Over the past several years, few irrigated regions of the country have escaped the impacts of drought and in many places this has translated to increased costs and reduced availability of irrigation water. California growers in particular have experienced dramatic reductions in surface water deliveries and have increasingly turned to using groundwater to make up the surface water shortfall.  Consequently growers have experienced unprecedented water table level reductions that have increased the price of pumping and maintenance on their water wells. Growers have also responded to the drought by increasing personal and personnel resources dedicated to water management and have increased their efforts to better understand the complexities of irrigation water management. Efforts to improve on-farm water management practices must include these key elements:

• Elevating irrigation system design and management expectations.
• Better exploit our understanding of crop development and physiology including crop sensitivity to water stress.
• Increasing our capacities to measure and manage soil water storage.

Proper evaluation and integration of these key water management elements can be complex but can result in operating whole farm and field irrigation systems at peak efficiency over time. For example, it may not help us to increase the monitoring and measurement soil moisture if we do not have a more complete understanding of how specific changes in soil moisture content influence crop water stress and crop performance.  Similarly it would not be difficult to misinterpret plant water stress or soil moisture information in a field where water is not applied in a uniform manner. And how might field indicators of soil water availability be used in different field management settings? Interpreting soil moisture readings in a drip irrigated field that applies water multiple times per week will differ from that of a furrow or flood irrigated field that is irrigated using two to three week intervals. These complexities help to point out that each field is unique from a water management standpoint and that irrigation decision makers should not rely too heavily on any one piece of information to guide their water management program without also considering broader systems information.

Irrigation system design and management
Improperly designed irrigation systems are incapable of achieving high performance levels making efficient water management an impossibility regardless of how well water might be scheduled. Application efficiency is a fundamental measure of irrigation system performance defined as the amount of beneficially applied water in relation to the amount of total irrigation water applied to the field. One of the biggest obstacles to developing field systems with high application efficiency comes from the fact that many irrigation systems do not apply water uniformly.  Water applied in a non-uniform manner which commonly leads to larger soil water deficits on low water application areas and over-irrigating areas of the field that have higher than average application rates. To maintain preferred soil moisture levels in all areas of the field, water managers typically compensate by over-applying water on portions of the field which causes losses to leaching or run-off both considered to be non-beneficial uses of water. Properly designed irrigation systems work to achieve a high degree of uniformity and deliver near equal amounts of water throughout the field.

One of the more challenging design issues in drip irrigation systems is achieving near uniform pressures throughout the field. Even when fitted with pressure compensating emitters, fields that have large differences in line pressure are susceptible to significant differences in emitter output causing non-uniform applications.  This problem is more commonly associated with long field lengths which stretch the design capacities of our current drip irrigation products. And while many drip irrigation fields have relatively high distribution uniformity, maintenance and management issues continue to leave many fields with less than optimal efficiencies.  Field lengths of less than 800 feet are less prone to this concern while it is a much more common issue in field lengths exceeding 1100 feet.  This emphasizes the need to carefully consider the products being purchased and the pressure requirements of that product. Uniformity issues can also be exagerated as the system ages or in systems that are not properly filtered and maintained to avoid biological and mineral contamination.

Occasionally simple modifications in surface irrigation systems can also result in significant improvements in distribution uniformity and application efficiency. Many flood and furrow irrigated systems experience slow water advance times down the furrow or irrigated strip before the irrigation is completed resulting in long infiltration periods at the head end of the field relative to the infiltration period at the lower end of the field resulting in higher amounts of applied water at the head end of the field. Solutions to this problem have been achieved by increasing the on-flow rate of irrigation water, reducing the size of the irrigation check and by modifying the surface soil roughness to allow water to more freely move to the bottom of the field. These relatively modest system design changes can have a significant impact on delivering water with greater uniformity and efficiency.

Crop development, physiology and water stress management
Crop sensitivities to water stress are not constant throughout the growing season depending on crop type, growth stage and atmospheric conditions.  Most field and row crops are highly susceptible to the impacts of limited water availability during the germination and seedling development stages and require high soil moisture availability in the surface soil during this period. However, as the early vegetative growth period is initiated crops like cotton and small grains can go many weeks before the first seasonal irrigation water is required.  This is partially due to the rapid root growth that occurs in these plants and soil water is more easily extracted during these low evapotranspiration days. Alternatively, many vegetable crops including tomatoes, carrots and the brassicas require more frequent irrigation events early season to relieve mild water stress during this period needed to support a rapidly expanding plant canopy. Later in the season when roots are well established and the lower portions of the soil profile are exploited, many deep-rooted crops are less sensitive to water stress and water management strategies can be incorporated that take advantage of deep soil water reserves resulting in delayed irrigation scheduling.

