Abstract: Based on daily maximum and minimum temperature observation data from 110 surface meteorological stations in the Dongting Lake basin from 1961 to 2022,the percentile threshold was used to determine the warm day threshold and the warm night threshold for each station.The spatial-temporal variation characteristics of warm days,warm nights,compound high temperature,high temperature heat wave frequency,high temperature heat wave days,and high temperature heat wave intensity in the Dongting Lake basin were analyzed.The results show that the warm day threshold and the warm night threshold in the Dongting Lake basin are gradually increasing from southwest to northeast,which is obviously affected by the topography of high in southwest and low in northeast.The average annual warm days and warm night days in the basin show a significant increasing trend.The former has a climate tendency rate of 1.6 d per decade,while the latter reaches 2.6 d per decade,indicating that the number of warm night days increases faster than the number of warm day days.The trend of changes in the number of warm days and warm nights in the watershed shows a low center and a high surrounding area in space.In addition,the number and intensity of composite high temperature days in the watershed also show a significant increasing (enhancement) trend,with climate tendency rates of 1.3 d per decade and 3.2 ℃ per decade,respectively.Since 2000,the number and intensity of composite high temperature days have shown an accelerated growth (enhancement) trend.The frequency and number of days of high-temperature heat waves in the watershed show a spatial distribution pattern of increasing from southwest to northeast,while the intensity of high-temperature heat waves increases from south to north.During the study period,the frequency,number of days and intensity of high temperature heat waves in the Dongting Lake basin showed a significant increasing trend,and the climate tendency rates were 0.2 times per decade,1.3 d per decade and 2.0 ℃ per decade,respectively.The variation trend of heat wave frequency and heat wave intensity show differences in the three zones of north,middle,and south.The heat wave frequency and intensity in the two zones of north and south are significantly increased (enhanced) compared to those in the central region.The significantly increased stations are mostly distributed in the north and south,while there are fewer in the central region.The change trend of annual average heat wave days in the basin increases from the center of the basin to the surrounding areas,with the most pronounced increase in the northeastern Dongting Lake,and the climate tendency rate as high as 3.0 d per decade.

Key words: Heat wave, Warm day threshold, Warm night threshold