Sodicity, conventional drainage and bio-drainage in Israel

Abstract

Years of intensive irrigated farming in the inland valleys of northern Israel have caused secondary salinity in vast areas. Soil salinity/sodicity surveys, conducted after the problem was recognised, showed relatively high levels of sodium adsorption ratio (SAR) in the active soil layers. In a sample of 7 affected fields, 92.5% of the 1584 SAR measurements on cored soil samples had values >5, and 39% had values >15. Two ameliorative approaches were conceived to reverse the evident salinity/sodicity trends in the Yizre’el Valley: (a) a gravitational, newly engineered, subsurface drainage system; and (b) bio-drainage, using eucalypts as a means to control the high water tables. Both approaches were very successful according to the chosen criteria. The drainage system, comprising 3 drain components, effectively controlled excess water from 3 sources: direct infiltration, lateral subsurface flow, and a deep, presumably upward-seeping, artesian aquifer. The groundwater table response to the installation of the applied drainage system was immediate. In the next rainy season, there was an associated and appreciable deep leaching of Cl^– and somewhat less of Na⁺. More than 3000 ha of land has been drained in this way in the past 10 years in the northern, salinity-prone, inland valleys.

The bio-drainage approach, tested in 5 different waterlogged and salinity-affected sites in the Yizre’el Valley, also proved very successful. The rates of growth of Eucalyptus camaldulensis provenances was impressive from the first year. In the best plot, in the fifth year, biomass production reached 30 t/ha.year for the Broken-Hill ecotype (NSW) and peaked at 57 t/ha.year for the Hadera (Israel) ecotype. The annual transpiration of selected trees at Nahalal was 1360 mm, or 3 times the local average annual precipitation. The groundwater table dropped to below 3 m in the summer of the fifth year—sufficient to provide saltflushing conditions. In a related study at the Nahalal site, it was demonstrated that E. camaldulensis screens out the salts while consuming soil water. That, coupled with prolonged stresses of salinity and flooding, can be detrimental to eucalypts. In conclusion, despite sodic conditions, which are ‘formal’ by definition and which prevail in much of the area of our northern valleys, drainage, whether conventional or biological, if well-designed and implemented, is a viable means for controlling local hydrology and restraining salinitysodicity trends.