The Spatio-temporal Distribution Of Dried Soil Layer And Its Available Soil Moisture In A Fragmented Catchment Of Northern Loess Plateau

The northern Loess Plateau is characterized by dense gullies and fragmented topography.The status of soil water shortage in this region is complicated under the combined effects of topography and vegetation.Soil water not only an important part of terrestrial water resources,but also a link between surface water,groundwater and atmospheric water.Gully is a typical geomorphological unit on the Loess Plateau,its development not only cuts the surface,destroys the cultivated land,but also silts up the river channel and fills the reservoir.Research on spatial-temporal distribution of soil water in fragmented area can help to evaluate the function of soil water reservoir and in-depth understanding the distribution pattern of dried soil layer in the fragmented area of the Loess Plateau.In this research,we designed 15 observational sites on 3 dividing waterlines in the fragmented area of Liudaogou catchment in Shenmu country.Based on long-term observation with neutron probe,we studied the spatial-temporal distribution of deep soil moisture,the characteristics of dried soil layer and the spatial-temporal distribution of available soil moisture.The specific results were showed as follow:(1)Soil moisture was in deficit in 0~3 m soil layers in the growing season,then it was recharged above 1.2 m depth.Soil moisture under 1.2 m depth did not recover and soil water deficit was serious in 2.6~6.4 m soil layers.The profile distribution of soil water content and relevant depths of extreme values as well as the occurred range of dried soil layers were inconsistent among different observational points.Moreover,the profile distribution of dried soil layers along the dividing waterline was discontinuous.The thickness of dried soil layers varied within the range of 0.4~8 m,the average formed depth of dried soil layer and mean soil moisture within the dried soil layer were 2.03 m and 9.03%,respectively.The former had negative linear correlations with the latter two quantitative indices.(2)The dynamics of DSL is characterized with somewhat complexity in the fragmented terrain;The distribution of DSL is temporally stable.The coefficients of Spearman rank correlation of DSLT and DSL-SWC were 0.95 and 0.90,respectively.To some extent,it is consistent with the characteristics of permanent DSL;Broken terrain does have impacts on the prediction accuracy of DSL,but we still obtain acceptable prediction results.(3)The magnitude of temporal variability was moderate in 0~9.6 m soil profile,coefficient of variation(CV)initially increased with depth,the maximum and minimum soil layer of variability was 7~8 m and 0~1 m,respectively.The mean SWS was correlated with CV in convex curve relationships in 0~3 m soil layers,and concave curve relationships were observed between them in others except 6~7 m layer.Soil moisture was in deficit at 68.42%of the observation periods,the maximum evapotranspiration(316.50 mm)was in September2016 and accounted for 77.11% of precipitation.In space,with the increase of DSL-SWC,the spatial heterogeneous of ASM decreases,and its content initially decreased with DSL-SWC and then increased.ASM becomes heterogeneous and then tends to be more homogeneous when DSL-SWC has more extensive heterogeneity,the soil water can be absorbed by plants will be more abundant.In time,ASM tends to be more abundant when DSL-SWC becomes wetter,and the magnitude of variation for them also in a positive relationship.ASM first tends to be more homogeneous and then becomes heterogeneous with the DSL-SWC increasingly.The temporal variability of DSL-SWC increasing will reduce the ASM content.