干旱作为一个极端天气现象，对农业生产、水资源利用以及社会经济层面具有严重的破坏性，干旱通常分四个类型为气象干旱、农业干旱、水文干旱（地表水干旱和地下水干旱）和社会经济干旱，在一定条件下气象干旱会发展为其它类型干旱对生态环境，社会经济等造成巨大损害，因此研究不同干旱类型之间的相关性和传播时间有助于制定抗旱措施和理解水循环过程。植被在干旱的发展过程中也起到了重要作用，植被生长可能通过直接影响蒸散量和间接调节降水来反馈土壤水分，地表径流和地下水，水分资源的变化也同时会影响植被生长，研究多水分资源类型与植被双向依赖关系对于我们理解气候变化下陆地生态系统的碳-水相互作用，为制定碳封存（植被绿化）政策和水分资源管理提供理论支持。以往的干旱传播研究空间尺度较小且忽略了地下水干旱，并且研究水分资源对植被的影响往往是一种水分资源类型对植被的单向影响，为了弥补这一研究空白本文研究了全球尺度的多干旱类型传播规律和多水分资源类型和植被的双向依赖关系。

本研究使用CRU降水数据、MERR2再分析土壤水分数据、GLDAS和GRACE数据来计算SPI（标准化降水指数）、SSI（标准化土壤水分指数）、SRI（标准化径流指数）和GDI（地下水干旱指数），分别表征气象、农业、地表水和地下水干旱；采用Pearson相关系数研究了这四种类型干旱的传播时间；进一步运用格兰杰因果关系模型研究了降水、土壤水、地表水、地下水和植被的双向依赖关系。主要结论如下：

（1）地表以上干旱类型的平均传播时间分别为：气象干旱-地表水干旱（3.5个月）、气象干旱-农业干旱（5.7个月）；不同干旱类型向地下水干旱传播中，时间从短到长依次为农业干旱到地下水干旱（12.97个月）、地表水干旱到地下水干旱（13.78个月）、气象干旱到地下水干旱（14.47个月）。气候条件对不同干旱类型的传播时间有显著影响。寒冷气候中的低温导致干旱传播时间最长，而温带气候中的干燥夏季气候减少了干旱传播时间。在干旱气候中，传播关系较弱。在热带气候中，降水可能不是干旱传播的主要驱动因素。

（2）不同的土地覆盖类型在地下水干旱的传播方面表现出显著差异，森林（5.9个月）从气象干旱到农业干旱或地表水干旱的传播时间比草地（5.5个月）和农田（4.8个月）长，而三种干旱向地下水干旱的传播时间中，森林最短（12.4个月），其次为耕地（13.8个月）和草地（14.0个月）。木本植物的根系比草本植物更深，可以吸收更深的地下水。与草原和农田相比，森林具有更强的蓄水能力和较弱的地下水补给，导致森林更能抵抗农业和地表水干旱，而当气象干旱发生后森林对地下水干旱的抵抗力较弱。

（3）在全球范围内，植被与水分资源以双向依赖关系为主。植被与水分资源双向依赖，植被单向依赖水分资源，水分资源单向依赖植被的占比分别为48.25%、39.5%和12.25%。这表明有48.25%植被覆盖区域的植被与水分资源相互影响，39.5%的植被覆盖区水分资源单向影响植被，12.25%的植被单向影响水分资源。降水与植被以单向依赖关系为主（65%），土壤水与植被以双向依赖关系为主（80%），地表径流和地下水与植被的双向依赖关系占比分别为44%和40%。森林植被与水分资源的双向依赖关系通常要高于耕地和草地，这表明森林与水分资源类型的相互影响能力要强于耕地和草地，这可能是森林的蒸腾能力和截取水分能力更强大导致的。

