Computation of advance and intake opportunity time

The difference between an evaluation and a design is that data collected during an evaluation include inflows and outflows, flow geometry, length and slope of the field, soil moisture depletion and advance and recession rates. The infiltration characteristics of the field surface can then be deduced and the application efficiency and uniformity determined. Design procedures input infiltration functions (including their changes during the season), flow geometry, field slope and length, and determine the rates of advance and recession as well as the field performance levels for various combinations of inflow and cutoff times.

The calculation of advance time is possibly the most important design step. At the beginning of the design process, this procedure is used to test whether or not the maximum flow will complete the advance phase within a prescribed time. Then it is used to find the minimum inlet discharge, and in the case of cutback or reuse systems to find the desired flow for the system operation. It is suggested that after the maximum inflow is determined and the associated t_L checked, the flow be incrementally decreased and additional values of t_L determined so that a relationship between flow and advance time can be established. At the end of this procedure, the minimum flow will also have been identified as that which fails to complete the advance phase in a set time, 24 hours for example. Finally, the t_L computation is used repeatedly in the search for the flow which maximizes the application efficiency.