Identify the relationship of meteorological drought and ecohydrological drought in Xilin Gol Grassland, China

摘要

Abstract Understanding how drought affects ecosystems requires us to identify how drought affects ecohydrological processes, and thus take more effective measures to reduce ecosystem vulnerability and risk in the context of climate change and intensive human activities. This case study detected the relationship between meteorological drought (AI = P/PET, precipitation/potential evapotranspiration) and ecohydrological drought (ESI = ET/PET) during 2005–2017 in the Xilin Gol grassland in northwestern China. The relationships between the two drought indexes (AI or ESI) and four biophysical drivers, i.e., PET, P, ET, and leaf area index (LAI), were also examined using Causality Test Model, correlation analysis and MK trend test to explain their causal relationships. Results showed that during 2005–2017, although AI had no significant trends in the study region, ESI had significant increasing trends (i.e., getting wetter) in both relatively humid eastern area and arid western area. The reasons for the significant increase in ESI in eastern and western areas are different. In the eastern area, AI and ESI have insignificant two-way causal relationship, which indicated that AI was not the only factor for the increase in ESI. The improvement of LAI due to vegetation restoration policies amplified the influence of AI on the increase in ESI, that means ET affect ESI through LAI in the relatively humid southeast region with more vegetation. In the northwest, although LAI decreased due to urbanization, mining, and other factors, ET increased due to more precipitation and rising PET, and thus increased ESI. We conclude that although local climate condition did not getting wetter significantly, the improvement of vegetation has significantly alleviated local ecohydrological drought, especially in the southeast where the climate is relatively humid and the vegetation has great improvement owing to local ‘Returning Grazing Lands to Grasslands’ project. The differences in AI and ESI distribution pattern and dynamics and their correlations with biophysical factors found in this study indicated that the ecohydrological drought can be either mitigated or exacerbated by vegetation depending upon how grassland dynamics respond to meteorological drought or human activities.