Hi,
I have a question for all the soil scientists and geo-hydrologists around.
We want to model the runoff behaviour of a couple of watersheds where some soil and water conservation measures will be implemented (something like small trenches to retain runoff).
Our model is something simple and we are building it for an area with little data available (thus the need to stay simple). We want to roughly estimate how much additional water (runoff and direct rainfall) can be captured by the soil and water conservation structures. The model is a simple water balance for which we need infiltration values (measured with mini disk infiltrometer), which in turns is influenced by soil texture.
We have collected quite a number of samples at surface (0 to 20 cm) and subsurface (20-50 cm) for this. Here is my question for you:
When on a budget constrain, which depth would you prioritize to characterize texture - surface or sub-surface?
Our gut feeling is to prioritize the surface samples, but i would love the opinion of some scientists with hands on experience (possibly with some references to prove the point).
Any thought on this would be much appreciated!
Thanks a lot everyone,
Francesco
Surface cover and soils probably more important to runoff than subsurface. Small trenches, if overloaded on sloping terrain, could fail or overload steep, disturbed or unstable soil types or landforms. If terraces are not maintained and fill with leaves or other debris, that too can reduce capacity. As long as terraces are constant elevation (contour), the chance of failure lessons. Some soil types tend to severely erode, cavitate, settle if disturbed, so monitoring and maintenance may be needed to ensure continuing function. Maintaining quality soil cover and root density, organic content help promote infiltration. Forests tend to ave high infiltration. Relative to subsoils, it is good to know if there is restrictive pan or bedrock limits to consider. If the soils have been classified and mapped by a soil scientist, this should help answer some of your questions specific to watershed conditions. If your area is subject to high rainfall events from tropical storms, monsoons, hurricanes, etc., it may be important to factor in both surface and subsurface. Severe wildfires, frequent soil disturbances, compaction and roads/trails are some of the factors that can affect infiltration and runoff. The development of rills and gullies can also promote runoff.
The priority would be for the surface layers. If low hydraulic conductivity layers are expected in the subsurface layer, the importance of characterizing these strata would be increased.
With budget constraints, it might be a valid idea to prioritize field infiltration testing to calibrate empirical models, in addition to soil density and macroporosity.
An additional note: in certain oxisols, texture is not a determinant of infiltration capacity, but rather soil structure (granular), which results in high macroporosity. In these cases, even for clay soils but with granular texture, the infiltration capacity is high.
Just houghts I'm afraid.
It seems to me that there will be a balance between evaporation and penetration below the depth affected by evaporation. I don;t have any idea how to parameterise such a model but others will
It's hard to give a definite/definitive answer to you question/problem. The surface layer might be limiting water infiltration, but clayey B horizons or even subsurface compacted layers may be even more restrictive to water flow (drainage). The best, of course, woule be considered both: surface and subsurface permeability.
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