Abstract:

Drought is one of the most prominent natural disasters in the Loess Plateau. Its spatiotemporal evolution has direct effects on the stability of regional ecosystems, the security of agricultural production, and the high-quality development in the Yellow River Basin. Based on the daily observed meteorological data of 218 stations from 1961 to 2020, the reference crop evapotranspiration (ET₀) was firstly calculated with the Penman-Monteith formula. Then, the daily water balance sequence was calculated based on daily precipitation and ET₀. Lastly, this study calculated the 30-day sliding cumulative daily standardized precipitation evaporation index (SPEI-30). Based on SPEI-30, this study investigated the spatiotemporal evolution of droughts’ lasting days, droughts’ frequency, and the serious levels of droughts in the Loess Plateau in the last 60 years. This study also adopted random forest regression models to quantify the relative contributions of meteorological factors to the evolution of drought. The results show that the daily SPEI-30 can effectively identify the occurrence and lasting days of drought, catch droughts’ fluctuation at the 30-day scale, and reflect the dynamic evolution process of drought in the Loess Plateau. There were significant interdecadal fluctuations and seasonal differences on droughts’ lasting days and its frequency. In specific, droughts’ lasting days were relative more from 1961 to 1980 and from 2001 to 2020, but less from 1981 to 2000. The lasting days and frequency of drought in spring and winter showed a downward trend but an upward trend in summer and autumn. The effects of meteorological factors on the variation of the duration of drought were different among seasons. Precipitation was the dominated factor for spring and autumn droughts (contributions reaching 43% and 42%, respectively). Summer and winter droughts were dominated by ET₀ (contributions of 36% and 30%, respectively). In summary, the spatiotemporal evolution of drought showed significant seasonal differences in the Loess Plateau, and meteorological factors were the main factors affecting the duration of regional drought.

Key words: Loess Plateau, daily SPEI-30, seasonal variation, driving factors

摘要：

干旱是黄土高原最主要的自然灾害之一，其时空演变对区域生态系统稳定、农业生产安全及黄河流域高质量发展具有关键影响。基于黄土高原218个气象站点1961—2020年逐日观测数据，利用Penman-Monteith公式计算参考作物蒸散发（Reference Crop Evapotranspiration，ET₀），结合降水量构建水分盈亏序列，计算30 d滑动累积的逐日标准化降水蒸散指数（SPEI-30），分析黄土高原近60 a干旱持续日数、频次及不同等级干旱的时空分布与变化趋势，引入随机森林回归模型量化各气象要素对干旱演变的相对贡献。结果表明：逐日SPEI-30能有效识别干旱发生和持续状态，适用于表征30 d尺度的短期干旱演变特征，可较好地反映黄土高原干旱的动态变化过程。黄土高原干旱持续日数及发生频次存在显著年代际波动与季节差异：1961—1980年与2001—2020年干旱持续日数整体偏长；1981—2000年干旱持续日数相对偏短；春、冬季干旱持续日数与频次呈下降趋势，夏、秋季则均呈上升趋势。影响干旱持续日数变化的气象要素存在显著季节分异特征：春、秋季干旱以降水量为主导驱动因素，贡献率分别为43%、42%；夏、冬季干旱则由ET₀主导，贡献率分别为36%、30%。

关键词: 黄土高原, 逐日SPEI-30, 季节变化, 驱动因素