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

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

Mountain water cycle & climate warming oF 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 1901-2000 mean 2 1 -1 -2 oF 15 11 7 4 -4 3

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

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. 2001.Response of mean annual evapotranspiration to vegetation changes at catchment scale. WATER RESOURCES RESEARCH, VOL. 37, NO. 3, PAGES 701-708, 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)

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

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

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

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

Sierra Nevada long-term average water yield ft 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

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

Increase in water yield w/ elevation, from rain to snow dominated 950 37 1800 71 1950 77 750 30 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

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

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

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

Annual evapotranspiration mm in 800 32 700 28 600 24 500 20 400 16 300 12 0 1000/ 2000/ 3000/ 3300 6600 10000 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

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

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

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.

Building the knowledge base to enhance forest & water management