Abstract
Abstract. Multi-year droughts in Mediterranean climates may shift the water balance, that is, the partitioning rule of precipitation across runoff, evapotranspiration, and sub-surface storage. Mechanisms causing these shifts remain largely unknown and are not well represented in hydrologic models. Focusing on measurements from the headwaters of California's Feather River, we found that also in these mixed rain–snow Mediterranean basins a lower fraction of precipitation was partitioned to runoff during multi-year droughts compared to non-drought years. This shift in the precipitation–runoff relationship was larger in the surface-runoff-dominated than subsurface-flow-dominated headwaters (−39 % vs. −18 % decline of runoff, respectively, for a representative precipitation amount). The predictive skill of the Precipitation Runoff Modeling System (PRMS) hydrologic model in these basins decreased during droughts, with evapotranspiration (ET) being the only water-balance component besides runoff for which the drop in predictive skill during drought vs. non-drought years was statistically significant. In particular, the model underestimated the response time required by ET to adjust to interannual climate variability, which we define as climate elasticity of ET. Differences between simulated and data-driven estimates of ET were well correlated with accompanying data-driven estimates of changes in sub-surface storage (ΔS, r=0.78). This correlation points to shifts in precipitation–runoff relationships being evidence of a hysteretic response of the water budget to climate elasticity of ET during and after multi-year droughts. This hysteresis is caused by carryover storage offsetting precipitation deficit during the initial drought period, followed by vegetation mortality when storage is depleted and subsequent post-drought vegetation expansion. Our results point to a general improvement in hydrologic predictions across drought and recovery cycles by including the climate elasticity of ET and better accounting for actual subsurface water storage in not only soil, but also deeper regolith that stores water accessible to roots. This can be done by explicitly parametrizing carryover storage and feedback mechanisms capturing vegetation response to atmospheric demand for moisture.
Highlights
Regions with a Mediterranean climate receive the bulk of precipitation during winter, while summers are dry (Klos et al, 2018)
By showing that ET is the only waterbalance component yielding statistically different performances during drought vs. non-drought years for a hydrologic model (PRMS), we quantitatively demonstrated that ET–drought feedback mechanisms are the likely driver of these shifts in water supply
To test the sensitivity of western slope Sierra Nevada rivers to shifts in precipitation–runoff relationships in response to droughts, we extended our analysis to the 12 other major rivers
Summary
Regions with a Mediterranean climate receive the bulk of precipitation during winter, while summers are dry (Klos et al, 2018). This seasonal imbalance in precipitation distribution, coupled with asynchronicity between precipitation input and potential-evapotranspiration demand (Fellows and Goulden, 2017; Rungee et al, 2018; Feng et al, 2019), complicates understanding and management of multi-year droughts and their impact on water supply (He et al, 2017). Water supply is the output of a water balance, that is, Q = P − ET − S, where Q is runoff, P is precipitation, ET is evapotranspiration, and S is the change in storage (in this paper, it is assumed that S is predominantly dictated by sub-surface storage). Avanzi et al.: Evapotranspiration feedbacks shift water supply during droughts pact of droughts on the water balance of Mediterranean climates is relevant to ecosystem services and water security (Bales et al, 2018), especially because droughts are likely to be more persistent than other water risks (He et al, 2017; Rungee et al, 2018) and increase in frequency and geographic extent under a warming climate (Cayan et al, 2010; Woodhouse et al, 2010)
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