Dynamics of soil wetting

The process of soil wetting goes through two stages:

During the wetting stage a soil depth ZI is wetted in excess to a maximal and relatively uniform with depth soil water content ӨI.

During the drainage stage, following cessation of irrigation, excess water from the upper soil layers drains downward at a dissipating rate

Eventually it reaches a quasi stable soil water content distribution ӨF down to a soil depth ZF. The soil water content distribution at the end of the drainage stage is defined
as the field capacity representing the maximal long time water storage capacity of the profile.

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When estimating the optimal quantity of water to apply in an irrigation, the objective is to eventually wet the soil profile to the water content of field capacity to a soil depth ZF equal to the depth of the bottom of the active rooting zone.

For a given soil profile and an initial soil water distribution Өi there is only one quantity irrigation water that would wet a soil depth ZF to the water content of field capacity of the profile, areas A and C in the figure. A larger quantity of water would result in the wetting of a larger soil depth, below the rooting depth, and visa versa.

The difficulty with using this approach for irrigation control is that the final depth, ZF is reached some houres after stopping irrigation. Thus the challenge is to estimate a value for stopping irrigation, Z‘I such that the drainage front would stop at the final depth, ZF, at the end of the drainage stage.

The procedure to estimate a value for the position of the wetting front to stop irrigation calls for an understanding of the relationships between ZI and ZF. It calls for a system that can measure and identify the location of the wetting and drainage fronts in real time.

 
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