When neither canals, nor springs, nor flood waters are available for the supply of water, it is yet possible to obtain a limited supply by so arranging the roof gutters on the farm buildings that all the water that falls on the roofs is conducted through the spouts into carefully protected cisterns or reservoirs.  A house thirty by thirty feet, the roof of which is so constructed that all that water that falls upon it is carried into a cistern will yield annually under a a rainfall of fifteen inches a maximum amount of water equivalent to about 8800 gallons.  Allowing for the unavoidable waste due to evaporation, this will yield enough to supply a household and some live stock with the necessary water.  In extreme cases this has been found to be a very satisfactory practice, though it is the one to be resorted to only in case no other method is available.

It is indispensable that some reservoir be provided to hold the surface water that may be obtained until the time it may be needed.  The water coming constantly from a spring in summer should be applied to crops only at certain definite seasons of the year.  The flood waters usually come at a time when plant growth is not active and irrigation is not needed.

The rainfall also in many districts comes most largely at seasons of no or little plant growth.  Reservoirs must, therefore, be provided for the storing of the water until the periods when it is demanded by crops.  Cement-lined cisterns are quite common, and in many places cement reservoirs have been found profitable.  In other places the occurrence of impervious clay has made possible the establishment and construction of cheap reservoirs.  The skillful and permanent construction of reservoirs is a very important subject.  Reservoir building should be undertaken only after a careful study of the prevailing conditions and under the advice of the state or government officials having such work in charge.  In general, the first cost of small reservoirs is usually somewhat high, but in view of their permanent service and the value of the water to the dry-farm they pay a very handsome interest on the investment.  It is always a mistake for the dry-farmer to postpone the construction of a reservoir for the storing of the small quantities of water that he may possess, in order to save a little money.  Perhaps the greatest objection to the use of the reservoirs is not their relatively high cost, but the fact that since they are usually small and the water shallow, too large a proportion of the water, even under favorable conditions, is lost by evaporation.  It is ordinarily assumed that one half of the water stored in small reservoirs throughout the year is lost by direct evaporation.

Available subterranean water