Aside from refinements to the method , performing this technique in different seasons, meteorological conditions, and during mixing events would enhance our understanding of the variability in emissions from liquid manure management on dairy farms. For the mobile laboratory, road access was a challenge at times. Large plots of surrounding cropland typically had a limited number of roads crossing through them, with those available often being private or undeveloped. In order to collect plumes adequately downwind of each site on accessible public roads, the ground-based ARI team required winds to come from certain directions. Being able to fly above the site eliminates these challenges. However, the aircraft flew a set pattern at each site, circling at a particular radius to optimize the established mass balance method, and did not explore downwind like the vehicle. As seen in Fig. 7, plumes used for determining emission rates were clustered in areas above each site that typically agreed with the dominant wind directions along the looping flight path. Wind rose plots for each site represent the wind conditions observed by the aircraft during the midpoint of each plume event . Onsite wind measurements during these events provided additional insight as to how the wind evolved between the site and aircraft. Other plume events sometimes occurred inside of the dominant downwind fetch, plastic growers pots especially during calm wind conditions, but lacked the prerequisites to be included in emission estimations.
Future work towards refining the tracer release method with an aircraft will require several improvements to the current experimental design. Instead of flying around the perimeter of a dairy farm or other emission source in a circle as part of an established mass balance approach , the aircraft could mimic the driven transects of the mobile lab via long horizontal transects at varying distances perpendicular to the dominant wind direction . Conducting downwind transects at greater distances would allow for better comparisons between platforms but may not be feasible in conditions similar to those experienced in this study , as it could be difficult to encounter the plume. Rather than relying on only a couple point source releases, tracer gas could be released as a line or grid source along the border of liquid manure management areas or animal housing fence lines . Increasing the flow rate of tracer gas from 15 slpm by several factors would improve signal-to-noise ratios of tracer enhancements. Furthermore, an aircraft carrying a second instrument on board that quickly and precisely monitors a second tracer gas would provide a check on the observed tracer concentrations or could aid source identification. With two tracer gases, the initial ratio of release rates ought to persist throughout the migration of the plumes and be reflected in the ratio of downwind enhancements . Deviations from the expected value indicate loss of tracer gas and inadequate representation of a source. It should be noted that the two tracers used in this original study were employed as independent tracers for better coverage over large multisource areas, while the scenario described above applies to overlapping use of tracer gases .
Benefits of adding a second tracer are described further in Roscioli et al. . Overall, combining these measurement techniques through aircraft-observed tracer release promotes positive aspects of each method. Low-flying aircraft measurements occur rapidly on a versatile platform with no road access restrictions. Tracer gases can indicate sources, identify interferences, and enable quantification without relying on modeling or highly accurate wind measurements. Using this method, an aircraft can have greater confidence identifying sources and can confirm ground-based observations.Pesticides occur in some California waterways as dissolved materials or attached to suspended sediments that settle out and accumulate in mud deposits. Certain pesticides may persist for only a few days, but others, such as DDT, may last for decades. Although pesticide concentrations in waterways are usually within drinking water standards, they are still high enough to cause toxicity to aquatic life. The pesticides found primarily include currently registered materials, although prohibited products such as DDT and its byproducts are also still being found . Pesticides can enter surface waterways directly from specific sources such as drift or spills. However, more general and widespread sources from both agricultural and urban uses are of increasing concern. Certain pesticides, including many organophosphates such as diazinon and chlorpyrifos, are readily picked up from the soil and dissolve in irrigation or storm water runoff as it moves across treated areas. Other pesticides, such as pyrethroids, are relatively insoluble in water. These pesticides move offsite attached to soil particles in water runoff, where they eventually settle out and contaminate downstream areas. Fine soil particulates are of particular concern because the pesticide concentration of soil-sorbed pesticides can be ten times higher than that in coarser particles such as sand, and the particulates tend to stay suspended in water the longest, potentially contaminating water at a greater distance from their source.
