1. Forests and Water in the Sierra Nevada
Roger Bales,
Sierra Nevada Research Institute,
UC Merced

2. Some motivating points
Water is the highest-value ecosystem service associated with Sierra Nevada conifer forests
Precipitation & temperature trends are changing the timing & amount of runoff
Many second-growth forests have dense canopies & growing fire risks

3. Mountain water cycle & climate warmingoF
Warming by 2–6oC (4–11oF) drives significant changes:
rain-vs-snow storms *
snowpack amounts *
snowmelt timing *
flood risk
streamflow timing *
low baseflows
growing seasons *
recharge?
drier soil in summer
Precipitation changes uncertain
Already observed (*)
Land surface temperatures
5-yr average
Departure from mean 2 1 -1 -2 oF 15 11 7 4 -4 3

4. Sierra Nevada precipitation & snow water equivalent (SWE) – climatological estimatein
>80 70 50 35 25 15 10 in
>28 24 16 8 3

5. A lot of precipitation falling on dense forests never gets into the streams
Grass
~300 mm (1 ft)
L. Zhang, W. R. Dawes, and G. R. Walker Response of mean annual evapotranspiration to vegetation changes at catchment scale. WATER RESOURCES RESEARCH, VOL. 37, NO. 3, PAGES , MARCH 2001
Current forests in the upper catchments were once dominated by ‘fewer but bigger’ trees due mainly to fire. The canopy was more of a mix of trees and grass, rather than a dense canopy of trees at both the top canopy and smaller ladder fuel trees. Forests that are currently at risk for big wildfires also use about 300 mm (or 1 acre foot of water per acre) more than a dense forest canopy. Some of this ‘lost stream runoff’ would be released if forests were restored to a more more fire resilient state.
Currently every acre foot of water that runs through the full set of PCWA turbines generates about 2.8 MWh which would be worth ~$130 at the average wholesale electricity price over the past 5 years.
Every acre foot of water that runs through the full set of PCWA turbines generates about 2.8 MWh which is worth ~$130 (5 yr avg price)

6. Mountain water balance
precipitation
Myth:
We can, with a high degree of skill, estimate or predict the magnitude of these quantities
transpiration
evaporation
sublimation
runoff

7. Forest management – principles & assumptions
Produce different stand structures & densities across the landscape using topographic variables to guide varying treatments
Higher density & canopy cover for local cool or moist areas, w/ less-frequent or lower-severity fire, providing habitat for sensitive species
Low densities of large fire-resistant trees on southern-aspect slopes
Thinning based on crown strata or age cohorts & species, rather than uniform diameter limits
Such treatments can also enhance water yield & timing of runoff

8. How much snow gets to the ground & how fast does it melt?
3 scenarios for solar & infrared radiation
1. Dense canopy
2. Small gaps
3. Large gaps
Shortwave (solar) radiation
Canopy longwave (infrared) radiation
Lowest shortwave
Low shortwave
High shortwave
High longwave
Low longwave
Lower longwave

9. Snow depths in mixed-conifer forest
D J F M A M in 59 39 20
2009
Snow depth under canopy only about half to two thirds of that in the open
Differences of about 40 cm (16 in)
Mean & standard deviation of snow depth over 6-mo period, Southern Sierra Critical Zone Observatory

10. Sierra Nevada long-term average water yieldft
3.3
2.6
2.0
1.3
0.7
In order to verify the impact of forest management, need to accurately estimate the precipitation, discharge & evapotranspiration
General N – S decrease
Decreasing precipitation
Increasing snow

11. A closer look at water yield: 8 KREW instrumented headwater catchments
Providence
1800 m
2400 m
Mean elevation range
Rain + snow  snow
6000 ft
8000 ft
Bull

12. Increase in water yield w/ elevation, from rain to snow dominated
Precip, mm inch in 47 39 31 24 16 8
Increase in water yield w/ elevation, from rain to snow dominated
Kings River basin
6000’
7000’
8000’
Decreasing temperature
Increasing snow fraction
Decreasing vegetation
Coarser soils
Hunsaker et al., JAWRA 2012

13. 50% more runoff in snow dominated vs. mixed rain-snow catchments 5oF
3oC
0.1 increase per 350 m (1150 ft)
50% more runoff in snow dominated vs. mixed rain-snow catchments
5oF
Implication for 2oC warmer climate:
Reduce runoff by 10-40% in mixed conifer forest (assuming ecosystems adapt)
6000’
7000’
8000’
Decreasing temperature
Increasing snow fraction
Decreasing vegetation
Coarser soils

14. The effect of snowpack storage on runoff timing
Cumulative over one water year
O N D J F M A M J J A S
In this rain-snow transition catchment, stream discharge lags precipitation by about 2 months
This lag is expected to decrease by about 1-2 weeks per 1oC (2oF) of warming
How forest management will affect the lag depends on how the energy balance changes
134
150
197
KREW/CZO, P-301 headwater catchment

15. Sierra Nevada research infrastructure –
evapotranspiration measurements
CZO
N-S transect of research catchments
MODIS image
Main CZO site
600
1200
1800
2400
3000
Elev., m
San Joaquin Experimental Range
400 m
1300 ft
Shorthair Creek
2700 m
8900 ft
CZO P301
2000 m
6600 ft
Soaproot Saddle
1100 m
3600 ft
E-W transect of flux towers
Southern Sierra Critical Zone Observatory

16. Annual evapotranspiration
mm in
/ / /
Elevation, m/ft
Annual evapotranspiration
Highest current evapotranspiration in rain to rain-snow transition region of mixed conifer forest – year-round growth
Lower elevation is water limited
Higher elevation is cold limited
Goulden et al., submitted

17. Hydrologic research in progress – American River
Sierra Nevada Adaptive Management Project (SNAMP)
Two instrumented headwater catchments in Forest Hill/Duncan Peak area
Sierra Nevada Framework treatments
Sierra Nevada Watershed Ecosystem Enhancement Project (SWEEP)
Phase 2 research to develop treatments & project effects
Phase 3 to carry out & evaluate treatments
Additional phase 2 planning needed
American River basin hydrologic observatory
National Science Foundation (NSF) supported infrastructure
CA-DWR supported infrastructure

18. A new generation of integrated measurements
lidar
eddy correlation
embedded sensor networks
isotopes & ions
satellite snowcover
sap flow
low-cost sensors
sediment

19. Basin-wide deployment of hydrologic instrument clusters – American R
Basin-wide deployment of hydrologic instrument clusters – American R. basin
Strategically place low-cost sensors to get spatial estimates of snowcover, soil moisture & other water-balance components
Network & integrate these sensors into a single spatial instrument for water-balance measurements.

20. Building the knowledge base to enhance forest & water management