Hydrological Effects Of Ground Cover Under Dominant Arbors Forests In The Center Part Of The Qilian Mountains

The increasingly serious shortage of water resources threatens the food security,ecological security and sustainable development of economy and society in Northwest China.The region of Qilian Mountainous is a high-value area of precipitation and runoff in the northwest of China.It is the birthplace of Heihe,Shiyang and Shule Rivers,and plays an important role in the sustainable development of Hexi Corridor’s society and economy.Therefore,water security in the Qilian Mountains is an important guarantee for the ecological security and the sustainable socio-economic development in the arid areas of northwest China.The abundant vegetation communities in the Qilian Mountains profoundly influence the hydrological cycle of the Qilian mountainous,especially,the ground cover layer between the forest plant layer and the soil layer can not only play an important role in runoff by retaining precipitation,but also affect soil evaporation by regulating the surface temperature of the soil,which in turn affects soil water content（SWC）and influences the eco-hydrological process of the region in many ways.Therefore,it is important to study the eco-hydrological function of the ground cover layer to bring into play the function of forest reservoirs in mountainous areas and to ensure water security in northwest China.In this study,the ground cover layer（all covering on the soil surface after removal of living vegetation）under the dominant arbor forest（Qinghai spruce and Qilian cypress）in the middle part of Qilian Mountains was selected as the object of study to determine its thickness,storage,water-holding capacity,and physical properties and infiltration performance of the soil layer under the ground cover,and to clarify the effective rainfall threshold and rainfall transport paths under different slope and ground cover conditions by measuring the difference between before and after rainfall of SWC.At the same time,a control group was set up to observe the dynamic changes of SWC in two cover conditions（with and without ground cover）,and to analyze the response of SWC to precipitation under the two cover conditions.The main results are as follows:（1）The SBD（soil bulk density）of Qinghai spruce forest and Qilian cypress forest both increased with soil depth.The SBD of Qinghai spruce forest ranged from 0.46–0.74 g·cm^-1 at 0–30 cm depth,while that of Qilian cypress forest ranged from 0.87–1.06g·cm^-1.The soil porosity of Qinghai spruce forest ranged from 72.8–83.1%at 0–30 cm depth,while that of Qilian cypress forest ranged from 61.0–68.1%.The soil infiltration rates and infiltration velocities ranged from 15.12–23.69 mm·min^-1 and 3.92–14.57mm·min^-1 in the Qinghai spruce forest,while that of Qilian cypress forest ranged from1.64–2.94 mm·min^-1 and 19.06–11.89 mm·min^-1,respectively.The average amount of cover storage in Qinghai spruce forests was 33.27 t·hm^-2,and the effective water retention in ground cover was 55.97 t·hm^-2,while the average amount of cover storage in Qilian cypress forests was 23.27 t·hm^-2,and the effective retention was 28.93 t·hm^-2.There were significant power and logarithmic function relationship between ground cover water-absorption rate,ground cover water-holding capacity and soaking time.In indoor tests,if other retention is not considered,the ground cover can retain almost all of the precipitation within 1 h of occurrence.（2）In the two sample plots,the slope levels were set as 0°,13°,18°,23°,28°,33°,38°,and 43°.In different slopes and ground cover conditions,the in-forest rainfall threshold for soil moisture supplementation in the Qinghai spruce ranged from 3.7 to6.6 mm,with the largest effective in-forest rainfall threshold for soil moisture supplementation at 43°（6.6 mm）and the smallest effective in-forest rainfall threshold for soil moisture supplementation at 23°（3.7 mm）.The in-forest rainfall threshold for soil moisture supplementation at the beginning of the Qilian cypress ranged from 2.1 to5.3 mm,with the maximum effective in-forest rainfall threshold for soil moisture supplementation at 18°（5.3 mm）and the minimum effective in-forest rainfall threshold for soil moisture supplementation at 33°（2.1 mm）.Among the different natural rainfall events,surface runoff was produced only during heavy rain events with in-forest rainfall heavier than 27.7 mm in Qinghai spruce,whereas surface runoff was already produced at a slope of 38°during a rainfall event of 13.4 mm within the forest in Qilian cypress.（3）In June and July,when transpiration is intense,the SWC of surface with ground cover is generally greater than that without ground cover.In May,when the snowpack is slowly melting,and in August and September,when rainfall is abundant and transpiration is more moderate,the SWC of surface without ground cover is generally greater than that with ground cover.The vertical variation of SWC at each observation point with ground cover was smaller than that at the observation points without ground cover.（4）On the day after the rainfall event,the degree of soil moisture variability showed a trend of spruce with ground cover（20.21%）>cypress with ground cover（19.12%）>spruce without ground cover（16.84%）>cypress without ground cover（15.82%）.After water fading and redistribution,the degree of soil moisture variability showed opposite trends,with the magnitude of cypress without ground cover（21.83%）>spruce without ground cover（17.70%）>cypress with ground cover（17.61%）>spruce with ground cover（14.33%）.In general,the degree of soil moisture variability after rainfall was less than that before rainfall in both sample sites.The normal distribution curve of SWC in cypress showed a“low peak”distribution and that of spruce showed a“sharp peak”distribution within three days after the rainfall.The presence of ground cover did not have a smoothing effect on standing soil moisture heterogeneity and increased it to some extent,whereas after moisture redistribution,the presence of ground cover had a decreased effect on standing soil moisture heterogeneity.The k means the depletion coefficient of SWC,which values were influenced by many environmental factors.In general,the k values of soils under spruce forests were larger than those under cypress forests,and the k values of soils with ground cover were larger than those without ground cover.The k values of different soil layers under the same cover conditions was arranged in the order of 10–20 cm>20–40 cm>0–10 cm.In summary,the ground cover layer under the dominant arbor forest in the central Qilian Mountains can intercepting precipitation and block runoff during rainfall events,among which the ground cover under the Qinghai spruce is more capable of regulating precipitation and retaining water and soil.The ground cover layer has a profound effect on soil moisture:it retains soil moisture during months of strong soil depletion and traps regulated moisture during months of heavy rainfall and water input.During a rainfall event,the presence of ground cover on the day of rainfall increases soil moisture spatial heterogeneity,reduces spatial heterogeneity after moisture redistribution,and slows the rate of soil moisture receding after the rain.