As the drought continues on the central coast, growers are trying to utilize water as efficiently as possible to produce their crops.  Retaining and reusing sprinkler runoff, also referred to as tail water, can be an important strategy to increasing water conservation.  Also, retaining runoff prevents suspended sediments, pesticides and nutrients from impairing rivers and estuaries downstream of agricultural fields. 

Many ranches in the Salinas Valley have retention basins and infrastructure that can capture runoff and reuse tail water for irrigating crops.  Most growers use this water during the pre-germination or the germination stages to avoid food safety risks from microbial pathogens.    However, updates to the leafy green marketing agreement (LGMA) now require that water stored in open reservoirs and used for irrigating leafy greens maintain generic E. coli levels less than 10 MPN/100 ml. In most cases, tail water in open reservoirs will need to be treated with chlorine to achieve this low threshold for generic E. coli.  Fine sediments suspended in the tail water can greatly reduce the effectiveness of chlorine to control bacterial growth. 

Polyacrylamide (PAM), an inexpensive polymer molecule that has been used for controlling soil erosion in furrow irrigated fields since the early 1990s may be able to improve the efficacy of chlorine by reducing the suspended sediment concentration in sprinkler runoff. Additionally, if runoff is discharged from a ranch, treatment with PAM can greatly reduce the concentration of sediment-bound pesticides and nutrients that can degrade water quality downstream.   Past field trials that we conducted have shown that adding PAM to irrigation water at a low concentration (< 5 ppm) is an effective way to minimize erosion in sprinkler irrigated fields and remove suspended sediments from tail water.   However, for this strategy to be successful with sprinklers, we found that PAM must be injected continuously throughout each irrigation.  In other words, a single application of PAM cannot control suspended sediments in runoff during subsequent irrigations. 

New approaches to using PAM

Accurately injecting PAM into a pressurized irrigation system is not a simple process. Dry PAM powder becomes very gooey and viscous when moistened, and is almost impossible to uniformly dissolve into water.  Emulsified oil formulations of PAM that mix up uniformly in water are available but are more costly than dry PAM products and require sophisticated pumps to meter it into a pressurized irrigation system, as well as trained staff to assure that the application rate is accurate.   Another limitation of liquid PAM is that the mineral oil used to emulsify these products can be toxic to aquatic organisms.  In contrast, dry PAM is less than half the cost of liquid PAM and has been shown to have no toxicity to aquatic test organisms such as Hyalella azteca and Ceriodaphnia dubia, even at concentrations 20 times greater than would be typically used for treating irrigation water.   Hence, for these reasons, we have been developing and evaluating approaches of using dry PAM to control sediment in sprinkler runoff during the last several years. 

Treating pressurized irrigation water with PAM

The first method that we describe in this article uses an applicator to dissolve dry PAM into pressurized irrigation water.   The applicator consists of cartridges filled with PAM granules that insert into a series of cylindrical chambers (Fig. 1).  A small pump can be used to divert a portion of the irrigation water from the mainline into the inlet of the applicator.  PAM slowly releases from the cartridges (Fig. 2) as irrigation water streams through the space between the cartridges and the outer walls of the chambers.  Vanes surrounding the cartridges increase turbulence of the flowing water to maximize the dissolution of PAM.  The treated water then returns into the main line of the irrigation system where it distributes into the field through the sprinkler system.

Fig. 1. Field trials evaluated a prototype PAM applicator in sprinkler irrigated lettuce

Fig 2. PAM cartridge with static mixing elements.

Field-testing the dry PAM applicator

Field-testing of the prototype PAM applicator was conducted in commercial lettuce fields during 2020 and 2021.  Each field test occurred during the germination phase of the crop (5 to 6 consecutive irrigations) using overhead sprinklers.  The fields were divided into untreated, and PAM treated areas, where the PAM treated plots ranged from 1.9 to 4.2 acres.  Soil textures at the sites varied from loam to sandy loam.  A portion of the flow in the main lines was diverted through the PAM applicator.  Flowmeters were used to measure the flow rate in the mainline and the inlet of the applicator.   Another flowmeter monitored the volume of water applied in an adjacent untreated plot. 

