Spatio-temporal Dynamics Of Soil Water And Deep Seepage Of The Four Land Use Types In The Mu Us Sandy Land

Soil moisture is a critical linkage point between water cycle and water resources formation and transformation in terrestrial ecosystems,but also a limiting factor to the survival and distribution of the most plants in the drylands.Therefore,understanding the dynamic changes of soil moisture and the characteristics of deep seepage caused by rainfall under different vegetation cover conditions is of great significance for vegetation re-construction and ecological restoration in the semi-arid sandy areas.In this paper,our experimental sites were located at the Wu Shen Banner,in the hinterland of the Mu Us Sandy Land.The four land use types,including Salix plantation（Salix Psammophila）,farmland,natural grassland and bare sandy land,were selected as the research objects.In this site,soil moisture and temperature for each land use type were observed by the ECH₂O-5 sensor,and the meteorological factors have also been simultaneously monitored during the whole year of 2021.The Hydrus-1d model has been used to simulate the temporal and spatial dynamics of soil moisture and deep seepage under the different soil type condition.Lastly,soil water storage,evapotranspiration and water budget were compared under the different types.The result showed as follows:（1）The annual rainfall in 2021 was 314mm in the study area,during the observation period.The total potential evapotranspiration（ET₀）in 2021 was 930.8mm.And the seasonal trend of ET₀generally presented a single peak curve,with the maximum of the average monthly ET₀in July（5.14mm/d）.The soil temperature of salix plantation,farmland,natural grassland and bare sandy land all showed a significant positive correlation with atmospheric temperature（P<0.01）.While the response of soil temperature to atmospheric temperature showed a gradually weakened dependence with increasing soil depth.（2）The results showed that 0-200cm soil layer at salix plantation,farmland and 10,20,40,60 and 120cm layers at natural grassland could all be classified as sandy loam.While100,160 and 200cm at natural grassland could all be classified as sandy soil;all layers of bare sandy land were sandy soil.The variance analysis showed that there was no significant difference in the bulk density between soil layers in salix plantation（P﹥0.05）,and there were significant differences between soil layers in other plots.And there were significant differences in the bulk density for the same layer across all plots.Also,there existed a certain degree of difference in soil water-holding capacity between 0-200cm layers across the four sample plots,with the highest average water-holding capacity at salix plantation plot.（3）The dynamic of the four plots has all showed a strong seasonal pattern with a certain differences across the plots.During the whole study period,the dynamic of soil moisture for the four types could be divided into freezing period,replenishment period,consumption period and recovery period（farmland,natural grassland）,while salix plantation and bare sand were divided as a stabilization period substitute for the recovery period.The soil moisture of salix plantation showed a vertical distribution of first decreasing and then increasing;while the soil moisture of farmland showed a increasing-decreasing-increasing trend with soil depth.And the natural grassland showed an decreasing trend except for the 100cm layer with depth.The bare sandy land showed an upward trend except for the 100cm layer.（4）The response of soil moisture in each layer of the four plots to atmospheric rainfall gradually weakened with the depth of the soil layer,and the deep soil moisture had a certain hysteresis response.During the growing season（May-October）,rainfall events had a significant impact on soil water content in the 0-40cm layer of salix plantation and farmland,0-20cm in natural grassland and 0-100m in bare sandy land（P<0.01）,while they were not significant for other soil layers（P﹥0.05）.（5）The rainfall events during the growing season only recharged deep soil water storage at the bare sandy land plot（27.33mm）,while the other plots had all soil water deficits in the following order:salix plantation（30.26mm）,natural grassland（29.12mm）and farmland（17.28mm）,indicating that the rainfall season cannot meet the growth and development of salix plantation,farmland and natural grassland vegetation due to their strong demand of water use during the growing season in the year of less precipitation.（6）During the growing season,the total deep seepage of bare sandy land was 139.77mm,The total deep seepage of farmland was 11.92 mm,it was a result from much irrigation water input（292.4mm）in addition to rainfall during the study period,while deep seepage did not occurred at salix plantations and natural grasslands plots,indicating that the plots covering the vegetation such as grasses and trees could not produce deep leakage due to high evapotranspiration,when annual rainfall was low.The soil water balance equation was employed to calculate evapotranspiration during the growing season under different land use types in the Mu Us sandy land.And the evapotranspiration was ordered as follows:farmland（561.16mm）﹥salix plantation（293.66mm）﹥natural grassland（292.52mm）﹥bare sandy land（96.3mm）.