Winter Erosion Winter Erosion Processes Research atProcesses Research at

Washington State Washington State UniversityUniversity

Joan Wu, Shuhui Dun Joan Wu, Shuhui Dun Prabhakar Singh, Cory GreerPrabhakar Singh, Cory Greer

Washington State UniversityWashington State University

Don McCoolDon McCool

USDA-ARS-PWAUSDA-ARS-PWA

IntroductionIntroduction

Water erosion is a serious and continuous Water erosion is a serious and continuous environmental problem in the US PNW and environmental problem in the US PNW and many other areas nationwide and worldwidemany other areas nationwide and worldwide

In the inland PNW, winter rain season, cyclic In the inland PNW, winter rain season, cyclic freeze-thaw of soil, steep slope, and improper freeze-thaw of soil, steep slope, and improper management practices act together to cause management practices act together to cause high erosion ratehigh erosion rate

Soil freeze-thaw alters hydrological processes Soil freeze-thaw alters hydrological processes and reduces soil cohesive strengthand reduces soil cohesive strength

Modelers must properly simulate winter Modelers must properly simulate winter hydrology in order to adequately simulate hydrology in order to adequately simulate surface runoff and water erosion for cold areassurface runoff and water erosion for cold areas

IntroductionIntroduction cont’dcont’d

WEPP: Water Erosion Prediction ProjectWEPP: Water Erosion Prediction Project

– a process-based erosion prediction model a process-based erosion prediction model developed by the USDA ARS to replace the USLEdeveloped by the USDA ARS to replace the USLE

– built on fundamentals of hydrology, plant science, built on fundamentals of hydrology, plant science, hydraulics, and erosion mechanicshydraulics, and erosion mechanics

WEPP’s unique advantage: it models WEPP’s unique advantage: it models watershed-scale spatial and temporal watershed-scale spatial and temporal distributions of soil detachment and distributions of soil detachment and deposition on event or continuous basisdeposition on event or continuous basis

Equipped with a geospatial processing Equipped with a geospatial processing interface, WEPP has interface, WEPP has GREAT POTENTIALGREAT POTENTIAL as a reliable and efficient tool for as a reliable and efficient tool for watershed assessmentwatershed assessment

IntroductionIntroduction cont’dcont’d

WEPP winter routines were designed to simulateWEPP winter routines were designed to simulate

– Snow accumulation and snowmeltSnow accumulation and snowmelt– Soil frost and thawSoil frost and thaw

The routines include The routines include

– Adjustment for aspect in calculating incoming radiationAdjustment for aspect in calculating incoming radiation– Surface temperature estimation based on energy balanceSurface temperature estimation based on energy balance– Accounting for snow driftAccounting for snow drift– Snowmelt simulation based on a generalized basin Snowmelt simulation based on a generalized basin

snowmelt equationsnowmelt equation– Frost simulation considering thermal conductivity of the Frost simulation considering thermal conductivity of the

snow-residue-soil system as well as upward water snow-residue-soil system as well as upward water movement in the soilmovement in the soil

However, the model was unable to properly However, the model was unable to properly represent the winter processes at the PNW and other represent the winter processes at the PNW and other colder regions as previous studies have showncolder regions as previous studies have shown

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Long-term Research Long-term Research EffortsEfforts

GoalGoal– To continuously develop and improve the WEPP model To continuously develop and improve the WEPP model

for solving water quantity and quality problemsfor solving water quantity and quality problems

Objectives for winter hydrology studyObjectives for winter hydrology study– Experimentally identify and mathematically formulate Experimentally identify and mathematically formulate

in WEPP the mechanisms by which freezing and in WEPP the mechanisms by which freezing and thawing of soils affect runoff and erosionthawing of soils affect runoff and erosion

– Examine WEPP’s original winter routines and an Examine WEPP’s original winter routines and an alternative energy-budget based approachalternative energy-budget based approach

– Test the improved WEPP model using data sets from Test the improved WEPP model using data sets from different localities under different hydrological different localities under different hydrological conditionsconditions

Major Funding SourcesMajor Funding Sources

Wash. State Univ., USDA-ARS-PWA Wash. State Univ., USDA-ARS-PWA (in house)(in house)

USFS Rocky Mountain Research USFS Rocky Mountain Research Station (1998–far future???)Station (1998–far future???)

Inland Northwest Research Alliance Inland Northwest Research Alliance (2005–08)(2005–08)

USDA NRICGP (2001–05)USDA NRICGP (2001–05) USGS/SWWRC (2000)USGS/SWWRC (2000)

Major CollaboratorsMajor Collaborators

USDA-ARS-NSERLUSDA-ARS-NSERL

USFS Rocky Mountain Research USFS Rocky Mountain Research StationStation

USDA-ARS-PWAUSDA-ARS-PWA

USDA-ARS-CPCRCUSDA-ARS-CPCRC

Univ. Idaho, USAUniv. Idaho, USA

Univ. Bologna, ItalyUniv. Bologna, Italy

Laboratory and Field Laboratory and Field InvestigationInvestigation

Water erosion experimentation Water erosion experimentation using a tilting flumeusing a tilting flume

Field experimentation on water Field experimentation on water balance and erosionbalance and erosion

Experimental plots at PCFS

On-site weather station

Tilting Flume at PCFS

An Energy-balance ApproachAn Energy-balance Approach(Lin and McCool, 2006)(Lin and McCool, 2006)

The approach was based on the The approach was based on the principle of a balance between the principle of a balance between the model simplicity and rigor and adequacy model simplicity and rigor and adequacy in representing snow and frost dynamicsin representing snow and frost dynamics

