Uncertainty in measuring seepage from earthen irrigation canals using the inflow-outflow method and in evaluating the effectiveness of polyacrylamide applications for seepage reduction
Date
2015
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Abstract
Seepage losses from unlined irrigation canals account for a large fraction of the total volume of water diverted for agricultural use, and reduction of these losses can provide significant water quantity and water quality benefits. Quantifying seepage losses in canals and identifying areas where seepage is most prominent are crucial for determining the potential benefits of using seepage reduction technologies and materials. In recent years, polymers have been studied for their potential to reduce canal seepage, and the use of linear-anionic polyacrylamide (PAM) was studied as part of this analysis. To quantify seepage reduction, seepage rates must be estimated before and after application of linear-anionic polyacrylamide (LA-PAM). In this study, seepage rates from four earthen irrigation canals in the Lower Arkansas River Valley (LARV) of southeastern Colorado were estimated with repeated measurements using the inflow-outflow volume balance procedure. It is acknowledged that a significant degree of measurement error and variability is associated with using the inflow-outflow method; however, as is often the case, it was selected so that canal operations were not impacted and so that seepage studies could be conducted under normal flow conditions. To account for uncertainty related to using the inflow-outflow procedure, detailed uncertainty analysis was conducted by assigning estimated probability distribution functions to volume balance components then performing Monte Carlo simulation to calculate possible seepage values with associated probabilities. Based upon previous studies, it was assumed that flow rates could be measured with +/- 5% accuracy, evaporation at +/- 20% accuracy, and water stage within 0.04 to 0.06 feet (all over the 90% interpercentile range). Spatial and temporal variability in canal hydraulic geometry was assessed using field survey data and was incorporated into the uncertainty model, as were temporal variability in flow measurements. Monte Carlo simulation provided a range of seepage rates that could be expected for each inflow-outflow test based upon the pre-defined probable error ranges and probability distribution functions. Using the inflow-outflow method and field measurements directly for assessing variables, deterministic seepage rates were estimated for 77 seepage tests on four canals in the LARV. Canal flow rates varied between 25.8 and 374.2 ft³/s and averaged 127.9 ft³/s, while deterministic estimates of seepage varied between -0.72 and 1.53 (ft³/s) per acre of wetted perimeter with an average of 0.36 (ft³/s)/acre for all 77 tests. Deterministic seepage results from LA-PAM application studies on the earthen Lamar, Catlin, and Rocky Ford Highline canals in southeastern Colorado indicated that seepage could be reduced by 34-35%, 84-100%, and 66-74% for each canal, respectively. Uncertainty analysis was completed for 60 seepage tests on the Catlin and Rocky Ford Highline canals. To describe hydraulic geometry within the seepage test reaches of these canals, canal cross-sections were surveyed at 25 and 16 locations, respectively. Probability distribution functions were assigned to parameters used to estimate wetted perimeter and top width for each cross-section to account for measurement error and spatial uncertainty in hydraulic geometry. Probability distributions of errors in measuring canal flow rates and stage, and in calculating water surface evaporation also were accounted for. From stochastic analysis of these 60 seepage tests, mean values of estimated seepage were between -0.73 (ft³/s)/acre (gain) and 1.53 (ft³/s)/acre, averaging 0.32 (ft³/s)/acre. The average of the coefficient of variation values computed for each of the tests was 240% and the average 90th interpercentile range was 2.04 (ft³/s)/acre. For the Rocky Ford Highline Canal reaches untreated with LA-PAM sealant, mean values of canal seepage rates ranged from -0.26 to 1.09 (ft³/s)/acre, respectively, and averaged 0.44 (ft³/s)/acre. For reaches on the Catlin Canal untreated with LA-PAM, mean values of seepage ranged from 0.02 to 1.53 (ft³/s)/acre, respectively, and averaged 0.63 (ft³/s)/acre. For reaches on the Rocky Ford Highline Canal and Catlin Canal treated with LA-PAM, mean canal seepage rates values ranged from 0.25 to 0.57 (ft³/s)/acre, averaging 0.33 (ft³/s)/acre, and from -0.73 to 0.55 (ft³/s)/acre, averaging -0.01 (ft³/s)/acre, respectively. Comparisons of probability distributions for several pre- and post-PAM inflow-outflow tests suggest likely success in achieving seepage reduction with LA-PAM. Sensitivity analysis indicates that while the major effect on seepage uncertainty is error in measured flow rate at the upstream and downstream ends of the canal test reach, but that the magnitude and uncertainty of storage change due to unsteady flow also is a significant influence. Based upon the findings, recommendations for future seepage studies were provided, which have the ability to account for and reduce uncertainty of inflow-outflow measurements.
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Subject
irrigation
polyacrylamide
uncertainty
Monte Carlo
inflow-outflow
seepage