QUESTOR represents a river as a series of river reaches within which physical, chemical and biological processes operate.. The initial selection of reaches is based on the location of confluences, diffuse (catchment) sources, discharge points, abstraction points, monitoring sites, and weirs. Once this has been done, long reaches, say those greater than 10km, can be sub-divided. The flow and water quality out of a reach are calculated based on a mass balance of flows into the head of the reach plus contributions from point sources and diffuse sources minus abstractions. The determinands and processes modelled are not fixed within the QUESTOR modelling environment, but are tailored to the particular application. The QUESTOR model has sets of equations to simulates nitrogen and phosphorus species, pH and suspended sediment, the aspects of water quality of particular interest are those determinands indicative of the impacts on the river ecosystem, namely temperature, chlorophyll-a (Chl-a), BOD and DO. Chl-a is used as a surrogate for the algae using a fixed stoichiometry model whereby the ratio chl-a:C:N:P is 1:50:10:1. The growth of algae is limited by light, temperature and nutrient concentrations. The model was developed to help catchment managers assess the impact of their actions e.g. changing discharges consents, abstracting more water from a river or building a flood relief channel. The aim was to produce a physically realistic representation of a number of important measures of river water quality while being practical to run using generally available datasets.
QUESTOR is based on the earlier in-stream water quality models IHQM (Institute of Hydrology water Quality Model) and QUASAR (Quality Simulation Along Rivers). QUASAR was sold commercially for both VAX and PC systems, and is now available free of charge, but unsupported, as PC-QUASAR. The model formulation of the "basic" version of QUESTOR corresponds broadly with QUASAR.