF),and length index of preferential flow(Li)were found among the 16 preferential flow observation plots in the catchment,and the difference was affected by infiltration amount in different degree(P<0.05).Total staining ratio,matrix flow depth,and length index of preferential flow were larger in Sl-S4 than those of other plots in the catchment,while the opposite result was shown in preferential flow ratio.Infiltration amount,total staining ratio,matrix flow depth,and length index of preferential flow were positively correlated with each other,while all of them had significant negative correlation with preferential flow ratio(P<0.05).The morphological characteristics of the preferential flow in the catchment were affected by macropore percentage and vegetation characteristics in different degree.(4)Obvious spatial heterogeneity was also observed in the effect of preferential flow on nutrients migration.Maximum migration depth of Br-and NO3-was 60 cm,which was found in S8.At the bottom of the catchment,although the migration depth of the preferential flow was small,water could be rapidly transported into river along with interflow,polluting surface water.With the increase of soil depth,change of Br and NO3-content mainly showed two kinds of trends,i.e.,gradually decreased as well as increased first and then decreased(P<0.05).Moreover,they generally showed different changes in the samen plot.The influence of preferential flow on Br and NI3-migration was affected by the combined effect of preferential path transport capacity and infiltration stage.In the first stage,water mainly moved downward through preferential pathes.During the second stage,obvious lateral movement of water was found in S3 and S11,showing larger NO3-content.In the third stage,significant lateral movement occurred in more than half of the observation plots,showing larger Br content.In addition,soil retained in the preferential pathes in the previous stages could continue to migrate into deeper soil layers after water was added in the latter stage.In summary,preferential flow can be promoted to deeper soil by the increasing infiltration water amount,and the largest infiltration depth and degree of preferential flow were found in the middle and lower part of the catchment.Although the preferential flow infiltration depth was smallest at the bottom of the catchment,obvious lateral movement of water was found in the same area.These results can provide some theoretical guidance for forest management and regional water environment protection.To reduce the pollution of preferential flow to surface water and groundwater,fertilization should be forbidden before big rainfall events during the artificial forest management. |
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