146 DOMESTIC WATER SUPPLIES FOR THE FARM 



If rainfall were equally distributed throughout the year and 

 if the consumption of water were regular, a cistern large enough 

 to hold one or two months' supply would be sufficient, but, un- 

 fortunately, the rainfall is ordinarily very irregular, from one- 

 half to two- thirds of the entire precipitation of the year not 

 infrequently falling in three or four of the colder months, while the 

 remainder is distributed over the eight or nine warmer months. 

 This reason alone would make large cisterns desirable, but still 

 another reason is found in the larger consumption of water in 

 the warmer season, which is generally the period of deficient 

 rainfall. To be sure of a supply through long periods of drought 

 such as are likely to occur one or more times in each decade, it is 

 necessary to have a cistern that will hold when full two-thirds or 

 three-fourths of the total amount required during the year. For 

 drinking, cooking and washing each person on a farm ordinarily 

 uses from 5 to 10 gallons or more of water a day, and each head of 

 stock ordinarily requires from 6 to 15 gallons. The amount used 

 however, depends so largely on local conditions that it should be 

 carefully ascertained in each specific case before the size of a 

 cistern is determined. An allowance of 25 to 35 per cent should 

 also be made for loss due to evaporation, the deflection of the 

 earlier and more or less dirty washings from the roof, the overflow 

 of gutters in heavy storms, the loss by snow sliding or blowing 

 from the roof, the leakage of pipes and other minor causes. The 

 total amount needed and the allowance for loss having been de- 

 termined, it becomes possible to calculate the size of cistern or 

 cisterns required. The volume of a round cistern is approxi- 

 mately five-sixths of the product obtained by multiplying the 

 square of the diameter by the depth. 



For assistance in determining the amount of water annually 

 falling on a roof the following table, showing the number of gal- 

 lons falling on each square foot on roofs of gentle, medium and 

 steep slopes at different rates of rainfall, is presented. The annual 

 rainfall of any particular locality can be ascertained from the 

 United States Weather Bureau. (See also Fig. I.) 



