Background: Tailwater ponds, especially those of two-stage design, are changing how row crop farmers manage runoff water in Yolo County. While ponds built over the last decade may not have shared the same objectives, their cumulative success demonstrates the excellent multi-purpose benefits from well-designed tailwater ponds. Not only do ponds offer solutions to widely-recognized surface water quality problems, they provide valuable wildlife habitat and enhance ground water recharge.

Without intervention, irrigation water turns into unrestricted runoff, thus bringing about a series of problems. Irrigated row crop fields that drain to one or several main low spots often empty into an initial drainage system that then dumps into a main water conveyance channel (canal or stream). Silt-laden runoff results both from summer irrigation and, even more so, from winter storms. Runoff not only removes topsoil from farmland, but deposits this resource downstream, at unwanted places throughout the watershed and beyond.

Ditch maintenance is perhaps the most visible impact of this sediment-laden runoff, but not the only one. First, public works crews must constantly remove silt from roadside ditches. Streams and canals also need periodic expensive excavation. Furthermore, quality of water flowing into the Sacramento River Delta, and Bay has been seriously degraded, thus adding to non-point source pollution. A third result of unrestricted runoff is the loss of the water itself. For Yolo County farmers, it makes much more sense to recapture this lost resource or reuse it or return it to our groundwater storage systems.

Design: A straightforward, cost-effective solution to all these widespread problems is the construction of a small double-pond system that catches and stores at least part of the runoff water. The double-pond design works efficiently by making the first, small pond work as a sediment trap, engineered for easy excavation of silt that is easily replaced on the field or farm roads during fall groundwork. The second, larger pond serves many other purposes: water storage, ground water recharge, water return systems, and plant and wildlife habitat. 

The larger pond can be designed with a natural shape rather than the usual long, narrow trench of some return systems. A curved "L" shape is easy to construct with standard scrapers. A gradual 3:1 or 4:1 slope (meaning for every 3 or 4 feet of distance there is 1 foot of fall) for the waterline area (and above) is preferable, with subsurface slopes no steeper than 1:1 to the pond bottom. Also, the pond should not be less than 5 feet deep, to minimize the encroachment of unwanted weeds. As water percolates or evaporates, the pond surface simply decreases in circumference. Wildlife will continue to use it even when it turns into a puddle. The gradual slope also creates several moist soil planting zones for the establishment of wetland species that can compete against unwanted weeds..

The overall size of your pond could vary greatly, depending on whether it will be used to capture and hold tailwater for wildlife only, or to also recirculate that water for irrigation. If you are planning a tailwater recirculating system (tailwater return system), pond and pump sizing will depend strongly on how you manage crop irrigations. Factors to consider in the design of the pond and the sizing of the pump and flashboard risers are: the amount of irrigation water you will be running, (measured in cubic feet per second (cfs) or gal/min), whether you will be running half or full sets, and the anticipated rate and volume of runoff.  For pond design purposes, runoff is usually considered to be approximately 25 - 40% of the amount of water applied in a surface irrigation. Another important factor is whether you will be returning the water to the upper end of the same field or sending it downstream to another field. With such a variety of scenarios for recirculating systems, it is important to consult with someone that has experience in pond design, such as your local NRCS engineer or a private consultant.

If you plan to have your pond simply capture and retain irrigation runoff, sizing and design becomes simpler. The deciding factors may simply be how much space you have or how much land you are willing to take out of production. A common size would be a 1-acre area in a 100 acre field, usually in the lowest corner. Half of the 1-acre pond site would be occupied by the pond and the other half by a landscaped mound created by excavation spoil. The mound provides structural diversity to the landscape which will in turn allow for the establishment of a wider variety of plants and animals. Mounding on the site also reduces the expense of moving the dirt during excavation. However, in order to maintain slope stability, any berms or mounds created from the pond spoils should not be closer than 12 feet from the pond edge. If you want to minimize the  loss of farmable acres, the pond spoils could also be redistributed over the field.