Similar issues can be observed with the permanent crops as early season root flushes occur at different intervals allowing soil moisture to be exploited at varied depths depending on crop type. Generally, crop water stress sensitivities in permanent crops are often more acute during early leaf out periods if winter rains have not fully charged the soil profile and again during the rapid fruit growth periods.  Monitoring the developmental stage of the crop often provides insights to the physiological periods of the crop that are more sensitive to water deficits and points to periods when water deficiencies are more likely to result in impacts to yield and quality. Understanding these more sensitive water stress periods also allows us to better plan on the time of year to increase crop water stress monitoring by using tools such as the pressure chamber or canopy temperature tools that are used to evaluate the relative degree of crop stress. And while water stress limits may differ during the growing season, using established water stress guidelines when available, can provide sound guidance on irrigation scheduling decisions.

Managing and measuring soil water storage  
The measurement of soil water can appear to be an uncomplicated process that involves placing soil moisture sensors in the soil and reading the values to provide an irrigation management decision. And while this may be satisfactory for relatively simple, uniform and well understood field systems, there is much that should be considered in developing an approach to soil moisture monitoring if the goal is to maximize the beneficial uses of applied irrigation water. Successful growers and consultants that regularly monitor soil water and use the information as an irrigation scheduling tool agree that there are several important questions to consider before purchasing, installing and using the sensors to make informed irrigation decisions. Before investing the time and resources in soil moisture monitoring, establish basic achievable goals with the understanding that the higher the expectation and more detailed the goal, greater effort and resources will be required.

As goals are established for field monitoring, consideration of the type of sensor to be used is a good place to start as well as the price of those sensors and systems. Field soil moisture systems can start with an investment of a few hundred dollars or less and can run into several thousand dollars for a single monitoring site. Soil water sensors can monitor soil water content more directly or they can provide direct or indirect measures of soil water potential. Each sensor type can be useful depending on the goals and information needs established.  Identifying a field location or locations and placing the sensors at appropriate depths is also important and care should be taken to select locations that are representative of soil water conditions in the crop root zone. Monitoring multiple soil depths can also provide information on soil water storage and percent soil profile water depletion level. Depending on the time of year, many growers have turned to using different soil depths as triggers to initiate irrigation events using sensors placed at more shallow depths to schedule early season irrigation events.

Evaluating the readings from soil moisture devices also requires some experience and understanding of soil water retention characteristics of the field being monitored.  Although estimated values of field capacity, permanent wilting point and plant available water can be referenced for various soil textural classes, these values are very generic and can misrepresent actual site values that can be used for irrigation scheduling purposes making the information less reliable. Developing individual field or soil type data for your fields often aids in providing more specific and repeatable information that can improve irrigation decision making.

Integrating the information
Recognizing the need to integrate key water system information into a coherent field and farm water management plan system requires work and experience in evaluating multiple system elements and will assist in avoiding the tendency to place the focus and reliance on singular management indicators. Integration of these primary water management elements will have impacts on the time and amount of field applied water and can have significant impacts on farm water use by limiting the application of water that does not directly benefit the crop. Recognizing the importance of integrating appropriate  information from multiple sources to manage farm water requires that we begin using the tools available to us to document and interpret a wide variety of information of our irrigation systems, soil systems and our individual cropping systems.

Independent evaluations of an irrigation system performance can provide feedback on the current issues of concern for individual systems and can provide an assessment of whether system maintenance is badly needed or if help determine if the system requires improved pressure regulation, higher flow rates or other design modifications to operate more optimally.  Water applied uniformly and with high efficiency limits water and nutrient losses to the groundwater and results in more uniform crop yield and quality. But information related to application efficiency or distribution uniformity can also aid in targeting locations for soil moisture monitoring sites and locating sites to monitor plant stress and crop growth.
Using available knowledge of plant development and physiology can aid in identifying periods when the crop is particularly sensitive to water stress events and aid in irrigation scheduling decisions by either limiting the drawdown of soil moisture reserves during those periods or by extending the irrigation cycle. In a similar manner, the use of plant water stress indices can be used to hasten or delay irrigation events and help establish the intensity and duration of water stress events. Numerous university publications are available that provide sound information on the development and physiology of specific crops. This information often includes water management studies that can provide tools to identify crop developmental stage as well as information on water stress sensitivity and periods of relative tolerance to water stress.

