Prediction of long term drainage water salinity of pipe drains

Long term drainage water salinity of pipe drains is modeled with the advection–dispersion equation for the zone above drain level and stream functions for the zone below drain level. Steady-state water flow is assumed. The model is applied to two experimental pipe drainage sites in Haryana State, India. Calculations are conducted for different values of leaching fraction and drain spacing. On average, comparison between measured and predicted drainage water salinity is satisfactory for both the Sampla site and the Hisar site. Calculations show that it may take 15–50 years before drainage water salinity has reduced to equilibrium levels. Leaching fraction has considerable influence on the drainage water salinity. An increase in the leaching fraction from 0.2 to 0.4 will reduce the time to reach equilibrium drainage water salinity levels by about 50%. Drain spacing has little influence on drainage water salinity, provided the hydraulic properties below drain level are uniform (Sampla). Some influence of drain spacing might be expected if the zone below drain level consists of a less conductive layer underlain by a more conductive layer (Hisar). In the latter case, the larger the drain spacing, the longer the time to achieve equilibrium drainage water salinity levels.