crack
This module is defined in Classcrack and handles frozen soil infiltration during the spring snow melt. This module is based upon 15 years of study of the snow hydrology of the Prairie region and results reported in the former USSR.
Observations
- none
Variables
- snowinfil (mm/d) - amount of daily infiltration – expressed as an equivalent depth (m3/m2).
- cumsnowinfil (mm) - cummulative infiltration.
- runoff (mm/d) - amount of daily runoff – expressed as an equivalent depth (m3/m2).
- cumrunoff (mm) - cummulative runoff.
- infil (mm/int) - 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.
- crackstat () - HRO infiltration status. Values: 0 - not started, 1 to 6 - #events, >6 - Limited ended and 10 - ice lens.
- crackon () - indicates when the crack frozen soil routine is enabled for an HRU. 0 - disabled/1 - enabled.
- RainOnSnow (mm) - cumulative rain on snow.
Parameters
- 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.
- basin_area (km2) - area of watershed.
- hru_area(km2) - area of HRUs.
- major (mm) - threshold for major melt. Default is 5 mm/day.
- PriorInfiltration () - allow 'limited" melt to infiltrate prior to first major melt event. 0 - No/1 - Yes.
Variable Inputs
- hru_tmax (obs)
- snowmeltD (mm/d) (*)
- SWE (mm) (*)
- net_rain (mm/int) (*)
Notes
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.
Definitions
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.