Indicators of the seasonal cycle of total dissolved and adsorbed salts under irrigation

This article describes the evaluation of indicators computed with a numerical simulation model of soil water and solute flow against field measurements of the amount of adsorbed and dissolved salts. In the Mediterranean environment, characterized by hot, dry summers, a field experiment on irrigation of vegetable crops with saline and tubewell water was carried out between 1988 and 1996 at a site near Naples, Italy. The investigation was carried out on a clay-loam soil classified as Argiustolls. Irrigation treatments were: tubewell water (no addition of NaCl), addition of 1.25, 2.5, 5 and 10 g NaCl per litre. Three irrigation intervals were applied until 1995: 2, 5 and 10 days. In 1995 all plots were irrigated on pre-selected days of the year, but the amount of water was equal to 100%, 75% and 50%, respectively, for the three irrigation treatments. Except 1996, all plots received consistently the same irrigation and salinity treatment, although different crops were grown on the same plot during the eight-year experiment. In 1996, the indicator of soil salinity was used to evaluate the salt accumulation in the soil after a long time of saline irrigation. The indicator applied was the total amount of dissolved and adsorbed salts measured and simulated, in this way observation and model calculations are truly comparable. The aims were to evaluate the accuracy and relevance of salinity indicator computed by means of a numerical deterministic model describing water and solute transport, namely SWAP. Total amounts of salts were determined on soil samples, collected through the irrigation season on the treatments 0 and 5 g NaCl per litre. The samples were taken at three depths. The CEC and soluble salt were measured, beside the initial (prior to saturation in the laboratory) composition of the soil samples. Total dissolved and adsorbed salts were calculated throughout the irrigation season with the SWAP model. Calculated salt concentration was compared with observed soil composition to assess the accuracy of model calculations.