| Under the trend of global warming,drought has a serious impact on human survival and socioeconomic development.The development trend and causes of drought have become the focus of scholars' research.Affected by the uneven distribution of meteorological elements,the northwest region has become one of the driest regions in the world at the same latitude.Plagued by serious shortage of water in arid northwest region economic and social development,the quantitative study on the changing characteristics of drought time and space in northwest China,and analysis on its causes is of great importance for the northwest drought prevention and disaster reduction.Under the background of the trend of worsening drought,the phenomenon of water shortage has emerged.The waste and pollution of water resources make the shortage of water resources worse.A large amount of township wastewater is discharged into rivers and underground,which not only causes waste of water resources,but also causes pollution of surface water and groundwater.Constructed wetland is a low-cost wastewater treatment process which is suitable for urban and rural dispersed treatment.This paper proposes a type of ridge-shaped constructed wetland,which can be used in areas with abundant land resources.It can not only treat wastewater for water resources reuse,but also improve climate and conserve water and soil by constructing wetland environment.The main research contents and results are as follows:(1)Based on the data of precipitation and temperature at 113 meteorological stations in northwest China from 1960 to 2018,the SPEI index was used to analyze the temporal and spatial variation of drought at multiple scales in Northwest China.In the analysis of the evolution characteristics of drought time,it can be seen from SPEI-1,2 that the dry and wet conditions occur alternately in northwest China,and the drought distribution is uneven over time,and the annual SPEI value declines with a trend of-0.155/10 a.SPEI-3 was used to analyze the fluctuation state of drought in four seasons in the last 60 years.The variation of SPEI value in spring,summer and autumn all showed a significant trend of decline,and the decline rate was 0.18/10 a,0.13/10 a and 0.16/10 a respectively.The annual drought index showed a slow trend of decline,indicating that the drought situation was increasingly serious.M-K mutation test showed that except in winter,there was an obvious drought trend in northwest China,and the mutation point was not consistent.The mutation was most obvious around 2000.In the analysis of the spatial distribution characteristics of drought,the spatial distribution of drought years in northwest China was analyzed by using the 12-month scale SPEI.From the perspective of the chronological change of SPEI index,the drought situation increased with the interannual change,and the frequency of moderate and severe drought was higher after 2010 s.The SPEI on the March scale was used to analyze the spatial distribution characteristics of the seasons in the northwest region.From the analysis of degree of drought,the probability of "extreme drought-light drought" is increasing in each season.The SPEI data at the January scale was used to analyze the spatial distribution characteristics of drought frequency in the northwest region,the probability range of different drought degrees varies greatly,and the places where high frequencies occur are also different,which also reflects the complexity of the drought situation in the northwest.(2)The summer drought in northwest China is affected by monsoon.The SPEI summer index is similar to the absolute value of propensity of the east Asian summer monsoon and the plateau summer monsoon,indicating that the increase of the plateau summer monsoon and the decrease of the east Asian summer monsoon under the overall development trend play a promoting role in the northwest drought.The results of the statistical analysis of ENSO cold and warm events show that the drought before 2000 has a great correlation with the warm events,and the cold and warm events after 2000 have a great influence on the drought.There was a certain correlation between SOI and drought,and the large correlation coefficient was mainly distributed in the southeastern part of the study area and the northern part of xinjiang.Except for summer,some sites have passed the significance test of 0.05 and 0.1;SPEI and MEI have similar oscillation period characteristics,and there are three phases in the high energy region and a wide-range of 1.5~6a resonance period.The anti-phase changes of the two indicate that the SPEI index declines with the increase of MEI.(3)This paper proposes a kind of ridge constructed wetland combining surface flow and horizontal subsurface flow,providing an environment of aerobic and anaerobic alternations for the whole wetland system and improving the removal of pollutants.In order to verify the pollutant removal effect of ridge-shaped constructed wetland,a traditional horizontal subsurface flow constructed wetland with the same specifications was constructed.The experimental results showed that the average removal rates of BOD,TN and TP in ridge-shaped constructed wetland were 80.8%,83.5% and 80.3%,respectively.In traditional subsurface flow constructed wetlands,the average removal rates of BOD,TN,and TP are 60.5%,66.5%,and 71.2%,respectively.Compared with the traditional subsurface constructed wetlands,the removal rates of BOD,TN,and TP in the ridge-shaped constructed wetland increased by 20.3%,17.0%,and 9.1%,respectively.Compared with the traditional subsurface wetland,the ridge-shaped constructed wetland increases the concentration of dissolved oxygen in the water body through the surface reoxygenation of the surface flow part and the plant roots secrete oxygen,which reduces the aeration cost,decreases the interference of human factors on the wetland,and improves The removal efficiency of pollutants. |
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