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Fate and Efficacy of Polyacrylamide Applied in Furrow Irrigation: Full-Advance and Continuous Tr... yf'-_-_ :[ ~'I E!' '•~ ~~ .f~--- View larger version (39K): [in this windowl [in a new windowl Page 10 of 16 Fig. 5. Mean polyacrylamide (PAM) mass-loss rate for treated furrow streams by sampling position and time. Notey axis break and scale change for Initial-10 2 h. Polyacrylamide mass-loss rate also changed with time. After furrow runoff began and before PAM application ceased (2 h), the PAM mass-loss rate at Initial-10 furrow-bottom positions averaged 95 mg min ~ over the four irrigations (Fig. 5). This level of loss rate was permitted for 0.5 to 1.0 h in our experimental furrows because PAM application was extended in order to reduce inter-irrigation variability of tail-water measurements. During typical farm use, this stage of PAM treatment would be very brief since further application after advance is unnecessary and would decrease PAM-use efficiency. Thirty minutes after PAM application ceased (3 h), mean PAM mass-loss rate at Initial- lO funow bottoms had decreased to 1.8 mg miri ~, and the 7-h Initial-10 mass-loss rate was similar, 1.9 mg miri ~. Polyacrylamide mass-loss rates in Cont-1 furrows were generally slightly less at 3 and 7 h than at 2 h. The smaller PAM mass-loss rate at later sampling times was caused primarily by a decrease in furrow-stream flow rates (Table 4). The irrigation cutback approach used here reduced furrow inflows after furrow advance, resulting in smaller furrow- stream flow rates at 3 and 7 h than at 2 h. The 2-h PAM mass-loss rate declined dramatically at the collection ditch where dissolved PAM from treated furrows mixed with suspended sediment largely contributed from untreated furrows. Polyacrylamide flocculated and adsorbed to the sediment, which resulted in an 80% reduction in dissolved PAM loss rate in Irrigation 2 and an average 50% reduction in Irrigations 4, 5, and 6 (Fie. 6) .Some of the resulting flocculated and aggregated sediment continued to move downstream as bedload and, at 2 h, was clearly evident in the top tail-ditch flow. +... o.....~ ~~~. - .~.- -,..m.N- ao N Wt eq tl !a e e _ 8 ~ ...... a a e g - ~ - e ,~ Ols4nco Down Furor are hll 014 (,ol View larger version (49K): Fig. 6. Rates of overall polyacrylamide (PAM) mass loss at each furrow and tail-ditch sampling position for Irrigation 2 and Irrigations 4, 5, and 6, where furrow loss rates represent the total PAM loss rate from all furrows from each PAM treatment (12 total). http://jegscijoumals.org/cgi/content/fulU31/2/661 9/21/2006