Estimating the Vegetation Water Content on Different Drought Condition with MODIS Reflectance Data

• Main Authors: Wei-Li Yan, 顏瑋利
• Other Authors: Chaur-Tzuhn Chen
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/76395903073093449742

碩士 === 國立屏東科技大學 === 森林系 === 94 === The Normalized Difference Vegetation Index (NDVI) is a well-known measure for biophysical variables that has been widely used for vegetation monitoring. However, NDVI is not always an appropriate tool for real-time drought monitoring. Due to a lagged vegetation response to drought, NDVI cannot detect drought events instantaneously. In this study, we used the data of the CI-700 and CM-1000 experiment for chlorophyll variation and reflectance on different drought conditions, and discussed biophysical leaf properties under drought stress. The R, NIR, SWIR and thermal bands of MODIS images were used as data for calculating NDVI, Water Index (WI), Normalized Difference Water Index (NDWI) and Global Vegetation Moisture Index (GVMI). The land surface temperature and NDVI was assessed in order to estimate a drought index of Temperature-Vegetation Dryness Index (TVDI) in different seasons. The TVDI was based on an empirical parameterization of the relationship between land surface temperatures (Ts) and NDVI. The results showed that there were significantly different (p<0.05) in five land use indexes. As expected, there is gradual drought in the dry season in developed areas as well as in forest areas. From this result, we can conclude that the TVDI reflects the soil moisture status, and that it can be used as an index in future drought monitoring. The drought response of the grassland is more sensitive than forests. WI has a high correlation with vegetation water content, but affected by canopy structure and viewing geometry. GVMI of grassland detected changes in vegetation water content three months prior to NDVI. A combination of the SWIR and the NIR is required to calculate vegetation water content. GVMI is not related to the vegetation moisture content expressed as a percentage of water per quantity of biomass. NDVI provides different information (e.g., vegetation greenness), which is not directly related to the quantity of water in the vegetation.