Water Yield in Thinned Stands: Observations from the Sierra Nevada

Transcription

1 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

2 Hydrologic context Extremes are part of California s climate Extremes in the past yr may not be a sufficient guide to the future Mar 27, 2010 Mar 29, 2015 Apr 3, 2017

3 Basic water balance Precipitation = Evapotranspiration + Runoff + ΔStorage = + snow & rain Evapotranspiration refers to evaporation, sublimation plus water use by vegetation

4 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

5 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)

6 Topics in this talk Sierra Nevada hydrology Estimating evapotranspiration Drought impacts Thinning impacts Wildfire impacts

7 Elev., m 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)

8 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

9 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

10 Topics in this talk Sierra Nevada hydrology Estimating evapotranspiration Drought impacts Thinning impacts Wildfire impacts Photo: Margot Wholey Dec 14, 2015

11 Century-long experiment: suppressing fire 1896 Kyburz, S. Fork American R., We now know this was a bad idea; & sustainable solutions remain elusive Photos from G. Gruell

12 The recent experiment: drought Widespread drought-induced mortality in rain zone of Southern Sierra pine-oak forest Photo: Margot Wholey Dec 14, 2015

13 Kings R. basin water balance by elevation P = ET + Q - ΔS Before drought During drought Bales et al., submitted

14 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

15 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

16 Study locations NDVI & ET changes associated with forest treatments & fire? American Last Chance Blodgett Forest Kings Stanislaus Sugar Pine Choke Fire Fire perimeters Forest treatments J. Roche, 2017

17 Stanislaus-Tuolumne Experimental Forest: Variable Thinning Project, 2011 Control Even thin Variable thin 1 km Landsat ΔNDVI J. Roche,

18 Stanislaus NDVI 10 3 mm Basal-area decrease = 40-50% 1 km Control Even thin Variable thin mm/yr Control Even thin Variable thin Year ET, mm yr -1 Mean water-year precipitation J. Roche,

19 Summary of forest treatments Treatment Area % change basal area NDVI change Blodgett Moderate = Intensive = -45 to 68% to ET change (mm/yr) to -310 Stanislaus -45 to -47% to to -200 Last Chance Mastication = -7% Tractor thin = -9% Sugar Pine Mastication = -11% Tractor thin = -15% Bold indicates significant change Restoration treatments: significant change in central Sierra Light treatments: observed change in central, but not southern Sierra J. Roche, 2017

20 Net impacts could approach 10% of full natural flow Intensive thinning reducing basal area 40-50% corresponds to mm/yr reduced ET Impacts of thinning reduced by water limitation & recovery rate 20

21 Topics in this talk Sierra Nevada hydrology Estimating evapotranspiration Drought impacts Thinning impacts Wildfire impacts

22 Choke Fire, 1997 e) Burn Severity* Very low Low Moderate High 1 km Landsat ΔNDVI *Data source: Monitoring Trends in Burn Severity 1 km

23 Choke fire, 1997 NDVI Burned Unburned ET, mm yr -1 1 km Year J. Roche, 2017

24 Choke fire, 1997 NDVI 1 km Burned Unburned Year % basal-area 0.2 reduction 210 ET, mm yr -1 Fire-caused basal-area data: Miller and Quayle, 2015 J. Roche, 2017

25 Basal-area & ET reduction due to fire Whole basin 800 American ET, mm yr Pre-fire 0-25% 25-50% 50-75% % Unburned Elevation, m Kings 600 J. Roche, Elevation, m Years relative to fire year

26 American Kings Average net ET reduction, mm/yr Cumulative area burned, 1000 ha Year American: fire return interval < 20 years (95%) ~5% of mean annual flow (10% for drought years) 0 J. Roche, 2017

27 Net ET reduction, 10 6 m 3 Net ET reduction, Net ET r Cumu 100 Potential whole-basin increase in runoff from wildfires 0 70 b) 60 American Kings acre-feet yr -1 Year J. Roche, 2017

28 How can we increase the pace & scale of forest restoration? Monetizing downstream benefits of changing the water balance

29 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