Lecture Panel 5 Flashcards
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)
What does PRISM use?
- historic record of measured precipitation
- geographic input, especially terrain
- –prevailing wind direction, streamflow measurements
How can we use gauges with PRISM?
to estimate the quantitative precipitation for a given event or time period
hyetograph and hydrograph
hyetograph - rainfall intensity vs time
hydrograph - discharge vs time in a river, etc.
-obvious relation between these two
Temporal representation of rainfall graph
cumulative rainfall hyetograph (aka rainfall mass curve) - summation of rainfall increments over time
-slope gives an idea of intensity
What does the changing slope of a cumulative hyetograph mean?
changes in rainfall over time
What are intensity-duration relationships? what are they good for?
- self explanatory. they must be locally calibrated but applicable to global scale
- yield straight line on log-log graph
- eg McCallum model, I=kt^-n (k and n are location dependent constants)
- good for predicting runoff
- different types of storms have different slopes
WHat are intensity-duration-frequency curves? What are they good for?
- IDF curves are established for many regions. Statistical relationship between intensity corresponding to storm duration and frequency
- helps when constructing a neighbourhood to withstand a 100 year storm, for example
Probable Maximum precipitation
- hypothetical concept
- analytically estimated greatest possible depth of precipitation that is physically possible and reasonably characteristic over a geographical region at a certain time of year
- defined wrt a given area, including estimates of of moisture inflow over the basin and the amount that could be precipitated
- theoretical concept, hard to defend
