以多時期MODIS衛星影像推估臺灣地區土壤濕度之時空變異

Using multi-temporal MODIS satellite imagery to estimate the temporal and spatial variation of soil moisture in Taiwan

建立空間性土壤濕度的長期動態變化，為提供集水區經營的重要資訊與參考基礎，但如以測站或利用現地調查的方式，取得土壤濕度的實測資料，則無法顯示大面積土壤濕度的時空變化。本研究以臺灣地區為研究範圍，使用多期MODIS衛星影像，包括2008年（2008/03-2009/02）、2010年（2010/03-2011/02）、2012年（2012/03-2013/02）、2014年（2014/03-2015-02）共16期影像為材料，利用地表溫度（Land Surface Temperature, LST）與常態化差異植生指數（Normalized Difference Vegetation Index, NDVI），推導溫度植生乾燥指數（Temperature Vegetation Dryness Index, TVDI），並以Pearson 相關分析臺灣地區各地理氣候區於不同季節之TVDI與累積降雨量的相關性，探討具有時空變化的 TVDI對於相對性土壤濕潤度的解釋能力。研究結果顯示臺灣地區除東北部（NE）氣候區的春季與冬季，以及西北部（NW）氣候區、東南部（SE）氣候區之冬季，累積降雨量對TVDI的影響非主要因素之外，其他中西部（CW）、西南部（SW）、東部（E）等地理氣候區，不同季節間之TVDI與累積降雨量呈現顯著負相關，顯示累積降雨量越多，相對其土壤呈現越濕潤，顯示多期MODIS影像演算的土壤乾燥指標，可有效解釋土壤濕潤程度之時空變化。整體而言，臺灣地區土壤濕度東部因植生覆蓋關係，相較於其他地區四季皆呈現較為濕潤狀態，而中西部及西南部，因受到土地高度利用的關係，植生覆蓋相對較低，四季之土壤濕度皆呈現較為乾燥狀態。 Estimation of soil moisture dynamics and distribution patterns over a long period can provides important information for watershed management as well as a frame of reference basis. However, the data obtained from the meteorological station in-situ or fi eld surveys is not suffi cient to represent the temporal and spatial variation of soil moisture. In this study, we used Taiwan as the study area. The multitemporal satellite images of MODIS were collected from year 2008 (2008/03-2009/02), 2010 (2010/03- 2011/02), 2012 (2012/03-2013/02), 2014 (2014/03-2015-02), 16 different periods of images. Land Surface Temperature (LST) and Normalized Difference Vegetation Index (NDVI) were used to calculate the Temperature Vegetation Dryness Index (TVDI). The correlations of TVDI and cumulative precipitation were analyzed by Pearson correlation analysis in different seasons for each climate zone in Taiwan, and the suitability of soil moisture represented by TVDI were discussed. The results of the study showed that, except the spring and winter in the Northeastern (NE) climate region and the winter in the Northwest (NW) and Southeast (SE) climate regions in Taiwan, the influence of cumulative precipitation on TVDI is not a major factor. In other geographic regions such as the Central West (CW), Southwest (SW), and Eastern (E), TVDI shows a signifi cant negative correlation with cumulative precipitation in different seasons. The more accumulated precipitation, the higher soil moisture. The results indicated that soil-drying indicators estimated from the analysis of multi-period MODIS imagery could effectively explain the temporal and spatial variation in the degree of soil moisture. Overall, compared to other regions in Taiwan, the eastern region appeared to have high soil moisture in all seasons due to high vegetation coverage. In the central west and southwest, the soil moisture is relatively dry due to high intensity land-use and low vegetation coverage.