Drought is one of the most severe stress factors affecting terrestrial ecosystems at regional and global scales, and is also one of the most serious disasters affecting human society. Drought indices are quantitative indicators for evaluating drought. But different drought indices lead to great uncertainties when evaluating spatial and temporal variations in drought characteristics (e.g., drought event frequency and intensity), mainly due to the differences in time scales and climatic factors (temperature, precipitation, etc.) these indices considered. The uncertainty is even more pronounced in the study of large-scale areas such as China which has complex topography and weather conditions. Therefore, how to select the most effective drought index in light of specific region, climate and time scale has become a key issue that needs to be solved in drought assessment. 

In view of this problem, assistant Prof. Ma Tianxiao, Prof. Liang Yu and their colleagues from the Landscape Process Group of the Institute of Applied Ecology, Chinese Academy of Sciences, used the data collected from 695 meteorological stations in China from 1970 to 2010 for drought assessment. By using 6 different drought indices, they evaluated the trend of drought frequency in different regions of China, discussed uncertainty in drought assessment, and quantified the impacts of climate variable (daily precipitation, maximum surface temperature, minimum surface temperature, average surface temperature, wind speed, sunshine hours, relative humidity, atmospheric pressure, and altitude), time scale (3, 12, and 24 months) and their interaction on the drought assessment results. 

The researchers found that compared to climate variable, time scale has less influence on drought assessment results, but their interaction cannot be ignored. Climate and time scale have more obvious influences on drought assessment results in more arid or humid regions compared to less arid or humid regions. The drought assessment results are more affected by time scale in the Inner Mongolia grassland area and Northeast China compared to other regions, suggesting that one needs to pay more attention to the choice of time scale when conducting drought assessment in these two regions using the drought index method. 

The above findings may guide the selection of a suitable drought index, which can help drought early warning and drought prevention and mitigation. 

This study was funded by the National Natural Science Foundation of China and the Ministry of Ecology and Environment Major Project for Biodiversity Conservation. The results were published in International Journal of Climatology, entitled "Assessing the effects of climate variable and timescale selection on uncertainties in dryness/wetness trends in conterminous China."