Abstract

Climate warming may cause mountain snowpacks to melt earlier, reducing summer streamflow and threatening water supplies and ecosystems. Few observations allow separating rain and snowmelt contributions to streamflow, so physically based models are needed for hydrological predictions and analyses. We develop an observational technique for detecting streamflow responses to snowmelt using incoming solar radiation and diel (daily) cycles of streamflow. We measure the 20th percentile of snowmelt days (DOS20), across 31 watersheds in the western US, as a proxy for the beginning of snowmelt-initiated streamflow. Historic DOS20 varies from mid-January to late May, with warmer sites having earlier and more intermittent snowmelt-mediated streamflow. Mean annual DOS20 strongly correlates with the dates of 25 % and 50 % annual streamflow volume (DOQ25 and DOQ50, both R2 = 0.85), suggesting that a one-day earlier DOS20 corresponds with a one-day earlier DOQ25 and 0.7-day earlier DOQ50. Empirical projections of future DOS20 (RCP8.5, late 21st century), using space-for-time substitution, show that DOS20 will occur 11 ± 4 days earlier per 1 °C of warming, and that colder places (mean November–February air temperature, TNDJF <−8 °C) are 70 % more sensitive to climate change on average than warmer places (TNDJF > 0 °C). Moreover, empirical space-for-time based projections of DOQ25 and DOQ50 are about four and two times more sensitive to earlier streamflow than those from NoahMP-WRF. Given the importance of changing streamflow timing for headwater resources, snowmelt detection methods such as DOS20 based on diel streamflow cycles may constrain hydrological models and improve hydrological predictions.

Highlights

  • The role of earlier snowmelt in driving earlier streamflow timing is of great concern in a changing climate (Barnett et al, 2005; Harpold and Brooks, 2018; Musselman et al, 2017; Stewart et al, 2004, 2005)

  • Mean annual DOS20 strongly correlates with the dates of 25% and 50% annual streamflow volume (DOQ25 and DOQ50, both R2 = 0.85), suggesting that a one-day earlier DOS20 corresponds with a one-day earlier DOQ25 and 0.7-day earlier DOQ50

  • The new DOS20 metric describes the timing of early snowmelt-mediated streamflow based on the diel streamflow signal and 275 suggests that shifts in snowmelt timing in colder, sunnier watersheds due to climate change have a greater effect on streamflow volume timing than in warmer, cloudier watersheds where snowmelt is more intermittent and more interspersed with rain

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Summary

Introduction

The role of earlier snowmelt in driving earlier streamflow timing is of great concern in a changing climate (Barnett et al, 2005; Harpold and Brooks, 2018; Musselman et al, 2017; Stewart et al, 2004, 2005). It remains difficult to predict how much streamflow timing and amount will shift in future climates due to altered 40 snow accumulation patterns (Mote et al, 2018), melt rates (Musselman et al, 2017), and shifts from snowfall to rainfall (Klos et al, 2014). Snowmelt-mediated spring streamflow timing is more sensitive to climate change in watersheds with rapid subsurface drainage than in landscapes with deep groundwater reservoirs that drain slowly (Safeeq et al, 2013). The sensitivity of snowmelt-mediated summer streamflow volume to climate change has shown to be higher in slow-draining watersheds (Tague and Grant, 2009). 60 The complexity of these storage relationships is exemplified by isotopic evidence showing that the fraction of streamflow that is "young water" (less than three months old) is smaller in steeper watersheds (Jasechko et al, 2016), suggesting that interactions between CZ water storage and changing hydrometeorology will be challenging to predict in mountainous areas

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