Water control structures, such as drop pipes, flashboard risers or weir boxes are important for controlling water movement and water levels in the sediment trap and pond. A flashboard riser, for example, should be used as the entry point from the sediment trap to the pond and should also be used at the pond outlet. The pipe barrel should not be less than 12 inches in diameter to reduce clogging from debris. The riser, or upright part of the structure, is always larger (approx. 1.5 X barrel diameter) and is based on the maximum water expected to flow through the structure during a given storm or irrigation event. Riser heights are standard at three or four feet and up, but it should always be high enough to see in order to avoid equipment damage. A steel stake can be a good marker to improve visibility of the structure.

Vegetation:  The plant species incorporated into a tailwater pond system determine the system's functional and biological value. A pond initially established with the right plant materials becomes a self-sustaining, weed-free system. The planting areas within a system include the edge of  deep water that rarely goes dry (Zone 1), a moist soil area that is intermittently under water (Zone 2), edges adjacent to the high water level and dryland areas with or without mounds. Dryland areas with mounds have slope and exposure zones. The two zones in the moist soil area will vary considerably depending on the permeability of the soil and how often the pond is filled during the irrigation season. The accompanying plant list below names some of the species that can be established and managed.

Water Quality and Storage Benefits:  The degree of ground water recharge in these systems will vary according to soil permeability. One pond alone is probably not significant, but one pond for every hundred acres increases recharge range. Out of the volume of water that a singe small pond holds, perhaps half of this water may return to the ground every time the pond is filled either from winter storms or irrigation. If this happened six times in a season and there were ten 1-acre-foot ponds on 1,000 acres, 30 acre feet of water would recharge into the ground.

By acting as biological filters, vegetation in ponds absorbs excess chemical nutrients. Therefore ponds help to improve water quality. This has become a high priority mandate in California. Agricultural practices have been identified by EPA and State Water Board as a leading cause of poor water quality in the Sacramento-San Joaquin River systems. In short, the installation of tailwater ponds has excellent potential to improve the quality of agricultural water runoff while at the same time enhancing wildlife and recharging ground water.

 Yolo County RCD promotes this land stewardship practice and we encourage farmers and landowners to consider installation of these valuable systems. Cost-sharing is available through NRCS conservation programs. Technical assistance is available through the NRCS office.

Suggested Plant Materials for Tailwater Ponds

Moist Soil Vegatation Zone 1 (Waterline and below)

• Spikerush (Eleocharis macrostachya and others)
• Sedges (Cyperus species)
• Rushes (Scirpus and Juncus species)

(Establishing short-statured rushes and sedges will keep out unwanted species such as cattails and bulrushes, which will dominate a small wetland if allowed to proliferate.)

Moist Soil Vegetation Zone 2 (0' - 1' above the waterline)

• Species listed in Zone 1
  
• Barbar's sedge (Carex barbarae)
• Clustered field sedge (C. praegracilis)
• Meadow barley (Hordeum brachyantherum)
• Bentgrass (Agrostis exarata)
• Hairgrass (Deschampsia caespitosa)

Pond Edge

• Meadow barley (Hordeum brachyantherum)
• Bentgrass (Agrostis exarata)
• Hairgrass(Deschampsia caespitosa)
• Slender wheatgrass (Elymus trachycaulus)
• Creeping wildrye (Leymus triticoides)
• Clustered field sedge (C. praegracilis)
• Barbar's sedge (Carex barbarae)

Dryland Native Grass Mixture

• Blue wildrye (Elymus glaucus)
• Purple needlegrass (Nassella pulchra)
• Oniongrass (Melica californica)
• Pine bluegrass (Poa secunda)

Trees (organized from high to low moisture tolerance)

• Willows (Salix spp.)
• Cottonwood (Populus fremontii)
• Sycamore (Platanus racemosa)
• Black walnut (Juglans californica var. hindsii)
• Valley oak (Quercus lobata)
• Interior live oak (Quercus wislizenii)
• Buckeye (Aesculus californica)

Shrubs  (organized from high to low moisture tolerance)

• Mulefat (Baccharis viminea)
• Button willow (Cephalanthus occidentalis)
• Elderberry (Sambucus mexicana)
• Wild rose (Rosa californica)
• Coyote brush (Baccharis pilularis)
• Redbud (Cercis occidentalis)
• California lilac (Ceanothus spp.)
• Toyon (Heteromeles arbutifolia)

Note: The grass, tree, and shrub species listed are all commercially available. Many of the wetland sedges and rushes are also commercially available but can be otherwise obtained by transplanting from natural stands.