Have China's drylands become wetting in the past 50 years?

Recently, whether drylands of Northwest China (NW) have become wetting has been attracting surging attentions. By comparing the Standard Precipitation Evapotranspiration Indices (SPEI) derived from two different potential evapotranspiration estimates, i.e., the Thornthwaite algorithm (SPEI_th) and the Penman-Monteith equation (SPEI_pm), we try to resolve the controversy. The analysis indicated that air temperature has been warming significantly at a rate of 0.4? decade(-1) in the last five decades and the more arid areas are more prone to becoming warmer. Annual precipitation of the entire study area increased insignificantly by 3.6 mm decade(-1) from 1970 to 2019 but NW presented significantly increasing trends. Further, the SPEI_th and SPEI_pm demonstrated similar wetting-drying-wetting trends (three phases) in China's drylands during 1970-2019. The common periodical signals in the middle phase were identified both by SPEI_th and SPEI_pm wavelet analysis. Analysis with different temporal intervals can lead to divergent or even opposite results. The attribution analysis revealed that precipitation is the main climatic factor driving the drought trend transition. This study hints that the wetting trend's direction and magnitude hinge on the targeted temporal periods and regions.

Automated summary

Recently, there has been a high level of attention on whether the drylands of Northwest China have become wetter. This study compares two different methods of calculating the Standard Precipitation Evapotranspiration Index (SPEI) and finds that the air temperature has been warming in the last five decades, with more arid areas warming at a faster rate. While the annual precipitation of the entire study area has shown insignificant increase, the Northwest region has experienced significant increasing trends. The study highlights the importance of considering temporal periods and regions when analyzing wetting trends.