Spatiotemporal Dynamics of Meteorological and Agricultural Drought on the Northern Slope of the Tianshan Mountains: Trends, Propagation Processes, and Triggering Thresholds

The propagation of meteorological drought into agricultural drought is a critical stage in drought hazard development and risk accumulation. Characterizing its spatiotemporal evolution, propagation processes, and triggering conditions is therefore essential for agricultural drought mitigation and water-resource regulation in arid regions. Taking the northern slope of the Tianshan Mountains in Xinjiang, China, as the study area, this study used monthly ERA5-Land reanalysis data from the European Centre for Medium-Range Weather Forecasts for 1980–2022 to derive the Standardized Precipitation Evapotranspiration Index and Standardized Soil Moisture Index. Three-dimensional drought-event identification, the Mann–Kendall trend test, cross-wavelet transform, wavelet coherence analysis, and Copula analysis were integrated to investigate drought dynamics, multiscale propagation, and triggering thresholds. This event–process–threshold framework links individual drought-event evolution with lagged propagation characteristics and probabilistic triggering conditions, extending previous studies that focused primarily on correlation or propagation time. The results showed that a total of 92 meteorological drought events and 65 agricultural drought events were identified, with agricultural droughts generally exhibiting greater persistence and cumulative severity. Meteorological droughts were concentrated mainly in the central-eastern region, whereas agricultural droughts occurred predominantly across the western-to-central agro-pastoral ecotone. The two drought types exhibited predominantly in-phase coupling, with dominant propagation periods of 2–8 months, particularly 3–6 months, and agricultural drought generally lagging meteorological drought by 1–3 months. Propagation was fastest in summer, most persistent in autumn, and comparatively weak in winter. Propagation probability increased markedly with meteorological-drought severity, while the triggering threshold ranged from −0.81 to −0.71 and exhibited pronounced seasonal and spatial heterogeneity. These findings provide quantitative criteria for tiered drought early warning. The identified propagation periods and thresholds can support timely soil-moisture monitoring, irrigation preparation, and seasonal water allocation before meteorological drought develops into severe agricultural drought, particularly during sensitive crop-growth stages and in vulnerable oasis-agricultural areas.