Monitoring irrigation flows down each furrow while growing crops is, however, labor-intensive. This practice can eliminate surface runoff.Properly designed and maintained sprinkler and drip irrigation systems for field crops can enhance irrigation efficiency and help eliminate surface runoff. However, this conversion increases capital costs over that of furrow irrigation, so it is generally practiced only on high-value crops such as tomatoes and other vegetable crops. Growers making this conversion must weigh the costs versus benefits of installing these irrigation systems, since the impact of the conversion on yield may be unpredictable.Surface runoff can be eliminated by either recirculation or storage and reuse systems. Recirculation systems collect surface runoff in a small reservoir or tailwater pond at the lower end of the field and then recirculate the water onto the same field. The recirculated water should be used on a part of the field that has yet to be irrigated,to reduce the volume of water in the tailwater pond. Simply recirculating the runoff to the same irrigation set that generated the runoff only ponds more water on the soil surface, resulting in runoff. Storage and reuse systems collect all of the surface runoff from a field in a tailwater pond and then used to irrigate another field with the water at the appropriate time. This approach requires a farm with multiple fields, round pot a relatively large reservoir, and a distribution system to convey surface runoff to the storage reservoir and to convey the stored water to the desired fields.

Practices that can improve the quality of surface runoff from furrow-irrigated fields include sediment traps, vegetated drainage ditches, and adding polyacrylamide to source water to help flocculate suspended soil particulates and cause them to settle out of solution. These practices target offsite movement of sediment and sediment-associated pesticides, as well as other pollutants. Vegetated ditches also help reduce offsite movement of pesticides dissolved in solution, primarily through increased soil infiltration. The impact of these practices on receiving waters depends on how they are designed and implemented as discussed in this section.A sediment trap can best be defined as a basin constructed to collect and store sediment that would otherwise be carried from the field by surface water runoff . Sediment traps are different from tailwater collection ponds in that they may be smaller and there is no pump and return system to reuse the water for irrigation. Sediment traps have inlets and outlets: surface runoff flows from the outlet into the receiving waters. Sediment traps may be the next step to consider for improving tailwater quality after source control measures such as furrow irrigation system improvements and application of PAM have been implemented or at least considered. The concept behind a sediment trap is to use a basin or trap to temporarily retain the irrigation tailwater for a reasonable period of time to reduce the flow velocity and turbulence. This holding period may provide sufficient time for a significant portion of the suspended material to settle out of the water.

The trap would also capture larger particles that are not suspended but move along the bottom of the furrow as bedload. The result would be improved quality of the tailwater that exits the trap and flows into receiving waters. Two variables largely determine the design and effectiveness of sediment traps for reducing pesticides in tailwater runoff and improving water quality. The first is the characteristics of the pesticides involved. For example, fields treated with water-soluble pesticides have a low coefficient of adsorption to soils, so a sediment trap is ineffective. These pesticides are dissolved in the aqueous portion of the runoff and will simply flow through the trap into the receiving waters. The second variable is the makeup of the sediment particulate in the runoff, which is largely influenced by the soil texture and surface soil condition at the time the field is irrigated. Coarse-grained or large-aggregated soil particles settle out of the runoff much more rapidly than do fine-grained silt and clay particles . For example, medium sand particles would be expected to settle at a rate of 1 vertical foot in 2.3 minutes, silt particles in 2.6 hours, and clay particles in several days. If runoff comes from recently cultivated coarse-textured loamy sand and sandy loam soils, the tailwater is likely to contain higher fractions of sand and coarse silt particles and less clay. In this case, substantial amounts of sediment may be captured with a sediment trap. Tailwater from croplands consisting of fine textured silt and clay soils has a high fraction of suspended silt and clay particulates in the runoff. Under these conditions, sediment traps will not be as effective because of the slow particle settling rate.

