Efficiency of the Summer Monsoon in Generating Streamflow Within a Snow‐Dominated Headwater Basin of the Colorado River

Abstract

 Monsoon rains generate 10±6% of annual streamflow while late spring snowfall delivers twice as much for the same water input.  The influence of monsoon rain on streamflow is half of late spring snow due to evapotranspiration in the lower subalpine forest.  Monsoon efficiency in generating streamflow decreases in warm years with low snow accumulation.

Variability of Snow and Rainfall Partitioning Into Evapotranspiration and Summer Runoff Across Nine Mountainous Catchments

“…Specifically for the East River, our findings corroborate hydrologic modeling results that showed that most rain gets evapotranspired, and little rain becomes runoff to the East River (Carroll et al., 2020). Our estimate of ∼7% of total Q stemming from rain for the East River at Pumphouse is within the range of modeled estimates (∼10%, Carroll et al., 2020). Our findings support ET source water estimates from particle tracking simulations, which indicated that ET of a forested hillslope in the East River catchment was to large parts sourced by snowmelt (Maxwell et al., 2019).…”

“…That our results are at the upper limit is due to the relatively high P S / P R ratio in our high‐elevation study area compared to other regions in the western US (Li et al., 2017). Specifically for the East River, our findings corroborate hydrologic modeling results that showed that most rain gets evapotranspired, and little rain becomes runoff to the East River (Carroll et al., 2020). Our estimate of ∼7% of total Q stemming from rain for the East River at Pumphouse is within the range of modeled estimates (∼10%, Carroll et al., 2020).…”

“…We found that as ET magnitude increases, a greater percentage of that ET (significant for 2015, 2016, 2019, 2020, Figure 3a) and thus more ET flux volume (Figure S15a in Supporting Information ) was sourced from snow. We explain this result by belowground snowmelt storage subsiding to supporting ET during water limited conditions in the summer that occur in subalpine and montane elevations (Carroll et al., 2020). This dependency of ET on snowmelt contributions was further supported by the observed trend that in catchments with more snowfall, more of the snow became ET during low‐snow years (2015, 2016, 2018, Figure 3b).…”

Wetter Summers Mitigated Temperature Stress on Rocky Mountain Forests During the Last Interglacial Warm Period

“…While we can only qualitatively constrain the increased use of summer rain, this result highlights how access to this additional moisture source could have acted to mitigate stress even though snowpack was still the principal water source. The importance of summer rain outside of the core monsoon zone is often overlooked because it is snowmelt that predominantly recharges the region's aquifers and drives streamflow (Carroll et al, 2020). If monsoon circulation and summer rains increase in response to warming, as has occurred during other warm periods, then the results suggest that forest ecosystems in this region could be resilient to moisture stress associated with warming.…”

A Subseasonal Regime Approach for Assessing Intra-annual Variability of Evapotranspiration and Application to the Upper Colorado River Basin

“…During drought years, additional precipitation (especially monsoon) plays a vital role in restoring ET and compensating for the loss of ET during the preceding dry periods. Carroll et al (2020) found that drought conditions modify monsoon precipitation's contributions to ET and streamflow. Their study suggested that monsoon precipitation is mainly effective in moistening dry soil and sustaining ET at lower subalpine forests during dry years, while being the primary contributor to streamflow generation under normal/wet year conditions.…”