Abstract: A rain gauge includes an orifice which is of a diameter generating water drops of uniform mass. A sensor below the orifice senses individual water drops. An adapter for an irrigation controller having a master valve output and a rain switch input and a system therefor includes a rain gauge input, a weather station input receiving Evapotranspiration data (ET) and an input from the master valve output receiving irrigation program cycles. A routine for updating the current water balance value by reducing the sum of the rainfall and the irrigation cycles by the value of ET is used with an output to the rain switch input responsive to the determination of the current water balance value. A maximum water balance value and a minimum water balance value are maintained to compare with the current water balance value to control the irrigation program.