Hydrology BC ENV 3025
Global precipitation (evaporation) patterns and
variability
1.) Average annual precipitation onto the continents is a
function of:
- (a) latitude (precipitation highest in latitudes of rising
air-0° and 60° north and south-and lowest in latitudes
of descending air- 30° and 90° north and south);
- global circulation patterns in the
atmosphere (Fig)
- (b) elevation (due to orographic cooling, precipitation
usually increases with elevation) (Fig) (Fig)
- (c) distance from moisture sources (precipitation is usually
lower at greater distances from the ocean);
- (d) position within the continental land mass;
- (e) prevailing wind direction;
- global circulation patterns in the
atmosphere (Fig)
- (f) relation to mountain ranges (windward sides typically
cloudy and rainy, with leeward sides typically dry and sunny)
- (g) relative temperatures of land and bordering oceans
- global circulation patterns in the oceans (Fig)
Exercise: look at global (Fig) and US (Fig) patterns in annual
precipitation and relate them to the above trends. Find five
places that relate to any of those mechanisms mentioned above
and be prepared to present them to the class.
2.) Global patterns in evaporation
Exercise: Analogue to the above,
look at the global patterns of evaporation (Fig), and explain why it is
high or low for five locations, e.g. Antarctica, western north
Atlantic, northern Africa, the Amazon region. Note: the graph
shows modeled global precipitation and evaporation,
because there is no global network of stations that measures
these parameters everywhere.
3.) Global variability of precipitation
- world precipitation is extremely
variable in time
- use the National
Climate Data Center's database to study variability of precipitation
globally and explain the patterns that you see
- make a histiogram of the daily
precipitation for one of those stations, are the data normally
distributed?
4.) Discharge of the Colorado River
- One historical example illustrates
just how important it is to know the magnitude of hydrological
fluxes. In the early part of this century, rapid growth in the
western and southwestern United States led to efforts to
"reclaim" the desert, mostly through management of the
Colorado River. To apportion the flow of the Colorado among
the states that would use the water (the Colorado River
Compact of 1922), it was necessary to determine the amount of
water available each year. This was done by averaging the
annual discharge measured at a single point on the Colorado
River over the available period of record (1896-1921), which
turned out to be about 16.8 million acre-feet (an acre-foot is
the volume of water which would cover an acre of land to a
depth of one foot, and is approximately 1,233 m3).
Unfortunately, this period of time turned out to be a
particularly wet era (or, the following years were
particularly dry). From 1922 to 1976, the average annual
discharge of the Colorado River at the gaging station was 13.9
million acre-feet. When the budget was calculated for the
Colorado River Compact of 1922 and the water apportioned among
the states, there was not enough water to go around! It should
be clear that the temporal and spatial patterns of
precipitation and evapotranspiration within the Colorado River
basin strongly influence water availability and hence its use
and management.
- Find a precipitation station in the
catchment area of the Colorado River that shows this trend in
precipitation using the National Climate Data Center's
database (NCDC
Climate
Visualization (CLIMVIS)).