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Fate and Efficacy of Polyacrylamide Applied in Furrow Irrigation: Full-Advance and Continuous Tr... (in this windowl [in a new windowl Page 11 of Overall PAM mass-loss rates from all fun•ows as irrigation water flowed across the field and down the tail ditch are plotted in Fig. 6. In Inrigation 2 at 2 h, the overall PAM loss rate at the bottom of the tail ditch was 0.01 g miri ~, nearly two orders of magnitude smaller than that at the top (inflow end) of the furrow, 0.9 g miri ~. The pattern differed for Irrigations 4, 5, and 6, where the average 2-h overall PAM loss rate at the tail-ditch bottom was 0.26 g miri ~, about one-fifth that at the top of the furrow, 1.05 g min ~. The increase in PAM loss rate at the tail-ditch bottom from early to late season (Fie. 6) was caused by an increase in stream PAM concentration (FiQ. 4), since tail-ditch flow changed little between early- and late-season irrigations (Table 4). For all irrigations, overall PAM loss declined greatly after PAM application in Initial-10 fwrows had ceased, and PAM was being applied to Cont-1 furrows only (3 and 7 h sampling times}. This emphasizes the importance of stopping PAM application in Initial-10 furrows once runoff begins, in order to minimize PAM runoff losses and maintain high PAM-use efficiencies. At any given sampling time, overall PAM mass-loss rate in the furrow and tail-ditch flows decreased with distance downstream. The rate of decline was greater at 2 h, when dissolved PAM concentrations were greatest. Furrow infiltration rates were highest in the early hours of the irrigation. Hence, furrow flow rate declined more steeply with distance downstream at the 2 h sampling time than at 3 or 7 h. Since PAM mass-loss rate is a function of flow rate, loss rates declined more steeply with distance at the 2-h sampling time than at later times (data not shown). Cumulative Polyacrylamide Mass Losses Runoff at the tail-ditch end transported a total of 2.6 g PAM off the farm during each of Irrigations 2, 4, 5, and 6 (F 7) .Thus, only 1 % of the total PAM a.i. applied per irrigation exited the area as irrigation return flow. Had the Initial- l OPAM application been curtailed when furrow runoff commenced, PAM contributions from those furrows to the tail ditch would have been considerably reduced (see discussion below), and cumulative PAM mass losses at the tail-ditch end would have been about one-fifth of the 2.6 g measured in this study, or about 0.2% or less of the total PAM a.i. applied. ~..~_ +rrr r naa ~~• ~n ,n View larger version (38K): [in this windowl [in a new windowl Fig. 7. Cumulative polyacrylamide (PAM) mass losses from all treated furrow streams at each furrow sampling position, and at tail-ditch positions (mean values for Irrigations 2, 4, 5, and 6.) http://jeq.scijournals.org/cgi/contenUfulU31/2/661 9/21/2006