Water and salt distribution in a soil under trickle irrigation

Abstract

A field study was conducted to determine water and salt distribution patterns in a soil irrigated by pairs of double-chamber, perforated polyethylene tubes. The study consisted of two experiments: a water distribution experiment and a salt distribution experiment. Both experiments were conducted at the same site with experimental plots having two perforated lines 9 m long, spaced 0.60 m. The tubing had outer orifices 0.5 mm in diameter spaced 0.30 m along the tubes. The water distribution experiment consisted of water application to the bare soil for periods of time of 3, 6, 9, and 12 hours. After each test a trench was dug normal to the irrigation tubes and samples were taken to determine soil moisture on a dry weight basis. Moisture profiles are presented for the various tests. The salt distribution experiment was conducted in the Fall of 1973 and repeated in the Spring of 1974. It consisted of four irrigation treatments comprising two irrigation levels and two levels of salt in the irrigation water (327 and 2000 milligrams per liter of salts). Experimental plots were planted with lettuce and soil samples taken after planting and after harvesting the lettuce. Soil samples were analyzed for electrical conductivity of the soil saturation extract, pH, calcium, magnesium, sodium, potassium and nitrates. Saturation extract conductivity profiles in the soil are presented for different treatments. After planting and after harvest concentrations of calcium, magnesium, sodium, potassium, nitrates and pH values are also shown. Seasonal water application and lettuce yields are presented for both trials Water movement in the soil was 2 to 3 times greater in the horizontal than in the vertical direction. Wetted soil volume showed a high positive correlation with both the volume of water applied and with time of application. Salt accumulation occurred mainly at the soil surface between the irrigation tubes and away from the main root zone of the plants. The surface accumulation was followed by a leached zone. There were no significant differences in yield among plots receiving different treatments. Seasonal water application was less than half of the seasonal amount of water normally applied for furrow irrigated lettuce in the Tucson area. It was higher than experimental determinations of seasonal consumptive use for lettuce at Mesa, Arizona. The study indicated that trickle irrigation with water of high salt content is likely to cause a high surface concentration of salts. Application of extra amounts of water by the trickle system, or another method, is recommended to leach the salts to a depth below the crop root zone.