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Highlights • Very-high density olive orchards have been expanding rapidly in the Alentejo region (Portugal). • The long-term impact of irrigation practices on soil salinization/sodification was assessed using HYDRUS-1D. • Soil salinization/sodification risks from current monitored practices were found to be low. • Modeled scenarios further considered waters with lower quality, maize as an alternative crop, and climate change. • The variability and uncertainty of model predictions recommend the close monitoring of the problem at the local scale. Abstract Deficit irrigation practices carried out in very high-density olive orchards grown in the Alentejo region of southern Portugal can bring important economic benefits in terms of water savings, yields, and oils. They can also result in serious salinization/sodification problems without proper management of soil and water resources. The main objective of this study was to evaluate the long-term (30 years) impact of those irrigation practices on local soil resources using a multicomponent transport modeling approach embedded in the HYDRUS-1D model. Soil salinization and sodification risks were quantified for 160 soil profiles by considering eight different scenarios: current monitored irrigation practices (S1), using waters of variable quality (S2-S6), planting maize as an alternative crop (S7), and using climate change projections for the region (S8). Despite the large observed variability, simulations that considered current irrigation practices (S1) produced average values of the electrical conductivity of the soil solution (EC sw) at the end of the leaching seasons always below the threshold limit for crops moderately tolerant to soil salinity. In this scenario, the average values of the sodium adsorption ratio (SAR) were also kept within the same magnitude of those determined at the beginning of the simulation period (initial conditions). Irrigations with worse quality waters (S2-S6) led to higher EC sw and SAR values. Although annual rainfall amounts influenced the salinity build-up, the SAR evolution depended mainly on water quality. In maize soil profiles (S7), the simulated EC sw and SAR values were lower than in olive soil profiles, with irrigation practices contributing to salt removal during the seasons. Conversely, the climate change scenario (S8) resulted in slightly higher EC sw and SAR values than those simulated for current conditions, indicating a potentially greater risk of soil degradation in the near future. Although current irrigation practices seem to present relatively low soil salinization/sodification risks, the variability of results and the uncertainty associated with model predictions indicate that close monitoring to prevent further degradation of soil and water resources in the region should be recommended. [ABSTRACT FROM AUTHOR]

Secondary salinization has long been reported in the Roxo irrigation district (RID), southern Portugal, due to the use of saline-prone irrigation water and the existence of poorly structured soils. This study assessed 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 studied crops were almond, olive, citrus (orange, mandarin, and clementine), and pomegranate. The model successfully simulated soil water contents measured in the different fields but there was a clear underestimation of the electrical conductivity of the soil saturation paste extract (EC e) in some locations, while simulations of the sodium adsorption ratio (SAR) were generally acceptable. Modeling errors were mostly associated with missing information on fertigation events rather than related to the effects of irrigation water quality. The water and salt balances were also computed for the 1979–2020 period. Considering the probability of non-exceedance of salt accumulation during this period, the risk of salinity build-up was high to very high for the very dry years in most fields, except in the citrus sites. The factors influencing the salt accumulation were the irrigation strategy, the seasonal irrigation and rainfall depths, the duration of the crop growth period, the rainfall distribution in the late and non-growing stages, the soil drainage conditions, and the irrigation water quality. For the current climate conditions and irrigation water quality, the risk of soil salinity levels affecting crop development and yields was found to be minor. This means that, despite salts tended to accumulate in the rootzone over a season, under current conditions the salinity stress did not reach harmful levels for plants. Only in two of the study sites, there was a need to promote salt leaching. Hence, this study shows that soil salinization risks in the study area are low but, for given locations during drier seasons, there is a need for tailored irrigation solutions aimed at the conservation of soil and water resources. • Soil salinization risks in nine commercial orchards were assessed with HYDRUS-1D. • The probability of non-exceedance of salt accumulation was computed for 1979–2020. • The risk of salinity build-up was high to very high for the very dry years in most fields. • For current conditions, the risk of soil salinity levels affecting crop development was minor. • Tailored irrigation solutions should be considered in some cases in dry and very-dry years. [ABSTRACT FROM AUTHOR]

Soil salinization problems are widespread in the Hetao plain, Inner Mongolia, resulting from arid climate conditions, a shallow saline water table, poor irrigation water management, and insufficient drainage. This study follows previous research aimed at evaluating crop water use and controlling the salinity build-up in the region, namely using weighing and static water table lysimeters to parameterize a water balance model aimed at the development of appropriate irrigation scheduling. Two sets of five static water table lysimeters, which fixed depths ranged from 1.25 to 2.25 m, were used over two maize crop seasons. The mechanistic HYDRUS-1D model was further used to daily predict measured data on soil water contents, boundary fluxes in the interface between the saturated and unsaturated zones, the electrical conductivity of the soil saturation paste extract (EC e), and the actual crop evapotranspiration (ET c act). The soil water balance helped quantify the combined effect of water and salinity stresses on root water uptake as well as groundwater fluxes into the rootzone. The salt balance showed that the salinity build-up was much related to irrigation and capillary fluxes, and that the autumn irrigation carried out during the non-growing season was essential for controlling soil salinity. The efficiency of the autumn irrigation much depended on groundwater depth and the amount of water applied for salt leaching, with the best results found for the lysimeters with water table depths at 2.0 and 2.25 m (85–100%) for irrigation depths ≥ 220 mm. The lysimeters with shallower water tables never showed a leaching efficiency higher than 88%. This research shows that the sustainability of irrigation in Hetao depends on finding adequate solutions for controlling the depth of the saline groundwater, minimizing capillary fluxes to the rootzone, and developing consequent approaches for autumn irrigation leaching. • Soil water contents and fluxes, EC e , and ET c act were measured using water table lysimeters. • The HYDRUS-1D model successfully simulated lysimeter data during two seasons. • The water table depth influenced the effectiveness of autumn irrigation leaching. • The leaching efficiency only increased by improving soil drainage conditions. • Reducing autumn irrigation depth (<200 mm) promoted soil salinization. [ABSTRACT FROM AUTHOR]

