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Over estimated hydrograph

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I am trying to calibrate a GSSHA model, the hydrograph is over estimated, I calibrated the sensitive parameters even at the minimum or maximum to decrease the discharge and the hydrograph to the observed value or close to it but still over estimated, what else I have to look for? Thanks a lot.

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You may have some processes, such as infiltration, that you're not modeling or you have some parameters, such as Manning's roughness that you could change.  Note that overland Manning's roughness is significantly higher than normal channel flow Manning's roughness values, possibly by a factor of 10 because of such low flow depths.  See this page for suggestions of Manning's roughness values for use in GSSHA.  If you're modeling infiltration (which you should be if you're calibrating a model), you should be able to adjust the infiltration parameters such as Hydraulic Conductivity and Initial Soil Moisture so your runoff decreases.  Note that your initial soil moisture cannot be higher than your Field Capacity saturation.  Good starting values for Green and Ampt parameters are located on this page.  Let me know how things go.


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