Anthropogenic pressures driving the salinity intrusion in the Guadalquivir estuary: insights from 1D numerical simulations

Abstract. The study presents a dynamic analysis of the present-day behavior of salinity concentration in the Guadalquivir estuary and evaluates the impact of anthropogenic pressures on the dynamics of the horizontal salinity gradient. A one-dimensional (1D) hydrodynamic model with an advection and dispersion module is used to study the effects of human pressure involved in the salinity concentration along the estuary. The observations, which correspond to continuous measurements taken during different oceanographic campaigns from 2021 to 2023, show an excessive salt intrusion in the estuary (with salinities of 5 psu at 60 km from the mouth) compared to the idealized situation when anthropogenic water withdrawals are absent. This highlights the need to include a water withdrawal term in the simulations to accurately reproduce the system's real behavior, thereby reflecting the magnitude of anthropogenic pressures. The model successfully reproduces the observations when this forcing factor is included. Under constant low-flow conditions, experiments show that increasing water withdrawals leads to an increase in the horizontal salinity gradient upstream. Similarly, under constant water withdrawal conditions, a decrease in the horizontal salinity gradient is observed when freshwater flows exceed 40 m3 s−1. Variations in anthropogenic pressures, such as water withdrawals for agriculture or the saline industry and reductions in freshwater flow, play a fundamental role in the evolution of salinity along the estuary. Under the current circumstances, the Guadalquivir estuary requires an urgent regulation of these uses in order to avoid further damage to the aquatic ecosystems.