Hydrological and salinity impacts of a bio‐drainage strategy application in the Yizre'el Valley, Israel

Abstract

AbstractThe bio‐drainage–commercial forestry strategy was applied in five plots in the Yizre'el Valley, northern Israel, to evaluate the hydrological and salinity impacts of eucalypt plantations. Each plot contained a mix of 11 selected eucalyptus species/ecotypes. Two plots (Nahalal and Genigar), representing the two extreme waterlogging/salinity conditions in the valley, were selected for in‐depth monitoring over a 10‐year period to assess the likely environmental improvement through bio‐drainage. Despite impressive growth rates of genetically improved Eucalyptus camaldulensis in the year‐round waterlogged, slightly saline Nahalal site (650 mm annual rainfall), the water uptake by the trees was insufficient to control the rising water table caused by excessive water inputs, both natural and human. In the more saline, alkaline and drier Genigar plot (450 mm annual rainfall), where rainfall is the only water input, the ground water dropped to below 3 m from soil surface in the fourth year after planting, i.e. deeper than the adjacent ground water levels. Both sites showed appreciable rise in wells that penetrated the 3‐ to 4‐m confining layer. The 10‐year salinity (EC) trend of the top layer in Nahalal varied because the drainage was limited by the positive water balance and the above‐average spells of dry winters. In and below the 4 m deep layer the EC remained below 1·5 dS m−1 throughout the entire 10‐year study. The last EC measurement, taken in 2003, showed values not higher than 4 dS m−1 throughout the 6 m soil profile. In Genigar, there was significant leaching of salts from the top layer (1 m) during the 9‐year monitoring period, but recently a salts ‘bulge’ was gradually developed in the 1–5 m strata indicating that the expected downward movement of leached salts was impeded by the 3–4 m deep low‐permeability clayey layer that lies over a coarser, far more conductive and notably confined layer, which leads to a perched water body. The last EC measurement at the end of 2003 showed a maximum value of 5·5 dS m−1 at 3 m depth. No signs of tree stress were observed in either site, at any soil depth during the 10 years of monitoring. Theoretical considerations do not support the hypothesis that there would be a fatal long‐term accumulation of salts in the root zone. The Israeli experience has shown that the bio‐drainage technique can effectively lower a shallow water table and reverse salinity trends, provided that the overall water balance is negative, i.e. that the water inputs match the water use by the tree plantation and local drainage characteristics. However, the rate of improvement of the hydraulic, salinity, sodicity and soil physical properties is site specific. Excess fresh water inputs into the plantation, although they create waterlogging conditions, supply unlimited water to the trees, which, in turn, show exceptional growth rates, with usable commercial value. Copyright © 2007 John Wiley & Sons, Ltd.