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