Irrigation practices affecting land degradation in Sicily

Abstract

The available amount of fresh water for agriculture, and specifically for irrigation, is decreasing all over the world. The quality of irrigation water is deteriorating, and saline/sodic waters are increasingly used in many arid and semi-arid regions of the world. Salinization is closely associated with the process of desertification.Sustainable land management practices are urgently needed to preserve the production potential of agricultural land while safeguarding environmental quality. In cracking soils sustainable management should take into account the occurrence of bypass flow and the influence that land use may have on soil structure and bypass flow phenomena.Measurement of vertical and horizontal shrinkage in confined soil cores was found to be suitable for determining the Soil Shrinkage Characteristic Curve (SSCC) and for incorporating shrinkage in the soil hydraulic parameters/functions determined on confined undisturbed soil samples. An optimization procedure based on multi-step outflow experiments with inverse modelling was developed for determining the soil hydraulic characteristics (HC). The need for accounting for structural porosity and shrinkage processes was recognized on the basis of hydraulic conductivity values determined by the suction crust infiltrometer method and of the SSCC determined on confined soil cores.Analysis of the response of clay soils to ESP values up to 15, showed that the concept of critical threshold needs reconsideration, because increasing soil degradation upon increasing ESP appeared to be a continuum. A major hazard of deterioration of structural and hydraulic properties was recognized even at low ESP values (ESPThe relevance of bypass flow on the water balance in a Mediterranean climatic context as that occurring in Sicily, was evaluated by application of the FLOCR model. The results showed that bypass flow corresponded with about 70-74% of cumulative yearly rainfall, and that models not accounting for bypass flow may lead to a significant overprediction of crop evapotranspiration and underestimation of the hazard of land degradation and desertification.Results of bypass flow measurements performed in a Mediterranean cracking soil under alternated use of a high salinity solution to distilled water showed that exchange of solutes occurred at the contact surfaces between the macropores/cracks walls and the incoming solution in concomitance with bypass fluxes. These exchanges were effective in determining leaching of solutes and removal of Sodium, and in preventing salinization and sodification in part of the soil volume that is in contact with the roots.Combined use of morphometric and physical techniques made it possible to explore the effect of irrigation on soil structure and bypass flow phenomena of a Mediterranean cracking soil under two different irrigation systems, i.e. drip and micro-sprinkler. Different vertical distributions of cracks was found under the two irrigation systems. In agreement with these observations, different flow behaviour was observed in the laboratory in cylindrical soil cores taken from the irrigated micro-sprinkler field. No bypass flow or lower amounts of bypass flow in the micro-sprinkler irrigated field compared to the drip irrigation treatment. Chemical dispersion of clay particles and detachment of these particles from the surface and their movement into the cracks were the mechanisms responsible for the partial or total occlusion of the (macro) pores in the micro-sprinkler irrigated field.In conclusion, this study showed that drip irrigation alternatively using high and low salt water was most effective in maintaining the productive capacity of the clay soil being studied, particularly when this water was applied to a cracked soil. Combined use of morphometric and physical methods was necessary to understand the underlined highly dynamic flow behaviour in these complex soils.</p