Establishing reasonable goals and expectations for monitoring soil water status and using soil sensor information for irrigation decision making purposes can assist in tightening irrigation schedules thereby reducing the likelihood of excessive losses while also reducing the risk of crop losses that result from elevated crop water stress levels. This information is particularly useful when combined with other irrigation scheduling tools such as crop evapotranspiration estimates and crop water stress indicators.

Sprinkler and flood irrigation often generates runoff that transports sediment from agricultural fields to downstream rivers, lakes, and estuaries. Additionally, some classes of pesticides, such as pyrethroids, bind to the suspended sediments in tail water which can cause toxicity to aquatic organisms in these receiving waters. Currently numerous rivers and creeks in California are considered impaired by pesticides and sediment transported with drainage from agricultural land. As water quality regulations become stricter, growers will need to implement practices on their farms that treat potential pollutants in runoff.

Tail water can be a particularly challenging water quality problem on the central coast of California where overhead sprinklers are widely used for vegetable production. Sprinklers can contribute to high concentrations of suspended sediment in tail water because the force of the falling water droplets erode soil aggregates and allow fine particles to be carried with runoff water. Research that we have conducted on the central coast has shown that adding a low concentration of polyacrylamide to irrigation water can dramatically reduce sediment loads and sediment-bound pesticides in agricultural tail water (Fig. 1). Across a range of soil types, polyacrylamide treatment reduced sediment concentration in runoff by more than 90 percent on average. On some highly erosive soils polyacrylamide reduced sediment concentration in sprinkler induced runoff by more than 98 percent. Total phosphorus and nitrogen concentrations in sprinkler runoff were also reduced by 40 percent to 70 percent using polyacrylamide.

Despite dramatic improvements in water quality, polyacrylamide, also called PAM, has been slow to be adopted as a management practice on the central coast and in much of California. One reason may be because of misunderstandings about how to most effectively use PAM for treating runoff, especially for sprinkler irrigation. Another reason is that several of the physical properties of polyacrylamide make it challenging for handling and applying to fields.

Brief background on PAM

Polyacrylamide (PAM) is a simple polymer molecule made of carbon, nitrogen, and oxygen. Various forms of PAM exist, but the type used to stabilize soil and prevent erosion is a very large, mostly negatively charged molecule (12-15 megagrams per mole). Agricultural PAM is commercially available in dry powder (granular), emulsified liquid, and dry tablet forms, and costs as little as $4 to $6 per pound of active ingredient depending on the formulation, supplier, and cost of the raw materials used for manufacturing PAM (i.e. natural gas). PAM is used for many nonagricultural purposes such as a flocculant for waste and potable water treatment, processing and washing of fruits and vegetables, clarification of juices, and paper production. It is also a component of makeup.

Use of PAM for Irrigation and Erosion Control

Because PAM is a very long, linear molecule it easily binds to soil aggregates, thereby preventing soil erosion during irrigation events. Beginning in the early 1990’s numerous studies demonstrated that low application rates of PAM (1 to 2 lb/acre) reduced runoff and improved water quality in furrow systems by stabilizing the aggregate structure of soil, improving infiltration, and flocculating out suspended sediments from irrigation tail water. Most of the research and demonstrations of PAM were conducted in furrow systems in Idaho and Washington states where soils are very erodible. By 1999, almost 1 million acres of land were annually treated with PAM in the northwest of the United States. Additionally, growers in the San Joaquin Valley and the Bakersfield areas of California used PAM to reduce soil erosion under furrow and flood irrigation.

Working with PAM

PAM can be very difficult to use if it is not handled correctly. Wet PAM is very slippery, and because it solubilizes slowly in water, PAM spills should be cleaned up with a dry absorbent rather than washing it with water. Although it is not toxic to humans, some precautions should be taken when handling PAM: Use gloves to avoid irritation to skin. Goggles will prevent eye exposure. Also, a dust mask is recommended when pouring or handling dry granular or powder forms of PAM to avoid inhalation.

