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COLORADO EXPERIMENT STATION 
grown below a ditch, they may receive additional waters no less effec¬ 
tively than by surface irrigation. To say they are raised by the 
natural rainfall is misleading, to say the least. 
Many inquiries are asked concerning the amount of rainfall on the 
plains, and some misunderstandings are prevalent. These may best 
be answered by giving the records of a number of places where the 
rainfall has been measured for a series of years. Those which have 
the longest record are chosen. 
To interpret the rainfall records, the distribution and character 
of the rainfall must be taken into account as well as the amount. It 
scarcely needs to be said that a moderate rainfall which falls mostly 
in the growing season, without long intervals between rain, moderate 
showers, is far more effective than a large rainfall coming principally 
in the cold season, or which comes in heavy shower^j at irregular 
and infrequent intervals. Sudden and heavy downpours are of little 
agricultural value for the effectiveness of a rain is .largely measured 
by the amount which the soil absorbs and this absorption takes place 
at a slow rate, varying with the soil, but, say, one-tenth inch per hour. 
Showers of a few hundredths of an inch are of little value, for they 
penetrate only the surface of the soil, evaporate almost immediately, 
and thus are of practically no benefit. Hence moderate rainfalls, well 
distributed, during the growing months, are of the greatest value. A 
favorable feature of the distribution of the rainfall in Colorado, is 
that nearly 50 per cent comes in these growing months. On the 
divide between the Arkansas and the Platte riversi, more falls in'the 
summer months of July and August. Speaking generally in respect 
to the plains, the storms of long continuance are mostly in the months 
•of April and May or perhaps the early part of June and October, 
Avhile the intermediate months of the summer have their precipitation 
principally in the form of thunder showers. 
Even the average rainfall for any given month may in itself be 
misleading, and especially where the rainfall is moderate or small, 
and subject to occasional violent storms. The average may be far 
from indicating the probable quantity to be expected. More than 
half of the years will be below the average. A better index to the 
agricultural value of the rainfall is to know the certain surety of a 
given rainfall which can be depended on. We may take the record of 
April at Fort Collins to illustrate the difference. While the average 
for 25 years is 2.31 inches, the record shows that practically three out 
of four years (18 out of 24) have been below the average. Hence so 
far as this is a guide for the future, the probability is about three 
to one that in any given April, the rainfall will be below the average. 
If, however, we count the Aprils with reference to the amount, 
we find that, for a quantity of about 1.44 inches, half of the Aprils 
have been above and half below. Tihs amount is the safer guide of 
what to expect. For lack of a better term, I have called this the 
agricultural mean as distinct from the 'Ordinary average, and it is 
indicated at the foot of the table. Where there is little difference 
