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Models relating sediment supply to catchment properties are important in order to use the geological record to deduce landscape evolution and interplay between tectonics and climate. Water discharge (Q(w)) is an important factor in the widely used BQART’ model, which relates sediment load to a set of measurable catchment parameters. Although many of the factors in this equationmay be independently estimated with some degree of certainty in ancient systems, water discharge (Q(w)) certainly cannot. An analysis of a world database of modern catchments with 1255 entries shows that the commonly applied equationrelating catchment area (A) to water discharge (Q(w)=0075A(08)) does not predict water discharge from catchment area well in many cases (R-2=05 and an error spanning about three orders of magnitude). This is because the method does not incorporate the effect of arid and wet climate on river water discharge. The inclusion of climate data into such estimations is an opportunity to refine these estimates, because generalized estimates of palaeoclimate can often be deduced on the basis of sedimentological data such as palaeosol types, mineralogy and palaeohydraulics. This paper investigates how the relationship between catchment area and river discharge varies with four runoff categories (arid, semi-arid, humid and wet), which are recognizable in the geological record, and modifies the coefficient and exponent of the above-mentioned equationaccording to these classes. This modified model yields improved results in relating discharge to catchment area (R-2=095 and error spanning one order of magnitude) when core, outcrop or regional palaeoclimate reconstruction data are available in non-arid systems. Arid systems have an inherently variable water discharge, and catchment areais less important as a control due to downstream losses. The model hereis sufficient for many geological applications and makes it possible to include variations in catchment humidity in mass-flux estimates in ancient settings.