Rice Irrigation Systems for Tailwater Management (3/7)
J.E. Hill, S.C. Scardaci, S.R. Roberts, J. Tiedeman, J.F. Williams
Recirculating Irrigation System
Tailwater recovery systems facilitate the reuse of drainage water and help keep pesticide residues out of public waterways. Early tailwater recovery systems for rice were used for water conservation and were installed primarily in areas where water was in short supply or expensive. When pesticide use restrictions mandated longer water holding periods, the transition to completely closed systems was relatively easy for growers who already had recirculating systems. Although many of these systems have been developed for single farms, some neighbors share systems and some irrigation districts have developed districtwide recirculating systems.
 Recirculating systems have been installed on only a small portion of the approximately 400,000 acres of rice in production in California. They are, however, gaining greater acceptance because they provide maximum flexibility for rice irrigation. Growers with recirculating systems have a shorter water holding period, after pesticide applications, than do growers who use conventional systems.
Small, closed, recirculating systems consist of a lowlift pump that picks up tailwater from a sump and delivers the water to the top of the field by pipe or ditch (fig. 3). Larger multi-field and multi-farm recirculating systems use pumps to pick up tailwater from the lowest elevation of the system and return it to supply ditches.
| Figure 3. Schematic diagram of a recirculating tailwater recovery system. |
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Power for tailwater pumps may be supplied by electric motors, or internal combustion engines where electricity is not available. Electric motors have the advantage of automatic start-and-stop control operated from float switches. Pumps are used to lift the tailwater either directly to the field or into a highline ditch. Water then flows via gravity back through the irrigation system.
To obtain optimal performance from a larger recirculating system, fields should be laser-leveled and the flow of water must be directed to drainage ditches leading to the main drain. The depth of water in each basin is controlled by conventional rice boxes.
The cost associated with construction and operation of a recirculating system depends upon the total land area of the farm, the slope of the land (the smaller the lift between the farm's low and high point, the less the water lifting expense), and the layout of the fields (whether ditches will serve to recirculate all the tailwater, or if pipelines are needed). Farm size has been found to greatly affect per acre cost of recirculating systems. For example, observed costs have ranged from $20 per acre for a 1000-acre system to $150 per acre for an 80 acre system (1990 costs).
Table 2 presents the advantages and disadvantages of a tailwater recovery or recirculating irrigation system.
| Table 2. Tailwater recovery systems for rice production in California |
| Advantages: |
 Tailwater and pesticide residues can be contained on farm |
| Best water management flexibility of all systems, especially during water-holding period |
| Recirculation reduces cold water effects on rice and the need for a warming basin |
| Fewer problems than a flow-through system with seasonal shortages of irrigation water |
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| Disadvantages: |
 High cost of purchase, construction, and operation of tailwater recovery system |
| Requires land set aside for tailwater storage (pond or drainage canal) |
| When many basins are interconnected, the large water surface area makes precise water management difficult |
| Lacks independent control of each basin |
| High degree of management required to balance intake with use, since drainage is eliminated as a "safety valve" |
| Weeds must be controlled in tailwater storage area |
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Comments to jayoung@ucdavis.edu