Evolutionary characteristics and spatiotemporal trend of meteorological drought in the Huang-Huai-hai Plain

Droughts have posed large threats to crop growth and economic development in the crucial grain-producing region, such as the North China Plain (Huang-Huai-Hai Plain). It is crucial to fully understand the drought characteristics for national food security. Previous studies have relied primarily on the single drought index rather than the multiple drought indices. The single drought index makes it relatively difficult to provide comprehensive and consistent information, compared with the multiple drought indices. However, drought monitoring often varies significantly, because the multiple factors are considered by different drought indices. In this study, four drought indices were utilized to evaluate the spatiotemporal variation of drought in the Huang-Huai-Hai Plain from 1970 to 2020. These indices included the standardized precipitation index (SPI), standardized precipitation evapotranspiration index (SPEI), self-calibrating Palmer drought severity index (scPDSI), and aridity index (AI). Drought events were fully examined from four aspects: intensity, area ratio, frequency, and duration of drought. Data resources were obtained from 564 meteorological stations in the National Meteorological Information Center, including the average, minimum, and maximum temperature, precipitation, wind speed, relative humidity, and sunshine hours. The effective soil water content was collected from the Chinese soil dataset using the World Soil Database provided by the National Tibetan Plateau Research Center. The research results indicate that the better consistency of four drought indices was achieved to characterize the drought intensity and area. The overall drought intensity shared a decreasing trend from 1970 to 2020. The downward trend was ranked in the order of the SPI<SPEI<AI<scPDSI; with a notable decline since the 1980s. In spatial distribution, the four drought indexes showed that the drought intensity increased the most in the eastern and western regions, whereas, the central region showed a gradually moistening trend. The drought area ratio exhibited an increasing trend from 1970 to 1980, followed by a decreasing trend from 1980 to 1990, an increasing trend from 1990 to 2000, a decreasing trend from 2000 to 2010, and an increasing trend from 2010 to 2020. Most drought areas accounted for 10% to 60% of the total area. Among them, scPDSI and AI detected the larger drought areas with significant fluctuations. SPEI was close to the average of monitored drought areas, with the proportions ranging from 20% to 60%. In contrast, the SPI recorded the smallest proportion of drought areas with the less variation, approximately ranging from 20% to 40%. Spatial heterogeneity was found in the different types of drought indices, according to the drought frequency and duration. The SPI and AI detected the drought more frequently in the northern regions than in the southern. Specifically, the highest drought frequency was located in Hebei Province (more than 60%), whereas the drought frequency was lower (less than 20%) in the southern Jiangsu and Anhui Province. However, there was little difference between SPEI and scPDSI monitoring, where the drought frequency was concentrated at approximately 25%-40% and 30%-50%. There was no significant spatial distribution in the SPEI and scPDSI on drought duration. The longer drought durations of the SPI were found in the northern region, compared with the southern. In addition, there was a relatively consistent duration of drought that was monitored by SPI and SPEI, usually between 2 and 7 months, while the duration of drought indicated by scPDSI was between 10 and 20 months. Overall, the droughts were more severe, frequent, and longer-lasting in the northern regions, whereas, the opposite was observed in the southern. This finding can also provide an objective evaluation and valuable insight into drought characteristics in the North China Plain.