Effluent salinity of pipe drains and tube-wells : a case study from the Indus plain

Keywords: anisotropy, aquifer, desalinization, effluent salinity, groundwater, irrigation, salt-water upconing, soil salinity, stream-function, subsurface drainageIrrigated agriculture in arid and semi-arid zones often suffers from waterlogging and salinity problems. Sub-surface drainage systems can be used to control the groundwater table and to facilitate the leaching of salts from the rootzone. In the Indus plain, pipe drains and tube-wells are used for this purpose. Regional water management requires that the development of the effluent salinity with time of these systems is known in advance. Numerical models based on the Darcy equation and the mass balance equation for water flow and the advection-dispersion equation for solute transport are powerful tools to predict the effluent salinity of pipe drains and tube-wells at field level. In advection-dominated transport problems, however, solute impulse response functions based on stream-functions constitute a more computationally efficient approach.A new modelling approach is presented that combines the one-dimensional vertical finite-difference SWAP model for the variably saturated zone with a solute impulse response function for the saturated zone. This approach is applied to the Sampla experimental pipe drainage site in Haryana, India, the S-I-B-9 pipe drainage unit of the Fourth Drainage Project, Punjab, Pakistan and the Satiana tube-well Pilot Project, Punjab, Pakistan. Results show that the effluent salinity of pipe drains and tube-wells changes only gradually with time due to the low percolation from the irrigated fields and due to the large quantities of salts stored in the groundwater. Areas with relatively high percolation and a shallow depth of the impermeable layer (pipe drains at Sampla) still require 10 years before the effluent salinity has reduced to equilibrium levels. In contrast, desalinization of the rootzone generally takes only 1-3 years. The implication is that farmers will benefit quickly from the installation of a drainage system. However, for the safe use and disposal of the effluent, long term solutions are required.In the Indus plain, groundwater salinity usually increases with depth. In water scarce areas, the shallow fresh groundwater may be an important source of irrigation water. In waterlogged areas, where sub-surface drainage is installed to control the groundwater table, the presence of fresh groundwater bodies may result in a relatively low effluent salinity. The finite-element model SUTRA is used to study the behaviour of skimming wells and pipe drains in fresh-saline groundwater systems. The model is calibrated on two documented experiments with a skimming well and a scavenger well at Phularwan research farm, Punjab, Pakistan. Salt water upconing below the skimming well is particularly sensitive to the anisotropy factor of the aquifer. The relationship between aquifer anisotropy and the Electrical Conductivity ( EC ) of the pumped water is non-linear. The skimming well simulations show that water with an EC of1.7 dS m -1can be pumped from a thin fresh groundwater body, provided that the pumping rate is low. Under the same circumstances, pipe drains yield a better effluent quality ( EC of 1.2-1.3 dS m -1). With pipe drains, flow is restricted to the shallow fresh groundwater. The deeper saline groundwater is left untouched. The better effluent quality for pipe drains as compared to skimming wells, must be evaluated against the considerably higher installation costs for pipe drains.</p

Drainage Water Salinity of Tubewells and Pipe Drains: A Case Study from Pakistan

Drainage water salinity data from 71 public deep tubewells and 79 pipe drainage units near Faisalabad, Pakistan, were studied. Drainage water salinity of the tubewells and the pipe drains remained approximately constant with time. This was attributed to the deep, highly conductive, unconfined aquifer underlying the area, which facilitates lateral groundwater inflow into the drained areas. Tubewells alongside surface drains showed average electrical conductivity, sodium adsorption ratio, and residual sodium carbonate values of 3.2 dS m-1, 17.2 (meq l-1)(0*5), and 6.4 meq 1-1, respectively. For pipe drains, which are situated in areas with comparable conditions, the corresponding values were 2.5dS m--1, 12.2 (meql-J)°5, and 3.7meq 1-1, respectively. Tubewells have an inferior drainage water quality because they attract water from greater depths, where the water is more saline

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. (C) 2000 Elsevier Science B.V

Calibration of capacitance probe sensors in a saline silty clay soil

Capacitance probe sensors are a popular electromagnetic method of measuring soil water content. However, there is concern about the influence of soil salinity on the sensor readings. In this study capacitance sensors are calibrated for a saline silty clay soil. An electric circuit model is used to relate the sensor's resonant frequency F to the permittivity () of the soil. The circuit model is able to account for the effect of dielectric losses on the resonant frequency. Dielectric mixing models and empirical models are used to relate the permittivity to the soil water content (). The results show that the electric circuit model does not fit the F¿() data if the calibrated bulk electrical conductivity (EC) model is used. The dielectric losses are overestimated. Increasing the exponent c in the tortuosity factor of the bulk EC model and thereby lowering the bulk EC and the dielectric losses improves the performance of the model. Measured and calculated volumetric water contents compare reasonably well (R2 = 0.884). However, only 73 out of 88 data points can be described. The rejected points are invariably at high water contents where the high dielectric losses result in the sensor frequency being insensitive to ()