Submitted to: Transactions of the ASABE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/14/2017
Publication Date: 12/18/2017
Publication URL: http://handle.nal.usda.gov/10113/5922761
Citation: Bautista, E., Schlegel, J.L. 2017. Estimation of infiltration and hydraulic resistance in furrow irrigation, with infiltration dependent on flow depth. Transactions of the ASABE. 60(6):1873-1884.
Interpretive Summary: Knowledge of the infiltration and hydraulic resistance processes is essential for the hydraulic analysis of surface irrigation systems. The recommended approach for characterizing those processes is using parameter estimation techniques, in which an irrigation model is used to solve for the pertinent process parameters from flow variables measured as part of an irrigation evaluation. This article discusses the estimation of those parameters for furrow irrigation. Previous studies on the subject have assumed, almost exclusively, that furrow infiltration depends only on the contact time between the water and the soil. In contrast, this analysis uses a recently developed flow-depth dependent model of furrow infiltration, developed from porous media flow theory, which accounts for variations in the infiltrating surface and water pressure with distance along the field and time. The model also accounts, empirically, for flows through soil macropores, which is not described by porous media flow theory. Typically, macropore infiltration rates can be very large but last a very short time. The estimated parameters were the hydraulic conductivity and the macropore value. Results show that the proposed model described the infiltration process under the test conditions with equal or better accuracy than when using empirical infiltration equations, while using only two parameters. Thus, it is a viable and practical model of the infiltration process. The estimated hydraulic conductivity values were consistent with values reported in the literature for the soil texture conditions of the tests. These results should be of interest to users of surface irrigation modeling tools and other surface irrigation researchers.
Technical Abstract: The estimation of parameters of a flow-depth dependent furrow infiltration model and of hydraulic resistance, using irrigation evaluation data, was investigated. The estimated infiltration parameters are the saturated hydraulic conductivity and the macropore volume per unit area. Infiltration through macropores is assumed to occur instantaneously and depend on furrow spacing only. The infiltration estimation uses volume balance calculations, which are applied in two stages. An empirical infiltration function is estimated in the first stage. That solution is used in the second stage to generate the flow depth hydrographs needed to solve for the flow-depth dependent parameters. The procedure takes advantage of the non-uniqueness of infiltration solutions: similar surface flow conditions (advance and recession times, flow depths, and runoff rate) can be predicted with different infiltration functions. The procedure was tested with two irrigation data sets, each consisting of three furrows. With all tests, macropore flow appeared to contribute substantially to the infiltration process. The sums-of-squares of the differences between volume balance and predicted infiltration volumes, computed by the estimation procedure, was smaller with the proposed model than when infiltration was computed with completely empirical infiltration equations dependent on opportunity time only. The estimation produced parameters that were consistent for each data set, and the hydraulic conductivities values were consistent with published values for the soil texture. Also, the parameters were relatively easy to find. The analysis also compared empirical infiltration equations, and showed that the Mailhol-Gonzalez equation, which calculates infiltration as the sum of macropore and steady infiltration rate terms, provides a better description of the infiltration process than the commonly used Kostiakov or Kostiakov-Lewis equations, at least for the test conditions. Although estimates of the hydraulic resistance parameter, the Manning coefficient, were the same for any furrow with any of the infiltration equations used, the estimated parameters of the flow-depth dependent infiltration equation varied slightly depending on the empirical equation used in the first stage of the analysis. Finally, results show that the effect of flow-depth dependent on distribution uniformity of the infiltrated water depends on soil and hydraulic conditions. For one of the data sets, distribution uniformity computed with the proposed infiltration model was only slightly different than the uniformity computed assuming that infiltration depends on opportunity-time only, because of the large macropore flow contribution and the shallow flow conditions.