Assessment Of Impact Of Climate Variability And Land Use Change On Drought In Black Soil Zone Of Northeast China

The black soil region is located in the middle to high latitudes of the N orthern Hemisphere.It is one of the most significant areas affected by global warming;moreover,it is also one of the most disturbed areas,which is under constant threat from human activities.In recent years,the frequency of drought has increased significantly in the black soil region of Northeast C hina.The damage caused by extensive drought is increasing considerably.Moreover,the impact of drought on the environment has become progressively serious.Therefore,extensive research efforts are urgently required to be devoted to investigate the impact of climate variability and land use change on drought in black soil zone of Northeast China.Herein,the basin of upper and middle courses of Wuyuer River controlled by the Yi'an gauge station was selected as study area.Based on terrain,vegetation,soil,meteorology and hydrology observation,and investigation data,the monthly drought simulation model was constructed by using the distributed hydrological model,namely,Soil and Water Integrated Model(SWIM)and drought indices.Q uantitative evaluation of the relative role and contribution of climate change and LUCC on drought were systematically investigated.The main contents of this thesis can be concluded as follows:(1)Applicability of the SWIM in the upper and middle reaches of the Wuyuer RiverResults indicated that in the periods of calibration and validation,the Nash-Sutcliffe efficiency(NSE)coefficients of the monthly and daily runoffs were not less than 0.71 and 0.55,respectively,and the re lative errors were less than 6.0%.Compared to daily runoffs,the simulation result of monthly runoffs was better.Furthermore,the NSE coefficients of the potential monthly evaporation were not less than 0.81.Together,the results revealed that the calib rated SWIM can be utilized in various simulation analyses of runoffs on a monthly scale in the black soil region of Northeast C hina.In contrast,the model exhibited some limitations in simulating runoffs from snowmelt and frozen soil.Moreover,the simula tion data deviated significantly from the measured data when applied to the years with spring and summer floods.The simulated annual runoffs were considerably higher than the measured data in the years with abrupt increase in annual precipitation.However,the designed model is capable of reproducing the changes in runoffs during flood seasons.In summary,this model can provide fundamental hydrological information for comprehensive management of the Wuyuer River basin water environment,and its application can be potentially extended to other river basins in the black soil region.(2)The monthly drought evaluation model was established based on the water cycle model SWIM and assessment methods of drought.Based on the critical cycle factors,the eco-hydrological model SWIM and three types of drought indices,namely,Standardized Precipitation Evapotranspiration Index(SPEI),Palmer Drought Severity Index(PDSI),and Palmer Hydrological Drought Index(PHDI)were adopted and the monthly drought evaluation model was established.Based on historical drought records and meteorological drought Atlas,herein we used both the time and space scales for verification.The results of the drought identification evaluation of the established monthly scale drought assess ment model are consistent with those of the historical drought data and meteorological drought.This indicates that the determination of critical threshold and evaluation of constructed drought pattern are reasonable,and it is possible to reflect the temporal and spatial evolution characteristics of drought in the study area.(3)The K value of the climatic characteristic coefficient of the Palmerian drought model was revised in the study areaBased on the North American PDSI and the modified PDSM for the western Songnen Plain,we further modified the K value of the climatic characteristic coefficient of the model in stations and also the calculation of potential evapotranspiration and runoff.As a result,a revised model of drought severity was successfully established in this study.The spatial comparability of PDSI in the study area was found to be better.(4)Spatio-temporal characteristics of drought in the study area were analyzedThe characteristics of meteorological drought,agricultural drought,and hydrological drought in the study area were analyzed from the perspective of time and space.The results show that the duration of drought is longer,the number of years of drought is more than the number of normal year.Three types of drought,the occurrence of moderate drought above the number of months were increasing trend.This is,in particular,true for the years after the 1990 s.The interannual variability of meteorological drought rate is not large;however,the interannual variability of agricultural drought and hydrological drought is obvious.Analysis of drought at the inter-decadal time scale indicates that the drought rates of all types of drought were the lowest in the 1980 s.After the 1990 s,the rates showed an increasing trend.Moreover,the average drought in the four seasons exhibited the highest rate in spring.There is little difference in the frequency of the distribution of meteorological moderate drought.The area under moderate drought above the highest frequency of distribution includes the eastern part of Keshan County and Kaidong County,southwest of the study area.The distribution of drought intensity is roughly the same.The frequency distribution of drought in each grade of agriculture is basically the same.Only the frequency of occurrence of drought in a small part of the region has changed.The high intensity zone of each grade is basically distributed according to the edge of the study area,and the low value area is distributed along the main river channel.There were significant differences in the frequency distribution of drought in each grade of hydrological drought.Spatial distribution was low in the Southwest,and high in the Northeast.The area with high intensity above that of moderate drought includes the Northwest of the study area.It was roughly C-shaped distribution.The distribution of the high value area of other grades of drought intensity changed from C-shaped to bar.(5)The two-dimensional Copula function joint distribution model of drought duration and drought intensity was establishedTwo-dimensional(2D)joint distribution function of the duration of the drought and drought intensity in meteorological drought,agricultural drought,and hydrological drought corresponds to C layton-Copula.For 2D distributio ns,co-occurrence return period is much greater than the joint return period under the same condition.For 3D distribution,the co-occurrence return period is much greater than the joint return period when the drought variables acquire the same value under the condition of given one drought variable.The joint return period of two drought variables under the condition that other drought variable is greater than a certain value is longer than that under the condition of the other drought variable being smaller than the same certain value under the condition of given two variables.(6)The quantitative analysis method considering the influence of change of climate and land use on meteorological drought,agricultural drought,and hydrological drought was put forward.Based on the data from 1961 to 2010 and the change of climate and LUCC,the reference period and influence period were defined and the driving effects of meteorological drought,agricultural drought,and hydrological drought were analyzed.Climate change and LUCC are positive directions for meteorological drought,and they exacerbate drought.The influential values of climatic variation and land use change on the duration of meteorological drought are 85.93 and 14.07%,respectively,on the drought intensity of meteorological drought are 73.36 and 26.64,respectively,and on the drought rate of meteorological drought are 64.44 and 35.56%,respectively.The influence of climatic variation and land use change on the duration of agricultural drought is 15.31 and 84.69%,respectively,on the drought intensity of agricultural drought are 30.33 and 69.67%,respectively,and on the drought rate of agricultural drought are 61.69 and 38.31%,respectively.The influential values of climatic variation and land use change on the duration of hydrological drought are 7.60 and 92.4%,respectively,on the drought intensity of hydrological drought are 24.36 and 75.64%,respectively,and on the drought rate of hydrological drought are 60 and 40%,respectively.