Maximum Allowable Drought in Century Succession

[Paul Henne]

I am compiling species parameters for the Century Succession Extension, and am uncertain how to assign the Maximum Allowable Drought parameter.  Here is the description from the User Guide:

2.17.6 Maximum Allowable Drought
If available water falls below zero for a percent of the growing season greater than this value, a species cannot establish. Units: fraction of the growing season (0.0 – 1.0). Lower values indicate species whose establishment is more sensitive to drought.

The purpose of the parameter is clear to me, but how to estimate it is not.  From the description, it seems the parameter is calculated differently than drought indices that are more easy to find such as the annual average of AET/PET or that are used elsewhere by Century such as the ratio of Available Water to PET where growth stops (pprpts(3)). 

What I am confused by is how available water can fall below 0.  Does estimating this parameter require calculating the proportion of the growing season where there is no available water?  The proportion where AET/PET < 1?  Or actually where available water < 0 such as when soil water content < wilting point?  Any suggestions for how to estimate this parameter, such as from distribution data would be much appreciated. 

Thank you in advance and best wishes, Paul.  

[Robert Scheller]

Paul,

This parameter was borrowed from LINKAGES and is used to calculate the probability of establishment.  A drought day is one in which available water is less than the wilting point.  The fraction of days is based on the growing season, i.e., percentage of the growing season with available soil water less than the wilting point.

Rob