An irrigation system includes a valve that controls the flow of water through a sprinkler head, and a flow sensor that measures the water flow rate through the sprinkler head. In some embodiments, the valve is adjusted until the flow sensor measures a water flow rate that matches a first predetermined water flow rate. This is a first position for the valve. The valve opening is adjusted again by a set amount or until the flow sensor measures a water flow rate that matches a second predetermined water flow rate. This is a second position for the valve. The first and second predetermined water flow rates each correspond to a throw distance the water stream discharged from the sprinkler head will travel. This relationship between the water flow rate and distance is based on empirical data for the sprinkler. A second equation relating the valve opening to water flow rate, for a given pressure, is then used to estimate the water effective pressure. In some embodiments, the first and second valve positions and the first and second water flow rates are used to estimate the effective pressure using the second equation.