Numerical models in designing tube-well irrigation and drainage of an aquifer with a fresh/salt interface
Boehmer, W.K.; Mühlenbruch, W. (1989). Numerical models in designing tube-well irrigation and drainage of an aquifer with a fresh/salt interface. Natuurwet. Tijdschr. 70(1-4): 117-131
In: Natuurwetenschappelijk Tijdschrift. L. Walschot/Natuur- en Geneeskundige Vennootschap: Gent. ISSN 0770-1748, more
Also appears in:
De Breuck, W.; Walschot, L. (Ed.) (1989). Proceedings of the 10th Salt-Water Intrusion Meeting Ghent (Belgium), 16-20 May 1988. Natuurwetenschappelijk Tijdschrift, 70(1-4). Natuurwetenschappelijk Tijdschrift: Gent. 408 pp., more
This paper describes the determination of the drainable surplus and the design of a field of skimming wells pumping fresh water for irrigation, in combination with deep wells pumping saline water for drainage, in a part of a large irrigation scheme in the Indus basin of Pakistan. Groundwater there occurs in highly permeable alluvial deposits more than 1000 m thick and filled with saline water, partly replaced down to a depth of 300 m by fresh water in strips 5-15 km wide along both sides of the Indus and Panjnad rivers. Irrigation with river water and seepage from irrigation canal causes severe problems of water logging and salinity.Groundwater is being abstracted by more than 600 deep tube wells to lower the water table. This has resulted however in fast deterioration of the groundwater quality due to upconing of the fresh-/saline- water interface to within filter depth of the tube wells. The authors carried out a model study to optimize the abstraction of groundwater in order to meet the drainage requirements and the greater demand for irrigation water, while maintaining an acceptable quality of fresh groundwater for irrigation.This study was carried out with two numerical models. First the drainable surplus and its division over the irrigation area under historical and future irrigation regimes was determined using a one-layer finite difference model. This model was calibrated using historical series of water-level and water-balance data.In a second study the Badon groundwater model was used to determine the effect of pumping on the fresh-/saline-water interface and to find an optimum division of the drainable surplus between skimming wells pumping fresh water and deep wells pumping saline water for drainage.The model was built up using a calibrated aquifer characteristics, water-levels and water-balance data of the first model study, and was calibrated using data on water quality from different times and a map of the interface derived from well data and a geo-electrical survey. It proved posible to reproduce quality and interface data properly after the calibration process.The second model study led to a new well-field design. This design includes the installation of more high-capacity drainage wells in the saline-groundwater zone, no pumping in the transition zone between fresh and saline groundwater, reduced pumping by shallow, low-capacity skimming wells in the area with a shallow fresh-groundwater zone, and increased pumping and export of water through the irrigation canals in the northern part of the area with a deep fresh-water zone and high seepage rates from the canals.
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