Analysis and Comparison of Spatial and Temporal Patterns of Meteorological and Hydrological Drought Indices in the Upper Reach of the Heihe River Watershed,Northwest China
Drought is a natural phenomenon and a natural hazard characterized by complex formation and an extended scope of influence. With the expanding human population,water demands are increasing along with global climate change,leading to an increase in drought frequency. Drought indices are useful tools to quantify drought and can be used to analyze the frequency,severity,and spatial and temporal patterns of drought. Here we chose the standard precipitation index (SPI) and hydrological index (representing the meteorological and hydrological drought index respectively) to calculate meteorological and hydrological drought at monthly,seasonal and annual scales in the Upper Reaches of the Heihe River in northwestern China from 1968-2009. Meteorological data from four meteorological stations(Qilian,Yeniugou,Tuole and Sunan)were interpolated to the entire upper reaches by Kriging and Inverse Distance methods. Digital Elevation Model(DEM)data were used to delineate subbasins within the study area by geographical information system (GIS) to compare spatial patterns of the two drought indices. The meteorological drought index was calculated,then we calculated the hydrological drought index according to historical records and drought reflected by the meteorological drought index. Results show that the SPI and hydrological index are both sensitive enough to detect the severity and frequency of drought;it is feasible to use these indices to assess drought. The SPI and hydrological index are consistent with each other at annual and spatial scales. However,at the monthly scale,hydrological drought lags behind meteorological drought due to the time it takes for precipitation to transfer to runoff and water vapor through the hydrological process at the watershed scale.