Rice Irrigation Systems for Tailwater Management (2/7)
J.E. Hill, S.C. Scardaci, S.R. Roberts, J. Tiedeman, J.F. Williams
Conventional Irrigation System
The conventional irrigation system is also known as a "flow-through" system, because water is usually supplied in a series from the topmost to the bottommost basin (check or paddy) and is regulated by wooden weirs or "rice boxes" (fig. 1). Spillage from the last box, usually into a drain, is necessary to maintain water levels across all basins.
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Figure 1. Wooden rice box (weir) placed in a levee between adjacent basins. |
 Rice boxes are placed about 4 inches below field grade, usually at both ends of each levee separating the basins within each field (fig. 2). Water level within the basin is regulated by adding or removing boards in the weir structures.
Initial flooding may take 3 or more days at maximum water flow rates. Flow rates for field maintenance then decline to between 2 and 3 cubic feet per second per 100 acres.
Because of the large water surface area of the fields, precise water management can be difficult. To correct the depth in any particular basin, water must be introduced at the top of the field and then moved through all of the basins. To drain a basin in the middle of the field, the basins immediately above and below it must be drained. Such changes can require a number of days to complete since many basins are involved.
| Figure 2.
Schematic diagram of conventional flow-through irrigation system. |
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The constant addition of cool water in the top basins often delays rice maturity and adversely affects yield in areas close to the inlet. Occasionally, a warming basin is used to mitigate these adverse effects. Additionally, introduction of water into the field, too soon after an application of the herbicide Londax®, can result in poor weed control in the top basins.
Because the water needs of every field vary with temperature, wind, relative humidity, soil type and plant growth stage, spillage of water from the bottom basin is often necessary to maintain a desired water depth. In practice, to avoid underestimating water requirements, spillage rates can be high. It has been estimated that 20 percent or more of the water used for irrigation with a conventional system is spillage.
To keep spillage to a minimum growers precision level their fields to very flat slopes, thus improving water control, and by manipulating in-field water depth. Current state regulations require a no-flow (holding) period after pesticide applications. Producers manage this no-drain period by building up water depth above what is desirable for plant growth and restricting inflow, thereby creating a temporary static situation. Holding water can be difficult in conventional systems, because water tends to move down slope resulting in excessive water depths in the lower basins, while exposing the soil in upper basins. Occasionally spring rains may raise water levels even more.
In summary, the flow-through system was designed to be self-regulating and was not intended for holding water, as current regulations require. Growers can "block-up" fields and basins during mandatory water holding periods, but this limits water management options. Table 1 presents some of the advantages and disadvantages of flow through systems.
| Table 1. Conventional flow through irrigation systems for rice production in California |
| Advantages: |
 Low cost |
| Low management if water holding is not required |
| Flushes salt from fields |
| Easy to install, maintain, and remove |
| Works well with irregular slopes |
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| Disadvantages: |
 Flow-through spill carries agricultural chemicals into public water |
| When many basins are interconnected, the large water surface area makes precise water management difficult |
| Requires careful water management during water holding period |
| In some areas, constant addition of cool water slows rice development in the intake basin |
| Lacks independent control of each basin |
| Excess water may build up in bottom basins and water in the top basins may get too shallow during the water holding period |
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