7. Snow accumulation and melting

The FEST model can simulate the snow accumulation and melting. The algorithms implemented in the current version of the model are based on the air temperature. For the snow melting, the classical degree-day model is implemented. For snow accumulation, the precipitation is partitioned into liquid (rainfall) and solid (snow) components based on air temperature thresholds: a lower threshold defines the air temperature below which precipitation is totally solid, an upper threshold defines the air temperature above which precipitation is totally liquid. In between the two threshold values, a linear variation of the percentage of rainfall and snow is assumed. Snow simulation is configured through a specific file. In the configuration file the user must define 10 sections. Each section sets a parameter or variable that can be assigned as a scalar value or as a grid map. Some of the parameters can change within the simulation period: [melt-coefficient], [melt-threshold-temperature], [partitioning-upper-temperature], [partitioning-lower-temperature], and [hydraulic-conductivity]. In this case the user must use the NetCDF format for the input maps that contain changing parameter values.

Two sections define initial value of state variables: [snow-water-equivalent], and [water-in-snow]. These two sections are optional: if they are not defined, a map is created with value = 0. Two further sections are optional: [refreezing-coefficient], and [water-holding-capacity].

Keywords in the snow configuration file (usually named snow.ini) are listed and described in the following tables: Table 7.1, Table 7.2, Table 7.3, Table 7.4, Table 7.5, Table 7.6, Table 7.7, Table 7.8, Table 7.9, and Table 7.10.

Note

[melt-model] section configuration

Table 7.1 Configuration of melt-model section in the snow configuration file

Keyword	Description	Requirements
[melt-model]	Currently only the degree-day model based on air temperature data is implemented: 1=Degree-Day	Mandatory
scalar	Scalar value to be used on the entire domain	Optional.
file	Name of file of the grid map.	Required when parameter values are assigned through a grid map
format	Input file format	Available options: esri-ascii, esri-binary, net-cdf
epsg	epsg coordinate reference system code	Required when parameter values are assigned through a grid map
variable	Variable’s name in net-cdf file	Required when input map is in net-cdf format
time	Date and time of the first map to read from net-cdf file	Optional.
sync-initial-time	Sync the initial map with the simulation initial time.	Alternative option to time. Available options: 1 = map is synced, 0 = map is not synced

Note

[snow-water-equivalent] section configuration

Table 7.2 Configuration of snow-water-equivalent section in the snow configuration file.

Keyword	Description	Requirements
[snow-water-equivalent]	Snow water equivalent initial value (m)	Optional. Default value = 0.
scalar	Scalar value to be used on the entire domain	Optional.
file	Name of file of the grid map.	Required when parameter values are assigned through a grid map
format	Input file format	Available options: esri-ascii, esri-binary, net-cdf
epsg	epsg coordinate reference system code	Required when parameter values are assigned through a grid map
variable	Variable’s name in net-cdf file	Required when input map is in net-cdf format
time	Date and time of the first map to read from net-cdf file	Optional.
sync-initial-time	Sync the initial map with the simulation initial time.	Alternative option to time. Available options: 1 = map is synced, 0 = map is not synced

Note

[melt-coefficient] section configuration

Table 7.3 Configuration of melt-coefficient section in the snow configuration file

Keyword	Description	Requirements
[melt-coefficient]	Snow melt coefficient (mm/day/°C)	Mandatory
scalar	Scalar value to be used on the entire domain	Optional.
file	Name of file of the grid map.	Required when parameter values are assigned through a grid map
format	Input file format	Available options: esri-ascii, esri-binary, net-cdf
epsg	epsg coordinate reference system code	Required when parameter values are assigned through a grid map
variable	Variable’s name in net-cdf file	Required when input map is in net-cdf format
time	Date and time of the first map to read from net-cdf file	Optional.
sync-initial-time	Sync the initial map with the simulation initial time.	Alternative option to time. Available options: 1 = map is synced, 0 = map is not synced

Note

[melt-threshold-temperature] section configuration

Table 7.4 Configuration of melt-threshold-temperature section in the snow configuration file

Keyword	Description	Requirements
[melt-threshold-temperature]	air temperature above which snow melt starts (°C)	Mandatory
scalar	Scalar value to be used on the entire domain	Optional.
file	Name of file of the grid map.	Required when parameter values are assigned through a grid map
format	Input file format	Available options: esri-ascii, esri-binary, net-cdf
epsg	epsg coordinate reference system code	Required when parameter values are assigned through a grid map
variable	Variable’s name in net-cdf file	Required when input map is in net-cdf format
time	Date and time of the first map to read from net-cdf file	Optional.
sync-initial-time	Sync the initial map with the simulation initial time.	Alternative option to time. Available options: 1 = map is synced, 0 = map is not synced

Note

[partitioning-upper-temperature] section configuration

Table 7.5 Definition of partitioning-upper-temperature section in the snow configuration file.

