基于温度植被指数TVDI的拉萨地区土壤湿度特征分析

Characteristics of Surface Soil Moisture in Lhasa Based on Temperature Vegetation Dryness Index(TVDI)

土壤湿度变化是探索高寒地区陆-气系统耦合演变的重要指标。基于MODIS地表温度和增强型植被指数,本文构建了拉萨地区主要土地利用类型地表土壤干湿特征空间,有效验证了温度植被干旱指数(Temperature Vegetation Dryness Index,TVDI)在研究区的适用性。研究结果表明:总体上,TVDI与实测的地表土壤湿度拟合效果良好,呈显著负相关(r^(2)=0.71,p<0.05)。耕地LST(Land Surface Temperature)随着EVI(Enhanceel Vegetation Index)增加而显著减小(p<0.05),林地、草地和湿地受海拔影响,LST与EVI呈显著正相关(p<0.05)。7月-9月6期LST-EVI特征空间,其散点图呈三角形状,均能够较好地拟合出干边和湿边。从空间上看,大部分地区处于极湿润(0~0.2)和湿润(0.2~0.4)的状态,分别占研究区面积的57.01%和22.27%,干旱区域(0.6~1.0)主要分布在北部,西南以及中南部分地区,土壤总体上呈现为湿润并逐渐变干的趋势。4种土地利用类型下土壤湿度均随海拔的升高而显著增加(p<0.05),整体上TVDI变化与气温变化呈正相关,与降水呈负相关,降水对于土壤湿度的影响更为明显。

Soil moisture is a crucial parameter for studying the changes of land-atmosphere system,particularly in the alpine region.Based on the MODIS surface temperature(LST)and Land Surface Temperature(EVI)data sets,this paper first constructs the LST-EVI feature space in Lhasa.Then,it verifies the applicability of TVDI indictor effectively to retrieve the surface moisture in this area.Our results show that TVDI and the measured surface soil volumetric water content have a good fitting effect,and there is a significant negative correlation(r^(2)=0.71,p<0.05).Cultivated land LST decreases significantly with the increase in EVI(p<0.05).Forestland,grassland and wetland are affected by altitude,and LST and EVI are significantly positively correlated(p<0.05).In the LST-EVI feature space during the 6th period from July to September,the scatter plots are triangular in shape,and both dry edges and wet edges can be fitted well.From a spatial point of view,most regions are in extremely humid(0~0.2)and humid(0.2~0.4)states,accounting for 57.01%and 22.27%of the study area,respectively.Arid regions(0.6~1.0)are mainly distributed in the north.In the southwest and central south,the soil generally appears to be moist and gradually dry.Under the four land use types,soil moisture increased significantly with the elevation(p<0.05).TVDI basically changes in the same direction with temperature and reverses with precipitation,and the impact of precipitation on soil moisture is more obvious.