As an extreme weather phenomenon, drought has serious destructive effects on agricultural production, water resources utilization and socio-economic. Drought is usually divided into four types: meteorological drought, agricultural drought, hydrological drought (surface water drought and groundwater drought) and socio-economic drought. Under certain conditions, meteorological drought will develop into other types of drought, which will cause great damage to ecological environment, social economy and so on. Therefore, it is helpful to study the correlation between different types of drought and the propagation time to make drought-resistance measures and understand the water cycle process. Vegetation also plays an important role in the process of drought development. Vegetation growth may directly affect evapotranspiration and indirectly regulate precipitation to feedback soil water, surface runoff and groundwater. Changes in water resources will also affect vegetation growth. The study of the bidirectional dependence between water resource types and vegetation provides theoretical support for our understanding of the carbon-water interaction in terrestrial ecosystems under climate change and for the formulation of carbon sequestration (vegetation greening) policies and water resource management. Previous studies on drought propagation have a small spatial scale and ignored groundwater drought, and the impact of water resources on vegetation is usually a unidirectional impact of water resource types on vegetation. In order to make up for this research gap, this paper studies the propagation law of multi-drought types and the bidirectional dependence between multi-water resource types and vegetation on a global scale.

This study used CRU precipitation data, MERR2 reanalysis soil moisture data, GLDAS and GRACE data to calculate SPI (Standardized Precipitation Index), SSI (Standardized Soil Moisture Index), SRI (Standardized Runoff Index),and GDI (Groundwater Drought Index), respectively, to characterize meteorological, agricultural, surface water, and groundwater drought; The Pearson correlation coefficient was used to study the propagation time of these four types of drought; Furthermore, the Granger causality model was used to study the bidirectional dependence of precipitation, soil water, surface water, groundwater, and SIF. The main conclusions are as follows:

(1) The average propagation time of drought above the surface was as follows: meteorological drought to surface water drought (3.5 months), meteorological drought to agricultural drought (5.7 months);The propagation time of different drought types to groundwater drought was from short to long: agricultural drought to groundwater drought (12.97 months), surface water drought to groundwater drought (13.78 months), and meteorological drought to groundwater drought (14.47 months). Climate conditions had a significant impact on the propagation time of different drought types. Low temperatures in cold climates resulted in the longest drought propagation time, while dry summer climates in temperate climates reduced drought propagation time. There were weaker propagation relationships in arid climates. In tropical climates, precipitation may not be the main driving factor for drought propagation.

(2) Different land cover types show significant differences in the propagation of groundwater droughts, with forests (5.9 months) having a longer propagation time from meteorological drought to agricultural drought or surface water drought than grassland (5.5 months) and cropland (4.8 months), and forests (12.4 months) having the shortest propagation time than grassland (14.0 months) and cropland (13.8 months) when meteorological drought, agricultural drought, and surface water drought is propagated to groundwater drought. Woody plants have deeper root systems than herbaceous plants and can draw up deeper groundwater. Forests have greater water storage capacity and weaker groundwater recharge than grasslands and croplands, resulting in forests being more resistant to agricultural and surface water droughts and less resistant to groundwater droughts during meteorological droughts.

(3) On a global scale, vegetation and water resources are mainly interdependent in both directions. The bidirectional dependence of vegetation and water resources accounted for 48.25%, the unidirectional impact of water resources on vegetation accounted for 39.5%, and the unidirectional impact of vegetation on water resources accounted for 12.25%. This indicates that 48.25% of vegetation interacts with water resources, 39.5% of vegetation is unidirectional influenced by water resources, 12.25% of water resources are unidirectional affected by vegetation. The relationship between precipitation and vegetation is mainly unidirectional (65%), soil water and vegetation is mainly bidirectional (80%), and the bidirectional dependence between surface runoff and vegetation is 44% and 40%, respectively. The bidirectional dependence between forest vegetation and water resources is usually higher than that between cultivated land and grassland, indicating that the mutual influence between forest and water resource types is stronger than that between cultivated land and grassland, which may be caused by the stronger transpiration and water interception capacity of forest.

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