Response Of Rainfed Rice Evapotranspiration To Extreme Drought In Southwest China

Water resources are vital for all human survival,whereas the global climate change and intensified human activities are increasing global water scarcity.Rice is a staple food that feeds more than half of the world’s population.It is also a major consumer of freshwater resources,requiring 2-3 times as much water for production as any other crops.Southwest China covers four provinces and one municipality,including Yunnan,Guangxi,Guizhou,and Sichuan provinces,and Chongqing municipality.It is one of the China’s six planting regions,accounting for 16% of the country’s grain production.The frequency and intensity of extreme droughts are expected to increase due to ongoing climate change.Southwest China,in particular,suffered a “Once in a century” drought event from the winter of 2009 to the spring of2010 and the summer of 2011,with a severe impacting on the ecosystems,societies,and climate systems.As evapotranspiration(ET)is a critical process in global water cycle,even slight changes in rice ET due to the extreme droughts will have a significant effect on regional water use,farm water management,and national food security.However,the response of rice ET to extreme drought and the underlying mechanism of rice ET change during droughts are still unclear.Therefore,the Southwest China was selected as the study area,and the 1km surface temperature product was downscaled to 250 m using a random forest model.We estimated the daily ET with the modified Surface Energy Balance Algorithm for Land(SEBAL)model from2001-2011 and evaluated the performance using two eddy covariance flux sites.we used a random forest classifier on the Google Earth Engine(GEE)platform to obtain the rice distribution in Southwest China from the Sentinel-2 data.In addition,we used three independent drought indices to characterize the onset,end,spatial distribution,and duration of the drought events in 2009-2010 and 2011.Furthermore,we detected the impacted of two extreme droughts on rainfed rice ET using daily ET data and the spatial distribution of rainfed rice in Southwest China.Finally,we examined the sensitivity of rainfed rice ET to the meteorological factors using multiple linear regression in Southwest China,and investigate the dominant factors in the rainfed rice ET changes during 2011 drought period.The main conclusions of this study are as follows:(1)The annual actual evapotranspiration from 2001-2011 in Southwest China ranged from 47.06 mm to 1638.53 mm,with the high value areas mainly distributed in the regions of Southwest Yunnan and Guangxi Province,and the low value areas mainly distributed in the regions of Northwest Sichuan Province and the YunnanGuizhou Plateau.The annual rainfed rice ET from 2001-2011 in Southwest China ranged from 178.45 mm to 1510.52 mm,with the high value areas mainly distributed in Guangxi,Sichuan basin,and the Chongqing junction,and the low value area is mainly distributed in almost the entire Guizhou and Guizhou region.ET is slightly overestimated at both Xishuangbanna and Ailaoshan stations,and Xishuangbanna’s inversion accuracy is slightly better than Ailaoshan’s.Overall,the modified SEBAL algorithm has adequate station-scale accuracy,with RMSEs of 1.19 mm/d and 1.13mm/d,r RMSEs of 51.9% and 49.8%,and MAPEs of 0.65 mm/d and 0.53 mm/d for Xishuangbanna and Ailaoshan stations,respectively.(2)A significant declining trend in rainfed rice ET was observed in the 2009-2010winter-spring post-drought period,with rainfed rice ET indicating a 24.7%(44.39 mm)reduction relative to the baseline value(2001-2008).Spatially,rainfed rice ET showed widespread and significant declines(SA<2)across 64.4% of the study region,mainly distributed in the regions of Sichuan Basin,southwest of Chongqing,Guangxi,and central Guizhou.Whereas the rainfed rice ET exhibited a slight positive anomaly in Yunnan Province due to the prolonged drought.(3)The 2011 summer drought exhibited a significantly positive impact on the rainfed rice ET,with ET values 4.3%(8.7 mm)and 15%(30.3 mm)higher before and during the drought period than that of the baseline years(2001-2008).There was an early increase in rainfed rice ET in April 2011,peaked in September,and then decreased in October.Spatially,the positive effects of the 2011 summer drought events on rice ET were widespread throughout Southwest China(SA>0.5),accounting for79.3% of the rice area in the study area.An increase in rainfed rice ET(SA>0.5)was observed in 86% of the rainfd rice area in Guizhou,84% in Sichuan,82% in Guangxi,74% in Yunnan and 49% in Chongqing.(4)Rainfed rice ET is far more sensitive to variations in Vapour pressure deficit(VPD)than soil moisture(SM)in Southwest China.Rainfed rice ET to VPD and SM has a mean linear regression slope of 0.2216 and-0.094,respectively.High VPD can positively affect rainfed rice ET while low SM exhibits a negative effect during drought periods.The areas where VPD or SM was the controlling factor for rainfed rice ET accounted for 54.4% and 45.6% of the study area,respectively.(5)Based on the multiple regression model,VPD explained 43.01% of the increase in rice ET during the 2011 summer drought,compared to 30.05% for SM and26.94% for their interaction.VPD and SM showed a significant negative coupling,with most VPD showing positive sensitivity and SM showing negative sensitivity.However,the sensitivity of VPD and SM changed continuously as the drought progressed,indicating a nonlinear relationship between them.The sensitivity of rainfed rice ET to VPD decreased as the intensity of the drought grew,while the sensitivity to SM increased.