A new approach for the estimation and control of the quantity of water applied in an irrigation is presented in which irrigation is stopped when the wetting front reaches a critical depth, ZL. An expression for calculating the critical depth ZL was developed. A major parameter in this expression is the velocity of advance of the wetting front, V, which was shown to be directly related to the application rate, IR, and inversely related to the initial soil water content, 0 i. A depth probe (patent pending) was designed, constructed and tested for the purpose of monitoring the position of the wetting front during infiltration and redistribution and for computing the value of F. Equations developed for relating the velocity of advance of the wetting front to 01 as well as for estimating the value of the critical depth Z L were successfully tested under conditions of uniform distribution of the initial soil water content. An iterative learning process which utilizes the real time output from the depth probe during each irrigation and is therefore capable of handling realistic field conditions where nonuniformity is the rule is presented. The acquired information is used to estimate a critical depth of the wetting front, ZL, for a planned final wetted depth, ZF, during each irrigation. This process is incorporated in the depth probe and is used to stop irrigation and thus control the quantity of water applied.

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