Research On The Spatio-temporal Changes And Driving Forces Of Agricultural Drought In Yunnan Province

Under the background of global warming,extreme weather events are more frequent.Drought usually develops slowly and affects a large range,so it has become one of the natural disasters with the most serious impacts on social economy and the environment.Yunnan Province is located in the humid region of Southwest China;however,it has suffered droughts frequently in recent years.Especially from the fall of 2009 to the spring of 2010,it suffered a once-in-a-hundred-year drought,which caused a shortage of drinking water,huge economic losses,and threats to food security.The current research on local drought is mainly based on meteorological factors measured on the ground such as precipitation and evaporation to evaluate and analyze the causes of meteorological drought.There is still a lack of comprehensive research on the spatio-temporal changes and forcing mechanism of agricultural drought in the past few decades.Soil moisture based on multi-source remote sensing and total water storage retrieved by gravity satellites provide help to solve these problems.This study takes the humid Yunnan Province as the research area.Firstly,a multi-source remote sensing soil moisture product,CCI SM(Climate Change Initiative Soil Moisture),was verified and revised.Secondly,based on the revised CCI SM,the characteristics of spatio-temporal changes of surface soil moisture in Yunnan Province from 1978 to 2016 were analyzed,meanwhile its ability to detect agricultural drought events was checked.Then,the evolution of agricultural drought in Yunnan Province over the past four decades on multi-temporal and spatial scales was comprehensively analyzed.In addition,based on GRACE TWS(Gravity Recovery and Climate Experiment Total Water Storage)data,this study investigated the response of TWS to the past drought and wet climate cycles,as well as the indication of future drought-prone areas.Finally,the influence of natural factors and human factors on the temporal changes and spatial distribution of agricultural drought was investigated.The main conclusions are as follows:(1)The data quality of CCI SM products differs in time and versions,which prompts users to check before use.The study finally selected three periods with better data quality of CCI SM as the research period for analyzing the long-term changes of soil moisture and agricultural drought,which included from November 1978 to July 1987(termed as P1),from August 1991 to August 1991.February 2003(P2)and from January 2006 to December 2016(P3).CCI SM has a systematic overestimation of in situ soil moisture of about 0.12 m~3/m~3.The methods based on the unified revision of the whole province and the district revision based on 18 soil moisture stations can effectively eliminate this systematic deviation.In contrast,since the latter method fully utilizes the information of the 18 stations,the result is relatively better.(2)From 1978 to 2009,the monthly surface soil moisture in Yunnan Province showed no significant change trend.It decreased significantly during the severe drought disaster in 2009,and then gradually recovered in the following years.The soil moisture in most parts of the province showed a decreasing trend in winter and an increasing trend in summer,and the changes in Qujing District were more dramatic than in other regions.The soil moisture kept the spatial pattern of low in the northwest and high in the south.The monthly soil moisture could effectively detect agricultural drought events which was more likely to occur in spring than meteorological drought,while the seasonal soil moisture had limited ability to detect agricultural drought events.(3)From 1978 to 2016,on a time scale of about ten years,the intensity of agricultural drought in Yunnan Province decreased,but the drought frequency increased.Especially,the drought duration was significantly prolonged by 20 days after entering the 21st century,thus the drought severity has also risen significantly.In the 16 regions of Yunnan Province,the drought duration and severity had no significant trends in the time series of drought events.The drought intensity showed a downward trend in 13 regions,and the drought area showed an upward trend in 6 regions in the west and southwest.After comprehensive assessment,the agricultural drought in the Qujing District in the east part was the most serious,with all drought characteristics much higher than other regions;followed by Kunming,Chuxiong and Yuxi in the central part.It is worth noting that Pu'er and Xishuangbanna in the southwest were prone to long-term and large-scale agricultural droughts.(4)From 2002 to 2014,the TWS of Yunnan Province experienced a change process of first decreasing,then increasing and then decreasing.Based on these trends,the study period was divided into the first dry period,the wet period,the second dry period,and the fluctuating period.TWS had the largest values in autumn,followed by summer and winter,and the smallest values in spring.TWS increased spatially from northwest to southeast.In addition,there were significant spatial differences in the loss and recovery capacity of TWS in the climate dry-wet cycle.TWS in the eastern and northern regions had gradually recovered after the severe drought disaster.TWS in the western and southern regions eventually had a net loss,indicating that these regions had poor recovery capacity of TWS,and are more prone to drought in the future.(5)The temporal changes in the severity of agricultural drought in Yunnan Province from2006 to 2016 was affected by the combined effects of natural and human factors.The relative importance of each factor could be ranked as climatic factor(NAO index,temperature,precipitation),human factor(agricultural output,GDP and reservoir capacity)and vegetation factor(NDVI).In terms of the spatial distribution of agricultural drought,climatic factors(temperature and precipitation)played a leading role,followed by human factors(population and GDP),elevation and soil type,then land use,and finally NDVI,soil texture and slope.There were differences in the influence of temperature and precipitation under different climatic conditions.After interacting with climatic factors,the influence of human factors increased significantly;thus,the impact of human activities on agricultural drought events is worthy of attention.