Wednesday, November 15, 2006

Influence of Plant, Soil and Water Properties on the Leaching Fraction.

Lynn Dudley, Florida State Univ, 111 Carraway Bldg, Tallahassee, FL 32306, Alon Ben-Gal, ISRAEL, Agric. Research Org., Agricultural Research Organization, Gilat Research Center, Mobile Post Negev 2, 85280, ISRAEL, and Uri Shani, The Hebrew University of Jerusalem, Dept. Soil & Water Sci., Rehovot, Israel.

Irrigation delivery systems such as drip and high precision sprinkler systems make it possible to irrigate to a ratio of irrigation to evapotranspiration near 1.0. However, a recent review of high efficiency irrigation and the leaching requirement suggested that to do so risks salinization of the root zone.  The author stated that if the practice was carried on long enough, salt could accumulate to the point of complete loss of the soil resource.  Computer simulations of irrigation with saline water at a ratio of irrigation to actual evapotranpiration near 1.0 show a different result.

A 1-D model that contained subroutines with second order numerical solutions to Richard's equation (with a root extraction term) and the equation of continuity for solute transport, and crop response terms for water and salinity was used for simulations.  The model assumed conservative behavior of dissolved salt.  The upper boundary condition (irrigation and potential evaporation) was taken from field data for an experiment attempting to balance irrigation to actual evapotranpiration.  The model was executed for an irrigation season and the salt and water content at the end of the season was used as the initial condition for the subsequent simulation.  The process was repeated to steady state.

Simulations showed that a reservoir of saline water accumulated in the lower profile while the crop continued to extract water from a low salinity zone near the surface.  The root zone was progressively shortened by the accumulating saline water.  Steady state was achieved when a small yield loss was induced by accumulating saline water resulting in excess irrigation and drainage.  The system is self regulating with a leaching fraction that is a function of water quality, crop tolerance and soil texture.  Results are shown for different soil textures, water salinity and water and salt stress crop response factors.