In the newly incorporated algorithmIn the newly incorporated algorithm– Energy is balanced cross air-earth interfaceEnergy is balanced cross air-earth interface– Frost (thawing) depth is computed by dividing Frost (thawing) depth is computed by dividing

the net energy influx by soil water (ice) content the net energy influx by soil water (ice) content and latent heat of fusionand latent heat of fusion

G = Rn – LE – H

Gn = G – Ln – S + Ju

Energy Flow into Energy Flow into SoilSoil

Net Energy Flux into SoilNet Energy Flux into Soil

Rn – net radiationH – sensible heatLE – latent heat of vaporizationG – energy flow into the soil

S – heat storage changeLn – latent heat utilized by snow meltingJu – upward heat flux within soil

Lin, C. and D.K. McCool, 2006. Lin, C. and D.K. McCool, 2006. Simulating snowmelt and soil frost depth by an Simulating snowmelt and soil frost depth by an energy-budget approach. Trans. ASABE 49, 1383–1394.energy-budget approach. Trans. ASABE 49, 1383–1394.

Preliminary Results Using Preliminary Results Using Datasets in Lin and McCool Datasets in Lin and McCool

(2006)(2006)

The Alternative ApproachThe Alternative Approach (Pullman, WA)(Pullman, WA)

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Preliminary Results Using Preliminary Results Using New PCFS Datasets in Greer New PCFS Datasets in Greer

et al. (2006)et al. (2006)

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WEPP’s Original ApproachWEPP’s Original Approach

Snowmelt estimation following Hendrick Snowmelt estimation following Hendrick et al. (1971) using a modified et al. (1971) using a modified generalized basin snowmelt equation generalized basin snowmelt equation for open areas developed by the US ACEfor open areas developed by the US ACE

Frost formation is governed by the Frost formation is governed by the temperature on the surface of the snow-temperature on the surface of the snow-residue-frozen soil system and energy is residue-frozen soil system and energy is balanced across the freezing frontbalanced across the freezing front

Hendrick, R.L., B.D. Filgate and W.M. Adams, 1971. Hendrick, R.L., B.D. Filgate and W.M. Adams, 1971. Application of environmental analysis Application of environmental analysis to watershed snowmelt. J. Appl. Meteor. 10, 418–429.to watershed snowmelt. J. Appl. Meteor. 10, 418–429.

Surface TemperatureSurface Temperature

Thra – hourly surface temperature (°C)

Tave – hourly air temperature (°C)

Rnet – net radiation (Ly min−1)conht – convective heat transfer coefficient (Ly s min−1 cm−1)radco – radiation coefficient (Ly s min−1 cm−1)

vwind – wind velocity (cm s−1)efthco – effective system thermal conductivity (Ly min−1 °C−1)depth – system depth (m)

Frost SimulationFrost Simulation Heat flux from surfaceHeat flux from surface

Heat flux from soil belowHeat flux from soil below

Energy balance in the order ofEnergy balance in the order of– ConductionConduction– Heat of fusionHeat of fusion– StorageStorage

Ksrf – thermal conductivity(W m−1 °C−1)

ΔTsrf – temperature difference (°C)

Zsrf – depth from surface to frozen front (m)

Current ImprovementCurrent Improvement

Mixed use of energy flux and energy has Mixed use of energy flux and energy has been correctedbeen corrected

Coding mistakes in energy balance Coding mistakes in energy balance during frost formation have been during frost formation have been correctedcorrected

Thermal conductivity of the snow under Thermal conductivity of the snow under testingtesting

Snow-drift routines have been activatedSnow-drift routines have been activated Improved adjustment for aspect in Improved adjustment for aspect in

calculating incoming radiationcalculating incoming radiation

Current ConcernsCurrent Concerns

Standing residue currently not Standing residue currently not considered in frost simulationconsidered in frost simulation

Single value for thermal conductivity of Single value for thermal conductivity of flat residue without considering flat residue without considering residue type and percent coverresidue type and percent cover

Snow-drift influence appears smallSnow-drift influence appears small

Temperature set at 7 Temperature set at 7 °C at 1 m below at 1 m below frozen zonefrozen zone

Preliminary Results Using Preliminary Results Using Datasets in Lin and McCool Datasets in Lin and McCool

(2006)(2006)

The Improved WEPPThe Improved WEPP(Pullman, WA)(Pullman, WA)

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The Improved WEPPThe Improved WEPP(Morris, MN)(Morris, MN)

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SummarySummary

A simplified, energy-balance based A simplified, energy-balance based approach to modeling snow approach to modeling snow accumulation and soil frost and thaw accumulation and soil frost and thaw was incorporated into WEPP v2004.7was incorporated into WEPP v2004.7

The model simulated adequate timing The model simulated adequate timing for frost occurrencefor frost occurrence

The effect of snow insulation appeared The effect of snow insulation appeared insufficientinsufficient

Model testing using the new PCFS data Model testing using the new PCFS data showed consistent results with those showed consistent results with those from using the historical datafrom using the historical data

SummarySummary cont’dcont’d

Improvement of the original WEPP Improvement of the original WEPP winter hydrology codes is ongoingwinter hydrology codes is ongoing

The current improved version has The current improved version has potential in improved modeling of frost potential in improved modeling of frost depthdepth

Over-predicted frost duration and Over-predicted frost duration and frequent thawing for PCFS are being frequent thawing for PCFS are being examined (frost depth’s ceiling near examined (frost depth’s ceiling near 200 mm appears problematic)200 mm appears problematic)

Thank You!Thank You!