| Soil moisture links surface water and groundwater together and its spatial and temporal distributions significantly affects variability of hydrological and ecological processes.In Northwestern China,Qilian Mountain is the main runoff generation of the three inland rivers(terminal lakes): Shule River,Heihe River and Shiyang River in the Hexi Corridor.Since soil moisture is not only an important part of the underground water storage,but also the main source of groundwater recharge,estimating its spatial and temporal variability is critical to understanding hydrological processes but also to sustainable water resources management.This study aims to estimate soil moisture and its variability by remote sensing in the upstream of the Heihe River in the Qilian Mountain area.We established 31 in-situ observation stations with ECH2 O probe for monitoring soil moisture over five layers(0~10 cm,10~20 cm,20~30 cm,30~50 cm,50~70 cm).Soil samples were also collected for analyzing physical-chemical properties of the soils at each station.A soil moisture estimation model based on time series MODIS NDVI(Normalized Difference Vegetation Index)and LST(Land Surface Temperature)data was established.With this model,we calculated a five-layer soil moisture data set with the spatial resolution of 1 km?1 km and the temporal resolution of 16 d from October 16,2013 to October 14,2016.Subsequently,the characteristics of annual and seasonal variation of soil moisture,its spatial pattern,and the heterogeneity of soil profile were discussed in this study.Incorporating the downscaling TRMM precipitation data,soil physical-chemical properties and terrain factors,the Canonical Correspondence Analysis method(CCA)was used to identify the main-control factors affecting soil moisture variability.The main conclusions are as follows:First of all,the soil moisture estimation model developed in this study using MODIS time series NDVI,LST and measured soil moisture data was relatively accurate and suitable to the high elevation and topographically complex Qilian mountain areas,with multiple correlation coefficient of ranging from 0.47~0.94,RMSE about 0.03,and at 0.01 significance test.Secondly,over inter-annual scale,the whole 0~70 cm profile and each layer soil moisture had a rising yearly trend during the 2014~2016 growing season(April 23 rd to Oct 15th).At the season scale,soil moisture content was highest in summer,followed by autumn and spring,and the lowest in winter.The soil moisture variation was the highest in spring,followed by winter and autumn,and the lowest in summer.In addition,the soil moisture variations of the 0~30 cm layer were similar to the whole profile soil moisture,but the 30~50 cm showed the highest variation in winter,followed by spring and autumn,and the lowest in the summer,while the 50~70 cm had the highest variation in the winter,followed by spring and autumn,and the lowest in the summer.Thirdly,spatial distribution of the soil moisture showed that,at the inter-annual scale,eastern region soil moisture content was generally higher than the middle-western part.During the growing season,the western region had the higher variation of the soil moisture than the middle-eastern areas.At the 0~50cm,the soil moisture in the east and northwest areas was higher than that in the eastern area of the study region.The annual change of the 0~10 cm soil moisture in the west area from the 2014~2015 was higher than that in the eastern area,while during the 2015~2016,the soil moisture annual change was higher in the east than that in the west area.However,the soil moisture annual change over the layer 20~70 cm was higher in the west area than that in the middle-eastern areas.Fourthly,the layer 20-30 cm had the highest soil moisture content,the range of soil moisture changes decreased with the increasing depth.In the summer,the soil moisture content at the 0~20 cm was greater than that of the whole profile,and the 50~70 cm had the higher soil moisture in the winter.Fifthly,the precipitation,soil clay,silt and Soil Organic Carbon(SOC)were positively correlated with the soil moisture distribution.While soil sand and slope were negatively correlated with the soil moisture distribution.Soil moisture was also higher at the elevation range of 2750~3750 m than that at 1500~2750 m and 3750~5000 m,respectively.Analysis of the relationship between soil moisture and environmental factors showed that,the spatial distribution of soil moisture in Qilian Mountain region was mainly influenced by altitude,precipitation and SOC,followed by the soil texture types,and the Topographic Wetness Index,slope and aspect.Over the soil profile,the main factors affecting the soil moisture content were precipitation,soil texture,slope,altitude,aspect and Topographic Wetness Index,respectively.Hence,altitude,precipitation and SOC were the main factors affecting regional scale soil moisture distribution,while soil texture type and topography were the main factors affecting profile soil moisture. |
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