Impacts Of Climate Change On Flash Droughts In The Gan River Basin

As opposed to traditional drought events,flash droughts evolve rapidly over brief time scales(usually less than one month),and are characterized by high temperatures with abnormally low and quickly declining soil moisture.The general lack of high resolution soil moisture and evapotranspiration data makes identifying flash droughts at short-term scales nearly impossible,particularly at the basin scale.Flash droughts tend to occur suddenly with little warning in humid and subtropical basins that may be populated by communities that are unprepared for their severe impacts on society and the economy.It is necessary to further study flash droughts and their mechanisms for the sake of early prediction.There has been limited research to date in regards to identifying and attributing spatial variations in flash drought events,especially at the basin scale in China.This study used the Gan River Basin as an example to investigate the spatial and temporal characteristics of flash droughts and to explore their causes.Furthermore,this study provided an initial overview of the downscaled CMIP5-VIC models' capability in representing historical flash droughts and the future flash drought changes,especially at a 1.5? and 2.0? world.The most notable conclusions of this study can be summarized as follows:(1)A common variation trend in rainy season precipitation,and the variability in maximum value occurring around 28°N decreased to the north and to the south.The northern parts of the Poyang Lake Basin were more vulnerable to drought and flood.Nino-3.4 SST(sea surface temperature)in winter may positively impact the subsequent year's rainy season precipitation.Strongly positive correlation existed between RX5day(maximum consecutive 5-day precipitation amount)and rainy season precipitation,but the correlation between DTR(diurnal temperature range)and rainy season precipitation was negative.Further,the water vapor transport from western boundaries into the Poyang Lake Basin was substantial whereas the eastern boundary mainly output water vapor.Large input/output values were concentrated on about 850 hPa.A sizeable quantity of water vapor converged over the basin in wet years,forming effective precipitation with negative values of divergence over the basin.For dry years,positive anomalies in water vapor flux divergence implied that little water vapor had converged over the basin.The best correlation coefficients between SSI(standardized streamflow index)and SPEI(standardized precipitation evapotranspiration index)were found at timescales of 2-3 months,which better reflect the intensity and magnitude of seasonal dry/wet conditions.Severe dry/wet episodes were easy to occur in the northern part of the basin.Both a positive height anomaly at 500 hPa and a very little water vapor flux influence the basin during the July-September of anomalous years,which can lead to the occurrence of drought events in the basin.However,during April-June of anomalous years,geopotential height anomalies caused by areas of high pressure in the south(South China Sea)and areas of low pressure in the north(Mongolia),are responsible for the transport of copious amounts of water vapor from both the Bay of Bengal and the South China Sea.Under these conditions,the negative values of water vapor flux divergence dominate the basin and bring abundant precipitation,which often causes calamitous flood events.(2)The variable infiltration capacity(VIC)model can accurately reflect hydrological processes in the Gan River Basin at daily and monthly time scales;here,flash droughts were defined based on VIC outputs(soil moisture and evapotranspiration)and meteorological observations(maximum temperature and precipitation)during the growing season(March-October)from 1961 to 2013.Heat wave flash droughts are high temperature driven events,high temperatures(heat waves)cause evapotranspiration to increase and soil moisture to decrease rapidly.The main driver of precipitation deficit flash droughts is precipitation deficits,which cause soil moisture to drop and in turn cause evapotranspiration anomalies to decrease and temperature to increase.The northern part of the basin is apparently vulnerable to heat wave flash droughts,whereas precipitation deficit flash droughts tend to occur across the central and southern parts of the basin.Precipitation deficit flash droughts are more common than heat wave flash droughts in general.Both types of flash droughts became significantly more frequent from 1997 to 2013.These increases in both types of flash droughts are likely attributable to climate-related variables.As evidenced by our investigation of the evolution of the two types of flash droughts and the example of 2003 summer flash drought across the Gan River Basin,flash droughts can evolve into prolonged droughts.Precipitation deficit flash droughts occur over a lengthier time span than heat wave flash drought.In terms of both drought types in the Gan River Basin,the sunshine duration and surface net solar radiation increases substantially due to small amount of low-level clouds.fluxes,which increase the near-surface air temperature.The Bowen ratio is an important factor in distinguishing a heat wave flash drought from a precipitation deficit flash drought.(3)We evaluated the downscaled Coupled Model Intercomparsion Project Phase 5(CMIP5)coupled with the VIC model(CMIP5-VIC)in reproducing flash droughts in a humid subtropical basin in China.Most downscaled CMIP5-VICs capable of simulating the spatial patterns of flash droughts with respect to the benchmarks.The coupled models fail to readily replicate interannual variation,but most models can reflect the interannual variability(temporal standard deviation)and long-term average pentads of flash droughts.It is difficult to simultaneously depict both the spatial and temporal features of flash droughts within the same coupled model.The climatological patterns of the best multi-model ensemble mean are close to those of the all-models ensemble mean,but the best multi-model ensemble mean comes with shorter bias range and less computational burden.(4)The warming rate in drylands and high altitude areas in China is obviously greater than that in south China(e.g.,Gan River Basin),and the temperature increase at the Gan River Basin is close to the global average.The annual pentads of future(2006-2099)flash droughts in the basin show a near-linear increase.Climatological median pentads appear likely to be 9,6-17.6%more frequent in a 2.0? world than a 1.50C world with respect to the reference period(1961-2005).The greatest increase in flash drought pentads came from the 25th percentile values in a 1.5? or 2.0? world.We observed multifold increases in the low-value areas and significant increases in the high-value regions in regards to the spatial frequency of flash drought occurrence in the Gan River Basin.The substantial increase in flash drought prevalence may represent an entirely new climate regime for the Gan River Basin,especially under 2.0? of global warming.