modules_Greencrack (08/19/09)

This module is derived from two other CRHM modules, crack and Greenampt and is defined in ClassGreencrack. It handles frozen soil infiltration during the spring snow melt and during the remainder of the year uses Green-Ampt for unfrozen soil infiltration.

Observations

• none

Variables

• infil (mm/int) - depth of infiltration in time step, Dt – expressed as an average depth of water on an HRU.
• cuminfil (mm) - cumulative infiltration - expressed as an average depth of water on an HRU.
• snowinfil (mm/int) - amount of daily infiltration – expressed as an equivalent depth (m³/m²).
• cumsnowinfil (mm) - cummulative infiltration.
• runoff (mm/int) - amount of daily runoff – expressed as an equivalent depth (m³/m²).
• cumrunoff (mm) - cummulative runoff.
• meltrunoff (mm/int) - melt runoff.
• cummeltrunoff (mm) - cummulative melt runoff.
• crackstat () - infiltration status. Values:  0 - not started,  1 to 6 - #events, >6 - Limited ended and 10 - ice lens. State variable.
• crackon () - indicates when the crack frozen soil routine is enabled for an HRU. 0 - disabled/1 - enabled.
• RainOnSnow (mm) - cumulative rain on snow.

Parameters

• basin_area (km^2) - area of watershed.
• hru_area(km^2) - area of HRUs.
• fallstat () - fall status <= 0.0 or -1 for unlimited/cracked,  >= 100.0 for restricted and < 0.0 limited < 100.0. Pore saturation as a percentage.
• major (mm) - threshold for major melt.  Default is 5 mm/day.
• soiltype - 0 through 11 for water/sand/loamsand/sandloam/loam/siltloam/sasclloam/clayloam/siclloam/sandclay/siltclay/clay/pavement.
• soil_moist_init (mm) - initial soil moisture content --- used to set F1.
• soil_moist_max (mm) - maximum soil moisture content.
• PriorInfiltration () - allow limited melt to infiltrate prior to major melt.

Variable Inputs

• hru_tmax (°C) - from module obs.
• snowmeltD (mm/d) (*)  - from melt module, e.g.  ebsm, etc..
• SWE (mm) (*)  - Snow water equivalent from module, e.g.  pbsm, etc..
• net_rain (mm/int) (*) - depth of rain received by the ground surface in time step, Dt. Variable available from module intcp,  brushintcp etc..
• soil_moist (mm) - from module smbal, etc.

Notes on Crack.

The Division of Hydrology at the University of Saskatchewan (Granger et al. 1984; Gray et al., 1985), postulated that the infiltration potential of frozen soils may be grouped in three broad categories, namely: restricted, limited and unlimited.

Restricted - Infiltration is impeded by an impermeable layer, such as an ice lens on the soil surface or within the soil close to the surface. For all practical purposes, the amount of meltwater infiltration can be assumed to be negligible and that the melt goes directly to runoff and to evaporation.

Limited - Infiltration is governed primarily by the snow-cover water equivalent and the frozen water content of the top 30 cm. of soil.

Unlimited - A soil with a high percentage of large, air-filled macropores at the time of melt. Examples of soils having these properties are dry, heavily cracked clays and coarse, dry sands, organics and others.

Within the model there is not sufficient information to determine these classifications automatically and as a result the user is required to specify these properties for each HRU every fall and input this information to the model as a parameter. The one case the model does handle is when there is an early melt and the subsequent re-freezing causing an ice lens to form. This will change both Limited to Restricted. Implementation of the infiltration to frozen soils routines is described below.

1. Index = INF/SWE where INF=5(1-q p)*SWE^0.584.

2. Potential = INF/6.

3. MELT_THRESHOLD = 5 mm. Minimum daily meltwater at which the melt routine is enabled. Lower meltwater levels are not counted as one of the six major melt events.

4. A major melt is a day when the amount of meltwater generated is greater than the MELT_THRESHOLD.

5. Six major daily melts are allowed before the limited infiltration category is changed to Restricted.

The Frozen Soil Infiltration routine

1. The Frozen Infiltration routine is enabled in an HRU when its SWE is greater than 50mm.  Each HRU is handled independently.

