Mountain runoff vulnerability to increased evapotranspiration with vegetation expansion

Abstract

Climate change has the potential to reduce surface-water supply by expanding the activity, density, or coverage of upland vegetation, although the likelihood and severity of this effect are poorly known. We quantified the extent to which vegetation and evapotranspiration (ET) are presently cold-limited in California's upper Kings River basin and used a space-for-time substitution to calculate the sensitivity of riverflow to vegetation expansion. We found that runoff is highly sensitive to vegetation migration; warming projected for 2100 could increase average basin-wide ET by 28% and decrease riverflow by 26%. Kings River basin ET currently peaks at midelevation and declines at higher elevation, creating a cold-limited zone above 2,400 m that is disproportionately important for runoff generation. Climate projections for 2085-2100 indicate as much as 4.1°C warming in California's Sierra Nevada, which would expand high rates of ET 700-m upslope if vegetation maintains its current correlation with temperature. Moreover, we observed that the relationship between basin-wide ET and temperature is similar across the entire western slope of California's Sierra Nevada, implying that the risk of increasing montane ET with warming is widespread.water resources | plant migration R oughly 4 billion people globally and 20 million people in the state of California rely on mountain runoff for freshwater, and there is growing concern these water resources will prove vulnerable to climate change (1-6). River flow (Q) is a function of precipitation (P) minus evapotranspiration (ET) (P−ET); increased montane ET with warming, either because of the direct effect of temperature on evaporative demand or the indirect effect of warming on vegetation density and distribution, would reduce Q (5, 7-9). However, hydrologic model projections for California's Sierra Nevada have discounted this possibility, indicating little or no effect of warming on annual . This result appears linked to two model assumptions: (i) models have often assumed the properties of montane vegetation will remain static, and (ii) models have often implicitly assumed that current annual montane ET is almost entirely limited by water availability and that warming will simply hasten the beginning and end of the growing season.Recent evidence calls both of these assumptions into question. Widespread increases in subalpine tree growth, tree-line altitude, and species distribution with elevation have been reported with recent climate trends in California and elsewhere, implying that rapid vegetation shifts are possible (14-16). Time series of Sierra Nevada forest greenness indicate a transition from water limitation at low elevation to cold limitation at high altitude, implying that upper elevation ET is sensitive to warming (17). Nonetheless, the extent to which annual montane ET is currently temperature-limited, as well as the sensitivity of large-scale ET to vegetation redistribution, remain largely unquantified.We used the upper Kings River basin in California's Sierra ...

Tracers Reveal Recharge Elevations, Groundwater Flow Paths and Travel Times on Mount Shasta, California

“…On the other hand, an increase in the area of barren land that is free of snow cover for an extended time period could lead to an increase in recharge, offsetting to some degree the loss of recharge due to the increase in forest ET. This study's finding that elevations above 2000 m are disproportionally important to groundwater recharge is consistent with observations in the Southern Sierra Critical Zone Observatory [10,42]. NDVI derived estimates of annual evapotranspiration are below annual PRISM precipitation suggesting evapotranspiration is not water limited (Figure 11a).…”

“…CIMIS, which uses weather data to determine average daily Eto for short grass, shows ET for this section of Mount Shasta ranging from 2.35 to 3.39 mm/day, or 858 to 1237 mm/year. As an alternative more appropriate for a mountainous region with conifer forests, we use the relationship described in Goulden and Bales (2014) [10] for watersheds across the Sierra Nevada, which they applied to estimate ET from normal difference vegetation index (NDVI) data (measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua satellite and averaged for snow-and cloud-free periods). ET is calculated using the following regression between annual average NDVI and annual ET [10]:…”

“…As the dominant region of snowpack storage in California, the hydrologic system of the Sierra Nevada Range of California has received considerably more attention [10][11][12] than the Cascade Range to the north [13]. Extensive granitic rocks in the Sierra Nevada batholith are suggestive of a 'teflon basin' where infiltration and groundwater storage are on the order of a few meters.…”

Recent Patterns in Climate, Vegetation, and Forest Water Use in California Montane Watersheds

“…Despite the first year of the drought occurring in 2012, Providence showed an increase in mean NDVI. This result could have been from greater potential ET from higher energy availability, as suggested with climate warming by Goulden and Bales [5], or from carryover water storage from the wettest year of 2011, as suggested by Safeeq and Hunsaker [19], or a combination of both. Garcia and Tague [54] also showed a high sensitivity of Sierra Nevada annual ET to plant available water capacity in subsurface storage, which would then influence the NDVI value.…”

Vegetation Regrowth Following Wildfires in the Santa Cruz Mountains of Northern California Monitored Using Landsat Satellite Image Analysis

“…Results from Landsat image studies have shown that canopy green leaf cover typically increases rapidly over the first five years following a standreplacing disturbance, doubling in value by about 10 years after the disturbance, and then leveling off to approach pre-disturbance (mature) stand values by about 25 -30 years after the disturbance event (Potter, 2014a). In addition, annual vegetation evapotranspiration (ET) flux in montane and sub-alpine communities of the western U. S. has been closely correlated with satellite NDVI (Goulden & Bales, 2014).…”