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Fate and Efficacy of Polyacrylamide Applied in Furrow Irrigation: Full-Advance and Continuous Treat... Page 5 of 16 <br />remainder of the season. Runoff water was subsampled in triplicate at locations 0 (top), 93 (middle), and 154 m <br />(bottom) down the tail ditch, at the same times as those collected from furrow streams. During each irrigation, stream <br />water samples were taken at 2 h from the end of the tail ditch, located 530 m downstream from the top. Tail-ditch-end <br />samples were collected at 3 and 7 h times during Irrigations 4 and 5. Tail-ditch-end data were not included in the <br />statistical model because the sample set was not complete, but were used to estimate mean cumulative PAM loss per <br />irrigation at the tail-ditch-end position. A repeated measures analysis using means of triplicate subsamples produced <br />Huynh-Feldt e values >1, so a split plot analysis was employed to evaluate the three tail-ditch positions (top, middle, <br />and bottom), with time as the main plot, positions as the subplots, and irrigations as the random effect. Confidence <br />intervals (P = 0.05) were constructed on the position means. Early-season irrigation responses varied considerably <br />from those late-season Irrigations 4, 5, and 6. Thus, a separate analysis for late-season irrigations employed <br />orthogonal contrasts to test for tail-ditch position effects at the first sampling. For this analysis irrigations were <br />considered a random effect. Finally, trends in tail-ditch responses from Irrigation 2 were examined by plotting means <br />and confidence limits using variances from triplicate subsamples. <br />Irrigations and Monitoring <br />A gated pipe conveyed water to each furrow, and adjustable spigots controlled inflow rates. Initial irrigation inflows <br />were set high to speed irrigation advance (Table 1). When water in all furrows had traversed the field, inflows for all <br />treatments were simultaneously decreased to reduce runoff and sediment losses. Irrigation sets were 12 h long. Newly <br />formed furrows were irrigated early in the season. If furrows were undisturbed by cultivation since the previous <br />irrigation, these were termed repeat furrows. Repeat furrows were used mainly during late-season imgations. <br />Irrigation water supplied by the Twin Falls Irrigation District had an electrical conductivity of 0.05 S m ~ and sodium <br />adsorption ratio (SAR) of 0.5 [mmolc L-~ ]~•5. Polyacrylamide was added to irrigation water at a rate that produced the <br />desired furrow stream target concentration, either by injecting a 300 mg L-~ (Cont-1) or 2400 mg L-~ (Initial-10) stock <br />solution into the turbulent flow pouring from the gated pipe spigots. Furrow inflows, and stream flow rate and <br />sediment concentrations were measured throughout each monitored imgation at furrow top, middle, and bottom <br />positions. Measurements were made at 30-min intervals early in the irrigation, every hour during mid-irrigation, and <br />every three hours later in the irrigation, when outflows and sediment loads had stabilized (at >7 h into the set). <br />Inflows were measured by timing the filling rate of a known volume, and outflows were measured with long-throated <br />v-notch flumes (Trout and Mackey. 1988). Runoff sediment was measured using the Imhoff cone technique (So,ka et <br />al. 1992). Details of the flow and sediment monitoring procedure were given by Lentz et al. (1992). The computer <br />program, WASHOUT (Lentz and Sojka, 1995), calculated runoff and PAM loads using measured flow rates and <br />sediment and polymer concentrations. Runoff PAM loads were computed under the assumption that runoff <br />component concentrations were constant between sampling intervals. <br />Sample Handling and Analysis <br />Sediment was removed from PAM furrow stream samples within 90 min of field sampling by centrifugation (10.2 <br />RCF, 10 min, 10 to 15°C). We added small amounts of boric acid and 2-propanol to inhibit biologic activity and <br />stabilize polymer present in the samples (Lentz et al., 1996). Polyacrylamide polymer concentrations were determined <br />using a flocculation method (Lentz et al. 1996, protocol with Option 2). We employed the high-precision option, <br />which required preparation of additional calibration standards for waters with varying sediment concentrations. The <br />procedure could detect as little as 0.1 mg PAM L-~. Precision was t3% for solutions with <2.5 mg PAM L-~ and f6% <br />for solutions with >2.5 mg PAM L-~. <br />- RESULTS AND DISCUSSION <br />http://jegscijournals.org/cgi/contenUfulU31 /2/661 9/21 /2006 <br />