22 
The Colorado Experiment Station 
San Miguel counties, and a strip bordering San Luis Valley. The 
15-inch precipitation line follows quite closely the 6,000-foot con¬ 
tour line along the eastern mountain front and the 7,000-foot line 
' in the western portion of the state, except in the Gunnison dis¬ 
trict where it is elevated to 8,000 or 8,500 feet. The 15-inch line 
runs at an altitude of about 9,500 feet in the Wet Mountain Valley, 
North Park, and San Luis Valley. The eastern half of the Great 
Plains and also a strip along the southern border, except the cor¬ 
ners of Baca and Prowers counties, experience 15 to 20 inches of 
precipitation on the average, which gives this section an advan¬ 
tage over the western half of the Plains in dry land agriculture. 
The 20-inch isohyetal line varies in altitude from about 8,500 feet 
to 10,000 feet. The higher elevations in the state have over 20 
inches of rainfall. Although there are localities with an annual 
precipitation considerably over 25 inches, there are too few sta¬ 
tions to enable one to put in the 25-inch line with any degree of 
accuracy. 
In connection with this discussion, it should be pointed out 
that the total precipitation for the year may not give us a true idea 
of the effectiveness of such in increasing the moisture content of 
the soil, and in influencing p’ant growth. In addition, we need to 
know particularly the distribution of the precipitation throughout 
the year, and the frequency and nature of the same. For example, 
torrential rains, although they swell the total annual amount, may 
run off in large part as flood waters, and only a small fraction be 
left in the soil for plant use. Again, frequent light showers, fol¬ 
lowed by sunny skies, may moisten merely the surface of the soil, 
and the moisture be carried away shortly; moreover, light showers 
may effect the formation of a surface crust on the soil, which may 
actually be harmful in that it establishes a capillary connection 
with the soil below, and moisture from these lower layers is lost 
to the air. The minimum effective amount of rainfall will not be 
the same for different soil types, or for similar soil types when 
atmospheric conditions vary. Light showers followed by cloudy 
weather are more beneficial than when followed by bright, sunny 
weather. 
Variation in Precipitation from Year to Year .—Very wet and 
very dry years, or a consecutive series of these, are not uncommon 
in Colorado. The mean difference between the wettest and driest 
years is somewhat greater for localities east of the Continental 
Divide than for localities west. The following brief table gives 
one an idea of this fluctuation. 
