Method For Drought Identification Of Winter Wheat Based On Evapotranspiration In The Huang-Huai-Hai Plain,China

In this study,we are going to work on calculating potential evapotranspiration?ET₀?,crop evapotranspiration under standard conditions?ET_c?and actual evapotranspiration?ET_a?in winter wheat growing season on a daily basis in the Huang-Huai-Hai?3H?Plain,using meteorological data of 27 stations and the phenophases of winter wheat at 17 agro-meteorological stations,MODIS remote sensing products and historical disaster data in 2000-2013.The spatial and temporal distribution characteristics of ET₀,ET_c and ET_a of winter wheat are analyzed,which is spatially separated by the 3H Plain,the region planting winterness wheat and weak winterness wheat in the 3H Plain,espectively.According to the ratio of ET_a and ET_c,a CDII?Crop Drought Identification Index?for winter wheat at different developmental stages is established and evaluated the fitness by random drought samples for winter wheat reserved independently.On this basis,we analyze the drought disaster of winter wheat and reveal the temporal and spatial distribution characteristics of drought disaster at different development stages of winter wheat.Main results of the paper are showed as below:?1?The daily average ET₀ is 2.36mm·d^-11 in winter wheat growing season in the 3H Plain,which is lowest during the wintering stage.ET₀ in the before wintering stage,returning green–heading stage,heading–milky ripening stage and milky ripening–maturity stage increases in turn.The spatial distributions of ET₀ at before wintering stage,returning green–heading stage,heading–milky ripening stage and the whole growing season of winter wheat are all relatively higher in the nortern and lower in the southern part of the 3H Plain.The spatial distributions of ET₀ during the winter wheat growing season in the 3H Plain is most closely related to vapor pressure and wind speed.?2?Combined with remote sensing data?LAI and albedo?,it is reasonable to calculate ET_c using the Penman-Monteith formula.A set of daily ET_c data sets for winter wheat in the 3H Plain is generated based on this method.The spatial distributions of ET_c at wintering stage,returning green–heading stage and the whole growing season of winter wheat are all relatively higher in the southern and lower in the nortern part of the 3H Plain,while it varies little in other stages.Except for milky ripening–maturity stage,the spatial distributions of ET_c at other stages of winter wheat in the 3H Plain is most closely related to leaf area index.?3?The daily average ET_a is respectively 0.79mm·d^-1,0.78mm·d^-1,1.81mm·d^-1,2.99mm·d^-1and 3.27mm·d^-1 at the before wintering stage,wintering stage,returning green–heading stage,heading–milky ripening stage,and milky ripening–maturity stage.There is little difference between the spatial distribution in the before wintering stage and wintering stage,while the spatial distributions of ET_a at the other three developmental stages are all relatively higher in the central-southern part of the 3H Plain and Bohai Bay,and lower in other areas.?4?CDII for winter wheat is established by the ratio of ET_a and ET_c.The critical thresholds of drought?95%probability?at different developmental stages are determined by the actual disaster data.The CDII threshold at before wintering stage,wintering stage,returning green–heading stage,heading–milky ripening stage and milky ripening–maturity stage in the winterness wheat planting area of the 3H Plain is 0.80,0.45,0.85,0.70 and 0.50,respectively.The CDII threshold at the five developmental stages in the weak winterness wheat planting area of the 3H Plain is 0.85,0.50,0.85,0.80 and 0.65,respectively.The results identified by the CDII were basically consistent with the random drought samples for winter wheat reserved independently,with 86.2%of historical records strongly consistent with the CDII.The verification results of the current"Disaster Grade of Drought for Wheat"also show that the CDII could reasonably reflect the actual damage caused by drought disasters in winter wheat planting.In 2000-2013,drought frequency is higher at before wintering stage,returning green–heading stage,heading–milky ripening stage?the winterness wheat planting area of the 3H Plain?and milky maturity–maturity stage?mid-western Shandong Province?.The drought distribution in the 2006–2007 growing season and heading–milky ripening stages in 2001–2013 also indicate that the CDII can reasonably reflect the actual drought disasters in winter wheat planting,and it can be used in the monitoring and assessment of disaster damage at regional scale.