Modelling the dynamics of soil moisture and soil water salinity in tropical saltmarshes

Tropical saltmarshes share the intertidal zone with mangroves. In contrast to saltmarshes of temperate latitudes, these only occupy the uppermost niche of the intertidal zone, and are characterised by periods of severe drought and hypersalinity in the upper soil during dry seasons. Like mangroves, they show pronounced species zonation patterns along elevation gradients. The primary driver behind these soil water patterns is the variation in tidal flooding frequency, especially notable in the upper tidal zone. Precipitation is the second main source of water, and evapotranspiration and plant transpiration are further factors shaping soil water dynamics and driving plant growth. To describe and comprehend the soil water processes and their dynamics in tropical saltmarshes, we introduce the hydrological model SALTFRED. This model aims to mechanistically predict soil water salinity and root zone soil moisture, elucidating the intricate differentiation of drought and salt stress within the saltmarsh. The model explicitly describes processes of infiltration, seepage and evapotranspiration, along with their influence on the salinity of the soil water. We demonstrate its suitability with an application to a specific study site on the Bragança Peninsula (Pará, Brazil). Utilising complete time series of tidal data and precipitation (for the year 2015), we predict drought and salt stress and compare simulated levels of stress with tolerance levels of the dominant vegetation types and saltmarsh species. Extreme salt and drought stress are defined as salt concentrations of 90 parts per thousand (ppt) and plant available water of 0.5 vol% (0.005 cm³ water per cm³ of soil), respectively. The results suggest that mangrove vegetation thrives where these extreme stress conditions are persistently not reached due to regular flooding by the tides. In the saltmarsh, as elevation increases and the frequency of spring tide flooding decreases, longer periods without water infiltration occur. As a result, the duration of salt stress events increases. In the higher parts of the saltmarsh, additional drought stress is predicted. Our results confirm a robust correlation between simulated levels of seasonally varying drought and salt stress and tolerance levels of the dominant saltmarsh species Rhynchospora riparia, Fimbristylis cymosa, and Sporobolus virgincus at the study site.