2. Limited infiltration is triggered into operation by the first major melt. At this time, Index and Potential are calculated from the soil moisture (q p) and the SWE of the snowpack.

3. The Frozen Infiltration routine is disabled in an HRU when its SWE reaches zero.

The three frozen soil categories are described below.

LIMITED

1. Only six major over-winter snowmelt events are possible before the infiltration potential becomes Restricted.

2. Meltwater amounts less than the MELT_THRESHOLD is handled as runoff unless the parameter PriorInfiltrationare is set,  then all of the melt is permitted to infiltrate into the soil. Once the  MELT_THRESHOLD has been exceeded it is assumed that spring snowmelt has begun and only an amount of meltwater equal to MELT*Index will infiltrate and the remainder will be handled as runoff.

3. Index and Potential are recalculated if another major melt occurs with a greater SWE.

4. If the temperature the day following a major melt event is colder than -10ºC, the category is changed from Limited to Restricted assuming an ice lens has formed.

5. When the SWE of the snowpack is zero,  the model returns to the module that handles unfrozen infiltration.

UNLIMITED

1. All meltwater is allowed to infiltrate.

2. If the temperature the day after a major melt event is less than -10ºC, the category remains unchanged and all the melt water still infiltrates into the soil.

3. When the SWE of the snowpack is zero,  the model returns to the module that handles unfrozen infiltration.

RESTRICTED

1. No meltwater is allowed to infiltrate.

2. When the SWE of the snowpack is zero,  the model returns to the module that handles unfrozen infiltration.

Rain on Snow.

    The energy component of Rain on Snow was not handled when the Rain on Snow occurs before melt.  All the Rain on Snow is accumulated before melt and released when the first melt occurs, i.e. snowmelt is greater than zero.  This is can be seen by an inspection of RainOnSnow as it remains at zero until there is melt in the HRU.  The Rain on Snow is accumulated before melt in an internal variable RainOnSnowA which is not accessible to the user.

Notes on Green-Ampt.

This portion of the module replaces the module srunoff which is used to simulate prairie infiltration when the assumption is made that unlimited infiltration is possible.

When the surface ponding is negligible,

f (mm/hr)  =  k*(y*Dq/F + 1),    (eq. 1.)

where:

k (mm/hr) = saturated hydraulic conductivity.

y (mm)  = capillary suction at the wetting front.

Dq = (qs -  qi). change in volumetric soil moisture content.

When ponded,

F1 (mm) = k*Dt + y*Dq * ln( (F1 + y*Dq)/(F0 + y*Dq)).       (eq. 2.)

where F0 (mm) = cumulative infiltation at the beginning of the time interval i.e. t,

f0 (mm/hr) =  infiltration rate at the beginning of the time interval i.e. t,

F1 (mm) = cumulative infiltration at the end of the time interval i.e. t+Dt,

f1(mm/hr) =  infiltration rate at the end of the time interval i.e. t+Dt,

It (mm/hr) = rainfall intensity during the interval.

The cumulative infiltration (Fp) at instant of ponding time is

Fp = k*y*Dq/(It - k).     (eq. 3.)

The portion of the interval, after which ponding occurs is Dt',

Dt' = (Fp - F0)/It.    (eq. 4.)

Flow Chart

Calculate the maximum infiltration rates,  f0 and f1 at the beginning and end of the time step using equation 1. using F1 = F0+Dt*It.

Case 1:  When f1 is >= rainfall intensity,  all the rainfall infiltrates with F1 = F0+Dt*It.

Case 2:  When the rainfall intensity > f0,  all the rainfall ponds and F is calculated using an iterative solution of equation 2.

Case 3:  When the rainfall intensity <= f0 but the rainfall intensity > f1,   ponding occurs during the interval and the solution consists of  solving for Fp using equation 3, then using equation 4 to solve the time into the step during which ponding occurs. Next, using equation 2 with Fp as F0 the cumulative infiltration F1 after the ponding time of (Dt - Dt') is calculated.

Assigned Soil Properties

All, except fcap are specified at 0% saturation from Chow pg 115.

Value for wilt is from Chow.

Values for por, fcap is from Floods in Canada

Soil curves of form K = Ks / (A*Psi^3 + 1 )