利用遙測資料估算水稻田蒸發散量

Using Remote Sensing to Estimate Evapotranspiration of Paddy Field

簡子杰(Tzu-Chieh Chien) ； 劉說安(Yuei-An Liou) ； 張子瑩(Tzu-Yin Chang)

《航測及遙測學刊》 13卷1期 (2008/03) Pp. 1-18

https://doi.org/10.6574/JPRS.2008.13(1).1

摘要

蒸發散在水文循環、區域氣候中扮演重要角色，如何能獲得其觀測值，成爲重點課題。以水稻田爲例，水稻田區能吸收與儲存大量的熱能，經由蒸發散作用釋放出來，以調節周圍環境的溫度，因此在都市化的區域中，得有效降低熱島效應強度。本研究的目的，在於建立遙測資料估算水稻田蒸發散量的反演模式，以利未來應用於長時間、大面積的監測水稻田蒸發散量，及進行蒸發散作用調節熱島效應的評估及研究。所提出的反演模式是以地表能量平衡爲基礎，利用遙測資料配合地面氣象站的氣象資料，進行地表熱通量的估計。首先，利用大氣不穩定的修正模式，推估潛熱通量和可感熱通量的初始值，再以迭代方式自動取得乾濕控制點，獲得乾、濕控制曲線，進而重新分配淨可用能量，以推估最終的潛熱通量和可感熱通量，而潛熱通量即水稻田的蒸發散量。研究中先應用空載影像進行改良模式的測試，在反演水稻田的蒸發散量結果上與地面測站比對，蒸發散量比值的偏差僅有6.8%，因而將此方法推廣至較高時間解析度衛載的MODIS影像加以分析。比較MODIS影像所推估的潛熱通量與地面測站真值，相關係數爲0.66，均方根差爲97.81(Wm-2)；可感熱通量方面，相關係數可達0.76，均方根差爲124.33(Wm-2)。結果顯示，利用本研究所提出的反演方法結合MODIS衛星資料，進行水稻田蒸發散量的估算爲可行的，期望未來能推廣至各地，利用於進行水稻田長時期蒸發散量之監測。

關鍵字

潛熱通量 ； 可感熱通量 ； 蒸發散量 ； MODIS

並列摘要

Evapotranspiration is an important factor in hydrology cycle and regional climate. For example, rice paddy field absorbs and stores huge amount of energy, which is released to the air through evapotranspiration. As a result, ambient temperature is adjusted and the strength of the heat island effect is mitigated in the urbanization areas. The aim of the current study is to develop an evapotranspiration retrieval algorithm for rice paddy field by remote sensing technique in order to provide long-term and large-area observation and to investigate its role in heat island effect. The proposed retrieval algorithm is based on energy balance at the land-air interface. Surface heat fluxes are estimated by remote sensing data with in situ surface meteorological measurements. The initial values of latent heat and sensible heat fluxes are obtained using an unstable atmospheric correction method. By iterating the energy balance equation, radiation and evaporation controlled lines are determined, and hence, the net available energy is redistributed into latent and sensible heat fluxes, respectively. In this study, both airborne and satellite imageries are utilized. The former imagery covers a small area with high spatial resolution. The satellite imagery acquired by MODIS is utilized to estimate the evapotranspiration over the entire Taiwan. The proposed retrieval algorithm has provided a fairly good estimate of evaporation fraction by airborne imagery with bias of 6.8% when compared with the in situ measurements, while performed poorly with the satellite imagery. To improve the evapotranspiration estimate by satellite imagery, the Paulson and Dyer's corrections to unstable atmosphere were modified. The correlation coefficients for latent heat flux and sensible heat flux and the corresponding in situ observations were found to be 0.66 and 0.76, respectively. The root mean square errors for latent heat flux and sensible heat flux were 97.81 Wm-2 and 124.33 Wm-2, respectively.

並列關鍵字

Latent heat flux ； Sensible heat flux ； Evapotranspiration ； MODIS