WATER-MOVEMENT AND SALT LEACHING IN DRAINED AND IRRIGATED MARSH SOILSOF SOUTHWEST SPAIN

The Guadalquivir river marshes in southwest Spain, are situated in a f ormer estuary that has now been drained. They cover some 140 000 ha. S oils formed in this zone are alluvial, very clayey, salinesodic, and o f vertic character with a shallow, very-saline water table. An area wi thin these marshes, reclaimed in 1979, is the object of this detailed study which looks at some physical and chemical properties of soil, an d also the influences of sprinkler and furrow irrigation on drainage a nd salt leaching. These studies were conducted between 1988 and 1990. A 1 ha experimental plot was used for this purpose. The plot is situat ed within an area of the marshes being used for standard agricultural practices, equipped with a drainage system of ceramic pipes buried 1 m deep and spaced 10 m apart, ending at an open ditch collector which i s perpendicular to them. The following measurements have been carried out: soil bulk density changes with water content, hydraulic conductiv ity and sorptivity, soil water content and water tension profiles chan ges, water table level, drainage outflow and salt content of soil, soi l solution and drainage water. Cotton was used as the crop. The result s obtained show that after ca. 10 years in reclamation the electrical conductivity and exchangeable sodium percentage decreased in the 0-90 cm layer of this soil, particularly in the top 50 cm, that reduces the adverse effects of salinity on crop development. During both sprinkli ng and furrow irrigation a rapid response of the drain pipes was obser ved because of the soil fissures and crack network that resulted from the shrinking and swelling processes. Maximum drainage outflow was rea ched when the irrigation stopped. Water movement in this soil is chara cterized by an initial rapid phase, due to the soil fissures and crack s, followed by a second slow one controlled by the soil matrix. The ef ficiency of irrigation in salt leaching was higher in the case of furr ow irrigation (16 g of salt leached per litre of applied water) than i n the case of sprinkling irrigation (10 g l-1), owing to the use of mo re water in the latter. The salinity of soil and soil solution decreas es after irrigation starts, the time taken to reach a minimum value fo r the different soil layers and the irrigation method used were differ ent. In 1989 due to water restrictions at some time of the crop period , the irrigation schedule was different to that normally used, and the crop was affected by water stress and capillary rise of saline water.