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Agricultural fields in Central Illinois are predominantly drained by subsurface drainage systems, which often arein depressional areas. The prediction of outflow from drainage systems is a challenge. The SCS curve number (SCSCN) meth-odis a popular method for evaluating direct runoff from rainfall. The concept behind this method can be applied to subsurfacedrainage flow. The SCSCN method was modified through theoretical analogy and term redevelopment to estimate subsurfacedrainage flow from rainfall. The analogical theory is that when accumulated subsurface drainage flow is plotted versus accu-mulatedinfiltration, subsurface drainage flow starts after some infiltration has accumulated and the relationship becomesasymptotic to a line of 45slope, just as the generalized SCS rainfallrunoff relationship.Procedures are introduced for modification of the SCSCN method and determination of curve numbers for subsurfacedrainage flow. In the process of defining curve numbers for drainage flow, it was found that the curve number varied not onlywith season but also with rainfall amount. In addition, the curve number varied with previous rainfall condition as with thetraditional SCSCN method. The best previous rainfall adjustment is based on the 10day previous rainfall for subsurfaceflow prediction. The curve number is sensitive to the fraction of initial abstraction (k). However, this does not mean thatpredicted flows resulting from different k values are significantly different, since the equation for flow prediction changeswhen k is changed; in fact, the predicted flow is not sensitive to the fraction of initial abstraction to potential maximumretention of the watershed.The modified SCSCN method was applied to estimate subsurface drainage flow for five drainage monitoring stations inthe Little Vermilion River (LVR) watershed in EastCentral Illinois. Predicted subsurface flows using the modified SCSCNmethod were compared with observed subsurface flows. Statistical tests showed that the predicted subsurface flows using themodified SCSCN method were not significantly different from the observed subsurface flows. Validation performed on twoof the sites using modified curve number relationships from the other sites showed that the predicted subsurface drainage flowswere not significantly different from the observed subsurface drainage flows.
Yuan et al. (Mon,) studied this question.