Environmental Engineering Reference
In-Depth Information
Fig. 3.3
Standardized overland flow (OLF) and drain flow (DRAIN) from the instrumented hillslope for the period
November 2006 to December 2007 (data from Marshall et al. 2009).
subsurface layer prevents much downward move-
ment of incoming rainfall, resulting in a near-
surface perched water table over an essentially
hydrologically isolated groundwater system that
is normally relatively unresponsive. Dye tracer
studies have illustrated the importance of prefer-
ential flowpathways in promoting the rapidmove-
ment of water down through the soil profile and
into the field drainage systems.
During 3
1
/
2
years of groundwater monitoring
the only time when the groundwater became re-
sponsive to incident rainfall was in the winter of
2006/7, following an exceptionally hot, dry sum-
mer. Preferential flow paths in the form of inter-
pedal cracks developed in the clay-rich soil, which
allowedwater to bypass not only theAhorizon soil
matrix but the normally impermeable subsurface
layer. The development of such extensive prefer-
ential flowpaths also resulted in a change in runoff
with an increase in responsiveness of the drain
flow. However, the dynamic nature of the bulk soil
physical properties and resultant effect on runoff
is not confined to extreme climatic conditions
and appears to be a continuous annual cycle. The
long-termdataset fromthe instrumented hillslope
has indicated seasonal changes in runoff response.
An increase in the flashy nature of the drain flow
during the summer and into the autumn is
Fig. 3.4
Bowl runoff data, and corresponding pore water pressure data,
c
(cmH
2
O) (plots a, c, e), from tensiometers
installed at 10, 30, and 50 cm (plots b, d, f) (50 cm depth tensiometer not in operation during 01-02/12/07 event)
(data from Marshall et al. 2009).
