Potential increase in in ’s Sierra under future climate projections

Tapash Das Scripps Institution of Oceanography, UCSD Mike Dettinger USGS, Scripps Institution of Oceanography Dan Cayan Scripps Institution of Oceanography, USGS

Hugo Hidalgo University of Costa Rica

Acknowledgements: California Water and Environmental Modeling forum CALFED Bay-Delta program Annual Meeting 2010 California Energy Commission Research Objectives

Should we be expecting increased risks in California under climate change?

. Potential for intensification or amelioration of flood magnitudes? . Potential for increased, or decreased frequencies of floods? . Potential for changing seasonalities and mechanisms of floods? Study domain: California’s

Northern Sierra

46,080 Km2 Southern Sierra

Northern Sierra -Snow fed 15,120 Km2

Southern Sierra Snow dominated

Hydrographs drawn from VIC simulated streamflow forced by observed meteorology (Hamlet and Lettenmaier, 2005, J. Hydrometeorology), 1950-1999 Datasets, Models, Methodology

DATA, MODEL METHODS . Compute flood discharge .Hydrologic model: . Fitting 3-day annual Variable Infiltration Capacity (VIC) maximum streamflows to

Liang et al. (1994), JGR Log Pearson III distribution .Future climate projections: . Compute flood events Statistically downscaled IPCC AR4 . Frequencies of floods Hidalgo et al. (2009), J. Climate greater than historical threshold Projected Changes in Annual Temperature

Northern Sierra

Southern Northern Sierra GFDL Sierra CNRM PCM1

Increase of annual temperature of about 0.5oC to 1.0oC by 2049 About 2.0oC to 3.0oC by 2099

GFDL model is the Southern Sierra warmest, followed by CNRM

PCM1 projects moderate warming Projected Changes in Annual Precipitation

Northern Sierra CNRM GFDL Northern Sierra Southern PCM1 Sierra

GFDL model projects overall drying

CNRM projects wetter climate in the early 21st century, Southern Sierra but shows near neutral or dry climate by 2099

PCM1 projects moderate increase of precipitation Projected Changes in 3-day Annual Precipitation

Northern Northern Sierra Southern Sierra Sierra

Southern Sierra

Simulation suggests an increase of 3-day precipitation among the highest values (longer return intervals), except in PCM1 Projected Changes in extreme precipitation events

Northern Frequency of 99%-ile Sierra precipitation events Southern Sierra Extreme precipitation events Northern Sierra CNRM accumulated over winter GFDL PCM1

2001- 2049 2051- 1951- 2099 1999

Southern Sierra Climate model suggests more frequent winter (NDJF) extreme precipitation In general, there is an events increase in the extreme precipitation events, except for GFDL for SSN Projected Changes in 3-day Annual Streamflow

Northern Northern Sierra Southern Sierra Sierra

Southern Sierra

VIC simulation by downscaled climate model forcing suggests an increase of 3-day maximum annual streamflow for the longer return period (largest floods), except in PCM1 for 2001-2049 Projected Changes in 3-day Annual Streamflow (example from CNRM CM3)

Northern Sierra

Southern Sierra

90%-ile computed from 1951-1999 n = 5 n = 9 n = 12

Some of the simulated 3-day maximum flows are unprecedentedly large (under the climate n = 5 n = 7 n = 8 projections), as compared to model historical period Projected Changes in flood magnitudes (example from CNRM CM3)

Northern Sierra

Southern Sierra Northern Sierra Southern Sierra

2001-2049 + flood magnitude 3-day annual with 20-years return period maximum 1951-1999 streamflows are fitted to Log Pearson III distribution

2051-2099 flood frequency curves from 1951-1999 (blue curves) are compared 1951-1999 with the same from 2001- 2049 and 2051-2099 (red curves) periods

Frequency factor, K Projected Changes in flood magnitudes (comparing 2051-2099 flood magnitudes with historical period 1951-1999)

Northern Northern Sierra Southern Sierra Sierra Southern Sierra

CNRM

By 2099, there is an increase in the 3-day flows for all the 3- GCMs for both NSN Flood discharge computed from 2051-2099 and SSN, including GCMs that project 8- GFDL Flood 15% decline of overall discharge computed precipitation from 1951-1999 However, 2 of the 3 GCMs for NSN and 1 of the 3 GCMs for SSN PCM1 suggest significant increases of flood magnitude in the early 21st century

Return periods (yrs) Return periods (yrs) Projected Changes in Floods (example from CNRM CM3)

Northern Simulated 99%-ile flows under climate projections Sierra

Northern Sierra Southern Sierra Southern Sierra

1951-1999

Blue X: Rain-driven Red O: Snowmelt-driven f = 2.7 yr-1 f = 2.7 yr-1

Same number of

2001-2049 99%-ile flow events despite f = 5.9 yr-1 f = 5.1 yr-1 declining precipitation totals for 2051- 2099 for SSN 2051-2099

f = 5.9 yr-1 f = 2.7 yr-1 Projected Changes in flood events

Northern Simulated extreme streamflow (>99%-ile) events: Sierra

Northern Sierra Southern CNRM Sierra GFDL

PCM1

In general, frequency of flood events increases or stays constant, except for Southern Sierra GFDL which projects slight decreases Projected Changes in flood events (winter vs. summer events)

Northern Sierra

Simulated extreme streamflow (>99%-ile) events accumulated over winter and summer: Southern Sierra Winter floods

2001- 2049 2051- 2099 Simulation 1951- 1999 suggests more frequent winter rainfall generated floods

And, decrease of Summer snowmelt driven floods floods for SSN, particularly under Snowmelt driven the late 21st century floods Mechanisms associated with changes in flood risks: Precipitation vs. Fractional area of rainfall receiving areas in1951-1999 and 2051-2099

Northern Northern Sierra Sierra

Larger floods (larger circle size) have typically been associated with storms with large precipitation totals, but notably few larger floods are occurred with storm with smaller precipitation totals Mechanisms associated with changes in flood risks: Changes in rainfall receiving areas & rain-on-snow for 2051-2099 (compared to model historical period 1951-1999)

Northern Rainfall receiving areas Sierra

Southern In general, there is Sierra an increase of PCM1 rainfall receiving GFDL area during the CNRM floods days under climate change projections

Rain on snow

Considerable decreases of the rain-on-snow areas for NSN. Surprisingly, not much change for SSN Summary

. There is a general tendency toward the increase in the 3-day flood magnitude (particularly in the late 21st century)

. Frequency of flood events increases in two GCMs (CNRM and PCM1) projections, while in the third model GFDL remained constant or decreased slightly, owing to the drying trend in the GFDL

. Climate model suggests, in general, more frequent winter rainfall generated floods and fewer snowmelt generated floods as climate warms through 21st century

. Increased flood frequencies are appear to derive from several factors, including: increases in the sizes of the largest storms increased storm frequencies days with more precipitation falling as rain and less as snow

. Future research, involving GCMs, downscaling and hydrological model simulations is needed to objectively merge results derived from several GCMs into a single master estimate of flood frequencies and their associated uncertainties Future Research Northern (uncertainties: GCMs, downscaling methods) Sierra

Southern Sierra Northern Sierra Southern Sierra 50-yr flood magnitudes 50-yr flood magnitudes

dT=+3 dT=+3 dP=+0% dP= –9% GCMs CNRM dT=+3.5 dT=+2 dP= –15% GFDL dP= +3% PCM1

dT=+3 dT=+2 dP= –10% dP= +5%

These kinds of ensemble scatter are unlikely to decline much in the coming decade