field evaluation of drain spacing equations for roodasht region of esfahan

field experiments were conducted on an existing subsurface drainage system in roodasht region of esfahan, ian, to test the performance of drain spacing equations for transient flow. the water table profiles recorded from a series of observation wells indicated an essentially flat water surface. the effective hydraulic conductivity and the degree of anisotropy were calculated as 19.2 cm h-1 and 1.47, respectively. the variable drainable porosity as a function of water table height was computed by means of a two-dimensional flow concept. twenty three drain spacings were computed by eight equations consisting of glover, luthin, van schilfgaarde, hammad, glover-van schilfgaarde, dumm, bouwer and van schilfgaarde, and youngs. the results indicated that youngs and luthin equations had the best performance, contrasted to glover and hammad equations, which gave the most unreliable predictions. the other equations showed inferior performance. it follows that in other areas of roodasht, where the impervious layer is shallow, either youngs or luthin equation may be used for design of drain spacing.