以20年衛星測高資料研究青藏地區湖水位變化

Abstract

於氣候變遷研究中，青藏地區湖泊水位變化數據提供重要的資訊。但由於此地區大部分之湖泊地理位置不易到達，且多缺乏水位計量測，能獲得之湖水位變化資訊有限。利用衛星光學影像與衛星測高儀觀測湖水位變化，具有高空間覆蓋率及高時間分辨率特性。故本研究使用1992-2014年之TOPEX/Poseidon系列測高資料，研究青藏地區23個湖泊水位變化，及Envisat測高資料(2002-2008年)觀測青海省湖泊-鄂陵湖與青海湖水位變化。測高資料須經環境參數及波形重定改正，其中波形重定是利用次波形門檻值演算法改善測距精度來求得湖水位變化。湖水位計算過程，包含粗差剔除、同cycle之觀測量取其平均及濾波處理。對於測距品質不穩定之Jason-1測高資料，須先進行波形分類取得可靠的觀測量來計算湖水位。 研究結果顯示，青藏地區湖泊20年湖水位變化趨勢中，上升之湖泊多分布於東半部，下降之湖泊多分布於西半部。青海省湖泊水位變化趨勢皆於2005年前後經歷先降後升，此現象與2005年青海省生態保護實施有關。整體而言，湖水位變化分佈受到印度季風吹拂以及湖泊地理位置影響，部分湖泊於1992-2002年的年震盪較2008-2014年大，部分則相反。本研究中，將20年之湖水位變化趨勢可以分成三類：(1)湖水位受到聖嬰現象影響造成趨勢反轉、(2)湖水位持續上升、(3)湖水位持續下降。

Lake levels in the Tibet and Qinghai Plateaus provide valuable records for climate change studies. Most lakes here are hard to access, having only few lake level gauges that give in situ measurements of changes in lake level and lake area. Remote sensing sensors, such as satellite imagery and satellite altimetry, are able to measure lake level variations with a dense spatial coverage and with a high temporal resolution. In this study, we use the altimeter data from TOPEX/Poseidon (TP) and its follow-on missions, Jason-1 and Jason-2, to observe lake level variations at 23 lakes along their repeat ground tracks every 10 days over 1992-2014. We also use the Envisat altimeter (2002-2008) to observe lake levels at three lakes of Qinghai at an interval of 35 days. We use subwaveform retracking to improve the ranging precisions of satellite altimeters. We employ an optimal processing technique to obtain quality measurements, including outlier detection, space-time reduction of measurements to a common reference point, and optimal filtering. Jason-1 fails to deliver height measurements over most of the lakes. The altimeter waveforms are classified to ensure observations with a sufficiently good quality. Over 1992-2014, the lake levels of most lakes in eastern Tibet rose, while the lake levels in western Tibet dropped. In Qinghai, the lake levels dropped before 2005 and then rose afterwards, suggesting a sharp climate change in 2005, or that the measure to protect the Qinghai ecosystem (e.g., reducing live stocks) started to take effect in 2005. In general, the overall pattern of lake change in Tibet and Qinghai is related to the variation of the Indian monsoons and locations of lakes. Most lake levels show clear annual and inter-annual oscillations. At some lakes, the amplitudes of annual variation in the TP era (1992-2002) were large and then turned smaller in the Jason-2 era (2008-2014). At some lakes, the annual amplitudes were reversed between the TP and Jason-2 eras. Also, some lakes show phase shifts in the highs of lake level. The lake level trends from 1992 to 2014 can be classified into three categories: (1) lakes with rising or dropping levels before a major ENSO event and then with reversed trends afterwards, (2) lakes with continuous dropping levels, (3) lakes with continuous rising levels.