Drought Assessments Of Inland River Basin In Hexi Corridor Based On SPEI Index

The standardized precipitation evapotranspiration index?SPEI?considers the impact of precipitation and temperature on drought,and is suitable for monitoring and evaluation of meteorological drought.Since the estimation of potential evapotranspiration?PET?is very important for the calculation of the SPEI index,this paper first uses the SPEI index to evaluate the drought situation in the Hexi Inland River Basin from 1960 to 2015.On this basis,it discusses the impact of different PET estimation methods on the SPEI index and drought assessment.PET estimation methods include Thornthwaite?TH?,Blaney-Criddle?BC?,Hamon,Szász,Makkink,Priestley-Taylor?PT?,Penman and Penman-Monteith?PM?.The conclusions of the study are as follows:The average drought intensity in the study area ranges from-0.79 to-0.96,the drought peak ranges from-1.99 to-3.68,the total drought duration is 194-260 months,and the maximum drought duration is 22-123 months,after 2000 extreme drought events occur more frequently,and drought condition has intensified over time.The years of severe drought and extreme drought in the river basin are mostly concentrated in the early 1960s,1983¹985,1992,2007²009 and 2012²013.Spatially,severe drought and extreme drought events occurred more frequently in the middle and lower reaches of the basin,and the drought condition in the lower reaches of the Shiyang River Basin,the middle and lower reaches of the Heihe River Basin,and the Shule River Basin are increasing.The PET values obtained by different estimation methods are unevenly distributed throughout the year,and are similar in the inter-annual change trend,but the values are quite different.The PET estimated by the Szász is the highest,and the PET estimated by the Hamon is the lowest.Regardless of the seasonal or inter-annual scale,the PET values in the middle and lower reaches of the watershed are higher.SPEI_TH is most related to the SPEI sequence corresponding to the temperature method,and SPEI_PM is more related to the SPEI_Szász and SPEI_Makkink sequences.The SPEI_TH and SPEI_BC sequences detected a significant trend of watershed aridity.The SPEI_PM and SPEI_Penman sequences detected a watershed wetting trend,but different SPEI sequences all detected a significant drying trend in the Mazongshan station.The average drought intensity extracted from different SPEI sequences are close to each other,and the drought peaks are slightly different.The number of severe drought and extreme drought events extracted by SPEI_BC sequence are more,and the total drought duration was slightly longer.SPEI_Szász and SPEI_PM sequences were extracted less frequently,and the total drought duration was slightly shorter.Spatially,the results of the extraction of different SPEI sequences(except SPEI_BC)show that the number of severe drought and extreme drought events in the middle and lower reaches of the Heihe River Basin are relatively high,and the Shule River Basin is relatively low.The choice of PET estimation method has a great influence on SPEI and drought assessment.It is suggested to use Blaney-Criddle?BC?based on the temperature method and Makkink based on the radiation method as an alternative formula to estimate TH in SPEI,and Szász based on temperature method as an alternative formula to estimate PM in SPEI.