Alterations in drying and wetting spells and their determinants in the Loess Plateau under the influence of climate change

【Objective】 Frequent droughts have significantly impacted agricultural productivity and ecological health of the Loess Plateau. This study investigates the characteristics and driving factors of dry-wet evolution in the region. 【Method】 The analysis was based on meteorological data, including temperature, precipitation, relative humidity, sunshine hours, and wind speed, measured from 1961 to 2020 at 71 weather stations across the Loess Plateau. The changes in wet and dry spells and their underlying determinants were analyzed using standardized precipitation evapotranspiration index (SPEI), Mann Kendall test, and run length theory. 【Result】 Temporally, temperature showed a significant increase, while wind speed, relative humidity and sunshine hours showed significant decreases from 1961 to 2020. In contrast, precipitation showed a weak decrease, while ET0 (reference crop evapotranspiration) exhibited a first increase followed by a decrease during this period. Spatially, the Northwest saw increased precipitation and decreased ET0. Conversely, the Northeast experienced declines in both precipitation and ET0. From 1999 onward, the SPEI showed an initial decrease followed by an increase. The Southeast experienced high drought frequency but with shorter durations and lower severity, extreme drought events in which were rare. In contrast, the Northwest had fewer but more severe and prolonged droughts. Rising temperatures, increasing precipitation and decreasing ET0 were the primary drivers of dry-wet alterations. ET0 had a stronger influence on dryness and wetness patterns than precipitation. The correlation between the mid-latitude westerly belt, sunspot activity, and SPEI was weak, indicating limited influence of large-scale atmospheric and solar factors. 【Conclusion】 The Loess Plateau showed a warm-drying trend from 1961 to 1999, followed by a warm-wetting trend thereafter. These findings improve our understanding of hydrological changes and help develop mitigating strategies for managing drought and water resources in the region.