WaterSubject

Irrigatedagriculture in Pakistan’s Indus basin suffers from waterlogging andsalinity. Seepage from the extensive canal system and deep percolationfrom irrigated fields, combined with flat topography, have led to highwatertables. Salts are added to the soil with the irrigation water andthrough capillary rise from the shallow watertables. Since the s,subsurface drainage systems have been constructed in Pakistan to tacklethese problems. Subsurface drainage systems can control the watertable,limit salinization through capillary rise, and facilitate the leachingof salts.In areas underlain by fresh groundwater  public deep tubewells covering about . million ha of irrigatedland and over   private shallow wells have beenconstructed. The public wells are used for drainage and irrigation; theprivate wells are meant primarily for irrigation, but they alsocontribute to drainage. The number of private wells is stillincreasing, leading to over-exploitation of the groundwater in freshzones e.g. ,which solves part of the drainage problem here. In saline groundwaterareas   public deep tubewells covering about .million ha of irrigated land and pipe drains covering about  ha of irrigated land have been constructed fordrainage . These systems are meant primarily for drainage, as the drainage water is generally too saline to serve as irrigation water.UnderPakistani conditions, pipe systems are some times more expensivethan tubewell systems roughly US$/ha compared with US$/ha. Oneof the main reasons why pipe drains were introduced in salinegroundwater areas, despite the high costs, was the assumption that thelong-term drainage water quality would be better with pipe drains thanwith tubewells .Better drainage water quality reduces disposal problems and increasesthe possibility of using drainage water for supplemental irrigation.In this paper, we shall review the relation between drainage method anddrainage water salinity using data from the Satiana Pilot Projecttubewells and the Fourth Drainage Project pipe drains. Theseprojects overlap and are located near Faisalabad, in the Punjabprovince of Pakistan. The main objective of this paper is to assess thelong-term drainage water salinity of tubewells and pipe drains, and toidentify the processes that play a key role.