在不同水文氣候特徵下農業灌溉對陸地-大氣耦合強度之影響

Impacts of Agricultural Irrigation on Land-Atmosphere Coupling Strength Under Different Hydroclimatological Characteristics

廖淇雅(Chi-Ya Liao)

指導教授 ： 羅敏輝

共同指導教授 ： 莊振義(Jehn-Yih Juang)

國立臺灣大學/理學院/大氣科學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201801923

摘要

陸地大氣交互作用的「熱點」，也就是土壤濕度距平對當地降雨有顯著影響的位置，通常被發現位於乾燥與潮濕氣候交界之過渡帶。在這些熱點，地表蒸發散仍然受到可用水量的限制，因為此時可用能量比可用水量多，但同時蒸發散的實際量值也大到足以影響當地大氣的穩定度。陸地大氣交互作用的強度隨水文氣候而不同，顯示出它和兩個蒸發散的限制因素(地表可用水量和可用能量)之間的競爭有顯著關係。
近來由於灌溉的廣大面積與高用水量，灌溉對氣候的影響受到許多關注，例如對地表溫度、地表通量、大氣環流和降雨的影響。此外，灌溉對陸地大氣交互作用的影響也是其中一個氣候議題。陸地大氣交互作用強度的改變是重要且值得關注的，因為交互作用強度的減弱可能會使次季節降雨的可預報程度隨之降低。過去有研究發現在亞馬遜雨林和美國中部大平原的灌溉會導致陸地大氣交互作用強度的減弱。然而其中的機制以及灌溉是否在其他地區也會同樣導致陸地大氣交互作用的減弱仍然不清楚。本研究比較在五個不同的灌溉區域(北印度、中國華北平原、西南歐、美國中部大平原和中亞)之間，灌溉對陸地大氣交互作用的影響有何不同。本研究同時使用陸地模式和陸地大氣耦合模式，目的在探討灌溉對地表直接的影響(例如:地表通量的改變)和後續影響到陸地大氣交互作用。此外，我們使用三種陸地大氣交互作用指數以表示陸地影響大氣的三個過程:(一)土壤濕度和蒸發散的關係、(二)地表可感熱通量和邊界層高度的關係以及(三)蒸發散和降雨量的關係。
陸地模式和陸地大氣耦合模式的結果皆顯示在不同的水文氣候下，灌溉能夠增強或減弱陸地大氣交互作用。在陸地模式中由於固定的大氣邊界條件，灌溉造成的變化能夠被當地的水文氣候特徵清楚解釋。在乾燥情況之下，灌溉傾向增強陸地大氣交互作用；在半乾燥及潮濕情況之下，灌溉傾向減弱陸地大氣交互作用。
然而在陸地大氣耦合模式中的模擬結果和陸地模式不同，原因來自大氣的變化及回饋作用。在五個灌溉區中，大部分的情況下灌溉的冷卻效應導致大氣的沉降距平以及水氣通量的低層水平輻散，除了中國華北平原的夏季初期。由於夏季初期中國華北平原極度潮濕，灌溉後地表蒸發散的增加和冷卻效應皆不顯著。該特性可能進一步抑制灌溉當地直接藉由地表通量變化改變大氣的訊號，相對的使大尺度大氣環流改變的訊號較顯著。大氣環流對灌溉的反應會改變降雨，並對地面可用水量形成回饋作用。此外，地表可用能量也會被大氣環流影響，主要藉由雲和大氣溫度的改變。雖然灌溉前後的陸地大氣交互作用的強度皆顯示出相同的隨水文氣候特徵變化的趨勢，蒸發散可用能量與可用水量的改變卻無法只以當地的水文氣候解釋，因為它們同時被灌溉和大氣環流改變的回饋作用影響。大氣環流在時、空上的變異性也需要被納入分析。
總結來說，本研究結果顯示出灌溉對陸氣交互作用的潛在影響，雖然單以水文氣候特徵的不同無法清楚的解釋所有過程，需要同時考慮大氣的回饋作用。本研究的結論可以被延伸應用在陸地大氣交互作用在聖嬰/反聖嬰現象或氣候變遷下的變化，因為降雨或能量的改變可能和灌溉有類似的效應。

關鍵字

陸地大氣交互作用 ； 農業灌溉 ； 水文氣候 ； 土壤濕度 ； 地表蒸發散限制因素 ； 蒸發散可用水量(能量)

