Polymer charge and molecular weight effects on treated irrigation furrow processes

Application of 5-10 mg L-1 water soluble anionic polyacrylamide (PAM) to furrow irrigation water during flow advance substantially reduces sediment loss and increases net infiltration. We hypothesized that PAM_s solvated molecular conformation influences its irrigation-management efficacy. The study was conducted in Kimberly, Idaho, on Portneuf silt loam (Durinodic Xeric Haplocalcids); under furrow- irrigated beans (Phaseolus vulgaris ) at a 1.5% slope. Polyacrylamides with contrasting molecular weight (anionic: 4-7, 12-15 and 14-17 MDa, i.e. Mg mol -1 ), charge type (neutral, anionic, cationic), and charge density (8, 19, 35 mol %) were tested in two studies. Inflow rate was 23 L min d during furrow advance, and 15 L min-1 for the remaining set. Anionic and neutral PAMs were twice as effective as cationic PAMs for controlling sediment loss in new furrows. The order of effectiveness for overall soilloss control was: anionic > neutral > cationic PAM, and efficacy increased with increasing charge density and/or molecular weight. Net furrow infiltration increased by 14 to 19% when PAM treatment molecular weight was reduced from 17 to 4 MDa. General trends suggested that medium and high charge density anionic and neutral PAM produced the greatest increase in infiltration compared with controls. Compared with untreated furrows, neutral PAM gave the greatest season-long net infiltration gains (5%); while charged PAMs tended to increase net infiltration early in the season on new furrows but decreased infiltration on repeat-irrigated furrows later in the season.