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Water & Sierra Nevada forestsWhat do we know & what do we not
know?
1. Introduction: Montane water balances
2. Background: Forests and snow3. SSCZO
Martha Conklin,Sierra Nevada
Research Institute, UC Merced
Collaborators: R Bales, UC Merced; S. Glaser, UC Berkeley; & many others
Photo courtesy SSCZO
1. Introduction: Montane Water Balances & Forests
Image by Jenny Parks, courtesy SSCZO
Basic annual montane water balance
Precipitation = Evapotranspiration + Runoff
= +
(Evapotranspiration is mainly water use by vegetation)
MODIS-derived map of Sierra Nevadasnow cover, courtesy Robert Rice
Photos courtesy SSCZO
What elevations provide the most snowmelt?
Fraction of annual snowmelt by elevation band in Merced River basin.From Rice & Bales, 2013.
9000
7000
6000
10000
5000
12000
11000
8000
feet
Snowpack loss & water storage: 30-yr horizon for the Sierra Nevada
13.5 MAF
11 MAF
14 MAF
Likely loss of ~3.5 MAF of snowpack storage in next 1-3 decades
Sacramento Valley storage San Joaquin Valley storage
snowpack annual storage
MAF: million acre feet Storage data from CA Department of Water Resources
2. Background: Trees & snow accumulation
Image by Jenny Parks, courtesy SSCZO
Trees block low-angle winter sun, retarding snowmelt …
… but intercept snowfall, some of which sublimates (< 20%) …
… and emit longwave radiation that melts snow(see tree wells) …
Tree density affects snow accumulation
Photo courtesy SSCZO
Thinned unit w/ control in backgroundStanislaus-Tuolumne Experimental Forest
Photo by Eric Knapp, USFS
Measuring forest effects on snow accumulation
1200 measurements
3. UC Research: Forest Evapotranspiration & Runoff
Image by Jenny Parks, courtesy SSCZO
University of California catchment field sites
Yosemite NPSNAMP
American R & SNAMP
MODIS image
6001200180024003000Elev., m
San Joaquin Experimental
Range400 m
Shorthair Creek
2700 mCZO P301
2000 m
Soaproot Saddle1100 m
E-W transect of flux towers
CZO sitesFigure courtesy SSCZO
SSCZO conceptual model
Figure courtesy SSCZO. Pore-to-plot and catena illustrations by Jenny Parks.
3 headwater catchments w/ stream gauges & water-quality measurements
2 met stations60-m tall flux tower60-node wireless embedded
sensor network214 EC-TM sensors for
volumetric water content113 MPS sensors for matric
potential57 snow-depth sensorsMeadow piezometers & wellsSap-flow sensors
Providence Ck (2100 m) – instrumentation
Figure courtesy SSCZO
Drilling, deeper wells, additional geophysics
in progressPhotos courtesy SSCZO
Seismic survey results at Providence
Evapotranspiration (ET) across a Southern Sierraelevation transect
Mid-elevation forests show neither summer nor winter shutdown: ̶Q deep rooting & resiliency to moisture stress ̶Q warmer canopy-level temperatures despite snow
From Goulden et al., 2012
ET, ft
per
yea
r
1
2
3
Oak savannah
Mixed conifer
Red fir
Elevation, ft0 3000 6700 10,000
Winter dormancy
Summer moisture
deficit
Sweet spot for mixed conifer
Photo courtesy SSCZO
Jun Jul Aug Sep
Monthly dry season evapotranspiration at mixed conifer site (Providence Creek)
~7000 ft elev.
After Goulden et al., 2012, courtesy Roger Bales
Jun Jul Aug Sep
Monthly dry season evapotranspiration at mixed conifer site (Providence Creek)
~7000 ft elev.
The trees are moisture stressed & are vulnerable to insects, disease & fire starting in 2012
After Goulden et al., 2012, courtesy Roger Bales
Precip based on
PRISMRunoff by difference
Fraction of runoff by elevation band
From Goulden & Bales, 2014
Rain-snow transition
ET extended using satellite
indices
Kings River Basin: Precip = Evapotranspiration + Runoff
What we know: science 1. Vegetation removal generally results in more runoff, initially2. Vegetation regrowth means less runoff3. Clear cutting or wildfire means more sublimation & earlier
snowmelt – runoff could go up or down4. Less-dense forests (up to a point) can retain snow longer and
are more fire-resistant.5. Colder, snow-dominated areas produce more runoff that
lower, rain-dominated areasPath forward: Water Security1. Sustained forest management that provides measurable
benefits for water supply & forest health will require investment, verification, & maintenance
2. Better information is a critical foundation for water security, especially in a warming & more-variable climate
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