Reducing Furrow Irrigation Erosion with Polyacrylamide (PAM)

Irrigated cropping is a critical component of global agricultural production. Surface irrigation—most of it furrow irrigation—accounts for >60% of Earth's 600 million irrigated acres. Erosion threatens irrigation's ability to maintain its 2X average yield advantage and 3X value over rainfed agriculture, with serious environmental and food security consequences to the burgeoning human population. Furrow irrigation-induced erosion is nearly halted by small additions of water-soluble polyacrylamide (PAM) to irrigation water. PAM is an environmentally safe flocculent used extensively in municipal water treatment, paper manufacturing, food processing, and other sensitive applications. On freshly cultivated furrows, 1 lb/acre of PAM applied at 10 ppm in irrigation water before runoff began (only), reduced sediment loss in runoff 94% and increased net water infiltration 15% in 3 yr of Idaho tests on silt loam soils. Irrigation return-flow quality was improved by PAM-use, greatly reducing sediment losses, biochemical oxygen demand (BOD), total P, and various pesticides. Polyacrylamide products are now registered in most western states and the Natural Resources Conservation Service (NRCS) has published a PAM-use practice standard. Cost sharing is available in some locales. In 1995 about 50 000 acres of furrow-irrigated land used PAM, halting as much as 1 million tons of soil erosion in the first year. Farmers see PAM-use as an attractive alternative to more difficult conservation practices. The typically $15 to $35/acre per crop costs are partially or entirely retrieved by crop responses or savings in erosion-related field operations and water conservation. Pam-use in irrigation is expected to expand rapidly in 1996. Research QuestionThere has been a large shift to sprinkler irrigation in the USA. Beyond labor considerations, this has been driven by soil and water resource conservation. Some crops, however, do not tolerate wet canopies (e.g., seed beans). Furthermore, the capital cost, energy use, and technological requirements are out of the reach of many U.S. and third world irrigators. If inexpensive, effective, easy erosion control were available, many furrow irrigation farmers could improve resource conservation and water management. Literature SummarySmall amounts of a specific class of polyacrylamide (PAM) copolymer can virtually halt furrow irrigation-induced erosion. Net infiltration is also improved (infiltration rate does not decline, compared with untreated water) while management options are broadened. Various copolymer formulations of water-soluble PAM and other copolymers have been available for soil-stabilization in agriculture since the 1950s. While these conditioners were effective at stabilizing soil structure and improving infiltration, early materials and high application rates were prohibitively expensive for all but high-return uses. In 1991, work by the USDA's Agricultural Research Service in Kimberly, ID, demonstrated that 1 lb/acre of moderate molecular weight (12–15 Mg/mole) anionic polyacrylamide water soluble copolymer could virtually halt furrow irrigation-induced erosion. Treatment required dissolving PAM at a concentration of 10 ppm in the irrigation water during its initial traverse down the dry furrow (advance). The treatment has proven consistently effective for a variety of soils and textures on freshly tilled furrows having slopes of as much as 7%. Net infiltration rates are also improved, with increases ranging from 15% in silt loam soils to 50% in clayey soils. Extensive testing in Idaho, complemented by studies in several other western states, led in 1994 to commercial product registration in most western states. In January 1995, the Natural Resources Conservation Service (NRCS) published a western states interim conservation practice standard to describe preferred methodology. Practice adoption in 1995 was estimated at 50 000 acres in the USA, predominantly the Pacific Northwest. A number of locales now have cost sharing for the practice. There is intense user interest in implementation and further development of this approach. Emphasis of recent research and farmer inquiries has been focused on development of easy application technology, minimization of PAM requirement, and verification of PAM's inherent resistance to off-field migration. Applied QuestionsWhat is the best way to apply PAM? There are several effective ways to apply PAM. The NRCS standard calls for dissolving PAM in irrigation water, bringing the concentration to 10 ppm in the flow during water advance, and then halting PAM addition when runoff begins. Add PAM to the water slowly (never add water to PAM). Provide adequate turbulence in the flow to insure thorough mixing. Direct application of PAM at the furrow head, or use of large PAM blocks in the head ditch is being evaluated. Are there special needs for best results? With PAM, high sediments suspended in supply water may settle out almost immediately, deactivating some PAM. A small settling pond at the top of the field can prevent sediment from filling head ditches. Using larger siphon tubes prevents sediment from adhering to the inside of the tubes and increases initial inflow rates. Farmers should increase initial inflow rates (double or more) with PAM. Higher inflows will not cause erosion if PAM is in the flow, and will compensate for higher infiltration rates with PAM and shorten the advance time. Once runoff begins, cutting back to the minimum sustainable flow rate optimizes this practice. The first water reaching the furrow must contain 10 ppm PAM or else PAM effectiveness will be greatly diminished. Similarly, PAM effectiveness is less when surface soil is moist. Occasionally farmers irrigate when the soil profile is still somewhat wet, for example, after a rain shower. Irrigation is done to “stay on schedule” because water deliveries to several fields takes a number of days. If erosion or infiltration is an overriding concern, the best strategy, if at all possible, is to wait until the soil has dried enough to increase infiltration and PAM absorption. Farmers should buy only registered (labeled) PAM. There is little chance that familiar major agrichemical suppliers, will supply the wrong formulations. Crosslinked superabsorbent or gel-forming PAMs will not halt erosion. Cationic formulations can be toxic. RecommendationPAM is an effective and inexpensive way to prevent furrow irrigation-induced soil erosion. It should not preclude the use of other reliable conservation practices. But where these practices are not possible, or where farmers have resisted implementing other practices, PAM may provide a conservation option that meets their needs. Use of PAM should follow the NRCS practice standard, which recommends 10 ppm in the inflow during the advance phase (only) of furrow irrigation sets for those irrigations when the soil in the furrow is loose (e.g., after furrow forming or cultivation). Use of PAM during additional irrigations can be beneficial, but farmers should determine if full rate application is required for adequate control on their fields in these cases.