Hydrological Modeling

FISH 513

April 10, 2002

Overview:

What is wrong with simple statistical regressions of hydrologic response on impervious area?

Toward a more complete understanding of normal flows.

Distributed Hydrological Modeling

Example from applications of Distributed Hydrological Modeling at UW

Changes in impervious area.

Changes in forest cover.

Global Climate Change.

Percent Impervious Area0 100%

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Typical Representation of Effect of Impervious Area on Runoff Coefficient

What we really want to know is:

What is the change from normal?

Previous graph is 100% correct for dry initial conditions.

What if it has just rained nonstop for five days . . .Well that never happens around here?

Percent Impervious Area0 1

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Representation of Effect of Impervious Area on Runoff Coefficient for extremely wet initial conditions

Therefore, normal response depends:

Static Variables:Land CoverImpervious Area, etc

Dynamic Variables:Soil MoisturePrecipitation IntensityStorm Duration, etc.

Numerous Studies have shown decreased effects of land use Change as antecedent conditions become wetter.

Our task is to build a predictive model of what is normal…And that can’t be done without considering interaction of meteorology with land cover changes

Hydrological Modeling to the Rescue

DHSVM Snow Accumulation and Melt Model

• Terrain - 150 m. aggregated from 10 m. resolution DEM

• Land Cover - 19 classes aggregated from over 200 GAP classes

• Soils - 3 layers aggregated from 13 layers (31 different classes); variable soil depth from 1-3 meters

• Stream Network - based on 0.25 km2

source area

Land surface characterization required by DHSVM

Calibration Location (Snoqualmie)

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20-Nov 4-Dec 18-Dec 1-Jan 15-Jan 29-Jan 12-Feb

Testing: Cedar

•Calibration to two USGS sites•Split sample validation at over 60 sites•Parameters transfer extremely well to other watersheds without recalibration

Effects of Impervious Area

Application of DHSVM to lower Cedar River Watershed to assess impacts of changes in impervious area on basin hydrology

Fraction Impervious Area (1998)

100 %

75 %

50 %

25 %

0 %

Taylor Creek (14 km2) 5% imperv.

Madsen Creek (5.4 km2) 20% imperv.

DHSVM Calibration to determine baseline parameters.Taylor Creek (5% impervious area)

Feb 1991 Mar 1991 Apr 1991 May 1991

CF

S

Test of Impervious Area Representation (no re-calibration) Madsen Creek (20% impervious area)

4/1/91 4/4/91 4/7/91 4/10/91

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4/5/1991 4/15/1991 4/20/1991

Old Growth Forest: 58 cfs peak, 2.3 inches total runoff

100 % increase in peak

1991 Land Cover (20 % imperv.): 115 cfs peak, 3.2 inches total runoff

Observed (1991): 120 cfs peak, 3.6 inches total runoff

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Flow Duration Curves for Madsen Creek:

Red is 1991 LandCover, Blue is historic Land Cover (Based on

Continuous simulation from Oct 1 1988 to Sept 30 1996)

Percent of Time Flow is exceeded

5 4 3 2 1100 80 60 40 20

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Percent Increase in Peak Streamflow for Madsen Creek: (Based

on Continuous simulation from Oct 1 1988 to Sept 30 1996 and

comparison between 1991 landcover and historic landcover)

Historic Peak Streamflow

Effect of Climate Change

Predicted Change in Mean Monthly Temperature due to Increased Carbon Dioxide Levels (Mean of 4 GCM’s)

Tem

pera

ture

Inc

reas

e (d

egre

es C

)

2020’s (w.r.t mid 20th century climate)2040’s (w.r.t mid 20th century climate)

2020’s Mean Winter Increase = 1.6 C

2040’s Mean Winter Increase = 2.4 C

Methodology for Assessing Impacts of Climate Change onWatershed hydrologyObserved

MeteorologyAt Stations in and near

Target Watershed

Synthetic “Observed” Record:

Talt=Tobs + Delta TPalt = Pobs*(Delta P)

Change In Temperature

Change in Precipitation

DHSVM

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SWE

Reservoir Inflow

Sno

w W

ater

Equ

ival

ent (

mm

)Cedar River Watershed:

Retrospective Analysis of Average Snow Water Equivalent Under Current and Altered Climates

Current20252045

Drought

Current Low YearBecomes . . .2025/2045 Best Case

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Effect of Climate Change on Mean Monthly Inflow (1988 to 1996) to Cedar Reservoir

Month

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thly

Inf

low

(m

eter

s)

Higher Winter Flows: Increased PrecipitationHigher Freezing Level -> More Rain

126,000 acre-ft 90,000 acre-ft 78,000 acre-ft

Snowmelt

Current20252045

Effect of Forest Harvest

Urban / SuburbanGrass / Crop / ShrubDeciduous ForestDouglas-Fir / HemlockHarvestedRock / Ice CapWater

Basins for which streamflow was simulated for each vegetation scenario. GAP, 1991 is based on a 1991 LandSat image. Band Harvest has a total clear-cut area identical to GAP, 1991 but concentrated in the transient snow zone (700-900 m). The control simulation is the historic vegetation coverage (based on GAP with all clear-cuts regrown).

GAP, 1991

Band harvestHistoric Vegetation

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02/4/96 2/5/96 2/6/96 2/7/96 2/8/96 2/9/96 2/10/96

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HistoricGap 1991Band HarvestComplete Harvest

Flood Stage (560 cms)

87 %26 %10 %

11 %4 %2 %

Hourly Precipitation (mm)

Low elevation(<300 m) snow (mm SWE)

Historic VegetationComplete Harvest

Effect of forest canopy removal, Snoqualmie River at Snoqualmie Falls, February 1996 event

Questions?