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jbach

GSSHA - Long-term with GAR infiltration & Penman ET

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I'm attempting to model a small basin (~0.2 sq mi) using a long-term simulation. I can get the model to run without errors or warnings, but some of the model results are counter-intuitive. So I'd like to verify if I've set up my model correctly and throw out a couple concerns/questions that some of you might be able to help me out with.

Some quick background about my model setup: I'm using a single rain gauge located very near the basin boundary, defined using a .gag file with hourly data for the duration of November 2009. I've compiled WES format hydrometeorological data file that also gives hourly data for November 2009. My HMET file includes no radiation data, so I'm relying on the GSSHA calculated radiation from lat/lon/time&date. Active modules are as follows: Overland flow computed by ADE with Interception; Evapotranspiration via Penman method; Infiltration via G & A with Redistribution (Soil moisture depth = 1.0 m, Top layer depth = 1.0 m); Channel routing via Diffusive wave.

My primary concern with the results is the lack of evapotranspiration calculated by the simulation. Out of ~100,000 m^3 of precipitation during the simulation period, only ~100 m^3 are evaporated. (Roughly 20% of the precip is intercepted and the other 79.9% infiltrates.) 0.1% evapotranspiration seems alarmingly low, even for a very wet month with consistent cloud cover in the Pacific Northwest. I've tried manipulating all of the parameters that feed into the Penman ET calcs (including cloud cover and wind speeds in my HMET file), and I can only increase ET by about 50 m^3. Has anyone else had such a problem? Or can anyone think of a reason that ET would be so seemingly underrepresented?

Secondarily, I'm not sure if I've understood the GAR layer depth parameters. I've read the Downer paper which explains the GAR (http://www.dtic.mil/get-tr-doc/pdf?AD=ADA473824) but still am unsure about the physical meaning of "Soil moisture depth" and "Top layer depth". Can someone clear this up?

Ultimately, my basin of interest has approx. 1 m of silty loam resting atop an impermeable siltstone bedrock. A perched water table develops above the siltstone during the rainy season and it would be nice to recreate that with the model, but until now I've been unsuccessful with any of the Infiltration options. Could I model such a configuration with Richards Eq. or multi-layer G&A?

 

Thanks in advance to anyone who reads! Hope it's all clear :)

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Joel,

I answered a question in another post.  The answer to this question is that it's hard to answer without seeing your model.  You can make sure you have good starting values for all the evapotranspiration parameters in your model.  I'd recommend the GSSHA course or contacting tech support with your model as I described in your other post.

Let me know if you have any other questions.

Chris

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Joel,

Here's a message from the GSSHA developer, Chuck Downer:

It's pretty hard to say why he is not getting any ET.  If he is in perpetual event mode, there is going to be little or no ET because ET will not occur in a cell that has any rainfall in it.  So if he has some period that has constant rain, then the ET is going to be close to zero, and I can see no reason why that is not correct.  Also, if his soil moistures were very low for some reason, below wilting point, he would get no ET.  Other than that, it's hard to say based on the input.

He can capture the effects of a perched water table, more or less, by using the multi-layer G&A model, and can specify the layer depths and the vertical hydraulic conductivity of the layers.  If he puts a low permeability layer in there, the water will perch above, it won't move laterally of course.  From the sounds of it, he probably wants to do this.  If he uses GAR his SOIL_LAYER_DEPTH should be less than the impermeable layer depth, the top layer depth is typically a calibration parameter, as is the soil layer depth.  If you use MLGA, you don't have to worry with these.
 

Hope this helps,

Chris

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