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.