The Spatiotemporal Evolution Of Drought In Northwest China In The Past 34 Years

This study is based on the monthly precipitation data and monthly average temperature data of 145 national basic weather stations in Northwest China from 1982 to 2015.By calculating the monthly standardized precipitation evapotranspiration index(SPEI)and standardized soil moisture index under different time scales(SSMI)and GIMMS NDVI Vegetation Index,analyzed the spatial and temporal distribution of drought in Northwest China in the past 34 years,and discussed the characteristics of the drought in the northwestern region during 34 years by using Mann-Kendall test,wavelet analysis,unary linear regression trend and Hurst index..The main results are as follows:(1)From the perspective of meteorological drought indicators:In terms of inter-annual changes,the overall SPEI in the northwestern region shows a downward trend,indicating that the drought situation in this region is gradually increasing.From the perspective of seasonal changes,the frequency of extreme drought and severe drought in the Northwestern region has increased with the increase of time scale.However,with the increase of time scale,the frequency of moderate drought has been increasing in spring,summer,and winter.The drought frequency in the three winter seasons all showed a trend of decreasing first and then increasing.There are small-scale signals with a period of 3-8a on the interannual period in the northwest region,and large-scale signals with a period of 12-15a and 28-30a on the interdecadal period.The two periods of 7a and 14a are in the past 57a in the northwest region.The drought has played a major role.The SPEI value of the northwestern region suddenly decreased in 1995,indicating that the northwestern region changed from humid to arid after 1995,and the SPEI value of the northwestern region reached a significant level of 0.05 around 2000,indicating that the northwestern region was after 2000 The increasing trend of drought is obvious.Compared with historical data,it is found that the SPEI drought index has better applicability in characterizing drought conditions in the northwestern region.(2)From the perspective of remote sensing drought indicators:In terms of inter-annual changes,the annual average NDVI in Northwest China increased significantly from 1982 to 2015,and there was a trend of fluctuations and declines in some parts,indicating that the drought situation in the region has slightly improved.From the unary linear regression trend,it can be seen that the area with Slope>0 in Northwest China accounts for 95.68%,indicating that the NDVI value in Northwest China is increasing as a whole,and the probability and area of drought have decreased.The Hurst index analysis shows that the area with H<0.4 accounts for 57.1%of the vegetation coverage area,the area between 0.4 and 0.6 accounts for 34.1%,and the area with H>0.6 accounts for 8.8%of the vegetation coverage area,indicating that the vegetation change in Northwest China is reversed.Strong continuity.Vegetation NDVI has a strong spatial correlation with temperature and precipitation.The areas where vegetation NDVI has a significant positive correlation with temperature in northwestern China are mainly distributed in southern Shaanxi,northern Shaanxi,southern Gansu,and parts of southern Xinjiang,and are significantly positively correlated with precipitation The regions are mainly distributed in southern Shaanxi,western Inner Mongolia,southern Gansu and southern Qinghai.The deviation compared with the SPEI value is due to the pixel deviation of the vegetation index product itself.At the same time,there is no change in NDVI for areas where vegetation is not exposed for many years,which may affect drought monitoring.(3)From the perspective of agricultural drought indicators:in terms of inter-annual changes,the standardized soil moisture index of the four sub-climate regions in Northwest China from 1982 to 2015 showed a fluctuating upward trend,indicating that the frequency and intensity of drought in this region have declined.From the perspective of the fluctuations of the standardized soil moisture index,the entire Northwestern Region has fluctuated greatly in the past 34 years,indicating that the improvement of soil moisture in the Northwest Region has been relatively good in the past 34 years,and the probability of drought occurrence is low.From the positive or negative slope of the regression equation of the standardized soil moisture index,it can be seen that the area with slope>0 accounts for 93.72%of the total area,indicating that the SSMI value of the northwestern region has been increasing in the past 34 years,and the northwestern region has a tendency to become humid.From the spatial distribution point of view,after the 1990s,the aridification trend in northwestern China has eased,but the degree of aridification in most areas of Xinjiang and western Inner Mongolia has shown an increasing trend.There is a deviation from the SPEI value.First,the selected soil moisture data is calculated from soil moisture at a depth of 160cm,while the SPEI value calculation takes into account precipitation and temperature factors.Second,the area discussed in this article has irrigated agricultural areas.Artificial irrigation may be carried out when drought occurs,which affects the uncertainty of the results.