Subsurface drainage of vertisoil

A technique was developed for subsuface drainage of vertisoil. Drains were installed in 24.5 ha of the Mandacaru irrigation Project, state of Bahia, Brazil, first, at a spacing of 20.0 m and an average depth of 1.70 m; Subsequently, at 30.0 m and depths of 1.20 and 1.50 m. In the first area the drains were installed within the "C" horizon (regolith), or the "A"/"C" transition. The water table was kept at an average depth of 1.17 m, for a recharge of 0.5 mm/day. Desalinization of the soil occurred for a deep percolation of 452.7 mm. In the second area the drains were installed, generally within the "A" horizon, or in the "A"/"C" transition. To facilitate the flow of water from the regolith to the drains, vertical relief auger holes were made at the bottom of each ditch to reach and penetrate into the regolith. The holes, spaced at 3.0 m, were filled with gravel to connect the regolith with the gravel enveloped drains. The hydraulic conductivity of the "A" horizon was equal to less than 0.005 m/day, while for the regolith it was about 35 times higher.