eM ete we ee a ee ie 
0 = saa ts 
i, § 
RAINFALL IN MINNESOTA. 451 
gation is introduced, many valuable resources will not only lie 
dormant, but our present yields remain far below what they might 
be. 
Usually irrigation implies that the gain in crop production is ac- 
complished through the application to a lesser area of the scanty 
rainfall collected from a greater one; butin Minnesota, with its 
abundance of natural surface and subterranean reservoirs, it is 
quite probable that no section need ever be depleted to supply else- 
where, even if the inflow to our lakes and rivers was thereby dimin- 
ished and the active area of evaporation correspondingly increased, 
as will very likely be the case. 
The monthly distribution of rainfall is quite an important factor 
which should also be taken into consideration. In the Atlantic and 
Gulf states, the fall during one month averages about the same as 
another, while on the Pacific coast nearly the entire annual supply 
is received between October and April, with June, July and 
August comparatively rainless. In Minnesota our wettest months 
are during the growing season, from April to September inclusive, 
with an average deposit of between seventy and seventy-five per 
cent of the yearly amount. 
Heavy rains in winter are harmful rather than otherwise, as 
they leach out fertilizing ingredients from the soil and are of no 
benefit to crops, the seed of which has not yet been sown; there- 
fore, if our normal spring and summer supply could always be 
depended upon, no better plan of moisture distribution could be 
devised, as it is a natural system of irrigation in itself, which 
would not require any artificial annex. 
The average rainfall during the growing season in Minnesota is 
approximately twenty inches and is equal to that received in other 
Northern states to the east, except along the immediate coast; there- 
fore. when applied to crop production, our yearly rainfall should be 
increased one-half, if comparisons are made with those states re- 
ceiving their annual supply in equal monthly installments. This 
all goes to show that if the employment of irrigation is needed 
here, its introduction is of like importance to a number of Eastern 
states receiving an annual rainfall one-half greater than ours. 
The question of annual rainfall resolves itself into still another 
phase of variability besides that of monthly distribution before we 
can rightly judge its full meaning. 
A yearly fall of thirty inches may also mean a fall of forty inches 
for several years, followed by years of twenty inches or less, and 
such is unfortunately the case here. Our annual deviation aver- 
ages about fourteen per cent of the normal, which means that in 
those places having a yearly rainfall of twenty-five inches it is quite 
likely the actual quantity received during any one year will be 3% 
inches greater or less thanthat amount. Itis also rather a singu- 
lar fact that during a long period there will be more years with less 
_ than the normal rainfall than years with a greater amount, which 
shows that, as a rule, the departures are greater in wet than in dry 
years. 
Districts having the smallest variability in annual rainfall are 
least subjected to prolonged drouths, and we will now see how Minne- 
