WaterSubject

Nearly .million hectare of agricultural land in India is estimated to have goneout of cultivation due to development of waterlogging and soilsalinity/alkalinity .Thus, the issue whether irrigated agriculture can be sustained on along term basis has been raised at many forums. Studies in India haveconfirmed the technical and economical feasibility of horizontalsubsurface drainage to reclaim waterlogged saline lands, to improveproduction and productivity in irrigation commands and to establishsustainable irrigated agriculture. Although drainage at the field scalein India is still in its infancy, research and development on drainagesystem designs, drainage materials and new techniques to minimize costof drainage are in progress in a big way. It is in this context thattheory of bi-level drainage systems has been developed to understandand investigate the ground water table behaviour in this kind ofdrainage.The bi-level drainage is a system of horizontal drainsin which two adjacent drain lines are laid at two different elevationsor levels on an alternating basis . Ina conventional system, on the other hand, all the laterals are laid atthe same elevation. Amongst the many positive features, the cost of abi-level drainage system would be relatively less than a conventionalsystem. Thus. it could bring down the level of investments on drainagein developing countries. In this kind of drainage system, it ispossible to avoid excess drainage through adjustments in the level ofdrains which in a conventional system is achieved only through complexoperational schedules or through controlled drainage .This system offers a major advantage in cases where it is intended toimprove the drainage efficiency of an existing drainage system. In thatevent the new drain lines to be introduced in between two alternatelines could be laid at shallower depth than the depth of the existingdrains. The exact level could be determined through investigations andthrough application of theory on bi-level drains. As has been shownlater in this paper, the discharge of the system with bi-level drainswould be relatively less than a conventional system. Thus, the problemsrelated to disposal of saline drainage effluents would be much less ina bi-level drainage than a conventional drainage system. On the otherhand, complexity of the theory of bi-level drains and itsnon-availability to the designers in simple useable form has been amajor limiting factor in its widespread use. It is believed that withthe use of computers in drainage design, this limitation could beovercome. KFig.. Graphic illustration of a bi-level and a conventional level drainageproblem. When the water table reaches the depth of the shallow drain,the bi-level system changes into a conventional level system withdouble spacing. While scanning through the literature on bi-level drainage, authors came across a few references only. reported a steady state drainage design equation and used the approach to develop a non-steady state equation for a bi-level drainage system. presented an analytical and a numerical solution to predict thetransient movement of the water table in a bi-level drainage systemassuming that initial shape of the water table between the drains isparabolic. Finite difference solutions to the transient flow problem ofa bi-level drainage for various boundary conditions have been dealt by .Unlike conventional level drainage systems which have been wellinvestigated, there has been a general lack of a comprehensive study onbi-level drainage in the literature. The purpose of the present paperis to propose an analytical solution of the Boussinesq equation for thetransient case of a bi-level drainage system. In this study, it isassumed that initial shape of the water table between the two adjacentdrains is horizontal. Such a situation could be expected under theclimatic conditions in India. The proposed solution is verified usingexisting solutions and field data as reported by .On the basis of this solution, several applications of the proposedsolution are illustrated to reveal the usefulness of this drainagesystem.