| Traditionally, the estimation of Mean
Flow (MF) in ungauged catchments has been approached using conceptual water
balance models or empirical formulae relating climatic inputs to stream flow. In
the UK, these types of models have difficulty in predicting MF in low rainfall
areas because the conceptualisation of soil moisture behaviour and its
relationship with evaporation rates used is rather simplistic. However, it is in
these dry regions where the accurate estimation of flows is most critical to
effective management of a scarce resource. A novel approach to estimating MF,
specifically designed to improve estimation of runoff in dry catchments, has
been developed using a regionalisation of the Penman drying curve theory. The
dynamic water balance style Daily Soil Moisture Accounting (DSMA) model operates
at a daily time step, using inputs of precipitation and potential evaporation
and simulates the development of soil moisture deficits explicitly. The model
has been calibrated using measured MFs from a large data set of catchments in
the United Kingdom. The performance of the DSMA model is superior to existing
established steady state and dynamic water-balance models over the entire data
set considered and the largest improvement is observed in very low rainfall
catchments. It is concluded that the performance of all models in high rainfall
areas is likely to be limited by the spatial representation of rainfall.
Keywords: hydrological models, regionalisation, water
resources, mean flow, runoff, water balance, Penman drying curve, soil moisture
model |