C-GEM (v 1.0): a new, cost-efficient biogeochemical model for estuaries and its application to a funnel-shaped system

Abstract

Abstract. Reactive transport models (RTMs) are powerful tools for disentangling the complex process interplay that drives estuarine biogeochemical dynamics, for assessing the quantitative role of estuaries in global biogeochemical cycles and for predicting their response to anthropogenic disturbances (land-use change, climate change and water management). Nevertheless, the application of RTMs for a regional or global estimation of estuarine biogeochemical transformations and fluxes is generally compromised by their high computational and data demands. Here, we describe C-GEM (Carbon-Generic Estuary Model), a new one-dimensional, computationally efficient RTM that reduces data requirements by using a generic, theoretical framework based on the direct relationship between estuarine geometry and hydrodynamics. Despite its efficiency, it provides an accurate description of estuarine hydrodynamics, salt transport and biogeochemistry on the appropriate spatio–temporal scales. We provide a detailed description of the model, as well as a protocol for its set-up. The new model is then applied to the funnel-shaped Scheldt estuary (BE/NL), one of the best-surveyed estuarine systems in the world. Its performance is evaluated through comprehensive model–data and model–model comparisons. Model results show that C-GEM captures the dominant features of the biogeochemical cycling in the Scheldt estuary. Longitudinal steady-state profiles of oxygen, ammonium, nitrate and silica are generally in good agreement with measured data. In addition, simulated, system-wide integrated reaction rates of the main pelagic biogeochemical processes are comparable with those obtained using a high-resolved, two-dimensional RTM. A comparison of fully transient simulations results with those of a two-dimensional model shows that the estuarine net ecosystem metabolism (NEM) only differs by about 10%, while system-wide estimates of individual biogeochemical processes never diverge by more than 40%. A sensitivity analysis is carried out to assess the sensitivity of biogeochemical processes to uncertainties in parameter values. Results reveal that the geometric parameters LC (estuarine convergence length) and H (water depth), as well as the rate constant of organic matter degradation (kox) exert an important influence on the biogeochemical functioning of the estuary. The sensitivity results also show that, currently, the most important hurdle towards regional- or global-scale applications arises from the lack of an objective framework for sediment and biogeochemical process parameterization. They, therefore, emphasize the need for a global compilation of biogeochemical parameter values that can help identify common trends and possible relationships between parameters and controlling factors, such as climate, catchment characteristics and anthropic pressure.

Linking biogeochemistry to hydro-geometrical variability in tidal estuaries: a generic modeling approach

“…Certain hydro-geometrical characteristics, such as the tidal period T , the tidal excursion E and the estuarine depth at the mouth H 0 can be approximated by characteristic values. For instance, E is usually close to 10 km for a semi-diurnal (T ≈ 12 h) tidal estuary, while an alluvial estuary flowing in a coastal plain generally reveals a tidally averaged depth (H ) of about 7 m (Savenije, 1992;Volta et al, 2014). Note that, for estuaries in which the freshwater discharge is known, a new formulation proposed by can be used to estimate the estuarine depth.…”

“…Hence, important biogeochemical properties in estuaries might, just as salinity profiles, be predicted on the basis of hydro-geometrical features (Fig. 2;Volta et al, 2014).…”

“…2.1), is used to discriminate between the three estuarine types. First, a reference estuary, characterized by an idealized geometry resembling that tested in Volta et al (2014), is defined. Then, its width convergence length (b = 30 km) is decreased and increased by 50 % in order to intensify the marine (b = 15 km) and the riverine (b = 45 km) character of the system, respectively.…”

“…Henceforth, according to Regnier et al (2013b) and Volta et al (2014), the marine-dominated estuary, the reference case and the riverine-dominated estuary will be referred to as the marine, the mixed and the riverine estuary, respectively. The estuarine width at the seaward limit, B 0 , and the estuarine length, EL, of the marine and the riverine estuaries are adjusted so that their total, tidally averaged volume corresponds to that of the mixed estuary (V ≈ 1.5 × 10 9 m 3 ).…”

“…In particular, we build on the mutual dependency between geometry and hydrodynamics (Savenije, 1992(Savenije, , 2005(Savenije, , 2012 and further hypothesize that hydrodynamics exert a strong control on biogeochemistry in alluvial estuaries (e.g., Alpine and Cloern, 1992;Volta et al, 2014). Next, three idealized systems, characterized by variable riverine influence and covering the main hydro-geometrical features of tidal alluvial estuaries, are modeled using the recently developed C-GEM modeling platform (Volta et al, 2014). These systems are designed to represent a tidal estuary dominated by marine characteristics, a tidal estuary dominated by its riverine characteristics, and an intermediate case (so-called mixed system).…”