Numerical assessment of the saltwater propagation in the upper Tagus estuary during droughts

18:00 Tuesday 28 May




Marta Rodrigues (Portugal) 1; André B. Fortunato (Portugal) 1; Paula Freire (Portugal) 1

1 - Laboratório Nacional de Engenharia Civil

Estuarine uses and activities may be negatively affected by climatic variability and climate change. Numerical modelling approaches, combining climate, hydrological and oceanographic scenarios, can support decision-making regarding climate change adaptation. This study uses a numerical modelling approach to assess the salinity dynamics in the Tagus estuary resulting from changes in river flow and sea level rise.

The upper Tagus estuary left margin, in particular, includes an important agricultural area (Lezíria Grande de Vila Franca de Xira Public Irrigation Perimeter – LGVFX PIP), which main water abstraction is located in the estuary close to the salinity propagation limit. A reduction of the Tagus river flow or the sea level rise may foster saltwater intrusion and impact negatively agricultural activities in this area. During the most recent droughts (2005 and 2012) several emergency measures were undertaken to minimize the negative impacts in the LGVFX PIP.

A three-dimensional hydrodynamic model is validated and the sensitivity of the saltwater intrusion to the river flow is discussed. Errors in the river flow data used to specify the boundary conditions constitute a major source of uncertainty in the model results. To evaluate the influence of low freshwater discharge and of sea level rise in the salinity intrusion, five scenarios are then established. Results suggest that the salinity does not reach the water abstraction of the LGVFX PIP for the climatological river discharge. For the remaining scenarios, salinity reaches concentrations that are inadequate for irrigation during some periods and these effects are exacerbated by sea level rise. Salinity intrusion also increases with the duration of the droughts.

The results achieved herein demonstrate the applicability of the approach followed and are further used to evaluate the risk and discuss the effects of climate change in the LGVFX PIP, presented in a companion communication.