The Importance of Realistic Spatial Forcing in The Importance of Realistic Spatial Forcing in Understanding Hydroclimate Change--Understanding Hydroclimate Change--

Evaluation of Streamflow Changes in the Evaluation of Streamflow Changes in the Colorado River BasinColorado River Basin

Hydrology Conference 2010

Tapash DasScripps Institution of Oceanography

Dan CayanScripps Institution of Oceanography, USGS

Julie VanoUniversity of Washington

Dennis LettenmaierUniversity of Washington

David W. PierceScripps Institution of Oceanography

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)

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

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

Research Objectives

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

To investigate how streamflow and other hydrologic measures vary due to changes in the resolution of the temperature and precipitation input to VIC

c) Spatial distribution of VIC simulated variables

a) Spatial distribution of observed gridded forcing

Roughly 75% of runoff in

Colorado River Basin is

generated from about 25% of

the area, mostly at high

elevations b) Elevations, m

Colorado River Basin

River Basins: GREEN: Green River, Utah

GRNJC: Gunnison River near Grand Junction, Colorado

BLUFF: San Juan River near Bluff, Utah

GREEN

GRNJC

BLUFF

108720 km2

59472 km220880 km2

GREENGREEN

BLUFF

GRNJC

GREEN

There is a substantial decrease in runoff and streamflow production

using basin averaged model forcing compared to the baseline

simulation

1) r1: p, t observed gridded 2) r2: t averaged 3) r3: p averaged 4) r4: p, t averaged

Impact of model forcing resolution to VIC simulations: River Basins in Colorado

Runoff climatologies at Green RiverSubstantial changes in runoff volume, timing in baseline VIC run and VIC run

using averaged model forcings

Impact of model forcing resolution to VIC simulations: Green River, Utah

There is higher actual evapotranspiration

(AET) in the simulations using

coarse model forcing. The coarse resolution simulations produced

increased AET in early of the winter months

Changes in soil moisture volume

and seasonal patterns also occur in the

coarse resolution runs in

comparison to the baseline run

VIC simulated April 1 Snow Water Equivalent (SWE) as driven by climate model meteorology

from NCAR CCSM3 from two cases a) downscaled meteorology (black color)

coarsend meteorology (red color)

Impact of model forcing resolution to VIC simulations: Results from Climate model data at Green River,

Utah

Substantial changes in April 1 SWE in downscaled

climate model data driven VIC run and VIC run using

averaged p, t climate model forcings

Fine scale forcing driven

p,t averaged forcing driven

Sensitivity of runoff to Climate Warming

GREEN

GRNJC

BLUFF

NORSN

River Basins: GREEN: Green River, Utah

GRNJC: Gunnison River near Grand Junction, Colorado

BLUFF: San Juan River near Bluff, Utah

NORSN: Northern Sierra Nevada, California

Sensitivity of streamflow to warming varies across different River Basins

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

warming at Colorado River Basins.Californian Northern Sierra shows 1%

decline per 1°C warming

Hydrologic characteristics: Region by Region

In Californian Northern 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

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 warming: Non-linear effects

Changes in annual runoff as a function of the imposed year-round temperature change at Green River, Utah. Dashed lines show the tangent line at the origin, illustrating what

the response would be if there were no non-linear effects

BLUFF

LESFY

Streamflow changes to climate warming using multi land surface models

River Basins: BLUFF: San Juan River near Bluff, Utah

LESFY: Colorado River at Lees Ferry, Arizona

Colorado River at Lees Ferry

San Juan River near Bluff

Annual runoff change ranges between

apprx. -3% to -10% per 1°C warming

Summary

Sensitivities of Runoff to warming:

Colorado > Northern Sierra

The region with the highest evaporative demand has the greatest sensitivity.

Sensitivity of runoff due to temperature varies greatly across the South 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