Effect of rainfall spatial variability and sampling on salinity prediction in an estuarine system

Reliable and accurate forecasts of salinity changes are essential for the success of current and future management scenarios aimed at restoring and sustaining natural resources of coastal and estuarine ecosystems. Because of the physical complexity of such ecosystems, information on uncertainty associated with salinity forecasts should be assessed and incorporated into management and restoration decisions. This study focuses on the impact of spatial variability and limited sampling of rainfall on salinity prediction in an estuarine system. The analysis is conducted on the Barataria basin, which is a wetland-dominated estuarine system located directly west of the Mississippi Delta complex on the United States coast of south Louisiana. The basin has been experiencing significant losses of wetland at a rate of nearly 23 km2/year. Radar-rainfall data with high spatial resolution are used to simulate various scenarios of hypothetical rain gauge sampling densities over the basin. A mass-balance hydrologic salinity model is used to assess the effect of reduced rainfall sampling on salinity prediction in the basin. The results indicated that, due to the critical role played by rainfall in determining the overall balance of the basin freshwater budget, a high degree of uncertainty exists in salinity predictions when using typical average rain gauge densities (e.g., 1.3 gauges/1000 km2 in the US). These uncertainties decline sharply as the number of available gauges is increased beyond the typically available density. Uncertainties in salinity predictions in the Barataria basin are larger in inland locations and smaller near the mouth of the basin, where salinity conditions in the coastal waters of the Gulf of Mexico exert a large influence. Rainfall uncertainties also affected parameter estimation during model calibration, where the estimation of some parameters exhibited significant levels of bias and random scatter. The study highlights the necessity of improving rainfall monitoring especially in estuarine systems that are controlled by rainfall as a main source of freshwater and where the management of freshwater supply is a viable option.