Flood, furrow or surface irrigation is a method of passing a volume of water over a confined surface in order to achieve a desired soil infiltration. Modern laser-grading techniques have allowed large areas (or bays) to be defined and accurately graded so that improved efficiencies can be achieved. In addition, high flows of water onto bays are resulting in improved water application efficiencies on many soil types. Most bays are rectangular in shape and graded to a uniform slope.
The challenge is to apply the correct amount of water to an irrigated crop using flood or surface irrigation such that:                1. The depth of infiltration is consistent with the required depth of infiltration of the crop throughout the entire bay.        2. There is no over-watering such that no excess water runs off the end of the bay, and no under-watering such that water does not reach the end of the bay—the precise time to stop the flow onto the bay.        
The difficulties in this task are:                a. The infiltration rate of the soil is unknown, and so although the volume of water applied is known (by multiplying the time of application by the measured flow rate), the infiltrated volume is unknown and so the remaining volume of water above the surface must be measured.                    Although the infiltration rate is likely to be consistent across the bay, it can vary from irrigation to irrigation.                        b. To determine the volume of water above the surface, the depth of water above the surface must be measured and multiplied by the area of coverage. The difficulty in measuring depth is defining the surface level, or the datum above which the depth is measured. The depth of flow can vary from irrigation to irrigation due to factors such as crop density and resistance it makes to the flow or the existing soil moisture.        c. Traditional measurement of the ground level is not a simple or reliable process, as it requires a detailed survey to determine mean ground level. It is difficult to achieve the required accuracy due to the localized unevenness of the soil and to determine the point at which to record the surveyed surface level measurement. To reduce this error, many measurements need to be taken from which to derive the mean ground level.        