Table 1. Description of PAM field trials conducted in 2020 and 2021

Flumes were installed 30 ft from the far end of the fields to measure run-off volume in the PAM treated and the untreated plots during the irrigations (Fig. 3).  A stilling well and float mechanism were used to measure the height of the water in the flume.  A datalogger recorded the height of the water in the flume and converted it to a flowrate using a calibration curve.   The datalogger also automated sampling of run-off into collection containers using a peristaltic pump.  Composite samples of run-off were collected from the plots during 5 to 6 irrigation events and analyzed for turbidity, pH, electrical conductivity, suspended sediments, total nitrogen (N), nitrate-N, total phosphate (P) and orthophosphate at the UC Davis analytical laboratory.

Fig. 3. Flume and peristaltic pumping system used to monitor run-off volume and automatically collect run-off samples from test plots in a commercial lettuce field.

 Results of field tests

The average concentration of suspended sediments in the untreated sprinkler runoff ranged from 466 to 1256 milligrams per liter (mg/L) during each trial (Table 2).   Results of these field trials demonstrated that pretreating the irrigation water with the PAM applicator could reduce the concentration of suspended sediments carried in sprinkler runoff by 85% to 95%, depending on the soil type.  The average reduction in suspended sediment concentration in the runoff was 90% across all trials.  Turbidity of the runoff in the PAM treated plots was also reduced by an average of 95% across all sites (Fig. 4, Table 3). Runoff volume in the PAM treatment was reduced by an average of 26%, but the reduction in runoff volume varied from 8% to 67% depending on the site characteristics (Table 3).   

Table 2. Average effect of PAM on suspended sediments transported in sprinkler runoff and sediment loss for field trials conducted during 2020 and 2021.

Fig. 4. Sprinkler run-off from PAM treated (left) and untreated (right) plots in a commercial romaine lettuce field. Run-off is flowing through flumes positioned at the lower end of the plots.

Table 3. Average effect of PAM on volume and turbidity of sprinkler runoff for field trials conducted during 2020 and 2021.

Total and soluble phosphorus were reduced by an average of 65% and 14% respectively in the PAM plots compared to the untreated controls in the two trials conducted in 2021 (data not presented).  Total nitrogen and nitrate nitrogen concentration in runoff from the PAM treated plots were not reduced compared to the untreated plots. 

The combined effects of reduced runoff volume and suspended sediment concentration under the PAM treatment resulted in less loss of soil from these fields (Table 2).  Soil erosion was reduced by an average of 93% compared to the untreated control, varying from 89% to 96% reduction in erosion among field sites.  Cumulative losses of sediment during the germination phases of the crop were reduced from an average of 76 lbs per acre in the untreated plots to 5 lbs per acre in the PAM treated plots.  The sediment lost in the untreated plots would equate to 15 tons for a 200-acre ranch that was planted with 2 crops of vegetables per season.  This estimate is only for the germination phase of the crop and so total losses of sediment could be much higher for crops irrigated with sprinklers until harvest.   

Potential soil health benefits

Improving soil structure and increasing soil organic matter may be an additional benefit of applying PAM to the field through the irrigation water and preventing the erosion of fine sediments.  Much of the stable organic matter in soil is associated with the clay size particles, so preventing the erosion of these fine sediments would presumably help with retaining and building soil organic matter.   Fine sediments and organic matter are also important in the development of better soil structure which can improve crop growth.   

Summary

The dry PAM applicator that we field tested showed promise for greatly reducing soil erosion, as well as helping improve water quality.   Presumably, by removing the sediment from the tail water, less chlorine would be required for controlling microbial pathogens.   The six-unit PAM applicator tested in this study can treat up to 500 gpm.  The applicator would need more mixing units to treat all the flow from a typical agricultural well in the Salinas Valley (flow rates of 1000 to 1500 gpm).  To see a video showing runoff from PAM treated and untreated field plots during a sprinkler irrigation, follow this link.  The second part of this article will discuss an additional method to use dry PAM to treat irrigation runoff.  This second approach uses an applicator that directly treats runoff in drainage ditches.

Acknowledgments:  We greatly appreciate assistance in fabricating the prototype PAM applicators from RayFab. This project was funded by the California Leafy Green Research Board.

