Abstract
Abstract. An inconsistent relationship between precipitation and runoff has been observed between drought and non-drought periods, with less runoff usually observed during droughts than would be expected based solely on precipitation deficit. Predictability of these shifts in the precipitation–runoff relationship is still challenging, largely because the underlying hydrologic mechanisms are poorly constrained. Using 30 years of data for 14 basins in California, we show how the Budyko framework can be leveraged to decompose shifts in precipitation versus runoff during droughts into “regime” shifts, which result from changes in the aridity index along the same Budyko curve, and “partitioning shifts”, which imply a change in the Budyko parameter ω and thus in the relationship among water balance components that governs partitioning of available water. Regime shifts are primarily due to measurable interannual changes in precipitation or temperature, making them predictable based on drought conditions. Partitioning shifts involve further nonlinear and indirect catchment feedbacks to drought conditions and are thus harder to predict a priori. We show that regime shifts dominate changes in absolute runoff during droughts but that gains or losses due to partitioning shifts are still significant. Low aridity, high baseflow, a shift from snow to rain, and resilience of high-elevation runoff correlate with higher annual runoff during droughts than would be predicted by the precipitation–runoff ratio during non-drought years. Differentiating between these shifts in the precipitation–runoff relationship using a Budyko approach will help water resource managers, particularly in arid, drought-prone regions, to better project runoff magnitudes during droughts based on available climate data and, furthermore, understand under what circumstances and to what extent their forecasts may be less reliable due to nonlinear basin–climate feedbacks.
Highlights
Droughts can threaten human and natural systems worldwide, accounting for more than 50 % of all natural hazard deaths over the course of the 20th and early 21st centuries (Van Loon, 2015; Maskey and Trambauer, 2015)
The shifts in the partitioning of available water can be related to feedback mechanisms between climatic conditions and catchment characteristics that either exacerbate or mitigate drought (Bales et al, 2018; Teuling et al, 2013; Avanzi et al, 2020). (In this study, “exacerbation” and “mitigation” are used with respect to runoff.) We examined four basin characteristics and responses to drought that may relate to observed shifts in the precipitation–runoff relationship
Average maximum daily temperature shows no significant change between droughts in the northern basins, but droughts in the central and southern Sierra basins are progressively warmer
Summary
Droughts can threaten human and natural systems worldwide, accounting for more than 50 % of all natural hazard deaths over the course of the 20th and early 21st centuries (Van Loon, 2015; Maskey and Trambauer, 2015). The need to adequately understand and predict the water balance implications of droughts is becoming more acute as climate change makes basins susceptible to more severe and prolonged droughts (Dai, 2013; Trenberth et al, 2014; Woodhouse et al, 2010). Chief among these water balance implications is the relationship between precipitation and runoff, which has been shown to be impacted inconsistently by drought depending on the basin.
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