Study On The Characteristics,driving Conditions And Loss Models Of Seasonal Drought And Flash Drought In Grassland Ecosystem

Under the influence of climate change,droughts are occurring more and more frequently.Grassland ecosystem is extremely sensitive to drought,among which flash drought is a special phenomenon with rapid outbreak,short duration and high influence.Flash drought often occurs together with seasonal drought,and its loss is huge and difficult to evaluate.At present,the understanding and prevention ability of seasonal drought,especially flash drought,are not mature.The purpose of this study was to analyze the evolution law of seasonal drought and flash drought in grassland ecosystem,to deeply understand the ecological status of Xilinguole grassland,to determine the impact of drought on grassland ecosystem,and to focus on the type,intensity and stage of flash drought and the drought tolerance of various grasslands.From the perspective of meteorological driving,the response of drought to meteorological factors was deeply understood,and a quantitative prediction method was established for the induction,relief and mutual evolution of flash drought and seasonal drought with different intensities in different grassland under the influence of key meteorological factors.From the perspective of grassland productivity,the characteristics and relationships of sensitivity,anomaly degree,duration,recovery ability and drought tolerance of net primary productivity（NPP）and rainfall use efficiency（RUE）of different grassland types to flash drought response were revealed.A multi-factor quantitative evaluation model was established to quantitatively evaluate the productivity loss and economic loss of different types of grassland caused by seasonal drought and flash drought.The drought prediction model for grassland was discussed and the influence trend of seasonal drought and flash drought on ecosystem was revealed.This study can provide a scientific basis for understanding the driving effects of climatic conditions on multi-scale drought and improving ecosystem resilience and drought prevention and control.The main research results are as follows:1.A new definition,strong evapotranspiration flash drought（SEFD）,was proposed for identification of flash drought in middle temperate grassland.The frequency of SEFD is lower than that of heat wave flash drought（HWFD）,but its intensity and influence are larger than HWFD,and it is not easy to recover.The period of flash drought and high outbreak of grassland ecosystem was from May to August,and the intensity was above moderate drought.HWFD occurs most frequently in June,while SEFD occurs most frequently in May.There was a weak correlation between the frequency and intensity of HWFD and SEFD,and it was easy to erupt in meadow grassland.HWFD was more likely to occur in typical grassland and SEFD was more likely to occur in desert grassland except meadow grassland.2.The ecological condition of Xilinguole grassland still needs to be further improved,and the policy of grass-stock balance implemented by the government can obviously promote grassland restoration.The center of gravity of NPP shows an expanding trend from meadow grassland to the south of typical grassland year by year,and NPP will maintain an increasing trend in the future.The mean value of NPP and RUE in Xilinguole grassland is low and the recovery speed is slow.The ecological stability and environmental factors are not optimistic.The ecological status of desert grassland and typical grassland in the north and southwest is poor.In Xilinguole grassland,the fluctuation of drought and humidity was obvious,and the trend of drought was obvious.The probability of medium drought>light drought>severe drought>extreme drought.It is feasible to predict drought in Xilinguole grassland by Markov chain model.In the future,the seasonal drought and flash drought will continue to increase.Under the influence of climate warming,the drought situation and the loss to the ecosystem in the study area will be more serious,and the drought events will change greatly at the turn of the decade.3.A multi-factor quantitative evaluation method of seasonal drought and flash drought induced by key factors in different grassland types at different stages was established,and a quantitative prediction method of flash drought and seasonal drought relief and their mutual evolution was developed.Temperature,precipitation,water vapor pressure and sunshine are the key factors affecting seasonal drought.The short-term and long-term changes of precipitation and temperature have strong influence on seasonal drought,while the short-term changes of water vapor pressure have significant influence on seasonal drought.Drought is sensitive to climate extremes.The key driving factors（temperature,precipitation,water vapor pressure and wind speed）and driving characteristics of flash drought are basically consistent with seasonal drought.The most important influencing period of flash drought is 1 penad（5 days）before the outbreak of flash drought.The burst speed and threshold of the key factors are the driving conditions for inducing seasonal drought and flash drought,and the recovery speed can be used as the basis for predicting whether flash drought evolves into seasonal drought.Seasonal drought from May to September is sensitive to meteorological factors,flash drought from April to September is sensitive to meteorological factors,and the performance of meteorological factors in July is key.The key factors are more likely to induce seasonal drought in desert grassland and flash drought in meadow grassland.4.The grassland is sensitive to the occurrence and development of drought,and its production capacity decreases obviously.The impact varies with drought grade,grassland type,time stage and drought type.The loss of grassland productivity caused by flash drought was higher than that caused by seasonal drought,and the impact degree increased year by year.It was difficult for grassland to recover to the pre-drought state immediately after the drought was relieved.More than 90%of the NPP and RUE anomaly index showed moderate negative anomaly under flash drought,and the NPP loss rate(NPP_LR)could reach more than 50%in the early stage of light drought,and the average NPP_LRcould reach more than 60%.NPP_LRincreased significantly with the increase of drought grade.The increase of NPP_LRcaused by flash drought was lower than that caused by seasonal drought,and NPP_LRincreased by more than 20%at all levels of seasonal drought.The maximum lag time of NPP response to flash drought was 2 months.More than 40%of NPP anomalies occurred simultaneously with flash drought,and NPP anomalies lasted for nearly 2 months.As the main factor of abrupt drought affecting ecosystem,RUE is more sensitive to abrupt drought.RUE responds to flash drought within 10 days with a reduction of more than 80%,30%higher than NPP,and the duration of the anomaly is half that of NPP.NPP and RUE were most sensitive to flash drought in spring.NPP is affected by flash drought with the highest degree and longest duration in summer,while RUE is affected in spring.The dry area of grassland is more sensitive to flash drought than the wet area.Grasslands with high productivity sensitivity to flash drought had shorter response duration,lower degree of anomaly,better resilience and better drought tolerance.The drought tolerance of grassland has little relationship with the size of NPP and RUE.The higher the frequency and intensity of flash drought,the greater the effect on vegetation.Meadow grassland is the most risky grassland type with the worst drought tolerance.Once the flash drought breaks out,it is more difficult to recover and easy to change into seasonal or even multi-season drought.Desert grassland has the best drought tolerance.5.The productivity of meadow grassland suffered serious loss in almost all stages of flash drought,the fastest growth stage of flash drought damage in typical grassland was light drought,while desert grassland was severe drought and extra drought.Under seasonal drought,the typical grassland suffered serious losses in all drought stages.Meadow grassland was affected by light drought,and NPP_LRchanged slowly with drought intensification.Desert grassland was less affected by mild to moderate drought than by severee to extra drought.6.Economic loss caused by flash drought in Xilinguole grassland1232.67×10⁶-1712.05×10⁶yuan d^-1（HWFD）and 1550.56×10⁶-2058.50×10⁶yuan d^-1（SEFD）,the economic loss caused by seasonal drought was 7327.00×10⁶-61713.25×10⁶yuan a^-1.The degree of productivity and economic loss caused by drought was in the order of meadow grassland>typical grassland>desert grassland from high to low,with direct economic loss>indirect economic loss>investment premium loss.In addition to farming and animal husbandry,chemical product industries are also affected by the drought.The regional disaster loss is closely related to drought tolerance.West Uzhumuqin and Erenhot are the heaviest and least affected areas of Xilinguole.The protection and management of the ecosystem not only contributes to the restoration of the ecological environment,but also has a positive impact on the sustainable development of the economy and society.