Note: The changes to support this option in PyHELP have already been made in PR #5. So this PR is only about the changes to the FORTRAN code.
The goal of this project is the assessment of groundwater recharge at the regional scale with the HELP model. To do that, the study area is divided in thousands of cells and the HELP model is executed for each cell in batch. Calibration of HELP is then carried out against total and base flow estimates derived from measured river hydrographs by compiling the results from HELP over the corresponding watershed of the hydrometric station.
Preliminary results showed that the HELP model overestimated the period during which the soil was considered frozen in our region. For the sake of model calibration, it was thus necessary to have a mean to control the parameters that control the prediction of frozen soil conditions in HELP.
This PR address this by adding the capability in HELP to change the average air temperature below which the soil is assumed to be freezing (TFSOIL). A description of how HELP predict the freezing and thawing of the soil is provided below.
Some definitions
TFSOIL in the HELP model is the average air temperature, in Fahrenheit degrees, below which the soil is assumed to be freezing.
DFS is a constant for a particular set of climatic data that represents the number of days in which the soil is to remain frozen after the daily mean air temperature first rises above freezing. The DFS increases with latitude and decreases with solar radiation in the winter at the site and is calculated in HELP from the first year of solar radiation data using some empirical relations.
Prediction of frozen soil conditions in HELP3:
Freezing
In the HELP model, the soil is assumed to enter frozen state when the average temperature of the previous 30 days first drops below 32 °F. During the time in which the soil is considered to be frozen, the infiltration capacity of the soil is reduced by increasing the calculated runoff. This is done by increasing the curve number.
If the CNII for unfrozen soil is less than or equal to 80, the CNII for frozen soil conditions is set at 95.
When the unfrozen soil CNII is greater than 80, the CNII is reset to be 98 on days when the program has determined the soil to be frozen.
In addition, other processes are affected such as soil evaporation, vertical drainage in the evaporative zone and groundmelt of snow.
Thawing
The point in which the soil is no longer considered to be frozen is determined by calculating the length of time required to thaw frozen soil; that is, the number of days in which the soil is to remain frozen after the daily mean air temperature first rises above freezing. The thaw period in days, DFS, is a constant for a particular set of climatic data. The thaw period increases with latitude and decreases with solar radiation in the winter at the site and is calculated in HELP from the first year of solar radiation data using some empirical relations.
A counter in the program keeps track of the number of days of below freezing (one is subtracted for each day down to a minimum of zero) or above freezing temperatures (one is added for each day up until a maximum of DFS is reached, at which point the soil becomes unfrozen) since the soil became frozen. When the soil freezes for the first time during the season, the counter is set to 0. When a thaw is completed, the counter is reset to (DFS + 2)/3, but not less than 3 unless greater than DFS. When the counter returns to 0, the soil is refrozen if the average temperature of the previous thirty days is below freezing. As such, the value of the counter also limits the occurrence of a refreeze after a thaw (i.e. the soil is prevented from refreezing immediately following a thaw when the previous 30-day average temperature may not yet have increased to above freezing) (Dozier, 1992).
Note: The changes to support this option in PyHELP have already been made in PR #5. So this PR is only about the changes to the FORTRAN code.
The goal of this project is the assessment of groundwater recharge at the regional scale with the HELP model. To do that, the study area is divided in thousands of cells and the HELP model is executed for each cell in batch. Calibration of HELP is then carried out against total and base flow estimates derived from measured river hydrographs by compiling the results from HELP over the corresponding watershed of the hydrometric station.
Preliminary results showed that the HELP model overestimated the period during which the soil was considered frozen in our region. For the sake of model calibration, it was thus necessary to have a mean to control the parameters that control the prediction of frozen soil conditions in HELP.
This PR address this by adding the capability in HELP to change the average air temperature below which the soil is assumed to be freezing (
TFSOIL). A description of how HELP predict the freezing and thawing of the soil is provided below.Some definitions
TFSOILin the HELP model is the average air temperature, in Fahrenheit degrees, below which the soil is assumed to be freezing.DFSis a constant for a particular set of climatic data that represents the number of days in which the soil is to remain frozen after the daily mean air temperature first rises above freezing. The DFS increases with latitude and decreases with solar radiation in the winter at the site and is calculated in HELP from the first year of solar radiation data using some empirical relations.Prediction of frozen soil conditions in HELP3:
Freezing
In the HELP model, the soil is assumed to enter frozen state when the average temperature of the previous 30 days first drops below 32 °F. During the time in which the soil is considered to be frozen, the infiltration capacity of the soil is reduced by increasing the calculated runoff. This is done by increasing the curve number.
In addition, other processes are affected such as soil evaporation, vertical drainage in the evaporative zone and groundmelt of snow.
Thawing
The point in which the soil is no longer considered to be frozen is determined by calculating the length of time required to thaw frozen soil; that is, the number of days in which the soil is to remain frozen after the daily mean air temperature first rises above freezing. The thaw period in days, DFS, is a constant for a particular set of climatic data. The thaw period increases with latitude and decreases with solar radiation in the winter at the site and is calculated in HELP from the first year of solar radiation data using some empirical relations.
A counter in the program keeps track of the number of days of below freezing (one is subtracted for each day down to a minimum of zero) or above freezing temperatures (one is added for each day up until a maximum of DFS is reached, at which point the soil becomes unfrozen) since the soil became frozen. When the soil freezes for the first time during the season, the counter is set to 0. When a thaw is completed, the counter is reset to (DFS + 2)/3, but not less than 3 unless greater than DFS. When the counter returns to 0, the soil is refrozen if the average temperature of the previous thirty days is below freezing. As such, the value of the counter also limits the occurrence of a refreeze after a thaw (i.e. the soil is prevented from refreezing immediately following a thaw when the previous 30-day average temperature may not yet have increased to above freezing) (Dozier, 1992).