Variations of dryness/wetness and responses to large scale climate factors based on SPI in Luanhe River basin
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Abstract:
Luanhe River basin is located in the north of BeijingTianjinHebei area with obvious monsoon climate characteristics and frequent dryness/wetness variations.It is an ecologically fragile area and is sensitive to climate change.Under the background of climate warming,it is of great significance to study the characteristics of typical dryness/wetness field modes and their responses to large-scale climate factors.The existing research focused on the analysis of the evolution characteristics of dryness/wetness and the impacts of climate factors,or the decomposition of the spatial and temporal patterns of drought and flood.However,there are relatively few studies on the spatiotemporal decomposition of dryness/wetness characteristics and the periodic identification and impact of climate factors. The standardized precipitation index was used to analyze the dryness/wetness change trend of Luanhe River basin from 1957 to 2016.The MannKendall method was used to test the evolution trend of dryness/wetness characteristics.The empirical orthogonal function (EOF)decomposition was used to decompose the dryness/wetness field,and its temporal and spatial characteristics and periodicity were discussed.The crosswavelet analysis was used to analyze the response relationship between large-scale climate factors and typical dryness/wetness conditions.The reference was provided for objective assessment and accurate prediction of drought and flood disasters in Luanhe River basin,so as to reduce the losses caused by extreme climate eventsand enhance people′s adaptability to climate change. In the study on the trend of wet and dry changes in the Luanhe River basin,the MannKendall method was used to analyze the trend and abrupt changes of SPI12 (12-month SPI) from 1957 to 2016.The wet and dry conditions in the basin had abrupt changes in 1989 and the drought trend became more serious.Before 1989,there were 29 years of extreme waterlogging events in the basin,and 2 years of extreme drought events,affecting 1972 and 1980-1984.After 1989,the intensity of drought events in the basin increased,and interannual drought events occurred frequently,with a total of 27 years of severe and extreme drought events.In the study of the characteristics of dryness/wetness changes in the typical modalities of the Luanhe River basin,the EOF method was used to decompose SPI12 in time and space,and the cumulative contribution rate of the first two spatial function eigenvalues was 72.3%.The distribution of dryness/wetness fields from 1957 to 2016 is dominated by mode 1,and the slope of PC1 (the first principal component) is -0.054,indicating that the degree of drought in the basin has an increasing trend.The continuous wavelet transform power spectrum and the wavelet variance graph show that the time-frequency structure of PC1 and PC2 in the dryness/wetness fields are similar,and both have a 1.5 to 4.0 year main period.In the study of the influence of largescale climate factors on the dryness/wetness fields in Luanhe River basin,the resonant period and phase relationship between largescale climate factors and PC1 and PC2 in time-frequency domain are evaluated by cross wavelet transform.The two typical modes of dryness/wetness changes in Luanhe River basin are closely related to Nina 3.4 and SOI (Southern Oscillation Index),and they have similar 1.5–6.0–year period but opposite correlation to the dryness/wetness fields.The influence of AO (Arctic Oscillation) and NAO (North Atlantic Oscillation) on the dryness/wetness fields is general,which is mainly reflected in the phase relationship of 2-4 year period. Conclusions（1) The dryness/wetness conditions of the Luanhe River basin experienced abrupt changes in 1989.Before the abrupt change,the basin was relatively humid,and drought and flood events occurred alternately;after the abrupt change,the drought trend in the basin is further strengthened,and interannual drought events occurred frequently; （2) The dryness/wetness evolution of the Luanhe River basin showed two modes:the same type of dryness/wetness in the whole area and the reverse type of upstream and downstream in the basin,with a cumulative contribution rate of 723%.The dryness/wetness field of the river basin was of the same type of dryness/wetness in the whole area.The trends of the time variables of the two modalities show that the degree of drought in the basin has an increasing trend;the two time variables have a similar timefrequency structure,and both have a 1.5 to 4.0 year main cycle;（3) Nina 3.4 and SOI,which characterize ENSO (El Nio/Southern Oscillation) events,and the two modes both have a period of 1.5 to 6.0 years;ENSO has the most significant impact on the spatialand temporal distribution of dryness and wetness in the Luanhe River basin,followed by AO and NAO,which are related to the weakest AMO and PDO;the impact of the six climatic factors on the basin′s dryness/wetness fields is mainly reflected in changing the basin′s dryness/wetness consistency
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