I am a new hydrus1d user. I am simulating water flow and solute transport in a field with horizontal drains. The drains are located in 1 meter depth and my soil profile has a total depth of 5 meters. The impermiable layer is assumed to be in 5 meter depth. The field is located in an acid sulfate soil and we are interested in the drainage water quality.
I understood that the drainage flux is removed from the bottom cell of the simulated column and the same applies for the solutes. For example, I am interested in the soil pH that is lower in the upper soil layers and higher in the deeper layers. I am worried that if the solutes are removed from the bottom cell (at 5 meters depth), the drainage flux has higher pH than it should (pH in the drain layer is close to 4 and in the bottom cell close to 8.). This would also decrease the pH of the soil layers under the drain layer more rapidly than it should when more acidic water will flow towards the bottom and is not removed at the drain depth (1 meter). Can I do something with the model to fix this?
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The standard HYDRUS module (but none of the specialized modules, such as HP1 or UnsatChem) has an option to choose if the drainage flux (to horizontal drains, qdrain) is applied at the bottom of the soil profile or vertically distributed along the saturated part of the soil profile. This is obviously reflected in solute fluxes as well. J.