One rule of thumb to keep in mind is that it is much easier to add water to PAM than to add PAM to water. Dry PAM rapidly absorbs water, increasing its original volume many times to become a slimy, gooey substance. Dissolving dry PAM in water can be challenging. Because PAM is a very large molecule it does not dissolve readily into water and requires many hours of agitation to dissolve. It will often stick to the side of a tank when being mixed. Also, mixing up concentrations greater than 0.15 percent in water is nearly impossible because the solution becomes very viscous and difficult to agitate. Some manufacturers sell effervescent PAM tablets which aids dissolution in water, but still only relatively dilute solutions can be mixed up.

For these reasons it easiest to use liquid PAM products that have been emulsified with a carrier such as mineral oil or humectants, or work with dry PAM products, such as granular PAM or PAM in a tablet form. The emulsified liquid products generally have active ingredient concentrations ranging from 25 to 50 percent.

Application Methods

For applications in furrow systems dry or liquid product can be added to water flowing in a head ditch or main line (if gated pipe is used) at a rate to achieve a 2.5 to 10 ppm (parts per million) concentration. Automated equipment can be used to spoon feed granular PAM into flowing canal water. The application can be made continuously during the irrigation or until the water advances almost to the end of the furrows. An alternate application method, called the “patch method” involves spreading granular PAM to the first 3 to 5 feet of each furrow. The granular PAM slowly dissolves as water flows down the furrows. A similar strategy is to add a PAM tablet at the beginning of each furrow. Applications into sprinkler systems require specialized equipment for injecting either liquid PAM or dry PAM into pressurized pipe which will be discussed in more detail later.

Environmental and Food Safety

Only PAM products labeled for application to food crops should be used in agriculture. Also, the buyer/processor/shipper of the produce should be informed that PAM is being applied to the crop, especially if the application is made near harvest.

Agricultural PAM used for soil erosion is not toxic to mammals. Environmental studies of anionic (negatively charged) PAM have not shown any detriment to fish, algae, and aquatic invertebrates such as Ceriodaphnia dubia, and Hyalella azteca. Polyacrylamide is sometimes confused with acrylamide monomer, a precursor in the manufacturing of PAM. Acrylamide monomer, a potent neurotoxin, has a high, acute toxicity in mammals. The Federal Environmental Protection Agency (EPA) requires that PAM sold for agricultural uses contain less than 0.05 percent acrylamide monomer. In soil, PAM degrades rapidly by physical, chemical, biological, and photochemical processes, but it does not decompose into the acrylamide monomer. A previous study of the movement of PAM from agricultural fields showed that less than three percent of the applied product remained in the runoff leaving the field. The remaining PAM in the tail water was almost completely removed through adsorption to suspended sediments as the water flowed a distance of 300 to 1000 ft in the tail water ditch.

One concern with using emulsified liquid PAM is that the mineral oil carrier can have toxicity to downstream aquatic invertebrates. However, toxicity from the mineral oil can be avoided by using PAM formulations with either high concentrations of PAM (>50 percent active ingredient) or with non-oil carriers such as humectants.

Optimizing PAM for Sprinklers

Although many research studies have evaluated the efficacy of PAM in furrow systems, fewer studies have evaluated the use of PAM with sprinklers. Applications of PAM made before irrigating with sprinklers, such as by spraying PAM solution or broadcasting dry product on the surface of the soil were far less effective than continuously injecting PAM at a low rate into the irrigation water. Injecting PAM only at the beginning of an irrigation was also less effective in controlling sediment in runoff than a continuous application at a low concentration during the entire irrigation. Our studies on the central coast show that injecting PAM to achieve a 5 ppm concentration in the irrigation water provided the highest reduction in sediment, nutrients, and pesticides in the tail water using the least amount of product. In some fields 2.5 ppm PAM provided equal efficacy for control of suspended sediments as 5 ppm PAM. Treatment with PAM should be started with the first irrigation after planting and continue during the following two to three irrigations. PAM should be reapplied when sprinkler irrigating after the field has been cultivated. Product can be saved in fields where very little runoff occurs during the first few hours of an irrigation by making an initial application for the first half hour and then applying product again when runoff becomes significant.

