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TFree

HEC-HMS snowmelt

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TFree    0

I am using HEC-HMS to model snowmelt, and have several questions:

1. What are reasonable values for the ATI (antecedant temperature index) meltrate function? Similarly for ATI meltrate coefficient? Alternatively, what are some reasonable methods to determine these values? It isn't clear to me what these really are.

2. When I run the model, I get an error message that melt rate is undefined for my first time step (even though I have numbers for ATI function and coefficient. Any insight as to why I am getting this error message and how I can correct it?

Thanks,

TFree

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Hi TFree,

Question 1: What are reasonable values for the ATI (antecedant temperature index) meltrate function? Similarly for ATI meltrate coefficient? Alternatively, what are some reasonable methods to determine these values? It isn't clear to me what these really are.

I haven't used the snowmelt routines in HMS very much, but here is some information from the HMS help file. More information is available on the page in the help file:

"A meltrate must be calculated for time intervals when the precipitation rate is less than the rain rate limit. The calculation starts with the meltrate antecedent temperature index. A coefficient is used to update the antecedent meltrate index from one time interval to the next. This index generally ranges from 0.015 to 0.550 and is separate from the cold content index. A typical value for the coefficient is 0.98.

An antecedent temperature index meltrate function is used to calculate a meltrate from the current meltrate index. The function must be specified separately in the Paired Data Manager before it can be used in the snow melt method. The function should define appropriate meltrates to use over the range of meltrate index values that will be encountered during a simulation.

Optionally, you may adjust the meltrate computed from the index meltrate function. A meltrate pattern may be specified that defines the percentage adjustment as a function of the time of year. If no meltrate pattern is selected, the meltrate will be computed only from the antecedent temperature index and the meltrate function. The pattern must be specified separately in the Paired Data Manager before it can be used in the snow melt method."

The Hydrology handbook, by the ASCE Task Committee on Hydrology (2nd edition, p. 453), seems to have some useful information about the temperature index method, and there appears to be a snowmelt model, NWS Hydro-17, by Anderson (1973) that has a similar method. I don't have any sample numbers, but you could enter some approximate numbers and run a sensitivity analysis on your model.

Here is another detailed document about snowmelt.

Question 2: When I run the model, I get an error message that melt rate is undefined for my first time step (even though I have numbers for ATI function and coefficient. Any insight as to why I am getting this error message and how I can correct it?

Each sub-basin has temperature index parameters. You need to make sure all the data is defined for each sub-basin's temperature index data. The data that should be defined includes a temperature gage, a lapse rate (degrees/1000 ft/m), an index, and one or more elevation bands with several parameters for each band. Some of these elevation band parameters can be obtained from the NRCS "snotel" snow surveys. Other parameters need educated guesses. You add elevation bands in HMS by right-clicking on the "temperature index" data for each sub-basin and selecting the "add elevation band" option.

Hope that helps,

Chris

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TFree    0
Hi TFree,

Question 1: What are reasonable values for the ATI (antecedant temperature index) meltrate function? Similarly for ATI meltrate coefficient? Alternatively, what are some reasonable methods to determine these values? It isn't clear to me what these really are.

I haven't used the snowmelt routines in HMS very much, but here is some information from the HMS help file. More information is available on the page in the help file:

"A meltrate must be calculated for time intervals when the precipitation rate is less than the rain rate limit. The calculation starts with the meltrate antecedent temperature index. A coefficient is used to update the antecedent meltrate index from one time interval to the next. This index generally ranges from 0.015 to 0.550 and is separate from the cold content index. A typical value for the coefficient is 0.98.

An antecedent temperature index meltrate function is used to calculate a meltrate from the current meltrate index. The function must be specified separately in the Paired Data Manager before it can be used in the snow melt method. The function should define appropriate meltrates to use over the range of meltrate index values that will be encountered during a simulation.

Optionally, you may adjust the meltrate computed from the index meltrate function. A meltrate pattern may be specified that defines the percentage adjustment as a function of the time of year. If no meltrate pattern is selected, the meltrate will be computed only from the antecedent temperature index and the meltrate function. The pattern must be specified separately in the Paired Data Manager before it can be used in the snow melt method."

The Hydrology handbook, by the ASCE Task Committee on Hydrology (2nd edition, p. 453), seems to have some useful information about the temperature index method, and there appears to be a snowmelt model, NWS Hydro-17, by Anderson (1973) that has a similar method. I don't have any sample numbers, but you could enter some approximate numbers and run a sensitivity analysis on your model.

Here is another detailed document about snowmelt.

Question 2: When I run the model, I get an error message that melt rate is undefined for my first time step (even though I have numbers for ATI function and coefficient. Any insight as to why I am getting this error message and how I can correct it?

Each sub-basin has temperature index parameters. You need to make sure all the data is defined for each sub-basin's temperature index data. The data that should be defined includes a temperature gage, a lapse rate (degrees/1000 ft/m), an index, and one or more elevation bands with several parameters for each band. Some of these elevation band parameters can be obtained from the NRCS "snotel" snow surveys. Other parameters need educated guesses. You add elevation bands in HMS by right-clicking on the "temperature index" data for each sub-basin and selecting the "add elevation band" option.

Hope that helps,

Chris

Thanks a lot for the sources of info and ideas on how to address the temp index parameters

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