CHUM: a hydrochemical model for upland catchments

Abstract

CHUM (CHemistry of the Uplands Model) simulates soil-water chemical interactions and water flows in upland catchments with acid soils. The chemical interactions are described with the equilibrium speciation code WHAM, which emphasises the interactions of inorganic chemical species with solid-phase and dissolved organic matter, and also considers inorganic solution speciation and sorption reactions of fulvic acid. Of special significance is the treatment of aluminium chemistry; control of dissolved aluminium concentrations is assumed to be due primarily to complexation reactions with solid phase organic matter, whereas previous models have postulated equilibrium with mineral phases or simple ion-exchange. In addition, CHUM takes nitrogen transformations and weathering (dissolution) reactions into account. The catchment is conceptualised as a series of columns consisting of two soil horizons and an underlying permeable or fractured bedrock zone. Water flows are described with a hydrological submodel that distinguishes immobile water present in soil micropores (which is in chemical equilibrium with the soil solids) from mobile drainage water. The mobile water can move vertically from one horizon to another, from one column to the column immediately downslope, or as outflow from a column adjacent to a stream or lake. Solutes exchange between the mobile and immobile compartments. The model runs on a daily time-step. The fundamentals of the model are given, together with a description of its application to a site in the English Lake District.