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# Sensitivity of Potential Evapotranspiration Estimation to the Thornthwaite and Penman-Monteith Methods in the Study of Global Drylands

• Drylands are among those regions most sensitive to climate and environmental changes and human-induced perturbations. The most widely accepted definition of the term dryland is a ratio, called the Surface Wetness Index (SWI), of annual precipitation to potential evapotranspiration (PET) being below 0.65. PET is commonly estimated using the Thornthwaite (PET_Th) and Penman-Monteith equations (PET_PM). The present study compared spatiotemporal characteristics of global drylands based on the SWI with PET_Th and PET_PM. Results showed vast differences between PET_Th and PET_PM; however, the SWI derived from the two kinds of PET showed broadly similar characteristics in the interdecadal variability of global and continental drylands, except in North America, with high correlation coefficients ranging from 0.58 to 0.89. It was found that, during 1901-2014, global hyper-arid and semi-arid regions expanded, arid and dry sub-humid regions contracted, and drylands underwent interdecadal fluctuation. This was because precipitation variations made major contributions, whereas PET changes contributed to a much lesser degree. However, distinct differences in the interdecadal variability of semi-arid and dry sub-humid regions were found. This indicated that the influence of PET changes was comparable to that of precipitation variations in the global dry-wet transition zone. Additionally, the contribution of PET changes to the variations in global and continental drylands gradually enhanced with global warming, and the Thornthwaite method was found to be increasingly less applicable under climate change.
摘要: 干旱区是对气候变化和人类活动响应最为敏感的地区之一. 通常将地表湿润指数(SWI, 年降水量与潜在蒸散发PET的比值)小于0.65的区域定义为干旱区. Thornthwaite方法和Penman–Monteith方法是当前估算PET的两种常用算法. 本文在年代际尺度上比较分析了基于这两种PET算法时全球干旱区面积的时空变化特征. 结果发现虽然两种方法估算的PET在时空特征上存在显著的差异, 但基于这两种PET得到的全球和各大陆(北美洲除外)的干旱区面积呈现出相似的年代际变化, 相关系数为0.58~0.89. 二者均显示出1901-2014年全球总干旱区面积呈现出明显的年代际振荡, 其中极端干旱区和半干旱区显著扩张, 干旱区和干湿过渡带显著缩小. 这是因为全球降水的年代际变化主导了全球干旱区面积的年代际变化, 而PET变化的贡献次之. 同时也发现, 在干湿过渡带上, PET与降水变化的贡献相当, 这使得两种算法得到的全球半干旱区和干湿过渡带面积的年代际变化存在明显的差异, 且这种现象在北美最为明显. 此外, 上世纪80年代以后, 两种算法均显示PET的年代际变化对全球干旱区面积年代际变化的贡献逐渐加大. 因此在当前和未来情景下, 在全球干旱区面积变化的研究中, 采用Penman–Monteith方法估算PET更为合理.

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## Manuscript History

Manuscript revised: 28 April 2017
Manuscript accepted: 16 June 2017
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## Sensitivity of Potential Evapotranspiration Estimation to the Thornthwaite and Penman-Monteith Methods in the Study of Global Drylands

• 1. Key Laboratory of Regional Climate-Environment for Temperate East Asia, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
• 2. University of Chinese Academy of Sciences, Beijing 100049, China

Abstract: Drylands are among those regions most sensitive to climate and environmental changes and human-induced perturbations. The most widely accepted definition of the term dryland is a ratio, called the Surface Wetness Index (SWI), of annual precipitation to potential evapotranspiration (PET) being below 0.65. PET is commonly estimated using the Thornthwaite (PET_Th) and Penman-Monteith equations (PET_PM). The present study compared spatiotemporal characteristics of global drylands based on the SWI with PET_Th and PET_PM. Results showed vast differences between PET_Th and PET_PM; however, the SWI derived from the two kinds of PET showed broadly similar characteristics in the interdecadal variability of global and continental drylands, except in North America, with high correlation coefficients ranging from 0.58 to 0.89. It was found that, during 1901-2014, global hyper-arid and semi-arid regions expanded, arid and dry sub-humid regions contracted, and drylands underwent interdecadal fluctuation. This was because precipitation variations made major contributions, whereas PET changes contributed to a much lesser degree. However, distinct differences in the interdecadal variability of semi-arid and dry sub-humid regions were found. This indicated that the influence of PET changes was comparable to that of precipitation variations in the global dry-wet transition zone. Additionally, the contribution of PET changes to the variations in global and continental drylands gradually enhanced with global warming, and the Thornthwaite method was found to be increasingly less applicable under climate change.

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