Chemical modelling on the bare rock or forested watershed scale

Abstract

The simulation of weathering, solute distribution or acidi®cation at the catchment scale is predominantly done with either a mass balance or process level model. The former redistributes total elemental concentrations between known points with measured total concentration, but does not explicitly include catchment hydrology. The latter includes compartmental hydrological models and detailed descriptions of spatially averaged chemical reactions. Interestingly, the model applications tend towards hydrologically dierent watershed structures: mass balance modelling favours bare rock watersheds similar to the Apache Leap Research Site, whereas process level models are applied most often to forested watersheds, among which the Hubbard Brook Experimental Forest is an example. Although constrained either by mineral or water compositions on the one hand, or calibrated against stream or lake water chemistry on the other, both approaches basically ®t parameters to the geochemical circumstances of the speci®c watershed of interest. Limited success is attained if the hydrological conditions remain within the circumstances of the parameter ®tting. The principal dierences in model formulation and approach to mass balance modelling are discussed. Without advancements in model calibration and rigorous model testing, and the development of methods for optimizing the important reactions and pathways, the transportability of models between watersheds or the simulation of extreme events will continue to be inadequate.

Mineralogic controls on the composition of natural waters dominated by silicate hydrolysis

“…Ion exchange.-Because clay minerals are a primary source of exchange capacity in most natural materials, cation exchange on clays has been invoked to account for varying degrees of compositional change in silicate-solute mass balances. However, in contrast to many models proposed for forested catchments (Bassett, 1997), we have shown that the role of cation exchange is minimal, except where significant mixing of solutions of different reactive history is involved (none of which are included in our six examples). The basic assumption is that new silicate surface is created by weathering more slowly than cation exchange rates.…”

“…Ever since the work of Garrels and Mackenzie (1967) in the Sierra Nevada of California and Bricker, Godfrey, and Cleaves (1968) in the Maryland Piedmont, the majority of the watershed modeling studies described in the literature from bare rock catchments in the western United States or the forested experimental watersheds in Europe and the eastern United States have utilized mineral mass balance in some form (Bassett, 1997).…”

Water source characterization through spatiotemporal patterns of major, minor and trace element stream concentrations in a complex, mesoscale German catchment

“…Analysis of these relationships has been used across scales in a wide variety of conditions. This includes chemical modelling on the watershed scale with foci such as catchment response to acidification and mass balance studies of chemical weathering (Bassett, 1997). Associated with this, tracers have been used to apportion stream runoff to geographic runoff sources using hydrochemical tracers and to separate event and pre-event water using water isotope composition (Genereux and Hooper, 1998).…”

Multicriteria parameter estimation for models of stream chemical composition

“…These models of catchment hydrology and hydrochemistry are uncoupled from rigorous hypothesis testing because calibration data do not contain enough information to uniquely determine model parameters and there is not a one‐to‐one relationship between state variables in catchment biogeochemical models and field observations (e.g., the model represents average field conditions, while field measurements are point‐scale measurements) [ Christophersen et al , 1993]. The decoupling of these models from rigorous hypothesis testing limits their use to comparative studies between models or between catchments with the same model [ Bassett , 1997]. Without improved methods of model calibration and validation the useful application of catchment hydrochemical models will remain limited.…”