Deep Soil Water Dynamics And Their Influencing Factors Analysis In A Gully-hilly Region Of The Loess Plateau

Deep soil water content(DSWC)is an important component of water resources in the gully and hilly region of loess area.It is not only one of the main sources of water for the growth of deep-rooted plants,but also a key factor in maintaining the ecological effects of above-ground vegetation.To evaluate and predict the DSWC movement and its related eco-hydrological processes,it is of crucial importance to investigate the spatial-temporal variability of DSWC and its influencing factors.This thesis takes the DSWC in the gully and hilly region of loess area(Gutun watershed)as the main research object.Combined with high-density field sampling strategy and in-situ monitoring,we monitored DSWC(0-500 cm)at 41 plots,installed one standard weather station,and six groundwater level monitoring sites.Moreover,in the laboratory,we determined the data sets of soil particles composition(n=1400),unsaturated soil hydraulic conductivity(n=12),and fresh plant root morphology(n=945),and then evaluated the spatial-temporal dynamics of DSWC and their influencing factor.The main conclusions are below:1.Quantitative characteristics of DSWC in 0-500 cm depth at the slope scale were investigated.The maximum soil water content(SWC)in slope and gully was 12.9%(10 cm)and 22.4%(480 cm),respectively,with different vertical distributions(slope:stable;gully:increased→decreased→increased).The maximum values of SWC in shallow(0-100 cm)and deep(100-500 cm)layer in slope were found in October and January,respectively.2.Analyzed the temporal stability of DSWC under three site conditions:forestland on slope,grassland on slope and farmland on gully,and assessed the spatial dependence of soil water content at different depths in slope.Moreover,we also evaluated the distribution of deep soil water storage(DSWS).The representative soil layers for DSWC under three site conditions were 120 cm(9.5%),200 cm(12.8%),and 320 cm(25.0%),respectively,which can be used to predict the mean soil water content within 0-500 cm profile.The spatial autocorrelation of SWC was strong for 0-10 cm and 30-40 cm layers,was moderate for 10-20 cm and 460-480cm layers,and was weak for 200-220 cm layer.In September,DSWC was continuously infiltrated and recharged.The depth of replenish at the end of December can reach to 200 cm or more.In late January,the replenish depth of rainfall to DSWS reached to 500 cm.3.Clarified the relationship between DSWC dynamics and soil particle composition,micro-topography,root system,unsaturated soil hydraulic conductivity and other factors.The vertical change of SWC in the deep soil was similar to that of sand content.In the horizontal direction,the spatial heterogeneity of clay content was closely related to the spatial variability of SWC.Among different slope aspects,SWC on shady slope was higher than sunny slope,and there was significant difference below 130 cm(P<0.05).The root weight density was highly related to DSWC,and at 0-120 cm,the coefficient of correlation was 0.95.There was a positive correlation between SWC and unsaturated soil hydraulic conductivity at surface(0-10cm)and near surface(30-40 cm)of the grassland and forest land(R~2>0.84).In addition,SWC varied with slope length in different soil layers(i.e.,200 cm,300 cm,400 cm,and 500cm).4.Revealed the response of DSWC in slope to precipitation,preliminary analysis of the relationship among fluctuations in groundwater level in gully,precipitation and DSWC.The response depth of DSWC in slope to a rainfall(88.4 mm)was mainly concentrated on 0-50cm depth.The maximum response depth to the rainy season reached to 220cm.SWC at 10 cm responded quickly to rainfall;and while there was a significant hysteresis effect at 50 cm;at100 cm depth only a certain response to the cumulative precipitation in the rainy season was found;below 200 cm no obvious response was detected.The DSWC in gully increased gradually during the early rainy season(2017.6.1-8.15),and there was a slight fluctuation in groundwater level in the later period(2017.9.9-10.13).After the rainy season(2017.10.14-12.15),precipitation recharged to groundwater which rised quickly.The water depth was the shallowest(5.57 m)in mid-December;after that,the water level gradually decreased.