Authors: Ben Lee, Daniel Hasegawa, Ian Grettenberger

We would like to announce the creation of a new web-based tool that will provide more seamless access to pest population data from a pest trapping network. Pests included are thrips, diamondback moth, and aphids.

We wanted to provide growers with a tool to view the most up-to-date lettuce pest population data available and have developed an app to track pest populations over time throughout the Salinas Valley. Our app can be used to quickly view current thrips, diamondback moth, and aphid abundances, where pest populations are increasing the fastest, and how previous years' pest populations responded to changes in temperature. We hope to add more features in the future and are looking forward to feedback from the community for ways to improve our app or what data presentations would be useful in making management decisions.

The trapping efforts were started by former IPM advisor Alejandro Del Pozo-Valdivia and are now operated by Daniel Hasegawa with USDA-Salinas . Pests are trapped on a weekly basis using sticky traps (diamondback moth: pheromone trap, thrips and aphids: yellow sticky card). Ben Lee created this app as a member of the Grettenberger lab at UC Davis.

Click on the links (below) to open the app in your browser or on your phone, and be sure to bookmark for the most up-to-date pest data. These links can also be found on the Entomology section of the UCCE Monterey website Salinas Valley pest monitoring - Monterey County (ucanr.edu)

Desktop version: https://salinaspestmap.shinyapps.io/salinas-pestmap/

Mobile version: https://salinaspestmap.shinyapps.io/salinas-pestmap-mobile/

Once again, we are experiencing a prolonged heat wave in the Salinas Valley.  Maximum air temperature in the King City area reached 112 °F earlier in the week (Fig. 1).     Recent maximum air temperatures in South Salinas have been far greater than the average temperatures recorded for the same period during previous years (2015 through 2019).

Figure 1. Maximum air temperatures recorded at the South Salinas, Soledad, and King City CIMIS weatherstation from August 20 – September 6, 2022. Average maximum air temperatures for the same period during years 2015-2019 for South Salinas are also shown

Although this heat wave will probably wane in the next several days, the central coast region will likely experience periods of record setting temperatures in the future.  There are several concerns about how prolonged elevated temperatures affect cool season vegetables.  Heat can cause immediate damage to plant tissue when temperatures of the plant surfaces become too high and cause cells to die (Fig. 2).  In addition, sustained high temperatures can affect plant growth and development. For instance, in lettuce damage can vary from obvious burning on the edges of leaves from too much heat load (Fig. 3) to more physiological issues that result in poor head formation in iceberg (e.g. puffy heads). In broccoli, if heat damage occurs when heads are forming, it can result in uneven bead sizes when the head matures (Fig. 4).  Excessive heat can result in wilting in cauliflower (Fig. 5) during high temperatures and expose curds to sunburning or cause discoloring (Fig. 6.)  In the past two years, we have observed that excessive heat can stress lettuce plants and make them more susceptible to infection with Pythium Wilt (Pythium uncinulatum). That was particularly evident in the 2020 heat spells. If there is inoculum Pythium Wilt in the soil, stress caused by heat on the plant can set off infection (Fig. 7).  

Figure 2. Heat damage on broccoli leaves

Figure 3. Burning along the edges of leaves on romaine

Figure 4. Heat stress during the development of the broccoli heads can result in uneven bead sizes when heads mature.

Figure 5. Insufficient soil moisture during hot periods can lead to severe wilting of cool season vegetables such as cauliflower.

Figure 6. Sun exposure during a period of wilting in cauliflower can cause discoloration and sunburning of curds.

Figure 7. Pythium wilt infection in lettuce set off by excessive heat.

A previous article presented strategies for maximizing evapotranspiration rates to keep crops cool.  Evapotranspiration (ET) is the process in which liquid water vaporizes from plant leaves and moist soil surfaces and is lost to the surrounding air.  As liquid water vaporizes, heat is also lost from the surfaces of leaves and soil and from the surrounding air, which cools the crop. Assuring that crops have adequate soil moisture during the hottest period of the day (generally 11 am to 4 pm) can keep plants as cool as possible.  Insufficient moisture to meet crop water requirements can result in stomates of the leaves closing and decrease transpiration rates.  Limiting transpiration would raise leaf temperatures, potentially to temperatures greater than the surrounding air.  

