PERFORMANCE EVALUATION OF SUBSURFACE DRAINAGE SYSTEM UNDER UNSTEADY STATE FLOW CONDITIONS IN COASTAL SALINE SOILS OF ANDHRAPRADESH, INDIA

摘要

The performance of an executed subsurface drainage system was evaluated under unsteady flowrnconditions. The impulse-response relation has been studied for two different conditions of drainrnspacings, namely, the executed drain spacing based on steady state flow conditions and the drainrnspacing proposed on unsteady state flow conditions, incorporating the effects of drainablernporosity. It is found rational to use the "Dezeeuw-Hellinga model" for prediction of impulsernresponse relations in terms of temporal water table fluctuations against rainfall – recharge underrnunsteady state flow conditions. The responses of a sub-surface drainage system for the impulsernof incessant rainfall have been studied. The values of calculated drain spacings varied from 11 torn15 m. However, due to economic conditions, the practical drain spacings of the layout have beenrnfixed at wider value of 35m and 55m. It is found that the drain spacings adopted for unsteadyrnstate flow conditions might have resulted in a better performance of the drains compared tornsteady state drain spacing as depicted by Dezeeuw-Hellinga model run. The drainable porosityrnbeing the vital parameter in an unsteady state equation, the Dezeeuw-Hellinga model was alsornused for varying levels of drainable porosity under given drain spacing conditions. Generally, thernreference drainable porosity value is taken as 10 per cent for most of the drainage studies and therninfluence on drain outflows were compared for an increased value of 20 per cent and decreasedrnvalue of 5 per cent, since the drainable porosity value in the study area varied from 5 to 20 perrncent. It was found that the change in drainable porosity significantly influence the drainrnperformance as depicted by Dezeeuw-Hellinga model run over all the3 standard week of year.rnThe executed sub-surface drainage system has been found satisfactory in bringing down the soilrnsalinity levels to desirable limits below 4 dSm~(-1). The executed sub-surface drainage system hasrnalso resulted in appreciable crop productivity improvements in the locality.