8.6.2.2. Full Supply Level (or Full Supply Line) Since the channel has been aligned on the watershed, the available transverse slopes would be sufficient for flow irrigation provided that the full supply level (FSL) is slightly above the ground level. The full supply level of a channel should, therefore, be about 10-30 cm higher than the ground level for most of its length so that most of the command area of the channel is irrigated by gravity flow. It is not advisable to keep the FSL higher than the isolated high patches of land existing in the command area. These may be irrigated either by lifting water from the watercourses or from the upstream outlets, if feasible. Too high an FSL would result in uneconomical channel cross-section of higher banks. Besides, the available higher head would increase the seepage loss, wasteful use of water by the farmers, and the chances of waterlogging in the area.
The FSL of the offtaking channel at its head should always be kept at least 15 cm lower than the water level of the parent (or supply) channel. This provision is made to account for the head loss in the regulator and for the possibility of the offtaking channel bed getting silted up in its head reaches.
