What does water interception refer to?
water that never reached the ground
What things could happen to a raindrop falling in a forest?
- throughfall (to the ground)
- interception loss (canopy)
- interception loss (stem) to stemflow
- interception loss in leaf litter
What is Net Rainfall?
Net Rainfall = Gross Precipitation - Interception Loss
What is the hydrological significance of interception?
- results in reduction of precipitation and transpiration
- drop sizes are changed (larger under tree)
- spatial variability of precipitation is changed (distribution on ground)
- water chemistry is changed
How much precipitation does different foliage intercept and then lose to evaporation?
coniferous: 30-35%
deciduous: 15-25%
Tropical forest: 10-15%
What special adaptations do tropical forests have?
adaptations to move water to roots, eg. waxy leaves
What is the leaf area index?
-how much of a cylinder cross section would be covered by foliage (from ground up)
how much can a forest canopy store?
up to 25mm of precipitation in forests with multiple overlapping canopies
How does intensity affect interception?
lower intensity means higher interception
-high intensity overwhelms storage capacity of plants
how does precipitation duration affect interception?
-a large proportion of short duration precipitation is intercepted
How does wind speed affect interception?
- blows precipitation further into trees, promoting interception
- promotes evaporation as taller vegetation presents a rougher surface, thereby generating eddy convection necessary for vapour transport
How does precipitation type affect interception?
- liquid water adheres readily due to surface tension
- snow is easily blown off, but sticky snow can accumulate greatly on branches
how does precipitation frequency affect interception?
- there is only a certain amount of precipitation storage capacity
- maximum interception occurs when there are short duration events with enough time for evaporation between them
What is horizontal interception by conifers?
- needles act as condensation nuclei, intercepting water droplets in clouds
- conifers have massive surface area which helps this
what is stemflow
water from leaves that gets sent down the stem to the roots
What intercepts precipitation in the arctic?
Shrubbery - 10% in sparse to 30% in dense
What increases areal precipitation estimation?
density, time, and area
methods for areal precipitation estimation
- simple arithmetic average
- thiessen polygons
- isoheyetal method (line of equal water)
What is the isoheyetal method?
- most basic method of representing spatial distribution of rainfall
- most accurate but most laborious method
- locate all rain stations on a base map and record rainfall amount
- draw lines of equal water by proportioning the distances between adjacent gauges according to different catch
- calculate mean precipitation in each isoheyetal area
- sum over all areas for average
What is the inverse distance weights interpolation method
- for any point you choose, you can use data from nearby gauges to extrapolate rainfall at that point, giving each gauge a weight based on distance that you choose
- weight parameter is p, inverse exponent - set it large if you want far distances to have low weight
- computer can help optimize p
- user also can also set a distance limit for what points should be considered, and add directional preference for points
Kriging
- use semivariance between points on map for interpolation
- asks “how well are points of a certain distance related to another point” (covariance)
- could change temporally and directionally
How can we compare interpolation models?
could leave out certain data points and see which one gets closer
Spatial interpolation rules of thumb
- the point values may contain error - determine sources of error and make corrections if necessary
- don’t use a method that is more complicated than is warranted given the data quality
- often the statistical criteria for the interpolation method are not valid given other spatial controls (eg. topography) on precipitation
What methods try to account for topography effects on precipitation?
- hysometric method - approximates the effects of topographic change on precipitation statistics (linear regression)
- PRISM (parameter-elevation regressions on independent slopes model)
