The Study On Spatial-temporal Distribution Of Vegetation Canopy Rainfall Interception In The Qilian Mountains

As an important process in terrestrial hydrological cycle,vegetation canopy interception is the first part of rainfall redistribution,which can provide a series of important ecological functions,such as changing the distribution patterns of ground rainfall,relieving the surface erosion,reducing flood peak,regulating climate,and so on.The Qilian Mountains region is located at the inland arid and semiarid region in the northwest China,the spatiotemporal distribution research of vegetation canopy rainfall interception is important to understand the regional ecohydrological function of vegetation and to maintain the ecological security at the northwest China.However,the observation researches of vegetation canopy interception in the Qilian Mountains are most conducted at the plot scale with short observation period,lacking of multiyear continued observations.The existing global vegetation canopy interception products are fairly limited and have great uncertainties in the complicated mountain areas,which are hard to exactly reveal the spatiotemporal distributions of vegetation canopy interception in the Qilian Mountains.Besides,the spatiotemporal simulations of vegetation canopy interception are concentrated on small catchments in the Qilian Mountains,the research covered the whole region is still lacking.And the understanding of driving mechanism of vegetation canopy interception in the region is not clear yet.Hence,this thesis takes the Qilian Mountains as study area and vegetation canopy interception as study object,first based on the long-term plot observation data of forest(Qinghai spruce forest and Qilian juniper forest)in a typical small catchment(Tianlaochi)in the Qilian Mountains,the mechanism of forest canopy interception is studied.Then,according to the results of plot observations,the regional spatiotemporal variation patterns of precipitation and vegetation cover in the Qilian Mountains are studied using 3S technology.Finally,the vegetation canopy interception is quantitatively simulated,the vegetation canopy interception dataset in the growth season(from May to September)during 2000–2020 in the Qilian Mountains are developed,and the driving mechanism of vegetation canopy interception spatiotemporal variation are quantitatively explored at the regional scale.The study aims to lay the foundations for ecohydrological process research in the Qilian Mountains,and to provide theoretical support for rational utilization and management of water resources in the region.The main research results are as follows:(1)The results of plot research in the Tianlaochi catchment in the Qilian Mountains show that the throughfall amount,throughfall rate,stemflow amount,stemflow rate and interception loss are increasing with the increase of rainfall amount,while the canopy interception rate is decreasing with the increase of rainfall amount.The average interception rate of Qinghai spruce forest and Qilian juniper forest are 36.49% and27.94%,respectively.When the rainfall amount reaches 1.4 mm and 1.3 mm,the throughfall occurs in the Qinghai spruce forest and Qilian juniper forest,respectively;when the rainfall amount reaches 4.7 mm and 5.6 mm,the stemflow occurs in the two kinds of forests,respectively.The influencing factors significantly related to forest canopy interception ratio include LAI,rainfall amount,rainfall intensity,temperature and vapor pressure deficit.Among them,the correlation coefficients of rainfall amount and canopy interception ratio is-0.33(p<0.05),and that of LAI and it is 0.36(p<0.05).(2)The geographically weighted regression model is used to downscale GSMa PGauge products to obtain precipitation dataset with high resolution(1 km).The results show that the downscaled precipitation data have met reliable accuracy(for annual precipitation,R=0.91,Bias=0.11,RMSE=64.62 mm).In the growth season(from May to September)during 2000–2020,the rainfall amount and rainfall days have similar spatiotemporal variation trends.Spatially,they are gradually decreasing from the southeast to the northwest;temporally,they are increasing,except for September.(3)Using EVI time-series data,topographical factors and random forest algorithm can correctly extract the vegetation types in the Qilian Mountains.The overall accuracy and Kappa coefficient of vegetation cover classification are 88.39% and 0.87,respectively.The producer accuracy and user accuracy of each vegetation type are higher than 84%.Among different vegetation types,the area of grass(31.29%)is the largest,followed by shrubland(11.77%)and cultivated land(4.77%),while that of forest is the smallest(3.23%).The spatiotemporal variations of vegetation LAI in the Qilian Mountains are analyzed based on the vegetation cover classification results.Spatially,the vegetation LAI is decreasing from the southeast to the northwest in the Qilian Mountains.In the growth season from 2000 to 2020,the LAI value almost has an increasing trend,the average LAI value of each vegetation type from high to low are shrubland,cultivated land,forest and grass.(4)3S technology and RS-Gash model are used to simulate vegetation canopy rainfall interception in the Qilian Mountains,which have met reliable accuracy.The simulated results have good correlation with existing plot research results in the area and PML interception(R is between 052 and 0.84,p<0.01),which indicated that the results in the study can effectively reveal the real vegetation canopy interception in the Qilian Mountains.Temporally,the high values of canopy interception occurs in July and August;spatially,the high values region is concentrated in the southeastern Qilian Mountains.In the growth season in 2020,the interception loss of shrubland(80.83 mm)is highest,followed by cultivated land(66.52 mm)and forest(61.27 mm),while that of grass(37.63 mm)is the smallest;the order of interception rate of shrubland,forest,cultivated land and grass are 17.62%,14.97%,14.04% and 12.14%,respectively.In the growth season from 2000 to 2020,the canopy interception loss and interception ratio of all vegetation types are at an increasing trend.(5)In the view of the vegetation canopy rainfall interception in the whole Qilian Mountains,the predominant factors of the vegetation canopy interception are fraction of vegetation cover(FVC),rainfall days,rainfall amount,potential evaporation,and LAI.In 2000–2020,the average values of all predominant factors are significantly increasing.The relationships of each factor and interception loss/ratio are almost positively correlated.To the vegetation canopy interception loss/ratio,the relative contribution rate of FVC is the largest,followed by potential evaporation,rainfall amount,and rainfall days,while that of LAI is the smallest.