"Max Depth of Water in Upper Soil Zone" aka Root Zone Depth: what is it & why is the default so low?

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"Max Depth of Water in Upper Soil Zone" aka Root Zone Depth: what is it & why is the default so low?

Post by rcoville » Tue Apr 25, 2017 8:01 am

This is the depth of the water column which can act as the interface between surface and subsurface flows. The Upper Soil Zone is involved in infiltration, saturation, percolation, and evapotranspiration processes. It represents an average for the entire project area. This parameter describes the maximum depth of the water column in the Upper Soil Zone, not the depth of that soil zone itself.

Synonyms for this parameter include Root Zone Depth, Max Storage Deficit, and Mean Subcatchment Deficit, the latter term being used in USGS TOPMODEL which i-Tree Hydro is based on.

This parameter represents the root zone 'storage' area, which evapotranspiration draws water from. It reflects an average for all vegetation types, from small grasses to large trees, so we set the default to represent a shallow area close to the surface (0.05m).

The acceptable range for that parameter is 0.001 – 1.0 meters, which is intended to capture the wide range and extremes of potential root zone stores. Though some tree roots (tap roots) can grow very deep, most roots of those trees still draw water and nutrients from the first few feet of soil depth. This parameter represents water column depth, which is even less than those first few feet of soil since water column depth excludes the depth of space occupied by soil particles. When tree roots are averaged with other plant roots we assume the average root zone storage water depth will not be greater than 1 meter deep.

We're not aware of how sensitive the Hydro model is to the root zone depth parameter at this time. It is likely that it can make a significant difference in model results due to its key role in driving evapotranspiration and influencing infiltration, which can in turn have significant indirect effects on soil moisture, pervious area runoff, and the overall water budget.
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