Download Current Winter Processes Modeling Approaches

Current Winter Processes
Modeling Approaches - WEPP
Dennis Flanagan
Agricultural Engineer
USDA-ARS NSERL
West Lafayette, Indiana
WEPP Winter Hydrology
• Developed by Reza Savabi, Bob Young, George
Benoit, John Witte, Dennis Flanagan, Jeanine
Ferris
• Computations on an hourly basis
• Determines whether precipitation is rain or snow.
• Adjusts snow depth and density, and determines
amount of snow melt.
• Determines soil frost depth and thaw depth.
• Determines water movement through
frozen/unfrozen soil layers.
Winter Routines are active when:
• A snowpack already exists, or
• A soil frost layer already exists, or
• Average daily temperature is less than
0oC
Hourly calculations
• For non-winter periods, daily water balance
is only updated once a day.
• For winter periods, a separate water balance
is tracked, and all calculations for water and
heat transfer are on an hourly basis.
• Input values of maximum and minimum air
temperatures are used to create hourly
temperature values.
• An hourly radiation value is also calculated.
Hourly temperature calculations
Between sunrise and 2:00 p.m.
All other times
Hourly adjusted surface temperature at top of residue-snow-frozen-layer system
Hourly radiation calculations
Net daily radiation
Adjustment factor to hourly radiation
Rslp – solar radiation on sloping surface (Ly min-1)
alb – albedo of soil or snow
atem – atmospheric emissivity suem – surface emissivity
SBC – Stephan-Boltzman constant (8.1247x10-11 Ly min-1 oK-4)
Tavek – hourly air temperature (oK)
SOLO – solar constant (0.082 MJ m-2 min-1)
RELD – relative distance of earth from sun (rad)
latrad – latitude (rad)
SUND – sun declination (rad)
HASR – position of sun at sunrise (rad)
HAST – position of sun at sunset (rad)
Snow Melt Assumptions
• Any precipitation occurring in an hour when
temperature is <0oC is assumed to be snowfall
• No snowmelt will occur if maximum daily
temperature is < -3oC
• The snowpack will not melt until the density of the
snowpack exceeds 350 kg m-3
• The surface soil temperature equals 0oC during a
melt period.
• The temperature of the cloud base is
approximately that of the surface air temperature.
• The albedo of melting snow is 0.5
Snow Melt Calculations
hrmelt = 0.0254 (amelt – bmelt + cmelt + dmelt)
Soil Frost/Thaw Assumptions
• Heat flow in a frozen or unfrozen soil or snow system is
uni-directional.
• Average 24 hour temperature of the system surface-air
interface is approximated by average daily air temperature.
• Snow and soil thermal conductivity and water flow
components assumed constant.
• Heat lost through frozen zone is first balanced by heat flow
in the unfrozen soil as a result of the soil’s temperature
gradient and thermal conductivity.
• Additional heat loss is balanced by the heat of fusion
released by freezing water.
• Further heat loss is balanced by changes in soil heat
content of the unfrozen soil, the magnitude calculated by
difference.
Soil Frost/Thaw Calculations
Uni-directional Heat Flow through the soil or soil-residue-snow
system is calculated with:
▲Tsrf – temperature difference
across snow-residue-frozen soil
system thickness (oC)
Zsrf – thickness of system (m)
Average Thermal Conductivity
Ksnow – thermal conductivity of snow (W m-1 oC-1)
Kres – thermal conductivity of residue (W m-1 oC-1)
Kftill – thermal conductivity of frozen tilled soil (W m-1 oC-1)
Kfutil – thermal conductivity of frozen untilled soil (W m-1 oC-1)
Snowd, Resd, Tilld, Utilld – thicknesses of each layer (m)
Soil Frost/Thaw Calculations
Qsrf must be balanced by heat flow from the unfrozen soil below the
frozen layer (Quf).
Kuf – thermal conductivity of unfrozen soil (W m-1 oC-1)
Tuf – change in temperature from 0o isotherm to the depth of stable
temperature (oC)
Zuf – depth of unfrozen soil (m)
L – latent heat of fusion (W s m-3)
Kw – unsaturated hydraulic conductivity of the soil (m s-1)
P – change in total water potential (m)
Cuf - heat capacity of unfrozen soil (W m-3 oC-1)
Zc – depth of unfrozen soil that supplies heat (1.0 m)
Snow Drifting
• WEPP has a considerable amount of code
related to snow accumulation and loss due
to drifting, however…
• This code is not currently active, due to
questions about it performance and lack of
sufficient data for validation at the time of
model release.
Erodibility Adjustments
• Winter conditions can cause changes in the soil
erodibility.
• A soil that is completely frozen to the surface is
assumed to be non-erodible, and adjusted Ki and
Kr are set to zero for that day.
• A soil that has gone through a number of freeze
and thaw cycles is considered to be more erodible
while still in a thawing and wet state, and
erodibility adjustments are made to the soil on
those days of simulation.
Interrill Erodibility Adjustment
acyc – freeze thaw cycle factor
- matric potential of surface soil (KPa)
cycles - number of freeze-thaw cycles (max of 10)
Adjustments made until soil dries to less than 1/3 bar water content
Rill Erodibility and Critical Shear
Stress Adjustments
Adjustments only active until soil dries to field capacity,
then not active again until after a new freeze-thaw occurs.