Winter Erosion Processes Research at Washington State University Joan Wu, Shuhui Dun Prabhakar Singh, Cory Greer Washington State University Don McCool USDA-ARS-PWA

Introduction  Water erosion is a serious and continuous environmental problem in the US PNW and many other areas nationwide and worldwide  In the inland PNW, winter rain season, cyclic freeze- thaw of soil, steep slope, and improper management practices act together to cause high erosion rate  Soil freeze-thaw alters hydrological processes and reduces soil cohesive strength  Modelers must properly simulate winter hydrology in order to adequately simulate surface runoff and water erosion for cold areas

Introduction cont’d  WEPP: Water Erosion Prediction Project –a process-based erosion prediction model developed by the USDA ARS to replace the USLE –built on fundamentals of hydrology, plant science, hydraulics, and erosion mechanics  WEPP’s unique advantage: it models watershed-scale spatial and temporal distributions of soil detachment and deposition on event or continuous basis  Equipped with a geospatial processing interface, WEPP has GREAT POTENTIAL as a reliable and efficient tool for watershed assessment

Introduction cont’d  WEPP winter routines were designed to simulate –Snow accumulation and snowmelt –Soil frost and thaw  The routines include –Adjustment for aspect in calculating incoming radiation –Surface temperature estimation based on energy balance –Accounting for snow drift –Snowmelt simulation based on a generalized basin snowmelt equation –Frost simulation considering thermal conductivity of the snow- residue-soil system as well as upward water movement in the soil  However, the model was unable to properly represent the winter processes at the PNW and other colder regions as previous studies have shown

Snow and Frost Depth (Pullman, WA)

Snow and Frost Depth (Morris, MN)

Long-term Research Efforts  Goal –To continuously develop and improve the WEPP model for solving water quantity and quality problems  Objectives for winter hydrology study –Experimentally identify and mathematically formulate in WEPP the mechanisms by which freezing and thawing of soils affect runoff and erosion –Examine WEPP’s original winter routines and an alternative energy-budget based approach –Test the improved WEPP model using data sets from different localities under different hydrological conditions

Major Funding Sources  Wash. State Univ., USDA-ARS-PWA (in house)  USFS Rocky Mountain Research Station (1998–far future???)  Inland Northwest Research Alliance (2005–08)  USDA NRICGP (2001–05)  USGS/SWWRC (2000)

Major Collaborators  USDA-ARS-NSERL  USFS Rocky Mountain Research Station  USDA-ARS-PWA  USDA-ARS-CPCRC  Univ. Idaho, USA  Univ. Bologna, Italy

Laboratory and Field Investigation  Water erosion experimentation using a tilting flume  Field experimentation on water balance and erosion

Experimental plots at PCFS

On-site weather station

Tilting Flume at PCFS

An Energy-balance Approach (Lin and McCool, 2006)  The approach was based on the principle of a balance between the model simplicity and rigor and adequacy in representing snow and frost dynamics  In the newly incorporated algorithm –Energy is balanced cross air-earth interface –Frost (thawing) depth is computed by dividing the net energy influx by soil water (ice) content and latent heat of fusion

G = R n – LE – H G n = G – L n – S + J u Energy Flow into Soil Net Energy Flux into Soil R n – net radiation H – sensible heat LE – latent heat of vaporization G – energy flow into the soil S – heat storage change L n – latent heat utilized by snow melting J u – upward heat flux within soil Lin, C. and D.K. McCool, Simulating snowmelt and soil frost depth by an energy-budget approach. Trans. ASABE 49, 1383–1394.

Preliminary Results Using Datasets in Lin and McCool (2006)

The Alternative Approach (Pullman, WA)

The Alternative Approach (Morris, MN)

Preliminary Results Using New PCFS Datasets in Greer et al. (2006)

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WEPP’s Original Approach  Snowmelt estimation following Hendrick et al. (1971) using a modified generalized basin snowmelt equation for open areas developed by the US ACE  Frost formation is governed by the temperature on the surface of the snow- residue-frozen soil system and energy is balanced across the freezing front Hendrick, R.L., B.D. Filgate and W.M. Adams, Application of environmental analysis to watershed snowmelt. J. Appl. Meteor. 10, 418–429.

Surface Temperature T hra – hourly surface temperature (°C) T ave – hourly air temperature (°C) R net – 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 ) v wind – wind velocity (cm s −1 ) efthco – effective system thermal conductivity (Ly min −1 °C −1 ) depth – system depth (m)

Frost Simulation  Heat flux from surface  Heat flux from soil below  Energy balance in the order of –Conduction –Heat of fusion –Storage K srf – thermal conductivity(W m −1 °C −1 ) ΔT srf – temperature difference (°C) Z srf – depth from surface to frozen front (m)

Current Improvement  Mixed use of energy flux and energy has been corrected  Coding mistakes in energy balance during frost formation have been corrected  Thermal conductivity of the snow under testing  Snow-drift routines have been activated  Improved adjustment for aspect in calculating incoming radiation

Current Concerns  Standing residue currently not considered in frost simulation  Single value for thermal conductivity of flat residue without considering residue type and percent cover  Snow-drift influence appears small  Temperature set at 7 at 1 m below frozen zone  Temperature set at 7 °C at 1 m below frozen zone

Preliminary Results Using Datasets in Lin and McCool (2006)

The Improved WEPP (Pullman, WA)

The Improved WEPP (Morris, MN)

Summary  A simplified, energy-balance based approach to modeling snow accumulation and soil frost and thaw was incorporated into WEPP v  The model simulated adequate timing for frost occurrence  The effect of snow insulation appeared insufficient  Model testing using the new PCFS data showed consistent results with those from using the historical data

Summary cont’d  Improvement of the original WEPP winter hydrology codes is ongoing  The current improved version has potential in improved modeling of frost depth  Over-predicted frost duration and frequent thawing for PCFS are being examined (frost depth’s ceiling near 200 mm appears problematic)

Thank You!