Submitted to: Journal of Irrigation and Drainage Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2010
Publication Date: 9/15/2010
Citation: Bautista, E., Zerihun, D., Clemmens, A.J., Strelkoff, T.S. 2010. External iterative coupling strategy for surface-subsurface flow calculations in surface irrigation. Journal of Irrigation and Drainage Engineering. 136(10):692-703 Interpretive Summary: Modern analysis of the performance of surface irrigation systems is based on hydraulic simulation models. An important limitation of existing irrigation models, including the ARS WinSRFR model, is that they represent the infiltration process - the flow of water from the soil surface to the subsurface – with empirical equations. A more rigorous infiltration modeling approach will enhance our ability to study irrigation hydraulic performance and the transport of fertilizers and other agricultural chemicals by the irrigation stream. However, coupling physically-based surface flow to subsurface flow models is mathematically complex and existing procedures experience computational problems. This study proposes a new strategy for coupling such calculations. The proposed coupling approach is simpler, less prone to computational problems, and reasonably accurate in comparison with existing coupling procedures. Furthermore, the approach can be adapted to any combination of existing surface and subsurface flow models. Such an approach will be incorporated in future releases of WinSRFR and will be made available to NRCS and other users of the software.
Technical Abstract: Coupling the unsteady open-channel flow equations of surface irrigation with the equation of variably saturated porous media flow is a computationally complex problem, because of the dependence of infiltration on flow depths. Several models of this coupled process have been developed, all of which solve the surface and subsurface iteratively at each time step of the surface flow solution. This study presents an alternative strategy, in which surface and subsurface flow calculations are conducted sequentially and iteratively at the level of the duration of the irrigation event. At each iteration, the subsurface flow results are computed based on the surface flow depths generated by the surface flow model at the previous iteration. These infiltration results are then fed back to the surface flow model, as an empirical infiltration function. The proposed strategy, labeled external iterative coupling, was prototyped for border and basin irrigation systems using the WinSRFR and HYDRUS 1-D models. The proposed procedure produced irrigation performance results comparable to those generated with an internally coupled model. Results also show that the procedure can produce reasonable results even when using a single representative location to calibrate the empirical infiltration equation used by the surface flow model.