Injecting PAM into Pressurized Irrigation Systems  

The chemical characteristics of PAM that make it so effective as a flocculant, also make it difficult to inject into pressurized irrigation systems. Liquid PAM solutions are viscous and will clog chemigation equipment with valves such as diaphragm and piston pumps, as well as venturi injectors. Also, because the desired PAM concentration in the irrigation water is low, injection rates as low as one to three ounces per minute are needed to treat 10 to 15-acre fields. Most small, gas-powered centrifugal pumps usually cannot be easily calibrated to inject at these low rates. Peristaltic pumps can be adjusted to inject slowly but often are not designed to operate under high pressures that are common in sprinkler main lines. The best type of pump that we have identified for injecting liquid PAM is an auger pump (Fig. 2.) which has no valves and can inject viscous solutions at very low rates. These pumps also maintain a consistent injection rate at pressures as high as 100 psi (pounds per square inch).

Although liquid formulations of PAM are the best option for pressurized irrigation systems, we are currently exploring methods of injecting dry PAM into pressurized sprinkler systems. The advantage of dry PAM is that it is generally cheaper than liquid products and the possibility of introducing toxicity from the inactive emulsifying ingredients in the liquid products is eliminated. Another advantage of this approach is that it may require less labor since no pump must be calibrated and managed during an irrigation. The dry PAM applicator is loaded with cartridges containing either granular or tablet forms of PAM (Fig 3.). A portion of the water from the main line is diverted through the applicator chambers and then added back to the main water stream. Although the PAM concentration is lower than can be achieved with liquid PAM, preliminary tests have shown that as much as 90 percent of the suspended sediments can be eliminated in the runoff (Fig 4). Further studies during the upcoming season will evaluate the practicality of use this applicator in commercial fields.

Other Potential Benefits of PAM

In addition to water quality benefits, we have observed or measured agronomic benefits from the use of PAM. Because PAM stabilizes soil aggregates, soil is less likely to crust under the impact of sprinkler droplets, which improves infiltration and decreases the volume of runoff. In one field trial, PAM reduced runoff from four successive sprinkler irrigations from 4000 gallons per acre to less than 1500 gallons per acre. Less crusting of the soil may also improve germination of small seeded vegetables such as lettuce. In one of four replicated field trials conducted in commercial lettuce fields, we were able to measure an increase in yield and plant weight with the use of PAM. This yield increase may be a result of better infiltration of moisture or because the seed emerged earlier than in the non-treated plots. Although we have not conducted long-term studies, anecdotal reports from growers who applied PAM to their fields over successive years were that soil structure was improved by keeping the fine particles in place.

An additional benefit of PAM is saving costs associated with cleaning out ditches and retention ponds that become clogged with sediment during the irrigation season. Often once or twice per year growers on the east side of the Salinas valley who farm on soils prone to crusting and runoff must schedule a backhoe and crew to remove sediment from ditches and ponds and redistribute the material in their fields. To reduce costs with using PAM, growers also can receive cost-share payments under the United States Department of Agriculture (USDA) Natural Resources Conservation Service (NRCS) Environmental Quality Incentive Program) (EQIP).

In summary here are a few key concepts on using PAM for controlling sediment in runoff:

• Polyacrylamide is a long linear molecule that binds to soil and can flocculate suspended sediments in water.
• PAM does not readily solubilize in water and increases the viscosity of water (thickens).
• Concentrations of 2.5 to 5 ppm PAM in irrigation water are ideal for optimizing erosion control benefits under sprinkler and furrow irrigation.
• For agricultural purposes only use anionic PAM products for erosion control and labeled for use on food crops.
• Small amounts of granular PAM can be applied to the beginning of furrows before flood irrigating (1 to 3 lbs/acre).
• For sprinklers PAM needs to be injected into the irrigation water during the entire irrigation event.
• Applying PAM to the soil before sprinkle irrigating or only at the beginning of an irrigation will not maximize control of sediment in runoff.
• PAM should be applied during the first three to four irrigations after planting or transplanting and when irrigating after soil cultivation.
• Auger pumps are ideal for metering liquid PAM products into pressurized sprinkler water.
• PAM itself is not toxic, but the mineral oil in some liquid PAM products can be toxic to aquatic organisms.
• A prototype applicator is being developed to inject dry PAM into pressurized sprinkler systems, although it is not yet commercially available.

Further information on using polyacrylamide is available on the UC Cooperative Extension Website for Monterey County  (http://cemonterey.ucanr.edu/Custom_Program567/Polyacrylamide_PAM/)