CHUM - A HYDROCHEMICAL MODEL FOR UPLAND CATCHMENTS

CHUM (CHemistry of the Uplands Model) simulates soil-water chemical in teractions and water flows in upland catchments with acid soils. The c hemical interactions are described with the equilibrium speciation cod e WHAM, which emphasises the interactions of inorganic chemical specie s with solid-phase and dissolved organic matter, and also considers in organic 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 t o complexation reactions with solid phase organic matter, whereas prev ious models have postulated equilibrium with mineral phases or simple ion-exchange. In addition, CHUM takes nitrogen transformations and wea thering (dissolution) reactions into account. The catchment is concept ualised as a series of columns consisting of two soil horizons and an underlying permeable or fractured bedrock zone. Water flows are descri bed with a hydrological submodel that distinguishes immobile water pre sent in soil micropores (which is in chemical equilibrium with the soi l solids) from mobile drainage water. The mobile water can move vertic ally from one horizon to another, from one column to the column immedi ately downslope, or as outflow from a column adjacent to a stream or l ake. Solutes exchange between the mobile and immobile compartments. Th e model runs on a daily time-step. The fundamentals of the model are g iven, together with a description of its application to a site in the English Lake District.