Reclamation of a Coastal Saline Vertisol by Irrigated Rice Cropping, Interpretation of the Data with a Salt Leaching Model

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ABSTRACT

Data are available for the years 1970 to 1974 on the reclamation of a heavy, motmorillonitic, clay soil (vertisol) by means of irrigated, submerged, rice crops during three seasons in the experimental area of Chacupe in the arid coastal area of Peru near the city of Chiclayo. The area is under influence of upward seepage of saline groundwater from the uplands, therefore very saline and barren. Despite the difficult soil conditions the experiment has been successful owing to the installation of a subsurface pipe drainage system that intercepted the groundwater flow and drained the leaching water stemming from the irrigation water that percolated slowly downward through the soil to the drains. Thus the inflow of salt water was prevented and the soil was desalinized thanks to the removal of the saline percolation water. An earlier experiment in the Tagus delta, Portugal, revealed that the reclamation of vertisols was a slow process partly owing to the low leaching efficiency and the slow hydraulic conductivity of the soil. To verify this, the reclamation process in Chacupe is described and a salt leaching model, named LeachMod, is applied to explain the phenomena observed. It introduces the concept of leaching efficiency. In the period of the experiment computer software was not yet available, reason why it is used now. The model uses water and salt balances of the root zone of which sufficient data are available to make the model workable. It is found that the desalinization is faster at the start of the experiments than later on. One reason is that at higher salinity the salt concentration of the percolating water is higher in the beginning so that the removal of salts is faster. A second reason is that the leaching efficiency decreases with decreasing soil salinity, a phenomenon that is related to the expansion of the diffuse double layer of clay particles at lower soil salinity levels whereby the soil loses structure and the soil permeability for water reduces. Without the model, this conclusion could not be found. The consequence of the structure loss is that it will take many years before the soil salinity has gone down sufficiently to allow growing “dry foot” crops. However, the “wet foot” rice crops produced satisfactory yields already after one year owing to the good quality of the irrigation water in which the roots were submerged. The presence of the drainage system was important to remove the salty downward percolating water and to maintain a deep water table during fallow periods thus preventing capillary rise of the groundwater and re-salinization of the soil. An example is given of a situation with an inadequate drainage system using LeachMod with a reduced drainage capacity to demonstrate the danger of such re-salinization.

KEYWORDS

Saline soil reclamation, vertisol, leaching model, salinity simulation, water and salt balance, leaching efficiency, capillary rise

Cite this paper

R. J. Oosterbaan. (2019) Reclamation of a Coastal Saline Vertisol by Irrigated Rice Cropping, Interpretation of the Data with a Salt Leaching Model. International Journal of Environmental Science, 4, 48-60