Kurtzman, D., Insti tute of Soil, Water and Environmental Sciences, Agricultural Research Organization, Volcani Center, Bet Dagan 50250, Israel
Scanlon, B.R., Bureau of Economic Geology, Jackson School of Geosciences, Univ. of Texas at Austin, 10100 Burnet Rd., Austin, TX 78758, United States

Land use change from natural ecosystems to cropland influences groundwater recharge, including water quanti ty and quality. Soil core samples (0-11-m depth) from six boreholes beneath irrigated cropland and two boreholes beneath natural ecosystems, in Verti sols, were analyzed for parti cle size distribution, water content, and water-extractable Cl -.Chloride mass balance and numerical, one-dimensional unsaturated flow and transport modeling were used to assess average and transient recharge fluxes and to test matrix vs. preferential flow hypotheses. Water contents under irrigated cropland were significantly higher than under natural land with similar parti cle size distributions. Pore-water Cl - concentrate ons in deep vadose zones (>3 m) under irrigated cropland (900-2000 mg L -11) were similar to recent local groundwater Cl - and significantly lower than pore-water Cl - in deep vadose zones under natural land (3000-6000 mg L -11). Calibrated models' recharge rates through the soil matrix were much higher under irrigated cropland (90-230 mm yr -11) than natural ecosystems (1-3 mm yr -11) and were consistent with groundwater-balance esti -mates of average recharge (110-160 mm yr -11). In contrast, matrix recharge rates under natural ecosystems were considerably lower than those based groundwater balance (50-80 mm yr -11). While matrix unsaturated flow under irrigated cropland explains vadose zone and groundwater observations, under natural ecosystems preferential flow paths are required to support observations. Plowing and irrigation prevent development of crack networks and promote matrix percolation through the clay, which flushes salts from previously immobile vadose zone pore water. These phenomena may be applicable to similar land use changes in Verti sols globally. © Soil Science Society of America 5585 Guilford Rd., Madison, WI 53711 USA. All rights reserved.