Soil salinization risks under current climate and irrigation management in orchards of southern Portugal

Secondary salinization has long been reported in the Roxo Irrigation District (RID), in southern Portugal, due to use of saline prone irrigation water and the existence of poor structured soils. This study evaluates the soil water and salt budgets in nine commercial orchards located in the RID using the multiple ion chemistry module available in the HYDRUS-1D model during the 2019 and 2020 growing seasons. The study crops were almond, olive, citrus, and pomegranate. The model successfully simulated soil water contents measured in the different fields along two seasons. There was a clear underestimation of the EC_e in some fields while simulations of SAR were found to be acceptable. Modeling errors were mostly associated to missing information on fertigation events rather than difficulties in simulating the effect of irrigation water quality on soil quality. The water and salt balances were also computed for the 1979-2020 period. Considering the probability of distribution of salt accumulation during this period, the risk of salt accumulation was very high, except in the citrus areas. The factors influencing the salinity build-up in the study sites were the irrigation strategy, the seasonal irrigation and rainfall depths, the crop growing period, rainfall distribution in the late and non-growing seasons, soil drainage conditions, and irrigation water quality. On the other hand, for current climate conditions and irrigation water quality, the risk of soil salinity levels affecting crop development and yields were found to be minor. Only in two of the study sites, there was the need to promote salt leaching following strategies that differed between locations. This study further aims to promote sustainable irrigation management practices through the better use of soil and water resources in the Alentejo region of southern Portugal.