IRRIGATION PILOT STUDY LEADS TO INVISIBLE WASTEWATER RECYCLING FOR FODDER CROPS

The McKinleyville Community Services District (MCSD) historically irrigated the 750 acre Fischer Road pasture site with disinfected secondary pond effluent through spray irrigation. Nearby residents voiced concern over the potential for aerosols migrating across the buffer. The California Department of Health Services stated that there would be essentially no risk of aerosols if the fields were flood irrigated. Although flood irrigation is common for some situations, the requirements of flood irrigation of pasture grasses on a sloping site had not been previously developed.The MCSD embarked on an irrigation pilot study to determine the best method and configuration for a full size flood irrigation system for pasture grasses. The objectives of the study were to determine the type of system, the area requirements, special management considerations and the approximate costs. A pilot study was developed to test the feasibility of surface flood irrigation and compare the results to the existing spray irrigation system. A flat field, a mildly sloping field, and a steeply sloping filed were equipped with several types of flood irrigation equipment and tested over a period of several months.The results of the tests indicated that the flat field worked very well, but that due to porous soils an application rate equivalent to over three feet was necessary to cover a 100 by 500 foot field. The mildly sloping field took less water to cover the area but sheet flow tended to deteriorate into channelized flow after advancing 200 to 300 feet. The steeply sloped field was irrigated using gated pipe and tended to channelize quickly.Other findings included noting that water runs down gopher holes, edge berms need very mild slopes so harvesting equipment can cross them, and that a high flow rate over a short duration yielded the best coverage with the least infiltration.The design and construction of the full scale improvements using mildly sloping fields has been concluded. The construction experience and the data from the full scale operation yielded practical operating experience for this technique for recycled water irrigation to avoid aerosols. The project was clearly a success when residents within several hundred feet asked when the district would start irrigating even though irrigation had taken place for months.