Keyword	Description	Requirements
[partitioning-upper-temperature]	air temperature above which only rainfall occurs (°C)	Mandatory
scalar	Scalar value to be used on the entire domain	Optional.
file	Name of file of the grid map.	Required when parameter values are assigned through a grid map
format	Input file format	Available options: esri-ascii, esri-binary, net-cdf
epsg	epsg coordinate reference system code	Required when parameter values are assigned through a grid map
variable	Variable’s name in net-cdf file	Required when input map is in net-cdf format
time	Date and time of the first map to read from net-cdf file	Optional.
sync-initial-time	Sync the initial map with the simulation initial time.	Alternative option to time. Available options: 1 = map is synced, 0 = map is not synced

Note

[partitioning-lower-temperature] section configuration

Table 7.6 Definition of partitioning-lower-temperature section in the snow configuration file.

Keyword	Description	Requirements
[partitioning-lower-temperature]	air temperature below which only rainfall occurs (°C)	Mandatory
scalar	Scalar value to be used on the entire domain	Optional.
file	Name of file of the grid map.	Required when parameter values are assigned through a grid map
format	Input file format	Available options: esri-ascii, esri-binary, net-cdf
epsg	epsg coordinate reference system code	Required when parameter values are assigned through a grid map
variable	Variable’s name in net-cdf file	Required when input map is in net-cdf format
time	Date and time of the first map to read from net-cdf file	Optional.
sync-initial-time	Sync the initial map with the simulation initial time.	Alternative option to time. Available options: 1 = map is synced, 0 = map is not synced

Note

[hydraulic-conductivity] section configuration

Table 7.7 Configuration of hydraulic-conductivity section in the snow configuration file

Keyword	Description	Requirements
[hydraulic-conductivity]	Snow hydraulic conductivity (m/s) used to calculate intercell lateral flow	Mandatory
scalar	Scalar value to be used on the entire domain	Optional.
file	Name of file of the grid map.	Required when parameter values are assigned through a grid map
format	Input file format	Available options: esri-ascii, esri-binary, net-cdf
epsg	epsg coordinate reference system code	Required when parameter values are assigned through a grid map
variable	Variable’s name in net-cdf file	Required when input map is in net-cdf format
time	Date and time of the first map to read from net-cdf file	Optional.
sync-initial-time	Sync the initial map with the simulation initial time.	Alternative option to time. Available options: 1 = map is synced, 0 = map is not synced

Note

[water-in-snow] section configuration

Table 7.8 Configuration of water-in-snow section in the snow configuration file.

Keyword	Description	Requirements
[water-in-snow]	Liquid phase inside snowpack initial value (m)	Optional. Default value = 0.
scalar	Scalar value to be used on the entire domain	Optional.
file	Name of file of the grid map.	Required when parameter values are assigned through a grid map
format	Input file format	Available options: esri-ascii, esri-binary, net-cdf
epsg	epsg coordinate reference system code	Required when parameter values are assigned through a grid map
variable	Variable’s name in net-cdf file	Required when input map is in net-cdf format
time	Date and time of the first map to read from net-cdf file	Optional.
sync-initial-time	Sync the initial map with the simulation initial time.	Alternative option to time. Available options: 1 = map is synced, 0 = map is not synced

Note

[refreezing-coefficient] section configuration

Table 7.9 Configuration of refreezing-coefficient section in the snow configuration file.

Keyword	Description	Requirements
[refreezing-coefficient]	Coefficient to compute refreezing rate (-)	Optional. Default value = 0.05
scalar	Scalar value to be used on the entire domain	Optional.
file	Name of file of the grid map.	Required when parameter values are assigned through a grid map
format	Input file format	Available options: esri-ascii, esri-binary, net-cdf
epsg	epsg coordinate reference system code	Required when parameter values are assigned through a grid map
variable	Variable’s name in net-cdf file	Required when input map is in net-cdf format
time	Date and time of the first map to read from net-cdf file	Optional.

Note

[water-holding-capacity] section configuration

Table 7.10 Configuration of water-holding-capacity section in the snow configuration file.

Keyword	Description	Requirements
[water-holding-capacity]	Water holding capacity in snow defined as a percentage of current snow water equivalent (-)	Optional. Default value = 0.1
scalar	Scalar value to be used on the entire domain	Optional.
file	Name of file of the grid map.	Required when parameter values are assigned through a grid map
format	Input file format	Available options: esri-ascii, esri-binary, net-cdf
epsg	epsg coordinate reference system code	Required when parameter values are assigned through a grid map
variable	Variable’s name in net-cdf file	Required when input map is in net-cdf format
time	Date and time of the first map to read from net-cdf file	Optional.

An example of snow.ini configuration file follows.

[melt-model]
   scalar = 1  # 1 = degree-day
   
   
[snow-water-equivalent]
   file = ./data/initial_snow_depth.asc
   format = esri-ascii 
   epsg = 32632
 
[melt-coefficient]
    file = ./data/melt_factor.nc
    format = net-cdf
    variable = melt_factor
    epsg = 32632
    sync-initial-time = 1
   
[melt-threshold-temperature]
   scalar = 0. 
   
[partitioning-upper-temperature]
   scalar = 1. 
 
[partitioning-lower-temperature]
   scalar = -1. 
   
[hydraulic-conductivity]
  scalar = 0.01 
  
[refreezing-coefficient]
  scalar = 0.05 
  
[water-holding-capacity]
  scalar = 0.1