5.5: Irrigation System Capacity

In addition to meeting the cumulative seasonal irrigation requirement, irrigation systems must be able to supply enough water to prevent crop water stress during short time periods when plant water requirements are at their highest. The system capacity is the rate of water supply that the irrigation system must provide to prevent this water stress during peak demand. The system capacity must account for peak crop need and the efficiency of the irrigation system. The net system capacity (C_n) is determined by the supply rate needed to maintain the soil water balance above a specified level that will reduce or minimize water stress. The gross system capacity (C_g) is the combined effect of crop needs and system inefficiency. Net and gross capacity are related by the application efficiency and the percent downtime (D_t) for the system:

\(C_g = \frac{C_n}{\frac{E_{LQ}}{100\%}\left(1-\frac{D_t}{100\%}\right)}\) (5.20)

where: C_g = gross system capacity,

C_n = net system capacity,

E_LQ = application efficiency of low quarter (%), and

D_t = irrigation system downtime (%).

Here, system capacity can be expressed as depth per unit of time, e.g., in/d, or flow rate per unit area, e.g., gpm/ac. For the latter case gross system flow rate is determined by multiplying C_g by the irrigated area. A useful conversion is 18.86 gpm/ac = 1 in/d.

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