Water is released from field ditches through siphons or by cutting temporary gaps through the earthen ridge of secondary ditches. After filling a basin with water, the farmer removes the siphons or closes the gap and repeats the procedure at the next basin. Many crops are irrigated by furrows, which are ditches between ridges on which the crops are planted. The water, coming from the laterals, is admitted to each furrow by cutting away a small earthen dike, thus opening a gap. When the water in each furrow has reached the desired level, the supply is cut off by reclosing the dike.
Water seeps into the soil and feeds the roots of the plants.
Compared to flooding, this method is more expensive to build and to operate. It can be justified, however, for high-value crops such as vegetables. If soil conditions are favorable, and the groundwater table is near the surface, subirrigation, or underbed irrigation, is used. Here water is delivered to the field in ditches and allowed to seep into the ground to maintain the desired groundwater level to feed the roots of plants. Compared with the flooding method, the amount of irrigation water is reduced significantly, but subirrigation also requires water of good quality and with low salt content.