A method of method of calculating an ET value for an irrigation area is disclosed. The method includes a weather parameter measurement system collecting at least one weather parameter of a weather parameter area, the at least one weather parameter of the weather parameter area providing a representation of weather outside of the irrigation area. An irrigation area weather parameter is calculated providing a representation of the weather within the irrigation area from the at least one weather parameter of the weather parameter area. The ET value for the irrigation area is calculated from an N-dimensional continuous function using the at least one irrigation area weather parameter, wherein the N-dimensions includes at least a 3-dimensional spatial cube.
