Spatial And Temporal Variation Of Drought And Its Effect On Winter Wheat Yield In Winter Wheat Region Of Guanzhong

Affected by global warming,drought has become one of the most widely extensive coverage natural disasters,with the highest frequency of occurrence and the largest economic loss.Guanzhong area is located in the middle of Shaanxi,which is an important wheat production base in China.Frequent drought has a serious impact on food production in this region.Therefore,it is very necessary to carry out drought monitoring of Guanzhong area.With the help of Google Earth Engine powerful computing ability,a method based on winter wheat Normalized Difference Vegetation Index(NDVI)time series curve was developed and tested in this study for mapping winter wheat planting area over Guanzhong region.The extraction model was established based on NDVI remodel amplification and NDVI increase(decrease)slope threshold.And then,Crop Water Stress Index(CWSI)with better drought monitoring effect was chose to analyze the temporal and spatial changes of drought in Guanzhong area.In addition,using statistical data and combining with CWSI to analyze the effect of drought on winter wheat yield in different growth stages.Main results are as follows:(1)The results of winter wheat extraction by using statistical yearbook show that:area of winter wheat extracted by remote sensing agreed well with statistics,with the determination coefficient R~2 of 0.82 and 0.62,and the index of agreement of 0.95 and 0.84 at city level and county level respectively.Further,the spatial consistency accuracy of extraction results and field survey data was 93.4%.Winter wheat was mainly distributed in Guanzhong Plain,whose terrain is relatively flat.Winter wheat in Guanzhong area was widely distributed in Baoji,Xianyang,Xi'an and Weinan,and relatively sparse in Tongchuan.From 2011 to 2017,the winter wheat planting area in Guanzhong area showed a spatial-temporal change pattern of"increasing in the north and decreasing in the South".In detail,in Qianxian County,Yongshou county and Pucheng County,whose winter wheat planting area increased.While the winter wheat planting area in Fufeng County,Chang'an District,Zhouzhi County and Huxian County,Xianyang and Weinan City decreased.As a whole,winter wheat planting area in Guanzhong region was showing a downward trend from 2011 to 2017.There was a series decline by 8.47%(83.22?10~3 hm~2)in planting area of winter wheat in 7 years.(2)As for the temporal variation of CWSI,2001,2003,2007,2009,2011,2013 and 2014were relatively dry years with low soil moisture.The soil was wetter in 2002,2004 and 2012.During the growth period of winter wheat,soil drought mainly occurred in October,November,March and June.According to the spatial distribution map of drought grades,the spatial distribution of drought conditions in the wheat region of Guanzhong has obvious spatial heterogeneity,showing a trend of increasingly serious drought conditions from southwest to northeast.Among them,Fengxiang County,Qishan County,Fufeng County,Chencang District,Qian County,Wugong County,Zhouzhi County,Xingping County,Huxian County and Lantian County are in the state of no drought or light drought.The drought in Jingyang County,Sanyuan County,Yanliang District,Gaoling District,Fuping County,Weinan County,Pucheng County,Huayin,Huazhou District and Tongguan County is serious,and most of them are in medium,severe or even special drought.(3)From 2001 to 2014,there was a trend of increasing CWSI value in the wheat region of guanzhong,indicating that the wheat region of guanzhong was getting drier and drier.In terms of space,42.57%of the regions have significantly reduced soil moisture,including Wugong County,Xingping County,Hu County,Chang'an District,Lantian County,Sanyuan county and Gaoling district;Fengxiang County,Qishan County and Weibin District(accounting for about 10.81%)have increased soil relative humidity.46.62%of the regions have increased but not significantly drought,including some regions of Fengxiang county Wugong County,Qianxian County,Fufeng County,Jingyang County,Fuping County,Pucheng County,Yanliang District,Weinan City.(4)Drought in different growth periods had different effects on winter wheat yield.The occurrence of drought stress in September,March,April and May will lead to the decrease of winter wheat yield.In December,January,February and June,drought stress was helpful to the growth and grain formation of winter wheat.The response mechanism of winter wheat yield to drought stress in October and November is still unclear.