灌溉的濕化與冷卻作用對於近地表微氣候的綜合反應

Abstract

近年來許多研究探討灌溉導致近地表微氣候改變的程度，但目前尚未有文獻提出在乾濕季下，濕球溫度對於溫度及濕度改變的敏感度，且至今尚未了解灌溉對濕球溫度氣候平均態的影響。本文探討灌溉的冷卻效果與濕化作用對於濕球溫度的互補關係以及乾濕季下濕球溫度的改變特徵。前人已發現一年中最熱月份的平均最高溫隨著灌溉的擴大而降溫（冷卻效果），而濕化效果主要是強調灌溉增濕近地面空氣將提升濕球溫度與降低該地區的舒適度，本文將探討此兩大作用並結合不同的背景濕度條件，討論濕球溫度變化的主導因素。此研究分析美國國家大氣研究中心(National Center for Atmospheric Research)發展之耦合氣候模型(Community Earth System Model)以及非耦合之陸地模型(Community Land Model)所輸出的兩公尺高的日最高溫、日平均相對濕度與混合比，並計算該地區的濕球溫度。我們同時使用多重變數線性回歸技術從多重訊號中分離出單一強迫項，得出冷卻效應與濕化效應各自對濕球溫度的影響。 灌溉比例在美國中部、歐洲、南亞與華北地區在過去百年有顯著擴大，模型分析結果發現隨著灌溉範圍擴張，所有地區的最高乾球溫度皆下降。在分析影響濕球溫度的因子後，發現混合比對於濕球溫度較為敏感。本研究總結兩種情形，如果背景相對濕度較低時，則灌溉的濕化效果較高，可能會主導濕球溫度上升的過程。另外，如果背景相對濕度接近飽和，因為蒸發機制不顯著，導致灌溉冷卻與濕化效應無顯著發生，進而對濕球溫度無顯著影響。由於暖化下的氣候平均態改變，會改變灌溉的效應，因此未來討論乾季濕熱之熱傷害，需同時考量到灌溉與暖化下的共同效應。

Irrigation practices can have significant biogeophysical effects on the climate. Previous studies have shown that the change in average daily maximum temperature during the hottest month of the year has warmed less in regions with irrigation expansion in the past 100 years. Furthermore, the irrigation’s moistening effect may cause higher wet-bulb temperature due to higher near-surface water vapor from excess evaporation. However, the effects that dominate the change of the wet-bulb temperature in the dry and wet seasons are not well understood. This study investigates the competing effects of cooling and moistening on the wet-bulb temperature. We use the meteorological variables of daily maximum temperature (T2m); daily mean relative humidity (RH) and daily mean surface pressure are used to calculate the specific humidity (or mixing ratio) and wet-bulb temperature from NCAR CESM coupled climate model and the offline NCAR Community Land Model. The linear regression technique isolates an individual forcing from a lumped signal and analyzes the temperature change through irrigation cooling and moistening effects. The irrigation fraction expanded in the central USA, Europe, South Asia, and North China in the past 100 years, so the maximum temperature decreased over those regions. We further differentiate the wet-bulb temperature from the dry-bulb temperature and the mixing ratio, which is very sensitive when the mixing ratio changes. The results show that when the background relative humidity is low, the mixing ratio could change a lot, which means the amount of mixing ratio change has a high probability of staying in the dominant region. The wet-bulb temperature is non-linear with the T and RH. We conclude with two scenarios. If background RH is low, the irrigation moistening effect most likely dominates. On the other hand, if the background RH is high, the evaporation is less from the lower water gradient. Therefore, there is no apparent cooling or moistening effect to alter the wet-bulb temperature. In a nutshell, irrigation can worsen comfort and increase the danger of heat stress, especially in dry conditions. This is an essential factor needed to be considered in the future.