The Impact of Elevation and Baseflow on Long-Term Hydrological Drought and Wet Spell Characteristics in a Semi-Arid Basin in Southeast Arizona

Hydrological drought and wet spell identification has drawn worldwide attention in recent decades. It is hard to identify drought in semi-arid regions where runoff is zero for significant periods of time. Classical methods like the variable threshold level method work well in regions with perennial runoff but does not work for ephemeral rivers within semi-arid environments. Recently, a method was developed to specifically identify hydrological extremes within semi-arid regions by combining the classical threshold level method and cumulative dry period method. However, this approach does not seem to work for regions that yearly are impacted by a strong yearly precipitation season (e.g. Monsoon). In this paper, we therefore modified the combined method to make it suitable for these transitional domains. The new approach was applied to decade-long streamflow observations at several stations in the San Pedro river basin in southern Arizona, for catchments covering multiple elevation domains and runoff characteristics, and compared it to previous methods. The new method shows substantially more suitable and realistic drought identification.

Using our new method, we investigated the pattern of droughts and wet spells in southern Arizona, which is under strong influence of the summertime North American Monsoon (NAM). Results show that if a drought or wet spell starts during the NAM or post-NAM season, it will generally last longer as compared to one that starts in winter or spring. This specifically holds for watershed with no perennial flow. By increasing the flow averaging interval, the new method also allows us to observe multi-year drought and wet spells patterns. These results show that for the San Pedro, multi-year wet spell and drought are rare, which can be related to the strong impact of the NAM.