Last Updated July 12, 2001

July 12, 2001

Q:
	I have only just realised that one of the compulsory parameters for
	Rhone-Agg is that the total soil depth should be a function of
	vegetation class. Our model cannot do this!  ( In my mind I had
	thought soil depth was to vary spatially between gridboxes, rather
	than between classes, but obviously this was wrong.)  

A:
	   Yes, it is true that the total soil depth is probably not
	directly related to vegetation type in the "real world": but 
	we did not have measured soil depths at a high spatial resolution
	over the Rhone domain. The ISBA-Modcou 
	system was originally developed over the
	Adour basin in France, and in studies by Florence (in her
	thesis), she found that certain general total soil depths
	worked well for certain surface/veg types (while keeping
	the input dataset as simple/general as possible by
	limiting the number of different parameter values). We use the
	same basic relationships for the Rhone. The relationship
	between d2 (root zone depth) and d3 (total soil depth)
        is nearly constant.
	
	
Q:
	We have vegetation tiles, each of which can have its own root depth, 
	but they share a common soil model and so all must have the same 
	total soil depth.

	I propose to assume spatially uniform soil depth for our model.

A:
	   If your model requires a constant total soil depth everywhere,
	then go ahead and do this. Since the maximum rooting depth
	provided is 2 m (and since you'll be using the prescribed
	rooting depths), I would suggest you use a total soil
	depth of 2 < depth_total <= 3 m (3 m being the upper limit in 
	order to be consistent with the upper limit used by the 
	other modelers). I strongly suggest 2.25 m for the total soil depth
	(for Exps 1 and 2) as this is the average over the entire basin
	(so that you will be using a soil depth which is at least
	fairly consistent with the average of the other modeling groups:
	your total soil water holding capacity is not too far 
	from the other models using this value). For Exp3, you'll have
	to calculate the average total soil depth for each of the 3 sub-basins
	using the provided contributing area fractions.