Abstract:
Hydrology is yet to fully understand the role that catchment characteristics have in
determining a river’s response to precipitation variability. This thesis assesses the
influence that catchment characteristics have on modulating a river’s response to changes
in precipitation throughout the UK. Central to this aim is the concept of the precipitation-
to-flow relationship (the transformation of precipitation into river flow), which is
characterised using the Variogram, a way of indexing temporal dependence (i.e. the
average relationship between river flow on a given day and river flow on the previous
days). Firstly, 116 catchments were grouped into four clusters, based on the shape of their
variogram, which significantly differed in their catchment characteristics demonstrating
that catchment characteristics control how, on average, precipitation is transformed into
river flow. Furthermore, over 70% of un-gauged catchments could be clustered correctly
using information about their soil type, slope and the percentage of arable land. Secondly,
a new method which identifies the changes in the variogram parameters over 5-year
overlapping moving windows was developed to investigate temporal changes in the
variogram parameters. This method was successfully demonstrated to detect changes in
multiple aspects of artificially perturbed river flow time series (e.g. seasonality, linear
changes and variability). On average >70% of the variability in the catchment variogram
parameters was explained by the precipitation characteristics, although there was large
variability between catchments. Finally, the influence that the catchment characteristics
have on the temporal changes in the variogram parameters was analysed, demonstrating
that rivers in relatively impermeable upland catchments have a relationship with
precipitation which is closer to linear and less variable than lowland, permeable
catchments. This thesis contributes significant new knowledge that can be used for both
assessing how individual catchments are likely to respond to projected changes in
precipitation and in informing data transfer to un-gauged catchments.
