Vita.The overall goal of this investigation was to develop a methodology for incorporating crop drainage requirements into subsurface drainage design schemes. The methodology developed was linked to the North Carolina State University Water Management Model (Skaggs, 1976) and it provides a workable simple design procedure. The proposed crop response model is a simple linear expression based on the Stress-Day Index concept advanced by Hiler (1969) and the ideas presented by Sieben (1964). The expression indirectly takes into account the intensity of oxygen deficits caused by high water tables, as well as the relative sensitivity of the crop to oxygen deficits at different growth stages. Long term computer simulations were conducted using grain sorghum as the experimental crop and a Bladen soil (poorly drained) located in Greenville, North Carolina. The crop response model was used to evaluate the degree of crop damage for given drain depths and spacings for 24 growing seasons (1952-1975). These output data were analyzed by methods generally used in hydrologic frequency analysis. Six different design graphs each having probability versus crop damage with a family of straight lines representing different drain spacing values were developed to show how the procedure could be used practically. The methodology developed incorporates drainage requirements and drainage theory in a rational way and can be used for day-to-day design of subsurface drainage systems
