Hydrology is the study of origins and fate of the water
It describes the movement of water in all forms above, in and on the Earths surface.
Driven by solar power there are inputs, outputs, storage and series of flows.
Input = precipitation
Includes all the different ways moisture from the atmosphere can be transferred.
E.g. rain, snow, hail, sleet and dew.
generally the greater the intensity the shorter
the duration. Some may cover a small area e.g.
convectional rainfall or some a larger area i.e.
those associated with a warm front.
Evapotranspiration
The combination of the physical
process - evaporation and the
biological process - transpiration
Evaporation affected by
daylight hours, wind
speed, humidity and temp.
Transpiration affected by type
and amount of vegetation,
length of the growing season,
time of year and available
moisture.
can be described in two different ways: potential and actual
Potential evapotranspiration Ep is the amount
of water that could be lost in an area E.g. in
deserts Ep is high as the amount that can be
lost is greater than what is available.
Actual evapotranspiration Ea = The amount of water available to be lost.
Interception
rainfall that lands and settles on
trees (before it reaches the ground
it is known as interception storage)
If it is a short rainfall, it
may never reach the
ground as it may be
evaporated before
E.g. Woodlands 30% intercepted, helping to limit soil erosion.
If rainfall persists it may reach the ground via throughfall or stemflow
undergrowth may creates potential for secondary interception.
Surface storage and surface runoff (overland runoff)
When water can not penetrate the surface below it may
just sit on top e.g. puddles. Eventually it may flow away
over the surface known as surface runoff.
Infiltration
water being taken downwards into the soil.
Rate depends on: the amount of water already
in the soil and soil structure and porosity, nature
of the surface e.g. ploughed and the type and
amount of vegetation cover.
Throughflow
The lateral movement of
water within the soil
Capillary action
During dry periods, the process by which
water is drawn up to the surface.
Percolation
constant vertical movement of water
through unsaturated soil where water fills
the pore spaces between the soil particles.
Eventually the water may build
to fill all the pore spaces forming
a new zone of saturation, the
upper layer of which forms the
water table.
Groundwater flow/baseflow
The lateral movement of water below the water table.
Channel flow
water that enters the river channel and
then flows out of the drainage basin.
Inputs include rain that
falls directly in, surface
runoff, throughflow and
baseflow.
The water balance
Drainage basin equilibrium
Inputs are equal to outputs.
P = Q + E + Change in storage
P = precipitation measured by a rain
gauge Q = runoff measured as river
discharge E = evapotranspiration which
is hard to measure.
If E > P there may be a soil
moisture deficit E.g. may
occur in Europe in Summer
where there is a mild climate
and hardly any precipitation.
During Autumn the climate is wetter and
cooler allowing for soil moisture recharge
until the field capacity is reached.
Winter, P >E there may be a water surplus.
This water budget is usually displayed graphically.