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)).