Precipitation1
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.
Precipitation, in conjunction with temperature is a primary determinate of vegetation.
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).
Monthly Precipitation (mm) across the Prairies
Monthly precipitation (mm) within Saskatchewan.
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.
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.
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.
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
L03prec.docDept of Agricultural and Bioresource Engineering