Vlad,
If you can measure soil moisture content directly, you can bypass the ET calculations completely since they are used to estimate soil moisture. The core of the algorithm is the “checkbook” balance calculation. Instead of calculating the balance each day based on rainfall, irrigation and evaporation (ET), you can just measure the balance directly. The various zone calculations using factors like soil type, plant types and root depth would still be useful to determine how long to water. These calculations do not involve ET or any weather data. Good luck, and let us know how it goes.
Dan