Influences of removing linear and nonlinear trends from climatic variables on temporal variations of annual reference crop evapotranspiration in Xinjiang, China

Yi Li; Ning Yao; Henry Wai Chau

doi:10.1016/j.scitotenv.2017.02.196

Influences of removing linear and nonlinear trends from climatic variables on temporal variations of annual reference crop evapotranspiration in Xinjiang, China

Sci Total Environ. 2017 Aug 15:592:680-692. doi: 10.1016/j.scitotenv.2017.02.196. Epub 2017 Mar 22.

Authors

Yi Li¹, Ning Yao², Henry Wai Chau³

Affiliations

¹ College of Water Resources and Architecture Engineering, Northwest Agriculture and Forestry University, Yangling, Shaanxi 712100, China. Electronic address: liyikitty@126.com.
² College of Water Resources and Architecture Engineering, Northwest Agriculture and Forestry University, Yangling, Shaanxi 712100, China.
³ Department of Soil and Physical Sciences, Faculty of Agriculture and Life Science, Lincoln University, Lincoln, Canterbury 7647, New Zealand.

PMID: 28341465
DOI: 10.1016/j.scitotenv.2017.02.196

Abstract

Reference crop evapotranspiration (ET_o) is a key parameter in field irrigation scheduling, drought assessment and climate change research. ET_o uses key prescribed (or fixed or reference) land surface parameters for crops. The linear and nonlinear trends in different climatic variables (CVs) affect ET_o change. This research aims to reveal how ET_o responds after the related CVs were linearly and nonlinearly detrended over 1961-2013 in Xinjiang, China. The ET_o-related CVs included minimum (T_min), average (T_ave), and maximum air temperatures (T_max), wind speed at 2m (U₂), relative humidity (RH) and sunshine hour (n). ET_o was calculated using the Penman-Monteith equation. A total of 29 ET_o scenarios, including the original scenario, 14 scenarios in Group I (ET_o was recalculated after removing linear trends from single or more CVs) and 14 scenarios in Group II (ET_o was recalculated after removing nonlinear trends from the CVs), were generated. The influence of U₂ was stronger than influences of the other CVs on ET_o for both Groups I and II either in northern, southern or the entirety of Xinjiang. The weak influences of increased T_min, T_ave and T_max on increasing ET_o were masked by the strong effects of decreased U₂ &n and increased RH on decreasing ET_o. The effects of the trends in CVs, especially U₂, on changing ET_o were clearly shown. Without the general decreases of U₂, ET_o would have increased in the past 53years. Due to the non-monotone variations of the CVs and ET_o, the results of nonlinearly detrending CVs on changing ET_o in Group II should be more plausible than the results of linearly detrending CVs in Group I. The decreasing ET_o led to a general relief in drought, which was indicated by the recalculated aridity index. Therefore, there would be a slightly lower risk of water utilization in Xinjiang, China.

Keywords: Climatic scenario; Detrended climatic variable; Reference crop evapotranspiration; Temporal variation; Xinjiang.