Soil Moisture And It's Macropore Characteristic As Affected By Gully Microtopography On The Loess Plateau

The Loess Plateau of China is characterized as serious soil erosion and fragile ecological environment.Severe soil and water loss restrict the social and economic development of the region.In recent years,the large-scale reconstruction of artificial vegetation has achieved great progress in the Loess Plateau.The problem of water and soil loss has been controlled to some extent.However,the excessive soil water consumption by artificial vegetation aggravates the contradiction between the supply and demand of water resources and the status of soil moisture has become a nonnegligible factor in the ecological environment construction in the Loess Plateau of China.The soil pore structure determines water holding capacity,permeability,and water conductivity of the soil and a better soil pore structure can effectively increase soil infiltration and water storage capacity,which in turn reduce the water and soil loss and increase water storage in soil reservoirs.Studies about the soil water content and macropore features of the gully microtopography is beneficial for deeper understanding of the soil moisture movement process in the gully,and provides scientific basis for the research on soil moisture and soil erosion in the gully.In this study,the characteristics of soil macropores are quantitatively analyzed by CT scanning technique,the physical properties of soil and the dynamic characteristics of soil moisture change are analyzed.In addition,the development characteristics of the caverns in the gully and the relationship between them are analyzed.The main conclusions are as follows:?1?The spatial variability of soil bulk density was weak,with an average CV of 6.91%,and there was no significant difference in different microtopography?p>0.05?.The soil texture of the gully is mainly composed of sand?76.50⁸2.28%?,the water permeability and infiltration capacity of gully head and water-dropping is relatively higher.The root biomass was low which ranging from 9.57 to 18.07 g·m^-2,and there was no significant difference in different microtopography?p>0.05?.?2?In terms of soil structure,The characteristics of soil macroporosity were:1)The large pores in the gully heads are relatively small and the pore shape is more regular which mainly distributed in the upper layer of soil.2)The soil structure in water-dropping is loose,the number of macropores was the largest,and the pore distribution in the soil was relatively uniform.3)The macropores in the dam are mainly distributed in the middle layer of the soil and macropores were generally larger with obvious differences.Among different microtopography,large aggregates were the main soil mechanical stabile aggregates.With the decrease of aggregates size,there is a"W"trend.The content of water stable aggregates was relative lower with the maximum value of 36.43.The size of MWD and GWD in soil mechanical stable aggregates under different microtopography followed a decrease trend of dam>water-dropping>gully head,and in soil water stabile aggregates they followed a decrease trend of gully head>dam>water-dropping.The structural destruction rate was33.03⁶7.14%.?3?In terms of hydraulic properties,the overall water conductivity is poor with an average soil saturated hydraulic conductivity?Ks?of 0.27 mm·min^-1.The differences in Ks between different microtopography were significant?p<0.05?.With the increase of soil depth,Ks of the head and water-dropping decreased,and the Ks in the dam increased.The average saturated soil water content is 33.10%,and followed a decreased trend of dam>water drop>gully head.The spatial variability of saturated soil water content is weak,and showed a decreased trend with the increase of soil depth.The van Genuchten model showed a good fitting on the soil water retention curves of different microtopography.The soil water retention curves of different depths have the same shape,and the water holding performance followed a decreased trend of dam>gully head>water-dropping.?4?Under the effect by the evaporation on the gully side walls,soil water content in the gully head was obviously lower than that in the dam.The impact depth of evaporation on the gully side walls was as much as 3 m.In the vertical direction,as the depth increased,the soil water content gradually increased and the coefficient of variation decreased,and the effect of precipitation,temperature and the vegetation water uptake on soil water content were also decreased.During the study period,soil water storage increased first and then decreased over time.By the end of the study period,soil water storage shortage was emerged and this is mainly due to severe water consumption by vegetation transpiration and surface evaporation.?5?The scale of sink holes in gullies small.The main section shape of sink holes were round or elliptical,and There is a significant positive correlation between the morphological parameters?p<0.05?.The sink holes mainly develop in the upper middle of the gully and at the head of branch gully,and was more common on the sunny slope.The sink holes generally single,beaded or honeycomb distribution,and they were often accompanied by the loess bridge and water-dropping.On the one hand,the unique geomorphic features of the gully provide the favorable conditions for the formation and development of a large number of caves.On the other hand,soil erosion is exacerbated by sink holes which promote the further development of gullies,and in this way,the regional landscape was changed.