IM,
As I understand it, you have (grass?) growing in thin ( = ? inches? ) topsoil over (dune ?) sand.
You are on the right track -- "almost there", as it were , and as we almost always seem to be in trying to accurately predict the behavior of natural systems ;-)
The computational problem you have is not strictly speaking with the ET component. ET is always a negative term in the mass balance equation. You can get to the right answer computationally by refining your conceptual -- and resulting computational -- mass balance model.
Water In -Water Out = +/- Change in Storage.
I presume that your overiding overall objective is to keep soil moisture (generally speaking for most plants) greater than the wilting point.
Check to see whether the roots of the plants you need to irrigate extend significantly through the topsoil. If not, you can make the simplifying assumption that what you are trying to model (= predict) is the amount of water in the topsoil alone or, alternatively to the depth of the roots for shallow-rooted plants.
(((You might also ask yourself why you are growing this grass in this location with all the attendant problems. There are deep-rooted native grasses that would likely do jist fine albeit with a different aesthetic ...
http://www.duneland.com/news/2010/02/dunes...-plant-sale.php)))
Then estimate the specific retention ( ~ specific capacity) of the top soil.
You wrote correctly: "This is the factor that I have yet to characterize ". Specific retention is defined as the amount of water retained after a fully saturated porous medium is allowed to drain completely (gravity drainage). This is the maximum amount of water your newly-defined 'system' (the topsoil) can hold. So if the soil specfic retention is say, 1.0", you would take as an initial condition for your model of 1" of water in storage at the end of any rainfall in equal to or greater than 1.0".
You can detemine the specific retention by:
1) saturating the soil in place,
2) allowing it to drain,
3) digging up/coring a cylinder or plug of it with an carefully estimated soil volume prior to disturbance
5) removing (most emergent) vegetation
4) weighing the wet soil,
5) drying it,
6)-reweighing
7) calculating
If you choose a large enough piece with actively growing grass, you can also detemine the wilting point by not (5) drying it, but rather waiting for it to slowly and naturally dry to the point that the grass appears stressed. Weigh it at that point to determne the amount of water that is in your model volume at the 'wilting point',then, as before, 5), dry, 6) re-weigh and calculate to determine specific capacity.
Hope This Helps ... Marc
hult at hydrologist dot com
