| Soil moisture is the main environmental element for the maintenance and development of natural ecosystems and dryland farming on the Loess Plateau.Analyzing the temporal and spatial changes of soil moisture is of important reference significance for sustainable use of soil moisture,agricultural production,and ecological restoration.The on-site monitored data of top(0-10cm)soil moisture(SM)of cropland is used to access the local suitability of SM generated from Land Data Assimilation System.The monthly SM the three sets of model assimilation data of CLDAS,GLDAS-1 and GLDAS-2.1Noah in the Loess Plateau was compared and evaluated from the aspects of spatial seasonal distribution,classical statistics and time series methods with monitored data.Aiming to select a kind of SM data which can be used to analysis the spatiotemporal variation of SM in Loess Plateau,with large scale and long time series.Above all,based on the verification and evaluation results,the soil moisture from CMA Land Data Assimilation System were used to analyze the spatio-temporal distribution and change of soil moisture content in the 0-10 cm layer of the Loess Plateau during 2008–2014.The main results are as follows:(1)The soil moisture content of CLDAS land surface mode was compared with the other two sets of data(GLDAS-1/ glda-2.1),which can better simulate the soil water characteristics of loess plateau.And it has a significant advantage in resolution which can describe the spatial details of soil moisture in the region.Therefore,it can used in soil moisture research on loess plateau at the regional scale.At the site,CLDAS showed good correlation and low error with observational data.The site with a significant correlation of 0.01 was 60% of the total site(78)and higher overall.The mean deviation of the site was low,and the site deviations(80%)were in-0.05-0.05.85% of the site absolute error concentration is below 0.06.And the absolute error of 15 sites is less than 0.03.88% of site root mean square error is distributed below 0.07.For the whole region,the correlation coefficient between CLDAS data and observation data is 0.90(significance level of 0.01).The mean deviation and root-mean-square error were also smaller(0.01 and 0.02 respectively).From the time sequence,the CLDAS can better fit the seasonal variation trend of soil moisture in the region from march to September(2011-2013),but in the spring compared with the actual measurement,there will be a different degree of underestimation,and the data of the actual station is very close from May to September.(2)From 2008 to 2014,in terms of spatial distribution,the soil moisture of all scales on the Loess Plateau showed a decreasing spatial pattern from southeast to northwest.Among them,the northern areas of the Qinling Mountains,the Weihe Valley,and the Liupan Mountains are regions with high soil moisture content throughout the year.West of Yulin,north of Yinchuan,and south of Linhe are low soil and water areas.In the change of time,the annual average value of regional soil moisture content showed a linear growth trend.The inter-annual space changes in the region,except for the Qinling Mountains in the Northern Plains,the surrounding area of Xining in Qinghai Province,the northern part of Shanxi Province in the north of Taiyuan,and the northwestern part of Inner Mongolia,which accounted for 26% of the total area,were not significantly reduced.The remaining 74% of the areas showed an increasing trend,of which the non-significantly increased area reached 73% of the total area of the Loess Plateau and was widely distributed in most areas of the study area(Figure 4-2).In addition,it can be seen from the distribution of the coefficient of variation of the annual average soil moisture content in the region that the interannual differences between the soil moisture content in the southeast and southwest of the region are not obvious,and the amplitude is small,while in the northwestern regions,inter-annual changes such as Guyuan in Ningxia,Linhe in Inner Mongolia and south of Baotou are more volatile.In the seasonal scale,except for the spring season in 2008–2014,the interannual changes of soil moisture content in summer and autumn all showed a linear increase,and the area in which the soil moisture content decreased in spring was 59% of the total area,the areas that increased in summer and autumn accounted for 82% and 71% of the Loess Plateau respectively.In addition,the annual average variation coefficient of soil moisture in each season is known: The surrounding areas such as Guyuan,Yan’an,and Linyi in the spring season fluctuate greatly;in the summer,the interannual changes are larger in the areas surrounding Tianshui,Baoji,and Wuzhong;and in autumn,Guyuan,Pingliang,Baotou,and south of Linfen.On monthly scale,the interannual variation of soil moisture content in each month shows that the soil water content in the spring months of March,April and May fluctuated significantly.In addition,except for March and April,the soil water content in the other months of 2008–2014 showed a linear growth trend. |
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