• Regression models were developed for soil salinity assessment in Lezíria Grande, Portugal; • Multi-year maxima of 9 vegetation indices were correlated to rootzone salinity; • The CRSI, vegetation cover and soil type were the inputs selected by stepwise regression; • Remote assessment of soil salinity is a viable option in future soil monitoring programs. Lezíria Grande is an important agricultural area in Portugal, prone to waterlogging and salinity problems due to the influence of estuarine tides on groundwater dynamics. Simple, non-invasive, practical approaches are need for monitoring soil salinity in the region and preventing further degradation of soil resources. The objective of this study was to develop regression models for soil salinity assessment in Lezíria Grande based on the relationship between multi-year crop reflectance data derived from Sentinel-2 multispectral imagery and rootzone salinity. Nine vegetation indices (VI), computed from the annual averages of the spectral bands, were tested between 2017 and 2019. The multi-year maximum from each pixel was then used for correlating the VI with the ground-truth dataset. This dataset was composed of average values of the electrical conductivity of the soil saturation paste extract (EC e mean) measured in 80 sampling sites (0–1.5 m depth) located in four agricultural fields representative of the salinity gradient in the region. The Canopy Response Salinity Index (CRSI), which uses the blue (490 nm), green (560 nm), red (665 nm), and infrared (842 nm) bands, provided the strongest correlation with measured data (r=−0.787). Regression models further considered vegetation cover and soil type as explanatory variables, with predictions resulting in a coefficient of determination (R2) ranging from 0.63 to 0.91 and a root mean square error (RMSE) varying from 1.63 to 3.26 dS m−1. The use of remote sensing data for soil salinity assessment showed to be an interesting option to consider in future soil monitoring programs. Nevertheless, more detailed covariates are needed for improving salinity assessment models at the regional scale. [ABSTRACT FROM AUTHOR]

• Salinity impacts on soils, crops and the environment. • Modelling salinity and leaching requirements. • The FAO56 approach to assess crop ET and its partition when affected by salinity. • Appropriate water and irrigation management issues to cope with salinity in agriculture. Soil and water salinity and associated problems are a major challenge for global food production. Strategies to cope with salinity include a better understanding of the impacts of temporal and spatial dynamics of salinity on soil water balances vis-à-vis evapotranspiration (ET) and devising optimal irrigation schedules and efficient methods. Both steady state and transient models are now available for predicting salinity effects on reduction of crop growth and means for its optimization. This paper presents a brief review on the different approaches available, focusing on the FAO56 framework for coping with the effects of soil salinity on crop ET and yields. The FAO56 approach, applied widely in soil water balance models, is commonly used to compute water requirements, including leaching needs. It adopts a daily stress coefficient (K s) representing both water and salt stresses to adjust the crop coefficient (K c) when it is multiplied by the grass reference ET o to obtain the actual crop ET values for saline environments (ET c act = K s K c ET o). The same concept is also applied to the dual K c approach, with K s used to adjust the basal crop coefficient (K cb). A review on applications of K s is presented showing that the FAO56 approach may play an interesting role in water balance computations aimed at supporting irrigation scheduling. Transient state models, through alternative formulations, provide additional solutions for quantification of the salinity build-up in the root zone. These include irrigation-induced salinity, upward movement of salts from saline ground water-table, and sodification processes. Regardless of the approach, these models are now very much capable of supporting irrigation water management in saline stress conditions. For maintaining crop growth under salinity environments, soil-crop-water management interventions consistent with site-specific conditions are then discussed. Adequateness of irrigation methods, cyclic uses of multi-salinity waters and proper irrigation scheduling are further analyzed as examples of efficient means to obviate the effects of salinity. [ABSTRACT FROM AUTHOR]

Highlights: [•] Changes of soil water, salinity and yield were simulated using SWAP for wheat. [•] SWAP was modified by adopting EPIC crop model and osmotic head-dependent function. [•] Model was well calibrated and validated using experimental data in 2007 and 2008. [•] Fifteen treatments with different irrigation and GWD were predicted and evaluated. [•] A target GWD of 1.0–1.5m was suggested to be optimal for wheat's growth. [Copyright &y& Elsevier]