基于植被/温度特征的黄土高原地表水分季节变化

地表水分是监测土地退化的一个重要指标，是气候、水文、生态、农业等领域的主要参数。选择陕北黄土高原地区作为研究区域，首先采用基于植被覆盖特征的植被状态指数（Vegetation Condition Index，简称VCI）和基于地表温度特征（Land Surface Temperature，简称Ts）的温度状态指数（Temperature Condition Index，简称TCI）分别评价了区域地表水分状况的季节变化。在此基础上分析了植被指数与地表温度特征线性关系的季节变化规律，计算了基于两者经验关系的地表干燥度指数（Temperature Vegetation Dryness Index，TVDI）。该指数对Ts／NDVI特征空间的生态特征的解释，对土壤和作物的水分含量具有综合的指示意义。文中利用该指数综合评价了研究区域地表水分状况的时空分布差异，进一步对VCI、TCI与TVDI相关关系的季节变化进行比较分析，并结合气候因子进行了相关验证，从而对不同指数的应用范围做出判定。研究结果表明，单独采用TCI或VCI表征地表水分会受到明显的季节影响，而TVDI能在不同季节综合体现植被覆盖和地表温度特征对地表水分的影响，从而能较好的反映区域地表水分状况的时空变化特征。各区域的TVDI值季节分布上皆为4—7月份高于10-翌年1月份，但各区TVDI值的季节变化则存在显著不同，而各流域内部TVDI值的空间变异性也存在季节差异，其中在10月份较为显著。

Intra-annual surface soil moisture change based on vegetation & temperature characteristics in Loess Plateau area

Vegetation coverage and surface temperature are important parameters to describe characters of land covers, and can provide information on vegetation and moisture conditions at the surface. The present paper aims at demonstrating how Terra/MODIS data may be used to estimate spatial patterns of soil moisture, a key variable in distributed hydrological models. The basic approach is to interpret the so-called Ts/NDVI space in terms of surface soil moisture status. Three kinds of index, such as vegetation condition index (VCI), temperature condition index (TCI) and a simplified land surface dryness index (Temperature Vegetation Dryness Index, TVDI) based on an empirical parameterisation of the relationship between surface temperature and vegetation index are suggested. These indexes are related to soil moisture and, in comparison to existing interpretations of the Ts/NDVI space, are conceptually and computationally straightforward. These indexes are based on satellite-derived information only, and the potential for application is therefore large.
Using 36 Terra/MODIS images from 2003 of the semiarid North Shaanxi Loess Plateau, NDVI (MOD11) and Ts (MOD13) were investigated as indicators of vegetation abundance and land surface temperature. To examine soil moisture conditions at the surface, the spatial pattern and temporal evolution in VCI, TCI and TVDI was analysed with Ts and NDVI. The results were: (1) Land surface moisture represented by VCI or TCI separately could not eliminate the effects caused from seasonal change of Ts and NDVI. However, TVDI, derived from analysis of the temperature/vegetation index space and created by integrated analyses of these two kinds of data, helped to determine main principles of the temporal and spatial variation of surface soil moisture. (2) From temporal evolution of TVDI, it could be inferred that the trend in TVDI is high values in the dry season (spring or autumn) and low values in the rainy season (summer or winter). (3) Spatial evolution of TVDI indicates that surface soil moisture of each region has its own seasonal rhythm. Mean TVDI in the northern region increased rapidly from winter to spring and decreased from summer to autumn in south; in the middle and west, the increment of TVDI from winter to spring was almost equal to decrement from summer to autumn. (4) Similarly, spatial variation of surface soil moisture in watersheds was seasonal. Typically, in October the variation in TVDI was greatest in most watersheds, especially in north.