並列摘要

The “hot spots” of land-atmosphere coupling (LAC), where soil moisture anomalies strongly affect local precipitation, are usually found in the transition zones between wet and dry climates. The evapotranspiration of these transition zones is mainly limited by the available water because the available energy is more than the available water, but at the same time, the evapotranspiration is large enough to affect the local atmospheric stability. LAC’s dependence on hydroclimate indicates that LAC strongly relates to competition between two limiting factors on surface evapotranspiration: the available energy and the available water.
Recently, the impacts of agricultural irrigation on climate, including the aspects of surface temperature, surface fluxes, atmospheric circulation, and precipitation, have gained lots of attention due to irrigation’s large area and magnitude. In addition, the impacts of irrigation on LAC is also a crucial climate issue. The shift of LAC is an important issue since if the strength of LAC is weakened, the predictability of sub-seasonal precipitation might decline. Two studies in Amazon and the Great Plains of America both show that irrigation results in the decrease of the LAC. However, the mechanisms behind and whether the irrigation process can lead to the overall reduction of the coupling strength worldwide remain unclear. This study aims to compare the differences of irrigation’s impact on LAC among five selected locations undergoing intensive irrigation: North India, North China Plain, Southwest Europe, America Great Plains and Middle East. The spatial and temporal differences of the factors which limit evapotranspiration (i.e., either by the available energy or water) will be the focus here.
Both offline land surface model simulations and coupled land-atmosphere simulations of Community Earth System Model (CESM) are used to explore the direct changes and the subsequent shifts in land-atmosphere interactions. Also, three coupling indices (including the relationships between changes in soil moisture and evapotranspiration; sensible heat flux and boundary layer height; evapotranspiration and precipitation) are adopted to quantify the coupling strength between the land and the atmosphere.
Results from both offline and coupled simulations imply that irrigation can weaken or strengthen the LAC under different mean hydroclimate. In offline simulations, because of fixed atmospheric boundary condition, the impact could be explained well by hydroclimatological characteristics. Under dry conditions, irrigation tends to increase LAC; in contrast, under semiarid and wet conditions, irrigation tends to decline LAC.
However, the changes of LAC are different between offline simulations and coupled land-atmosphere simulations due to the feedbacks from the atmosphere. Mostly the cooling effect of irrigation causes subsidence and further the low-level divergence of water vapor, except for North China Plain during early summer among the five simulations. Since North China Plain is the wettest place in this period, the increase of evapotranspiration and surface cooling is less significant. This property might inhibit local irrigation signal and contrarily more significantly reflect the signal of changed large scale circulation. Thus, the atmospheric response leads to changes in precipitation and cause feedbacks on the available water. In addition, the available energy is also affected by the atmospheric circulation through changes in cloud and temperature. The LAC before and after irrigation show similar characteristics with the hydroclimate. However, the conversion of the available energy and water for evapotranspiration, which results from both irrigation and its subsequent shifts of atmospheric circulation, could not be explained perfectly only by their original hydroclimatology characteristics. The temporal and spatial diversity of atmospheric circulation also contribute to these differences.
In sum, the results of this study show the potential response of LAC after irrigation although the local hydroclimate could not explain all the process perfectly, and the atmospheric feedback should be considered. The conclusion could be applied to the shift of LAC under ENSO and climate changes because the alteration of precipitation and/or energy may have similar effects as irrigation does.

並列關鍵字

land-atmosphere coupling ； agricultural irrigation ； hydroclimate ； soil moisture ； surface evaporation limitation ； available water (energy) of evapotranspiration

參考文獻

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration-Guidelines for computing crop water requirements-FAO Irrigation and drainage paper 56. Fao, Rome, 300(9), D05109.

Alter, R. E., Douglas, H. C., Winter, J. M., & Eltahir, E. A. (2018). Twentieth century regional climate change during the summer in the central United States attributed to agricultural intensification. Geophysical Research Letters, 45(3), 1586-1594.

Badger, A. M., & Dirmeyer, P. A. (2015). Climate response to Amazon forest replacement by heterogeneous crop cover. Hydrology and Earth System Sciences, 19(11), 4547-4557. doi:10.5194/hess-19-4547-2015

Barnston, A. G., & Schickedanz, P. T. (1984). The effect of irrigation on warm season precipitation in the southern Great Plains. Journal of Climate and Applied Meteorology, 23(6), 865-888.

Budyko, M. I. (1961). The Heat Balance of the Earth's Surface. Soviet Geography, 2(4), 3-13. doi:10.1080/00385417.1961.10770761