農業灌溉於低緯度地區造成之冬季時期高土壤濕度對區域與全球氣候之可能影響

Abstract

了解農業灌溉對環境造成的影響一直都是重要的課題。人為的水資源管理（如灌溉）足以改變地表的能量收支與水循環。在本研究中，我們關注在亞洲低緯度地區的灌溉行為對冬季時期區域和全球氣候的可能影響。我們利用地球系統模式模擬陸地與大氣交互作用如何受到灌溉的影響，以及大氣環流如何改變。我們發現，不考慮灌溉的模式模擬低估了冬季在印度河─恆河平原的平均蒸發散量。比較有無考慮灌溉的模式模擬結果顯示，低緯度地區的灌溉使得冬季時期有較高的土壤濕度，從而降低了地表鮑文比，經由大氣的迴饋造成整個印度次大陸尺度的地表降溫。由於較大的海陸熱力差異，盛行季風亦有所增強。此外，熱帶地區的降水與中緯度的氣候皆有改變，顯示了熱帶與溫帶間的遙相關關係。冬季的阿留申低壓加深並向東移動，儘管在冬季，北美幾乎沒有進行任何灌溉，但實驗中在北美陸地則有增暖的現象。在前人的研究中曾發現此增暖現象，但並無加以解釋，因此本研究提供了一個合理的機制，亦即此北美的增暖現象是來自於在亞洲低緯度地區進行灌溉所造成的影響。

The effect of agricultural irrigation on the environment has long been a critical concern. Anthropogenic water management can change surface-energy budgets and the water cycle. In this study, we focused on the impacts of Asian low-latitude irrigation on regional and global climates during boreal wintertime. We used a state-of-the-art earth system model to simulate land–air interaction processes affected by water management and the consequent responses in atmospheric circulation. Modeling without considering irrigation underestimates evapotranspiration in the Indo-Gangetic Plain during winter. Perturbed experiments show that wet-soil-moisture anomalies at low latitudes can lower the surface Bowen ratio and reduce the surface temperature to a continental scale through atmospheric feedback. The intensity of prevailing monsoon circulation becomes stronger because of larger land–sea thermal contrast. In addition, anomalous tropical precipitation and midlatitude climatic changes indicate tropical–extratropical teleconnections. The wintertime Aleutian low is deepened and shifts eastward, and an anomalous warm surface temperature is found in North America, although there is little irrigation over North America in the winter. Previous studies have noted this warming but left it unexplained, and we provide a plausible mechanism for these remote impacts, which come from the irrigation over Asian low-latitude regions.