Predicting the hydrologic implications of land use change in forested catchments

Dennis P. LettenmaierDepartment of Civil and Environmental Engineering

University of Washington

Chapman Conference on

Ecosystem Interactions with Land Use Change

June 14, 2003

Santa Fe, New Mexico

Outline of this talk

• Background – the signature of land use change

• Example 1 – Logging and flooding in the Pacific Northwest

• Example 2 –Hydrologic effects of vegetation change in the upper Midwest

• Some outstanding issues in prediction of hydrologic effects of land cover change

1) Continental and regional signatures of land cover change

Source:

National Institute of Public Health and the Environment (RIVM, Netherlands) and the Center for Sustainability and the Global Environment (SAGE, University of Wisconsin).

Estimated 1850 and 1990 global land cover

Early Conifer

Middle Conifer

Late Conifer

Early Deciduous

Middle Deciduous

Late Deciduous

Brush

Agriculture

Water

Historical (1900) Current (1990)

Columbia River basin estimated 1900 and 1990 vegetation cover (from ICBEMP)

2) Example 1: Logging and flooding in the Pacific Northwest

Assessment approach – spatially distributed hydrologic modeling

Mechanisms for hydrologic change

• Rain-on-snow runoff generation

• Channel manipulation via forest roads

• Water table (hence saturated area changes) via altered evaporative demand

• Combinations of above

Investigation of forest canopy effects on snow accumulation and melt

Measurement of Canopy Processes via two 25 m2 weighing lysimeters (shown here) and additional lysimeters in an adjacent clear-cut.

Direct measurement of snow interception

SWE difference for February 1996ROS event; harvest - no harvest

More snow at beginning

of event

Less snow at end of

event

Simulated response to forest harvestSub-basins of the Deschutes River, WA

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Increase in 2.3-Year Return Flood (%)

Incr

ease

in 1

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ear

Ret

urn

Flo

od (

%)

Sources of road-derived runoff

Surface routes for road runoff

Effect of forest roads on water table

Drier with roads

Wetter with roads

Simulated streamflow w/ and w/o forest roads

Hard Creek

Ware CreekHard Creek

Ware Creek

Bottom line:

• Both vegetation removal and roads contribute to increased peak runoff

• Effects more or less superimpose• For the Deschutes basin, each effect represents

about a 10% increase in the ~10 yr flood• Relative magnitude of vegetation effect decreases

with return period, road effect increases

Sediment Modeling with the DHSVM Watershed Sediment Module

DHSVM

MASS WASTING

SURFACE EROSION

CHANNELEROSION

Watershed Sediment Module

OUTPUT

Q

Qsed

Portraying Watershed Change Sediment Model

Wildfire in the Icicle Creek basin

Mass Wasting Module

MASS WASTING

Multiple realizations of total failure locations

Multiple time series of sediment supply

Soil depthDEM

Soil typeVegetation type

P(F

) Soil cohesionRoot cohesionVeg. SurchargeFriction angle

Surface Erosion ModuleMultiple time series of sediment supply

Overland flowSURFACE EROSION

Roads and streams

SoilPrecipitationVegetation

DEM

Distribution of sediment delivery

to channels(roads and streams)

Channel Erosion Module

CHANNELEROSION

Channel flow Mean and standard deviation of sediment

load for selected channel reaches

Distribution of sediment delivery

to channels(roads and streams)

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Probability of slope failure before and after Fourth of July Fire

Icicle Creek Vegetation

Pre-fire

Post-fire

Approximate extent of August

2001 fire

3) Example 2 –Hydrologic effects of vegetation change in the upper

Midwest

Regional Land Use ChangePresettlement Land Use Modern Land Use

Land Use Changes:

Change in Forest Cover

Variable Infiltration Capacity (VIC) Macroscale Hydrologic Model

• Full Energy Balance• Full Water Balance• Mosaic Vegetation

Cover• Variable Infiltration

Curve Generates Runoff

• Arno Baseflow Curve

Evaporation Changes

Dif

fere

n ces

(m

m)

Eva

p ora

t ion

(m

m)

Presettlement Land Use Modern Land Use

Snow Cover Changes

Dif

fere

n ces

(m

m)

Snow

Wat

er

Equ

ival

ence

(m

m)

Presettlement Land Use Modern Land Use

Calibrated Flow Comparison

Chippewa River Flow

St. Croix River Flow

Flow Comparisons

Modern Land Use Pre-Settlement Land Use

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Days Since 1951

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Days since 1/1/1951

• Discharge was calibrated using:– Modern land use types

– Discharge Observations from 1980-1989

• Plots compare discharge for the first 5 simulation years (1951-1955)

• Discharge also generated for presettlement land use using the same parameters

Extreme Flow Comparison

• Annual peak and low flow events for Water Years 1951-1995

• Compares simulated flow with presettlement and modern land use

• Both peaks and low flows are greater with modern land use

Annual Low Flows

Annual Peak Flows

Peak and Low Flow Comparisons

St. Croix River Chippewa RiverWisconsin River Mississippi River

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Presettlement (m3/s)

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m3 /

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Presettlement (m3/s)

Cumulative Flow Comparison

• Cumulative discharge from 1951 to 1995

• Decreased evaporation from smaller forested areas yields more runoff with modern land use

Chippewa River Cumulative Flow

St. Croix River Cumulative Flow

Cumulative Flow Comparisons

Modern Land Use Pre-Settlement Land Use

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rge

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ay)

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Days Since 1950

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rge

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ay)

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Days since 1/1/1950

4) Some outstanding issues in prediction of hydrologic effects of

land cover change

1. The calibration problem

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Date

Flo

w (

cfs

)

Observed Modern Channel Modern Inflows

Presettlement Channel Modern Inflows Presettlement Channel Presettlement Inflows

1965 Flood Hydrograph (Flood of Record)

1969 Flood Hydrograph

Mississippi River at Anoka, MN

2. The model complexity problem

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Years since wildfire

Effe

ctiv

e R

oot C

ohes

ion

Root decay Root regrowth Total root strength

Effect of wildfire on simulated root cohesion