To protect surface waters from pollutants, the California State Water Resources Control Board has implemented requirements that allow farmers to discharge irrigation and storm water runoff from farms into state waters as long as the runoff water does not impair the beneficial uses of the receiving water. For pesticides and many other pollutants, these water quality standards seek to ensure that discharge water from agricultural sources does not cause toxicity to aquatic life, including organisms that live in the water, such as some crustaceans, algae, and fish, as well as invertebrates that dwell in the bottom mud deposits.The SWRCB currently grants waivers to allow growers to discharge water into state waterways as long as growers make efforts to meet state water quality standards. To help comply with water quality regulations, growers should follow best management practices to minimize pesticide use in crop production. This includes the use of resistant plant varieties, certified seed, and integrated pest management practices. The safe use, storage, and handling of pesticides, as well as reading and following the pesticide label carefully as state law requires, will also help protect water quality. Information on pesticide use for crop production, including IPM and best management practices can be found in the University of California IPM guidelines and in Long et al. 2005. This publication provides information on management practices to help reduce the impact of pesticides on surface water quality when runoff occurs from furrow-irrigated crops. The discussion includes the chemistry and toxicology of pesticides to better understand their potential impact to water quality, irrigation management practices that help reduce surface runoff, and strategies for keeping sediments and sediment associated pesticides from moving offsite in irrigation tailwater.The likelihood that a pesticide will move in irrigation or storm water runoff from an application site depends primarily on the properties of the active ingredient, including the pesticide’s field dissipation half-life, 5 gallon plastic pots soil adsorption coefficient, and aqueous solubility. Field dissipation half-life is the time required for half of a given quantity of a formulated pesticide to degrade or dissipate in the soil. The soil adsorption coefficient is the degree to which a pesticide will adhere or stick to soil particulates. Pesticides also differ in their toxicity to aquatic life. In general, insecticides tend to have high toxicity to fish and invertebrates, while herbicides can be toxic to aquatic plants. The standard indicator species that the U.S. Environmental Protection Agency uses for pesticides to assess water quality include the crustacean Ceriodaphnia dubia, the fathead minnow, and a green algae. Toxicity of sediment is usually assessed using the crustacean Hyalella azteca. These tests typically involve bringing the water or sediment sample to the laboratory, adding the test organism, and measuring how many organisms survive after 4 to 10 days of exposure, depending on the particular species used.
The tests may also measure a sublethal endpoint, such as the ability to reproduce, or the growth rate. The runoff potential is the likelihood that a particular pesticide will move offsite from the point of application with water. In general, when a pesticide has a high soil adsorption coefficient and low water solubility, it has a high potential to move offsite attached to soil particulates. Conversely, if a pesticide has a low soil adsorption coefficient and high aqueous solubility, it generally has a high potential for dissolving in water and moving offsite in solution runoff. The runoff potential of a given pesticide must be considered together with its half-life and aquatic toxicity to estimate its overall runoff risk. For example, a higher runoff potential combined with a higher aquatic toxicity and longer half life increases the overall risk for negative impact to water quality. Many sediment-sorbed pesticides used in field crop production in California that would benefit from on-farm soil erosion mitigation practices based on their high risk to water quality are summarized in table 1. Pesticides with low risk to water quality should be used when possible.Furrow irrigation is an inexpensive irrigation method that uses gravity to transport water across fields that slope approximately 1 inch per 100 feet . The disadvantage of furrow irrigation is that performance strongly depends on the infiltration rate of the water into the soil, which in turn depends on soil type and soil structure and is therefore difficult to quantify. Due to soil variability, a single field can have various infiltration rates within it, which makes it very difficult for irrigators to accurately control water being applied to the crops. Other factors affecting irrigation performance are the roughness of the soil surface, furrow length, furrow inflow rate, and slope. Normally, a trial-and-error approach is used in the management of furrow irrigation because of the difficulty in measuring some of these factors.Improving the efficiency of furrow irrigation involves reducing deep percolation below the root zone or reducing surface runoff, or both. However, caution must be used in implementing standard recommendations for improving furrow irrigation because measures that reduce deep percolation can have the undesirable effect of increasing surface runoff. This section discusses standard measures commonly recommended for improving furrow irrigation and their effect on surface runoff.Increasing the flow rate is commonly recommended to reduce deep percolation, but it can also increase surface runoff. The idea is that increasing the flow rate will reduce the amount of time the water takes to reach the end of the field, which will decrease differences in infiltration along the field. Yet field evaluations show that increasing the flow rate makes only a minor improvement in the performance of furrow irrigation because the higher flow rate also increases the depth of water ponded in the furrow, which in turn increases the infiltration of water into the soil. This deeper water in the furrow is likely to contribute to greater runoff.Reducing the length of furrows is highly effective in reducing deep percolation, but it can substantially increase surface runoff. Shorter furrows decrease the differences in infiltration along the furrow and reduces irrigation time. Studies have shown that shortening furrows by one-half can reduce percolation by at least 50 percent. However, irrigating a given area with shorter furrows may increase surface runoff due to the smaller time required for water to reach the end of the field.The cutoff time is the time at which an irrigation set ends and no more water flows down the furrow. Shortening the amount of time a field is irrigated can reduce the amount of surface runoff from furrow-irrigated crops. The amount of time needed to irrigate a given field depends on the time needed to infiltrate sufficient water along the lower part of the field; this may need to be determined on a trial-and-error basis. For example, in cracked clay soils, water flows rapidly into the cracks, resulting in a very high initial infiltration rate. After the cracks close, infiltration rates become very small. The cutoff time should occur about 2 to 3 hours after water reaches the end of the field in these cracked clay soils, thus potentially reducing runoff.Some irrigators make small dams in furrows using dirt, plastic furrow dams, or plant material to help slow the water. This practice can conserve water, increase infiltration, and help water flow more evenly across fields so that surface flow in all furrows occurs more uniformly.