Hydrology
Interception, effects of forestry on
snow hydrology
R. Hudson - VFR Research
Interception
Before precipitation reaches the soil, it
must pass through whatever vegetation
cover is present. (In urban areas,
buildings and other structures act to
intercept precipitation.)
Vegetation cover retains some of this
precipitation and returns it to the
atmosphere by evaporation and/or
sublimation - this is interception
R. Hudson - VFR Research
Precipitation interception by a
coniferous canopy
Coniferous canopies intercept both rainfall
and snowfall - this interception loss can be a
significant component of water balance.
Studies of rainfall interception in coastal BC
and elsewhere show that up to 30% of total
annual rainfall does not reach the ground
under a mature coniferous canopy.
A similar percentage of total annual snowfall
is intercepted by mature conifers.
R. Hudson - VFR Research
Rainfall interception
rain can fall directly through gaps in the canopy
(throughfall), or it can hit the foliage and
branches
once the foliage has been completely wetted by
the rain, droplets will begin to cascade down
through the canopy
water can drip off the canopy to the ground
(throughfall) or can run down stems (stemflow)
evaporative losses can occur from the canopy, or
from vegetation at the forest floor (interception)
R. Hudson - VFR Research
P Components:
P = precipitation
C C = canopy interception
F = forest floor
interception
T T = throughfall
T S = stemflow
S Itotal = F + C
C Itotal = P - (T + S)
T
F
R. Hudson - VFR Research
Rain interception has been studied in detail
– a forest canopy has a saturation capacity
of 0.5 to 2.0 mm of water depending on the
age and structure of the stand
– water can be evaporated by advective
energy even during storms, so the capacity
of the canopy is constantly replenished
– throughfall and stemflow are monitored
with appropriate collectors, subtracted from
rainfall on an area basis to calculate
interception loss
R. Hudson - VFR Research
Rain interception
Spittlehouse (1998) (from Spittlehouse, D.L. 1998. Rainfall
interception in young and mature conifer forests in British Columbia. In: Weather Data
Requirements for Integrated Pest Management. 23rd Conference on Agricultural and Forest
Meteorology. American Meteorological Society, Boston Mass. 171-174.)
– interception loss is a function of storm size
and the age of the stand (i.e., tree size)
– asymptotic relationship: (coastal sites)
immature forest (6-10 metre trees) reaches a
maximum at about 14 mm for storms > 100 mm
mature forest, interception loss maxes out at an
average of 25 mm for storms > 100 mm, but in
spring storms, interception loss was about
twice that for winter storms
Rainfall interception as a function of
storm size and season (coastal sites)
from Spittlehouse, 1998
Rain interception Coast vs. Interior
For mature spruce-fir-pine forests at
interior sites, results are similar:
– I is much less (max of 4 mm) due to
smaller rain storms.
– I expressed as a percent of annual rain is
similar in coastal and interior forests
Interception was 25-30% of annual rainfall for
mature forest depending on species and crown
density
Interception was 10-20% of annual rainfall for
immature forest
from Spittlehouse, 1998
Rain and snow interception
Studies show that interception is
proportional to canopy density or
canopy height for both rain and snow
Generally, interception loss of snow is
greater than that of rain for coastal sites
– interception for snow is as high as 50%
– interception for rain is as high as 30%
– I is different for mature and immature
canopies due to different size and
development of trees
R. Hudson - VFR Research
Effect of Forest Density on Interception
R. Hudson - VFR Research
Snow Interception vs. Canopy Height
Gray Creek 1997
R. Hudson - VFR Research
Effect of canopy structure
Deciduous vs. coniferous: interception is
greater for coniferous than deciduous forests
– needles can hold more water than broad leaves
due to greater surface area
– conifers maintain foliage year-round
Mature vs. immature:
– rain interception greater for mature forest
– snow interception is affected by stocking density
as well as canopy structure in terms of tree form
R. Hudson - VFR Research
Water balance implications
Forest harvesting always increases
water available for runoff by decreasing
interception
– this increased availability can be in the
order of 30-50% for newly harvested
coastal sites
Increases in peak flows
– in rain dominated watersheds, or for spring
storms, water yield and peak flow could be
increased by 25-40% for a 100 mm storm
R. Hudson - VFR Research
– In snow dominated watersheds in coastal
B.C., snow accumulation and average melt
rates for harvested sites during the melt
period can be up to twice that of uncut old
growth forest.
On a watershed scale, those potential
increases are proportional to the
proportion of the watershed logged
– e.g. 25% of watershed logged, logged
areas experience 34% increase in snow
catch, then there is 8.5% more water
available for runoff
R. Hudson - VFR Research
Dominance of rain vs. snow
Rain dominated zone = 0-300 metres
elevation on the coast
Rain-on-snow zone (transient snow zone)
– usually about 300-800 metres elevation on the
coast
– fluctuating freezing levels result in alternating
snow accumulation and ablation in winter
– rain on snow produces greater runoff rates
than snowmelt or rainfall alone
R. Hudson - VFR Research
rain-on-snow produces largest peak flows on
the coast where it occurs frequently, results in
flooding in the interior where it is much less
frequent than on the coast
– it is thought that logging in the rain-on-
snow zone has the greatest potential to
increase peak flows
Snow dominated areas: >800 metres
on coast, most of the interior
– logging results in greater water yield due to
decreased interception, and increased
peak flows due to accelerated melt
R. Hudson - VFR Research
Hydrologic Recovery
We have seen that logging alters the potential
to produce runoff at the site that is logged, by
reducing interception and increasing rates of
snowmelt
Hydrologic recovery is the process by which
forest regeneration restores the runoff
production to near pre-harvest condition
How quickly does a harvested area recover?
R. Hudson - VFR Research
Hydrologic recovery for coastal B.C.
R. Hudson - VFR Research
Equation - hydrologic recovery
Research shows that hydrologic recovery of a
regenerating stand can best be expressed as an
exponential function of mean canopy height of the
regeneration.
– There is a recovery threshold - minimum canopy height
before recovery will begin, in this case the threshold is 2.25
metres
– Assumes the stand is fully stocked; if patchy, the coverage
must also be accounted for.
R. Hudson - VFR Research
Equivalent Clear-cut Area
As an area that has been harvested begins to
regenerate, its original area is reduced by the
level of recovery
– A = original opening area
– e.g., a 10 ha opening has an average canopy
height of 8 metres. Hydrologic recovery is 80%, so
its ECA is 2 ha.
– a 20 ha opening has 6 m regeneration, but in
clumps with 50% coverage. Recovery is
(0.5X70%) so ECA is 13 ha.
R. Hudson - VFR Research