Comparative Study of the Spacing of Subsurface Drains in the Eastern Gandak Project Using Different Criteria

Irrigation is an age-old practice in India. A sizable proportion of irrigated areas have suffered due to waterlogging and salt-related problems. The proper provision of drains in irrigated command areas differed or was postponed in many projects, including the Eastern Gandak project in Bihar, India. Keeping this in view, the spacing of sub-surface drains in the Eastern Gandak project has been determined. A comparative study was made using different steady-state and unsteady-state criteria. The study reveals that the irrigation schedule has a very high influence on the estimation of drain spacing, especially for unsteady-state conditions. In addition, it is observed that the spacing using Donnan’s approach is consistently larger than that from Hooghoudt’s approach for steady-state conditions. Also, the spacing estimated from these two models is correlated with r² = 0.7648. Further, for the unsteady-state approach, the spacing estimated from the van-Schilfgaarde approach varies from 1.45 to 1.85 times the spacing determined using Glover-Dumm’s approach. However, these two approaches have a good correlation with r² = 0.9734. Hooghoudt’s approach uses the concept of equivalent depth (de), which is less than the actual depth of the barrier layer from the level of the drain. Therefore, it is recommended to use Hooghoudt’s equation for steady state condition and Glover-Dumm’s equation for unsteady state condition in the study area and other projects of the state of Bihar and elsewhere in India. The sub-drains do not require repairs and maintenance once the envelope is properly designed and placed around the drain tube.