| The Loess Plateau(LP)is drought prone area whose ecological environment is very fragile in China.Under the influence of climate change and human activities,the spatio-temporal characteristics of drought in the past are in dynamic change,and is likely to face more severe drought in the future.Therefore,it’s of great significance to systematically analyze the past drought characteristics,predict future drought conditions,and study the vegetation growth characteristics for drought warning,agricultural production,ecological environment protection and high-quality development of economy,society and people’s livelihood on the LP.However,there is still insufficient research on which drought index is more suitable for drought monitoring and the potential drought risk of vegetation on the LP.Different drought indices have different suitability in different regions.At present,it is not clear which drought index is more suitable for drought monitoring and drought potential risk assessment of vegetation on the LP.At the same time,there is a lack of in-depth research on the response relationship between vegetation and drought and the potential risks of vegetation facing drought in the future climate on the LP.In this study,the applicability of six drought indexes on the LP was quantitatively evaluated.Based on the optimal drought index,SPEI,the characteristics of spatial and temporal drought variations of the LP from 1980 to 2022 were analyzed by using spatial and temporal continuous three-dimensional drought identification and measurement method.At the same time,the spatial and temporal changes of net primary productivity and their correlation with drought were studied.In addition,the vulnerability assessment model and future climate models data were used to predict the future drought characteristics and risk under multiple climate change scenarios.The main conclusions are as follows:(1)The spatio-temporal variation differences of the six drought indexes were compared,and the applicability of the six drought indexes was evaluated.Through spatial and temporal analysis of the six drought indexes,it is found that there are significant differences in the interannual and decadal variation,dry/wet alternation and spatial variation trend of each drought index.Through correlation analysis with soil relative moisture,river runoff and the terrestrial water storage changes,it is concluded that Standardized Precipitation Evapotranspiration Index(SPEI)is more applicable to monitor drought on the LP.(2)SPEI was applied on three-dimensional recognition and spatio-temporal variation analysis of droughts in the study area.The results indicated that 79 droughts occurred on the LP from 1980 to 2022,among which the drought during Jane.1995-Mar.1996 was the most severe.The drought frequency decreased firstly and increased afterwards,meanwhile,the duration,severity and affected area of drought show a decreasing trend since 1980.Droughts with high severity,long duration and large area mainly located in the confluence region of the central semi-arid areas of the LP.Furthermore,the worst drought incidences occurred in 1980 s.After 2010,droughts with high severity,long duration and large area were rare.(3)Based on multi-source remote sensing data,climate regions and vegetation types are divided,and the variation characteristics of NPP and drought loss are quantitatively analyzed to evaluate the response relationship between drought and vegetation NPP on the LP.The annual average NPP of the semi-humid area was highest and the lowest in arid area.From 2001 to 2020,NPP in most regions showed an increasing trend.NPP in arid and semi-arid areas showed a significant increasing trend,while NPP in semi-humid areas showed a downward trend.Through the evaluation of drought loss of NPP,it was found that the greater loss of NPP in the wetter region caused by drought.Spatially,the drought loss of NPP in the south and east of the study area was greater.At the same time,there were significant differences in NPP loss among different types of vegetation under the same grade of drought.Under moderate drought,NPP loss of grassland was the largest.Under severe drought and extreme drought,NPP loss of forest was the largest.In the past 20 years,arid and semi-arid areas showed a wetting trend,and drought loss of NPP decreased,while semi-humid areas showed a drying trend,and drought loss of NPP increased.(4)Autoregressive model and residual analysis were used to explore the drought resistance of vegetation and the characteristics of changes in vegetation drought resistance before and after the implementation of the Grain for Green Program.The results show that the vegetation in the semi-humid,arid regions showed the strongest and weakest drought resistance,and the grassland in the arid area is the weakest vegetation type.The positive contribution of climate change and human activities leads to the enhancement of drought resistance of vegetation in most areas of the LP,and the weakening of drought resistance of vegetation in semi-humid regions is dominated by the negative contribution of climate change.The enhancement of drought resistance of crops in each climate region was the main change.The drought resistance of vegetation was increasing in areas where the land cover was converted from crops to forests,and the drought resistance of vegetation was weakening where the land cover was converted from crops to grasslands.(5)In scenarios of SSP126,SSP245 and SSP585,the exposure of each grade drought in the Loess Plateau will increase from 2023 to 2050.Spatially,it is expected that the drought exposure in the east and south of the LP will increase,while that in the west and central of the LP will decrease.However,there are different characteristics in different climatic zone,in which the drought exposure in semi-humid and semi-arid areas will increase,but that in arid areas will decrease.(6)In scenarios of SSP126,SSP245 and SSP585,it is expected that the drought risk of vegetation growth on the LP will increase from 2023 to 2050.The high drought risk area is mainly located in the eastern part of the semi-arid area and a few areas in the semi-humid area.The drought risk of vegetation in the arid area is relatively low.The middle and high drought risk area of the vegetation on the LP is relatively the smallest in the SSP126 scenario,and the highest in the SSP585 scenario.This study optimized the applicable drought index for the loess plateau,revealed the temporal and spatial evolution characteristics of drought in the Loess Plateau in the past 43 years,quantified the response relationship between NPP and drought of each climate area and vegetation type,and the future drought risk of the study area was evaluated in the context of a variety of shared socio-economic paths.The research results can provide scientific basis for drought early warning,eco-environmental protection and high-quality development in the Loess Plateau. |
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