1. A method of calculating an ET value for an irrigation area, comprising: 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; calculating an irrigation area weather parameter providing a representation of weather within the irrigation area from the at least one weather parameter of the weather parameter area; calculating the ET value for the irrigation area from an N-dimensional continuous function using the at least one irrigation area weather parameter, wherein the N-dimensional continuous function includes at least a 3-dimensional spatial cube.