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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 16 Jan 2018

Research article | 16 Jan 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Natural Hazards and Earth System Sciences (NHESS).

Identifying a Transition Climate Zone in an Arid River Basin using a Hydrological Drought Index

Libo Zhang1, Yongqiang Liu2, Lu Hao1, Decheng Zhou1, Cen Pan1, Peilong Liu1, Zhe Xiong3, and Ge Sun4 Libo Zhang et al.
  • 1Jiangsu Key Laboratory of Agricultural Meteorology, International Center for Meteorology, Ecology, and Environment, College of Applied Meteorology, Nanjing University of Information Science and Technology, Nanjing, China
  • 2Center for Forest Disturbance Science, USDA Forest Service, Athens, Georgia, USA
  • 3Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
  • 4Eastern Forest Environmental Threat Assessment Center, USDA Forest Service, Raleigh, North Carolina, USA

Abstract. Drought indices have been widely used in climate classification. However, there is not enough evidence for their ability in identifying the multiple climate types in areas with complex topography and landscape, especially in those areas with a transition climate. This study compares a meteorological drought index, the aridity index (AI) defined as the ratio of precipitation (P) to potential evapotranspiration (PET), with a hydrological drought index, the evaporative stress index (ESI) defined as the ratio of actual evapotranspiration (AET) to PET. We conducted this study using modeled high resolution climate data for period of 1980–2010 in the Heihe River Basin (HRB) in the arid northwestern China. PET was estimated using the Penman–Monteith and Hamon methods. The climate classified by AI shows two distinct climate types for the upper and the middle and lower basin reaches, while three types were found if ESI was used. This difference indicates that only ESI is able to identify a transition climate zone in the middle basin. This contrast between the two indices is also seen in the inter-annual variability and extreme dry/wet events. The magnitude of variability in the middle basin is close to that in the lower basin for AI, but different for ESI. AI has larger magnitude of the relative inter-annual variability and greater decreasing rate from 1980–2010 than ESI, suggesting the role of local hydrological processes in moderating extreme climate events. Thus, the hydrological drought index is better than the meteorological drought index for climate classification in the arid Heihe River Basin where local climate is largely determined by topography and landscape. We conclude that the land–surface processes and human disturbances play an important role in altering hydrological drought conditions and their spatial and temporal variability.

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Libo Zhang et al.
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Publications Copernicus
Short summary
A transition zone often exists between a moist upper river reach and an arid lower reach in a watershed with complex topography. This zone is more suitable for human activities but difficult to be identified in climate classification. We found that a hydrological index overpowers a meteorological index in identifying a transition zone of a watershed in northwestern China, implicating the important role of the land-surface processes and human disturbances in formulating the transition zone.
A transition zone often exists between a moist upper river reach and an arid lower reach in a...