Hence, a good strategy to prevent heat damage to vegetable crops is to water fields that have not been recently irrigated.  Also, keep in mind that during the last few days daily reference ET increased substantially due to the high air temperatures and so more water is needed than normal to replace the amount of moisture that crops transpiration.  In South Salinas, for example, the CIMIS station showed that daily reference ET increased from 0.18 inches per day in late August to 0.25 inches per day during the heat wave, approximately a 40% increase in water demand (Fig. 8). 

Figure 8. Daily reference evapotranspiration recorded at the South Salinas, Soledad, and King City CIMIS weatherstations from August 20 – September 6. Average daily reference ET for the same period during years 2015-2019 for South Salinas are also shown.

Irrigations do not need to be very long, as much as they should supply the crop with enough water to refill the soil profile to the depth of the root zone.   Irrigating more frequently for less time would be a better strategy than irrigating less frequently for more time, since the soil has a limited capacity to store water in the root zone. Over-saturating the soil during high soil temperatures through heavy irrigations could worsen infections from soil-borne pathogens.

The CropManage online decision support tool can assist with determining the amount of water to apply and frequency to irrigate for most vegetable crops produced in the Salinas Valley.  The software allows one to customize the recommendations for the development stage of the crop, soil type, and irrigation system characteristics.     

Finally, for crops irrigated by sprinklers, short irrigations during the hottest time of the day can reduce air temperatures.  This might be a good strategy for vegetables that are in a stage of development that is very susceptible for heat damage, such as cauliflower close to harvest.  

Richard Smith¹, Eric Brennan² and Patricia Love¹

1. University of California Cooperative Extension, Monterey County.  2. USDA-Agricultural Research Service, Salinas.

Fall-grown cover crops (planted August-September and incorporated October-November) provide a useful planting slot for a percent of vegetable crop acreage in the Salinas Valley. It is a time when some growers find an opportunity, after two crop rotations, to fit a cover crop in their operations. It has the particular advantage of allowing the grower to incorporate the cover crop and still have time to work the ground when it is still dry before the onset of winter rains.

In Ag Order 4.0 which was approved in April 2021, cover crops that meet the following criteria were granted a credit on the R side of the applied (A) minus removed (R) metric for nitrogen loading in vegetable production fields: 1) a non-legume cover crop grown for ³ 90 days during the winter fallow period (October to April); 2) accumulates more than 4,500 lbs/acre of oven-dry biomass; and 3) has a C:N ratio of ³  20:1 at incorporation. Unfortunately, fall-grown cover crops do not meet these criteria and therefore growers cannot claim a credit when reporting nitrogen loading in their fields.

In the fall of 2021, we conducted six on-farm evaluations of fall-grown cover crops to determine their productivity, nitrogen scavenging capability and C:N ratio at incorporation. Planting dates ranged from August 25 to October 3, and the average days to incorporation was 54 (ranged from 47 to 59). Two barley varieties UC 696 and UC 937, as well as Merced rye were planted in each evaluation. The barley varieties were included because we anticipated that they would reach the heading growth stage more quickly than rye when growers typically terminate cover crops. However, in these evaluations, barley did not reach this stage any faster than Merced rye as measured by the Feekes cereal growth and development scale (Table 1). In 54 days, all cover crops produced more than 4,500 lbs oven-dry biomass and took up from 150 to 161 lbs N/A. The C:N ratios of the cover crops ranged from 13.0 to 13.4.

Fall-grown cover crops grow and mature quickly due to the longer days and warmer weather that they experience in these early planting slots. An important question is, do fall-grown cover crops help to reduce nitrogen leaching during the winter?  To help answer this question, we intend to conduct mineralization studies of the cover crop residue to determine what amount of the residue remains unmineralized after twelve weeks. The nitrogen in the unmineralized portion of the cover crop residue is not immediately susceptible to nitrate leaching and could potentially be deserving of a credit. The Central Coast Regional Water Quality Control Board will update the criteria in the Ag Order each five years based on new scientific information. If there is evidence that fall-grown cover crops can help mitigate nitrate leaching, this may help to justify expanding cover cropping options for growers in the Ag Order.