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<br /> <br />~ The applied water which percolates into the subsurface <br />N <br />N plays the major role in the life-sustaining drama of the <br />..... <br />irrigation event. Normally, it is also the greatest <br /> <br />contributor to pOllution in return flows. A part of the <br /> <br /> <br />water is stored in the root zone where it is used <br /> <br />comsumptively by crops. The plant uses the pure~water <br />fraction of root~zone moisture and the remainder is left' <br /> <br />with an elevated mineral (salt) and soluble nutrient <br /> <br /> <br />concentration. That water not retained in the root zone may <br /> <br /> <br />continue to percolate downward, continuously acting as a <br /> <br /> <br />mineral solvent or leaching' agent. It may then move <br /> <br /> <br />laterally to seepage areas, be collected by artificial <br /> <br /> <br />drains, or ultimately find its way into the ground water <br /> <br /> <br />system. The percolating fraction of applied water increases <br /> <br /> <br />the concentration of salinity in the return flow. This <br /> <br />increase is inevitable and is an inherent part of the <br />. irrigation scheme that must be recognized by agriculturist, <br />hydrologist, engineer and environmentalist alike. The <br />concentration of mineral salts in irrigation return' flow. <br />from both leaching and evapotranspiration may range from <br />three to ten times that of the applied water. <br /> <br />20 <br />