Fine-grained particles such as silt, clay, and plant-derived particles not only remain suspended in the water for long periods of time, they also generally have far higher concentrations of pesticides on them than does coarse-grained sediments. Thus, as coarse-grained material having a low pesticide content settles out of the water. Meanwhile, a phenomenon known as pesticide enrichment occurs in which the remaining suspended material appears to contain higher and higher concentrations of pesticides . This phenomenon means that while a sediment trap may capture a great deal of sediment, it may capture little of the pesticide because most of it is transported on the fine material. Sediment traps should be constructed in rectangular or linear shapes that help spread the water in the trap and provide a relatively long, slow, turbulent-free flow path for suspended sediments to settle out . The features of the land available for constructing sediment trays may dictate shape and design considerations. When earthen sediment traps are constructed, banks should be sloped at 1:1 to minimize bank erosion and prevent sloughing, which might otherwise add suspended sediment to the tailwater. Small sediment traps designed for smaller runoff flows may be constructed of concrete.Table 3 provides guidelines for designing and constructing sediment traps. The volume of trap storage or resident holding time needed to effectively settle out suspended sediments in runoff is approximated for a range of runoff flow conditions and particulate sizes of the suspended sediments. These approximations are based upon Stokes’ Law, where the settling times of suspended sediments can increase 10, 100, or more than 1,000 times as the particle size of the suspended sediments decreases. The approximations in table 3 suggest that sediment traps be designed 2.5 times larger than that needed for the actual holding time. This 2.5 times adjustment allows for the bottom portion of the trap to accumulate sediments within or over several irrigation seasons, to make trap cleaning and maintenance less frequent. The middle and upper portion of the trap function as the active portion of the trap, where tailwater enters and flows through. Sediments settle out to the lower portion, and higher-quality tailwater flows out. The remaining 20 percent of the trap volume provides “free board” and a place to construct inlets and outlets. Free board is the distance between the crown, or top, of the trap and the surface of the water inside the trap, round planter pot which it protects against over topping the banks during periods of high flow or from wave action due to wind. The suggested design factor of 2.5 may be adjusted downward, to some extent, to reduce the sediment storage volume and the costs associated with building the traps. Sediment traps are more feasible for trapping coarser suspended sediments than fine sediments . Table 3 shows that approximately 40 cubic feet of storage volume per 50 gallons per minute of runoff flow is needed to trap medium-grain sand and coarse particles.

In comparison, for the same runoff flow rate of 50 gallons per minute approximately 324 cubic feet is needed to trap fine sand and coarser particles; 2,760 cubic feet is needed to trap fine silt and coarser suspended particles; and 6.3 and 57 acre-feet are needed to provide enough holding time to settle fine silt and clay particles, respectively, 1 vertical foot . Thus, sediment traps may be very useful to trap coarse sediments but are probably not feasible for trapping fine sediments. If the predominant suspended sediments in the runoff are fine silts and clays that pose a risk of transporting adsorbed pesticides, it may make sense to direct the runoff through vegetative filters or use PAM instead of a sediment trap.Vegetated ditches are drains planted with vegetation that are designed to capture and filter surface water runoff from crops to protect water quality . The three main processes responsible for the effectiveness of these filter strips in reducing pesticides in surface runoff are sedimentation, infiltration, and adsorption. Sedimentation is the tendency for particulates and sediment-sorbed pesticides to settle out of suspension as the water slows down and flows through a filter strip. Infiltration is the process by which pesticides in solution and on fine particulates penetrate the soil, where they are usually broken down to less-toxic forms than the parent material by microbial activity. Adsorption is the process by which pesticides are taken out of solution as the water flows through a strip by the above ground vegetation and soil surface, where they may be broken down to nontoxic forms by natural processes such as sunlight and microbial activity. Vegetated ditches have been documented to significantly reduce the offsite movement of water-soluble pesticides as well a those that attach to sediments . The degree to which these filter strips function depends on their size, slope, density, the height of the vegetation selected, and the degree of soil saturation. Their effectiveness is also influenced by the size of the cultivated field and slope, the erosion rate, the volume and velocity of tailwater, and the sediment inflow, including grain size, aggregation, and concentration. Specific pesticide properties such as the degree of water solubility and how tightly the pesticide binds to sediments also affect how well vegetated ditches can protect water quality: for example, sediment-associated pesticides such as pyrethroids are more likely to be trapped via sedimentation than water-soluble ones that may flow through the filter strip. As a result of the many variables involved in how well filter strips function, it is difficult to predetermine the size needed to effectively filter a given runoff volume from a given field. The best vegetated ditch would be one that captures all of the water coming off a field. However, this may not be practical because the size of the filter strip needed to filter a given volume of tailwater may take too much cropland out of production to make it economically viable. In research trials in the Sacramento Valley, a 5-foot-wide by 160-foot-long filter strip planted to fescue and ryegrass reduced sediment and pyrethroid loads in tailwater by 60 percent from crop sites that were 0.7 acres in size . Field length at the crop site was 650 feet, and the crop was furrow-irrigated ata rate of 15 to 20 gallons per minute per furrow. The runoff entering the filter strip reached a maximum flow of about 55 gpm and diminished to a very low flow that was difficult to measure due to the slow velocity at the end of the filter strip. As a rough approximation and guideline for sizing filter strips, this experience suggests that about 1,450 square feet of vegetative filter may be needed per 100 gallons per minute of tailwater to significantly improve the water quality of field runoff. Prior to establishing vegetated ditches on farms it is important to conduct a whole-farm analysis to assess how and where the water flows, along with volume and velocity, to determine how to best incorporate them on farms. Investing in a vegetated ditch is costly, both in time and money, and must therefore be evaluated carefully against to other water quality protection strategies that may be implemented on farms. One strategy would be to plant existing drainage ditches on farms with perennial grasses; another would be to create new vegetated ditches on farms in areas that would capture and filter surface water before being discharged into drains . The following discussion summarizes the practices involved in installing vegetated ditches on farms.The water would flow through the strips, following the grade of the field, and discharge into a main drain.