從陸地大氣交互作用熱點談北美中部大平原之灌溉效應

Abstract

許多研究指出北美中部大平原為陸地大氣交互作用之熱點，並視其為改進季內尺度預報的重點區域。然而北美中部作為全球主要糧倉之一、每年灌溉用水量僅次於季風亞洲區，在過去文獻中灌溉對降水的反應卻相當分歧。這份研究旨在透過探討陸地大氣交互作用熱點的概念，試圖解釋灌溉對於當地降水影響不顯著的原因。GLACE (Global Land-Atmosphere Coupling Experiment) 是一組透過全球氣候模式來探討陸地大氣交互作用強度的系集實驗，其後多份研究利用這組實驗定義出陸地大氣交互作用熱點。由於 GLACE 移除陸地大氣交互作用的作法是在未耦合實驗裡強制給定耦合實驗的土壤濕度，這樣的研究方法將改變兩組實驗的土壤濕度變異度，反映陸地大氣交互作用熱點的定義是建立在該地對於土壤濕度變異度的敏感程度。我們因此推測灌溉對於北美中部降水的影響結果分歧，是因為不同研究中考慮灌溉的方式不同，使得土壤濕度的變異度有所差異所導致。我們使用與 GLACE 相同的全球氣候模式 CESM (Community Earth System Model) 來進行灌溉實驗，並採用兩種灌溉情境來改變土壤濕度的變異度：其一是透過注水讓土壤水飽和的方式直接改變土壤濕度，其二是透過將灌溉水均勻灑在表層土壤作為有效降水。結果顯示，雖然注水讓土壤水飽和的方式能夠有效增加表層土壤水，但其在增加蒸發散量的幅度反而不如噴灑灌溉來得大。這與 GLACE 所推論的結果不同，顯示在灌溉實驗中蒸發散量之變化不完全由土壤濕度變異度所主導。除此之外，雖然灌溉實驗中土壤濕度的增加能夠顯著增加蒸發散量，但降水反應卻不如 GLACE 所顯示的顯著，也就是說增加的蒸發散量並未能有效轉換成當地降水，這可能跟北美中部大平原的降水主要由中尺度對流系統所貢獻有關。值得注意的是，儘管夏季平均降水並未增多，但在注水讓土壤水飽和的實驗中卻可觀察到降水在年際尺度上變異度的下降，亦即灌溉活動可能讓乾年變得較濕、濕年變得較乾，因而灌溉實驗中的平均降水沒有太大的變化。這顯示了土壤濕度變異度之重要性。

The Great Plains in North America has been identified as a “hotspot” of land-atmosphere coupling, but shown equivocal responses in various irrigation studies despite being one of the breadbaskets in the world. In this study, the concept of land-atmosphere coupling hotspot is explored using GLACE (Global Land-Atmosphere Coupling Experiment). We assume that the diverged irrigation responses among the modeling studies can be explained by the differences in soil moisture variability resulting from different irrigation representations. To compare with GLACE, we adopted the same global climate model, Community Earth System Model (CESM), for our irrigation experiments. We considered two types of irrigation method (flood irrigation vs. sprinkler irrigation) and modified the irrigation water amount to assess whether the effect of soil moisture (SM) variability would be obscured by the mean SM differences. The results show that, although flood irrigation can remarkably increase soil moisture values, sprinkler irrigation is more effective in enhancing evapotranspiration (ET), implying that the changes in ET is not purely determined by soil moisture variability as GLACE suggests. In addition, despite the general increase in ET in the irrigation experiments, precipitation response is not as significant as GLACE indicates, which shows that the increase in ET cannot translate into local precipitation. Previous studies also have indicated that ET might not have a strong influence on precipitation in this region where the rainfall is primarily nocturnal induced by mesoscale convective systems. Notably, although the JJA mean precipitation does not vary much, precipitation variability reduces significantly where the soil moisture variability is reduced. That said, irrigation possibly makes dry years wetter and wet years drier, so that the mean precipitation changes little. This study underscores the importance of soil moisture variability changes.