SIMULATING THE IMPACT OF IRRIGATION MANAGEMENT ON THE WATER AND SALT BALANCE IN DRAINED MARSH SOILS (MARISMAS, SPAIN)

Using the simulation model MACRO, this paper investigates the likely c onsequences of reduced irrigation inputs on the water and salt balance and crop growth in a drained, saline clay in a Mediterranean climate (Marismas, SW Spain). The model was first successfully validated again st field measurements of the soil water and chloride balance, water ta ble depths and drain outflows in the 1989 growing season. Three-year s imulations were then performed assuming two different irrigation appli cations (60 and 75% reductions from the 1989 amount) and two different frequencies (12 or 6 irrigations per growing season). The model predi ctions suggested that reduced irrigation may lead to up to a 15% incre ase in the chloride content of the soil profile after 3 years. Also, d espite overall reductions in water discharge, slight increases in chlo ride leaching via field drains (c. 4 to 8%) were predicted. The model demonstrated that encroachment of salt into the soil profile map be ex acerbated by the non-equilibrium nature of water flow and solute trans port ('by-passing flow') in structured clays. With reduced water suppl y for irrigation, more frequent applications may give marginally bette r crop yields for the same quantity of irrigation but at the expense o f slightly increasing salt concentration in the root zone.