| Compared with other types of natural disasters,drought has the characteristics of high frequency of occurrence,wide range of influence and large loss.The Lancang-Mekong River basin is characterized by uneven distribution of precipitation within a year and distinct dry and wet seasons.The basin is vulnerable to drought disaster in dry season when precipitation is short.Therefore,the study on the characteristics of meteorological drought and hydrological drought in the Lancang-Mekong River basin will provide reference for timely drought response measures to reduce drought losses in the Lancang-Mekong River Basin to reveal their evolutionary relationship.Based on the precipitation grid data of CRU(Climate Research Unit)and the meteorological and hydrological data of six hydrological stations in the main stream in the Lancang-Mekong River basin for 56 years,this paper calculated the drought index,and analyzed the meteorological and hydrological drought in the upper,middle and lower reaches of the Basin respectively.Mann-kendall test and wavelet analysis were used to analyze the trend and periodicity of drought in the Lancang-Mekong River basin,the characteristic variables of drought were identified by using run theory,and the time-delay relationship between hydrological drought and meteorological drought was discussed based on correlation analysis.The main conclusions are as follows:(1)Based on CRU monthly precipitation grid data,the standardized precipitation index(SPI)representing meteorological drought was calculated to analyze the spatial and temporal distribution characteristics of meteorological drought in the Lancang-Mekong River Basin.The results showed that: In the upper Reaches of the Lancang-Mekong River Basin from 1960 to 2016,the z-values of China-Qingsheng and Qingsheng-Vientiane test statistics were 0.41 and 0.91,showing a wetting trend,while the Z-values of the middle and lower reaches were-0.12 and-0.18,showing a drying trend,but not significant.The main cycle of meteorological drought in the upper,middle and lower reaches of the Lancang-Mekong River basin is generally 25-30 years,with 3 to 4 drought cycles alternating.In the Lancang-Mekong River basin,the frequency of mild and moderate drought is higher in the north than in the south.The frequency of mild and moderate drought decreases from 33.19% in the north to 31.43% in the south,and the frequency of moderate drought decreases from 9.35% in the north to 8.92% in the south.The frequency of severe and extreme drought is higher in the south than in the north.(2)Based on the runoff observation data in the Lancang-Mekong River Basin,the standardized runoff index(SRI)was calculated to represent hydrological drought,and the spatial and temporal distribution characteristics of hydrological drought were analyzed.The results showed that: During 1960-2016,the statistical values of hydrological drought at Qingsaeng station,Luang Prabang station and Vientiane station were-1.95,-2.28,-2.98,showing a significant drying trend,while the statistical values of Mudahan station,Basse station and Jujing station were 6.30,2.21,4.37,showing a significant wetting trend.The main cycle of hydrological drought in lancang-Mekong River basin is 16-24 years.The spatial distribution characteristics of hydrologic drought in the Lancang-Mekong River Basin are as follows: the frequency of light drought,moderate drought and severe drought decreases from34.07% to 25.72%,moderate drought frequency from 9.65% to 7.3%,and severe drought frequency from 4.08% to 2.78%.The occurrence frequency of special drought is the highest in the southern part of the Basin.(3)The comparative analysis of meteorological drought and hydrological drought in the Lancang-Mekong River Basin shows that the occurrence duration of meteorological drought in the whole Lancang-Mekong River Basin is higher than that of hydrological drought,but the severity of hydrological drought is higher than that of meteorological drought.The similarity of meteorological drought and hydrological drought characteristic variables in the Lancang-Mekong River Basin shows that the occurrence frequency of drought in the upper reaches is more but the drought is less,and the occurrence frequency of drought in the lower reaches is less but the drought is more serious.The response of hydrological drought to meteorological drought in the Lancang-Mekong River Basin was identified by time-delay cross-correlation analysis.The results show that: Standardized precipitation index(SPI)and standardized runoff index(SRI)scale the strongest correlation of 12 months,the lancang-Mekong river basin upstream maximum correlation coefficient in lag of 2 months,lagging river middle and lower reaches the maximum correlation coefficient in a month,the lancang-Mekong river basin hydrological drought on the transmission of meteorological drought time upstream 2months,respectively,One month in the middle stream and one month in the downstream.The seasonal response time of hydrological drought to meteorological drought in the Lancang-Mekong River Basin is significantly different,which is11-12 months in winter and spring with less precipitation,and 7-8 months in summer and autumn with more precipitation.The transmission time is shorter in the rainy season and longer in the dry season.In addition,the study shows that the construction of reservoirs in the Lancang-Mekong River basin has a positive impact on the evolution process from meteorological drought to hydrological drought by means of flood storage and replenishment,and alleviates the severity of hydrological drought.(4)The lancang-Mekong river basin,the frequency of extreme drought event is higher than the upstream region is closely connected with climate change and human activities,the cause of the whole basin production activities is given priority to with agriculture,irrigation water demand is big,affected by elnino drought intensifies,confronted with the drought,a hierarchical region increase agricultural water withdrawals,As a result,the lower reaches of the Yangtze River are affected by climate change and human water extraction from the middle reaches of the Yangtze River,which further aggravate the degree of drought and increase the possibility of extreme drought.The response time of hydrological drought to meteorological drought in rainy season is shorter than that in dry season.The reasons for the difference in response speed of drought evolution process in different seasons are as follows: temperature rises in spring,snow and ice ablation increases infiltration,and evapotranspiration is still weak,while precipitation and evaporation decrease in winter,and the impact of precipitation on runoff is weakened.The slow down of water cycle makes the spreading time of meteorological drought to hydrological drought relatively longer in dry season.However,in the Lancang-Mekong River Basin,it is hot in summer and autumn,evapotranspiration intensifies,agricultural water consumption is also large,and precipitation contributes more to runoff,so hydrological drought responds faster to meteorological drought in the rainy season. |
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