Inlet discharge control

Surface irrigation systems have two principal sources of inefficiency, deep percolation and surface runoff or tailwater The remedies are competitive. To minimize deep percolation the advance phase should be completed as quickly as possible so that the intake opportunity time over the field will be uniform and then cut the inflow off when enough water has been added to refill the root zone. This can be accomplished with a high, but non-erosive, discharge onto the field. However, this practice increases the tailwater problem because the flow at the downstream end must be maintained until a sufficient depth has infiltrated. The higher inflow reaches the end of the field sooner but it increases both the duration and the magnitude of the runoff.

There are three options available to solve this problem, at least partially: (1) dyke the downstream end to prevent runoff as in basin irrigation; (2) reduce the inflow discharge to a rate more closely approximating the cumulative infiltration along the field following the advance phase, a practice termed 'cutback'; or (3) select a discharge which minimizes the sum of deep percolation and tailwater losses, i.e., optimize the field inflow regime. Examples of these alternative practices are discussed and illustrated in Section 5. In this configuration, the head ditch is divided into a series of level bays which are differentiated by a small change in elevation. Water levels are regulated in two bays simultaneously so that the lower bay has sufficient head to produce an advance phase flow in the furrows while in the upper bay the head is only sufficient to produce the cutback flow. Thus, the system operates by moving the check-dam from bay to bay along the upper end of the field.

Two very recent additions to the efforts to control surface irrigation systems more effectively are the 'Surge Flow' system (Figure 6) developed at Utah State University, USA and the 'Cablegation' system developed at the US Department of Agriculture's Snake River Water Conservation Research Center in Kimberly, Idaho, USA. These systems will be dealt with in more detail in a later section.