『Abstract
Groundwater from unconfirmed chalk aquifers constitutes a major
water resource in the UK. The unsaturated zone in such systems
plays a crucial role in the hydrological cycle, determining the
timing and magnitude of recharge, and the transport and fate of
nutrients. However, despite more than three decades of study,
our physical understanding of this system is incomplete. in this
research, state of the art instrumentation provided high temporal
resolution readings of soil moisture status, rainfall and actual
evaporation from two sites in the Pang and Lambourn catchments
(Berkshire, UK), for a continuous two year period (2004/5). A
parsimonious, physically based model for the flow of water through
the chalk unsaturated zone, including a novel representation of
the soil and weathered chalk layers, was developed. The parameters
were identified by inverse modelling using field measurements
of water content and matric potential. The model was driven by
rainfall and evaporation data, and simulated fluxes throughout
the profile (including the discrete matrix and fracture components),
down to the water table (but not the water table response). Results
showed that the model was able to reproduce closely the observed
changes in soil moisture status. Recharge was predominantly through
the matrix, and the recharge response was strongly attenuated
with depth. However, the activation of fast recharge pathways
through fractures in the chalk unsaturated zone was highly sensitive
to rainfall intensity. Relatively modest increases in rainfall
led to dramatically different recharge patterns, with potentially
important implications for groundwater flooding. The development
and migration of zero flux planes with time and depth were simulated.
The simulations also provided strong evidence that, for water
table depths greater than 5 m, recharge fluxes persist throughout
the entire year, even during drought conditions, with important
implications for the calculation of specific yield from baseflow
estimates and the representation of recharge in groundwater models.
Keywords: Chalk; Groundwater recharge; Richards equation; Unsaturated
zone; Weathered rock; Zero flux plane』
Introduction
The field monitoring scheme
CUZ model development
Concentration of the profile
Quantitative representation of the profile
Additional model details
Parameter identification
Model calibration stage one
Model calibration stage two
CUZ Model application
Conclusions
Acknowledgements
Appendix A. Alternative models for the relative hydraulic conductivity
References