Streamflow/runoff sensitivity to warming and drying in the

Colorado (Western US) River Basin

Tapash Das, Dan Cayan, David Pierce, Mike Dettinger

Collaborators:

Robert Webb, Bradley UdallMartin Hoerling, Jonathan Overpeck Holly Hartman, Dennis Lettenmaier, Julie Vano, Dan Cayan, Tapash DasLevi Brekke, Kevin Werner

Reconciling Projections of Future Colorado River Stream Flow

Past Studies

Table from Colorado Water Conservation Board (CWCB) report “ Colorado Climate Change: A Synthesis to Support Water Resource Management and Adaptation.” Oct 2008 (available online at: http://cwcb.state.co.us/NR/rdonlyres/8118BBDB-4E54-4189-A354-3885EEF778A8/0/CCSection5.pdf)

1) Reconcile discrepancies in projected Colorado River flow changes.

2) Assess the basins sensitive in runoff to changes in temperature, in precipitation, or in both.

3) Identify the underlying mechanisms for these sensitivities (e.g. soil moisture, ET).

4) Provide meaningful information for water managers and policymakers that incorporate uncertainties in future climate change projections.

Project Objectives

Data, Model

Hydrologic model: Variable Infiltration Capacity (VIC)

Liang et al. (1994), JGR

Historic forcing: Observed Gridded meteorology

Wood and Lettenmaier (2006), B Am Meteorol Soc

Hamlet and Lettenmaier (2005), J. Hydromet.

Methods Compute streamflow/runoff change

Responses to prescribed changes in T, P imposed on historic climate

Temp sensitivity

Q ref+1 - Qref

Qref

deg C

=

Precip elasticity

Q ref-1% - Qref

Qref

%

=

DATA, Model, Methods

Colorado River Flows: highly sensitive to warming

Catchments: GN: Green River at Greendale

MA: Yampa River near Maybell

CA: Colorado River near Cameo

GA: Gunnison River near Grand Junction

CI: Colorado River near Cisco

BL: San Juan River near Bluff

LE : Colorado River at Lees Ferry

IM: Colorado River below Imperial Dam

Sensitivity of streamflow to warming varies across different

catchments

Overall sensitivity approximately 6% decline in streamflow per 1°C

warming at Colorado at Lees Ferry

-6%

GN

MA

CAGA CI

LEBL

IM

WarmingBaseline

Temp increases: streamflow decreases

annually, mainly because

decreases flow in spring/summer

Climatologies of monthly streamflow:

Colorado at Lees Ferry

The total decline in streamflow increases each

time the temperature increases, but the total

reductions decrease as the temperature increases

more and more

Streamflow changes to climate change: Colorado at Lees Ferry

Colorado River Flows: Sensitive to Drying

Catchments: GN: Green River at Greendale

MA: Yampa River near Maybell

CA: Colorado River near Cameo

GA: Gunnison River near Grand Junction

CI: Colorado River near Cisco

BL: San Juan River near Bluff

LE : Colorado River at Lees Ferry

IM: Colorado River below Imperial Dam

Sensitivity of streamflow to drying varies across different catchments

Overall sensitivity approximately 2% decline in streamflow per 1%

reduction of precipitation at Colorado at Lees Ferry

-2%

GNMA

CA GA CI LEBL IM

Runoff climatologies at Colorado at Lees FerrySubstantial changes in runoff volume, timing in baseline VIC

run and VIC run using average model forcings

Impact of model forcing resolution to VIC simulations: Colorado at Lees Ferry

Annual mean streamflow (MAF)(WY1916 through 2000)

Naturalized flow : 15.1VIC simulated flow (baseline) : 15.7VIC simulated flow (p, t basin wide) : 8.1VIC simulated flow (t basin wide) : 11.8VIC simulated flow (p basin wide) : 7.8

baseline

P,T basin average

Research Objectives: Sensitivity of Runoff to Climate Change Over The Western U.S.

Understand runoff sensitivity to warming, and drying in the Western United States

Compare major river basins in the West, and understand the underlying differences

Apply sensitivity method to IPCC AR4 climate change ensemble projection

Study domain: Western US

(+ part of British Columbia)

oC

Colorado River Snow dominated

Northern Sierra Rain-Snow fed

Columbia River Snow dominated

Southern Sierra Snow dominated

Hydrographs drawn from VIC simulated streamflow forced by observed meteorology, 1915-2002

%oC-1

Sensitivity of runoff to Warming varies greatly across the West

Major Basins: Upper Colorado shows the highest sensitivity to warming, followed by Columbia, and least sensitivity in the

Californian Nevada

Hydrologic characteristics: Region by Region

In Columbia and Sierra Nevada,

about 50% of total precipitation is

lost via ET

Runoff efficiency

(R/P)

Evaporative demand (AET/P)

However, in Colorado, about

80% of total precipitation is

lost via ET

Actual ET and runoff are computed using VIC as driven by observed meteorology, 1915-2002

Sensitivity of runoff to Drying varies across the West

Major Basins: Upper Colorado 2.2% reduction, Northern Sierra 1.4% for 1% reduction of precipitation

%%-1

Does snowpack produce higher Sensitivity?

Region considered as snow-dominated if VIC simulated mean April 1 Snow Water Equivalent (SWE) in the period

1915-2002 is larger than 50 mm (VIC simulation with varying snowline)

Other region

Snow-dominated region has higher

sensitivity in Colorado, Columbia, and Southern Sierra, but NOT in Northern

Sierra

Snow-dominated

region

Sensitivity of Streamflow to Warming:

Month by month

Warming in the warm season yield larger

reduction (as compared to reduction in cool season warming) for Columbia, Colorado and Southern

Sierra

Runoff fraction: Region by Region

In Columbia, Colorado and

Southern Sierra Nevada >70% of

total runoff produced in

warm season

Precipitation fraction: Region by Region

In Columbia and Sierra Nevada >65% of total precipitation falls in cold

season

Summary

Sensitivities of Runoff to warming:

Colorado > Columbia > Southern Sierra > Northern Sierra

The region with the highest evaporative demand has the greatest sensitivity. Greatest sensitivities appear in zones of snow and snow-rain transition

Warming in the warm season yield larger reduction (as compared to cool season) for Columbia, Colorado and Southern Sierra

Sensitivity of runoff, especially due to temperature, varies greatly across the West

Topography and hydrologic structure is crucial: resolving snow vs. rain and other processes in the Colorado River Basin is crucial. Distributing precipitation over the basin produces less runoff, but lesser effect when temperature is smoothed out