Water and solute movement under conventional corn in central Spain .1. Water balance

Partitioning temporal changes in soil water storage to water balance components (evapotranspiration [ET], drainage) under field conditions remains a vexing task. An in situ protocol using neutron probes and tensiometers was established on a sandy loam (Typic Xerofluvent) under irrigated, conventional corn (Zea mays L.) with the objectives to separate ET from deep drainage and to improve the accuracy of estimating the water balance components. An experimental research site (0.4 ha) at La Poveda Field Station near Madrid (Spain) was divided into 24 plots of 9 by 11.1 m each. The heterogeneity of the research site had been previously determined and the experimental held was instrumented accordingly. Vacuum soil-water extractors were placed in all plots, but neutron probe and tensiometers were installed in only eight plots based on soil texture. Plot management included nine furrow irrigations of 404 mm and rainfall of 250 mm throughout the growing season. Measurements of water flow and soil water storage for periods of a few days allowed the determination of ET and drainage with the soil water balance equation. Four water flow patterns and six water balance cases were identified. Results showed a cumulative ET of 532 +/- 70 mm and total drainage of 75 +/- 22 mm below the 1.4-m soil depth. Peak drainage took place 20 d before maximum ET and represented 19% of total irrigation water.