Water Yield in Thinned Stands: Observations from the Sierra Nevada Roger Bales, UC Merced Topics in this talk Sierra Nevada hydrology Estimating evapotranspiration Drought impacts Thinning impacts Wildfire impacts Photo: Margot Wholey Dec 14, 2015
Hydrologic context Extremes are part of California s climate Extremes in the past 100-1000 yr may not be a sufficient guide to the future https://earthdata.nasa.gov/labs/worldview Mar 27, 2010 Mar 29, 2015 Apr 3, 2017
Basic water balance Precipitation = Evapotranspiration + Runoff + ΔStorage = + snow & rain Evapotranspiration refers to evaporation, sublimation plus water use by vegetation
Calif. water sources We can manage evapotranspiration on a basin-wide basis Sierra Nevada N More precipitation & runoff north of Delta More water use south of Delta CA Water Plan, 2013
Applied water use Precip: 200 MAF Applied: 80 MAF Data from DWR, adapted from Nor. Cal. Water Assn. Water supplies: Agriculture: 80% (33 MAF) Urban 20% (8 MAF)
Topics in this talk Sierra Nevada hydrology Estimating evapotranspiration Drought impacts Thinning impacts Wildfire impacts
Elev., m 3000 2400 1800 1200 600 San Joaquin Experimental Range 400 m Field research measurements E-W transect of flux towers Soaproot Saddle 1100 m CZO Providence 2000 m Shorthair Creek 2700 m MODIS image Merced Southern Sierra Critical Zone Observatory Ground measurements of precipitation, evapotranspiration, discharge, soil-moisture storage, snowpack storage (Shorthair not available)
Scaling evapotranspiration (ET) Annual ET measured by flux towers, correlated with satellite NDVI (greenness) Conceptual High LAI Feedback over a few yr High ET & NPP R 2 = 0.9 High LAI High NDVI NDVI indicates ET needed to support the current LAI ET calculated across the southern Sierra using this calibration Goulden & Bales, 2014
Scaling ET across Kings R. basin Mixed conifer High Sierra Oak Significant advances in mesurements in past 5-10 yr can better inform decision making Evapotranspiration across the southern Sierra using ET-NDVI correlation from flux towers Goulden & Bales, 2014
Topics in this talk Sierra Nevada hydrology Estimating evapotranspiration Drought impacts Thinning impacts Wildfire impacts Photo: Margot Wholey Dec 14, 2015
Century-long experiment: suppressing fire 1896 Kyburz, S. Fork American R., 5000 1993 We now know this was a bad idea; & sustainable solutions remain elusive Photos from G. Gruell
The recent experiment: 2011-2015 drought Widespread drought-induced mortality in rain zone of Southern Sierra pine-oak forest Photo: Margot Wholey Dec 14, 2015
Kings R. basin water balance by elevation P = ET + Q - ΔS Before drought During drought Bales et al., submitted
Interpretation Parts of the Southern Sierra forest reached a tipping point Multi-year subsurface storage critical for drought resiliency in headwater forests Forest densities are higher than can be sustained through historical droughts in a warmer climate
Management response: restore (thin) forest reduce ET Topics in this talk Sierra Nevada hydrology Estimating evapotranspiration Drought impacts Thinning impacts Wildfire impacts E. Knapp photo Thinned unit w/ control in background
Study locations NDVI & ET changes associated with forest treatments & fire? American Last Chance Blodgett Forest Kings Stanislaus Sugar Pine Choke Fire Fire perimeters 1990-2008 Forest treatments J. Roche, 2017
Stanislaus-Tuolumne Experimental Forest: Variable Thinning Project, 2011 Control Even thin Variable thin 1 km Landsat ΔNDVI J. Roche, 2017-0.4 +0.2 17
Stanislaus NDVI 10 3 mm Basal-area decrease = 40-50% 1 km 0.8 950 Control Even thin Variable thin 150-200 mm/yr 0.7 0.6 0.5 Control Even thin Variable thin 0.4 1985 2.0 1995 Year 2005 2015 1.0 570 340 ET, mm yr -1 Mean water-year precipitation J. Roche, 2017 1985 1995 2005 2015
Summary of forest treatments Treatment Area % change basal area NDVI change Blodgett Moderate = Intensive = -45 to 68% -0.007-0.11 to -0.18 ET change (mm/yr) +1-220 to -310 Stanislaus -45 to -47% -0.09 to -0.11-150 to -200 Last Chance Mastication = -7% Tractor thin = -9% Sugar Pine Mastication = -11% Tractor thin = -15% -0.026-0.068-0.006-0.015 Bold indicates significant change -40-100 -10-40 Restoration treatments: significant change in central Sierra Light treatments: observed change in central, but not southern Sierra J. Roche, 2017
Net impacts could approach 10% of full natural flow Intensive thinning reducing basal area 40-50% corresponds to 150-200 mm/yr reduced ET Impacts of thinning reduced by water limitation & recovery rate 20
Topics in this talk Sierra Nevada hydrology Estimating evapotranspiration Drought impacts Thinning impacts Wildfire impacts
Choke Fire, 1997 e) Burn Severity* Very low Low Moderate High 1 km Landsat ΔNDVI *Data source: Monitoring Trends in Burn Severity http://www.mtbs.gov/ 1 km -0.6 +2.0
Choke fire, 1997 NDVI 0.8 950 0.6 0.4 Burned Unburned 570 340 ET, mm yr -1 1 km 0.2 210 1985 1995 2005 2015 Year J. Roche, 2017
Choke fire, 1997 NDVI 1 km 0.8 950 0.6 Burned Unburned 1985 1995 2005 2015 Year 570 0.4 340 50-75% basal-area 0.2 reduction 210 ET, mm yr -1 Fire-caused basal-area data: Miller and Quayle, 2015 J. Roche, 2017
Basal-area & ET reduction due to fire Whole basin 800 American 600 400 ET, mm yr -1 200 Pre-fire 0-25% 25-50% 50-75% 75-100% Unburned 0 800 0 1000 2000 3000 Elevation, m Kings 600 J. Roche, 2017 400 200 0 0 1000 2000 3000-5 0 +5 +10 Elevation, m Years relative to fire year
400 300 American Kings 40 30 Average net ET reduction, mm/yr 200 20 Cumulative area burned, 1000 ha 100 10 0 1990 1995 2000 2005 2010 Year American: fire return interval < 20 years (95%) ~5% of mean annual flow (10% for drought years) 0 J. Roche, 2017
Net ET reduction, 10 6 m 3 Net ET reduction, Net ET r Cumu 100 Potential whole-basin increase in runoff from 1990-2008 wildfires 0 70 b) 60 American Kings 50 40 30 20 10 0 1990 1995 2000 2005 2010 10 0 50 40 30 20 10 0 10 3 acre-feet yr -1 Year J. Roche, 2017
How can we increase the pace & scale of forest restoration? Monetizing downstream benefits of changing the water balance
Forest restoration is part of adapting to a changing climate Forest biomass changes result in real changes in evapotranspiration & runoff Treatments may need to be more frequent than in the past Those who benefit from forest services can be partners in restoration NSF Southern Sierra Critical Zone Observatory