GLOSSARY - Fundamentals of Hydrology - page 179

lateral flow See throughflow .
low flow A period of extreme low flow in a river
hydrograph (e.g. summer or dry season river
flows).
low flow frequency analysis A technique to
investigate the magnitude-frequency relationship
for low flows in a particular river. This is based on
historical hydrograph records.
lysimeter A device for collecting water from the
pore spaces of soils and for determining the soluble
constituents removed in the drainage. In evapora-
tion studies a lysimeter is a cylinder filled with soil
and plants used to measure evaporation from a vege-
tated surface. This can be done either as a weight
loss calculation or through solving some form of the
water balance equation.
macropores Large pores within a soil matrix,
typically with a diameter greater than 3 mm.
model A representation of the hydrological
processes operating within an area (usually a catch-
ment). This is usually used to mean a numerical
model, which simulates the flow in a river, based
on mathematical representations of hydrological
processes.
mole drainage An agricultural technique involv-
ing the provision of rapid subsurface drainage routes
within an agricultural field.
net radiation The total electromagnetic radiation
(in all wavelengths) received at a point. This
includes direct solar radiation and re-radiation from
the earth's surface.
neutron probe An instrument to estimate the soil
water content using a radioactive source of fast
neutrons.
open water evaporation The evaporation that
occurs above a body of water such as a lake, stream
or the oceans.
orographic precipitation Precipitation caused by
an air mass being forced to rise over an obstruction
such as a mountain range.

overland flow Water which runs across the surface
of the land before reaching a stream. This is one
form (but not the only form) of runoff.
oxygen sag curve The downstream dip in dis-
solved oxygen content that can be found after the
addition of organic pollution.
partial areas concept The idea that only certain
parts of a catchment area contribute overland flow
to stormflow; compare to the variable source areas
concept.
partial duration series River flow data used in
flood frequency analysis. It takes the highest flow
peaks from the period of record irrespective of the
year in which it occurs (compare with annual
maximum series ).
peakflow See stormflow .
perched water table Area where the water table
is held above a regional water table, usually due to
small impermeable lenses in the soil or geological
formation.
pH The concentration of hydrogen ions within
a water sample. A measure of water acidity on an
inverse logarithmic scale.
phreatic zone The area beneath a water table (i.e.
groundwater).
piezometer A tube with holes at the base that
is placed at depth within a soil or rock mantle to
measure the water pressure at a set location.
pipeflow The rapid movement of water through a
hillslope in a series of linked pipes. (NB these can
be naturally occurring.)
porosity The percentage of pore space (i.e. air)
within a dry soil.
potential evaporation Evaporation which occurs
over the land's surface if the water supply is unre-
stricted.
precipitation In hydrology this is the movement
of water from the atmosphere to the earth's surface.
This can occur as rain, hail, sleet or snowfall.