Precipitation
1
HYDROLOGY: PRECIPITATION
Introduction
Precipitation is the primary source of water supplies. The abundance and temporal
distribution of it controls vegetation type and growth and how we utilize the land..
World wide annual precipitation is about 1000 mm/year. This is not evenly distributed in
space and time over the world. Land areas average only about 720 mm, while oceans
average 1120 mm (Lin, C.A. 1994). Annual precipitation in Canada varies between less
than 200 mm (Arctic and southern Okanagan) to more than 2000 mm in the West Coast.
Annual
Precipitation
(mm)
< 200
200 - 400
400 - 600
600 - 800
800 - 1000
1000 - 2000
> 2000
Precipitation, in conjunction with temperature is a primary determinate of vegetation.
Land cover
Ice/snow
Tundra
Forest & Taiga
Wetland
Cropland
Rangeland
(The State of Canada’s
Environment, 1991:5-7)
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Dept of Agricultural and Bioresource Engineering
Precipitation
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Between The Rocky Mnts and southern Ontario is a vast region that is relatively isolated
form maritime influence and is subjected to the extremes of temp associated with a
continental climate. The praire zone forms part of this area, and experiences bitterly cold
winters, short but warm summers, and low precipitation. About 1/3 of the annual
precipitation falls as frontal snow with a uniform distribution during the winter; another
one-third falls as frontal rain during spring to autumn period; and a final third falls as
convective rain during early summer (Ripley, 1989).
Saskatoon
100
10
Ppt 60
(mm) 40
0
0
-20
J FM A MJ J A S O N D
Calgary
80
40
20
J FM A MJ J A S O N D
Winnipeg
20
10
60
0
Temp
(C)
-10
20
100
20
80
100
20
80
10
0
60
-10
40
-20
0
-10
20
-20
0
J FM A MJ J A S O N D
Monthly Precipitation (mm) across the Prairies
90
80
70
60
50
40
Estevan
Kindersley
North Battleford
Prince Albert
Regina
Saskatoon
Swift Current
Yorkton
30
20
10
0
Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec
Monthly precipitation (mm) within Saskatchewan.
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Dept of Agricultural and Bioresource Engineering
Precipitation
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Prairie Climate
The Prairie region is classified as between sub humid to subarid. This classification
accounts for precipitation between 300 to 450 mm and for evaporation between 500 and
700 mm Prairie snowfall is between 1/5 to 1/3 in snow. About half the year is frozen and
so no evaporation. Much of our ppt is light due to origin of the air masses (and thus
water vapour)
Causes of Precipitation
Psychrometrics
Precipitation occurs when the amount of water vapour in the air condenses and forms
precipitation. For condensation to occur the water vapour must be at saturation at a
surface. Frost forms on windshields, branches; dew forms on vegetation, and in air
masses condensation occurs on condensation nuclei - tiny particles of sea salt, dust, and
pollution aerosols that vary in diameter from 0.1 to 1.0 um. The formation of
precipitation considers that of psychrometrics (vapour content of air and how it changes
upon cooling). If a given air mass at a certain temperature and vapour content (red circle
in Fig. 3) cools without the vapour content changing, it will reach saturation (relative
humidity of 1.0 or dew point). Continued cooling will cause condensation and the vapour
content to decrease. In Fig. 2 the air mass decreased in vapour content by about 1000 kPa
- at the temperatures shown this is about 10 g of water vapour per m3 of air.
Cooling, vapour saturation,
condensation, and vapour content
reduction due to precipitation
3000
6
0.
2000
0.4
1000
Relative humidity
Vapour pressure (Pa)
0.8
4000
1.0
5000
0.2
0
0
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10
20
Air temperature (°C)
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Dept of Agricultural and Bioresource Engineering
Precipitation
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Fig. 2. Psychrometric graph illustrating cooling and condensation.
There are three causes of precipitation. They all involve 'lifting' the air mass so that there
is cooling and subsequent condensation and precipitation;
Orographic (uplift forced by land masses, eg mountains)
Convective (uplift forced by hot air rising, eg summer storms)
Frontal (uplift forced by another air mass)
1. Orographic
For the Prairies the Rocky mountains form a barrier to the eastward movement of mild
moist Pacific air, forcing the air to rise, cool, and lose a good deal of its moisture on the
windward slopes (Ripley, 1989). This explains the high amounts of precipitation in B.C.
and the low amounts in the Prairies; one could say that the Rocky Mountains 'robs' the
precipitation from the Prairies.
Orographic cloud
Precipitation
Forced
lifting
Lee wind
descent
Fig. 3. Orographic lifting and resulting precipitation
2. Convective - or Ppt Recycling
Another process, mainly oeprative in the summer, called free convection, is responsible
for about one third of the precipitation in the prairies (Ripley, 1989). Here the high
summer sun heats the land mass which in turn heats the nearby air. If the air mass near
the ground is moist (from high evaporation) then as it rises it cools and precipitation
occurs. Very hot days can cause high cumulus clouds and thunderstorms. Although
convective heating can produce anything from a light sprinkle of rain to a torrential down
pair, but is generally responsible for the short duration heavy downpour storms of July
and August.
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Dept of Agricultural and Bioresource Engineering
Precipitation
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3. Frontal
Air masses are extensive volumes of the atmosphere of fairly uniform temp and
huimidity, FRONTS are the narrow transitional zones between air masses. The collision
of two of two different air masses can cause one to rise. The rising air mass can cool and
result in precipitation. Air masses are classifed into 6 categories, two each of Arctic,
Polar and Tropical with two subdivisionos of each are continetnal (c) and maritime (m),
depending upon the source region and humidity of the air mass (Bruce and Clark,
1966:9). In the prairies, frontal activity produces about 2/3s of the annual precipitation,
including almost all of the snow (Ripley, 1989). The most common air masses found
over the pariires in the summer are maritime polar air massses from mid-lat oceans, while
in the winter they are maritime arctic air masses from arctic oceons. On rare occasions
does maritime tropical airmasses come to the prairies. (Ripley, 1989).
Interception
Interception is the amount of water caught by plant (or other surfaces). Intercepted loss is
the amount evaporated before it reaches the ground. Conifers intercept a large amount of
snow, but much of this slips or melts to the forest floor. Crops can intercept up to several
mm of rain.
Throughfall is precipitation falling through spaces in veg canopy or when precipitation
drips from leaves and twigs
Stemflow is water flowing along twigs and branches to ground
References
Dyck, G.E. 1977. Areal Precipitation Estimates from Point Measurements. Masters
Thesis, Dept. of Agricultural Engineering, Divsion of Hdyrology, University of
Saskatchewan, 161 pp.
Ripley, E.A. 1989. Agricultural meteorology class notes.Dept of Crop Science and Plant Ecology,
University of Sask
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Dept of Agricultural and Bioresource Engineering