The impact of elevation and flow dynamics on hydrological drought and wet spell characteristics in semi-arid southeast Arizona

Identification and understanding of the dominant control mechanisms of hydrological extremes has drawn worldwide attention in recent decades. However, detailed understanding of drought and wet spells within semi-arid regions has been hampered by the fact that identification is difficult for no flow conditions. Classification methods that have been developed for regions with perennial flow, do not work for ephemeral semi-arid rivers, while approaches for arid environments have difficulties to deal with seasonal runoff. Recently, a method was presented to identify hydrological extremes within semi-arid regions, by combining approaches developed for perennial flow and arid environments. However, this combined approach shows difficulties to identify drought and wet spells within semi-arid domains with a yearly precipitation cycle (e.g. monsoon). The current paper proposes to modify the combined method and make it suitable for these domains. The modified combined approach presented here to identify hydrological extremes was applied to decade-long discharge observations from 12 different locations within the San Pedro basin in southeastern Arizona. These locations correspond to catchments covering multiple elevation bands and runoff characteristics. Southern Arizona receives the majority of its rainfall from the summertime North American Monsoon (NAM), with frontal systems providing additional precipitation in winter. Using the modified method, the identified droughts and wet spells last longer compared to the previously defined combined procedure, and drought generally does not only start in spring at the end of the dry season. Furthermore, 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 catchments with no perennial flow. By increasing the flow averaging interval, the new method also enables to observe multi-year drought and wet spells patterns. For the precipitation limited semi-arid San Pedro basin results show that multi-year wet spell and drought are rare. This is caused by the strong impact of the summertime NAM that generally acts both as a start and reset button for both types of hydrological extremes.