In drainage for agriculture the field drains are usually located over the impermeable layer, or at a certain distance above it when the layer is at a great depth. The depth of installation of the drains, the depth of the profile, the hydraulic conductivity and the drainable porosity are parameters of fundamental importance in the calculation of the drain spacing. Several equations are suggested for the calculation of this spacing. However, there is not a single equation that adapts well to all the situations found in the field. This work was conducted at the Drainage Laboratory belonging to the Rural Engineering Department of the Escola Superior de Agricultura "Luiz de Queiroz"/USP, Brazil, using a reduced hydrologic model filled with sand, with the objective of defining the most appropriate equation in two different situations: when the drains are installed on or above an impermeable layer. Tests of flow (recharge) and hydraulic head (level of the water-table) under steady state conditions allowed the determination of the hydraulic conductivity using the Ernst formula. In a second stage, measuring the volume of water drained from the soil, the drenable porosity were determined. Later on, measuring the flow and water-table depth under conditions of non permanent state, the measured data were compared with those estimated by the formulas of Boussinesq-Glover, Schilfgaarde, Glover-Dumm, Terzidis, Hammad, Kirkham and Glover. The results indicated that the best spacing formula for drains to be used under conditions where the same are located over the impermeable layer is that of Boussinesq-Glover. On the other hand, when drains are located above the impermeable layer the Schilfgaarde equation followed by the Glover-Dumm equation gave the better results.
drains; equations; reduced model
