76 ENGINEERING ON THE FARM 



the extremes being greater in one period than in another. 

 Areas of land that in the wet period are considered swampy, 

 or are full of lakes, become in the dry season sufficiently dry 

 to be tillable and very productive. The beds of lakes are 

 frequently plowed up and two or three good crops raised 

 before the fields again become navigable lakes. Rainfall 

 data are usually collected along with temperature data, 

 and information is to be had in the publications of the 

 Weather Bureau. 



As with temperature, the amount of rainfall that affects 

 farm production in a humid country is not dependent upon 

 the average of a given month or year, but is determined 

 by the amount of rainfall in twenty-four hours and the 

 number of days in succession that the rain falls. An inch 

 rainfall in twenty-four hours in a dry season may be bene- 

 ficial, but an inch five days in succession might be destruc- 

 tive. On the other hand, an inch rainfall in five days would 

 have hardly any bearing on crop production, but an inch 

 coming at the end of several rainy days might be disastrous. 

 Losses during wet seasons usually occur as a result of a series 

 of heavy rains, coming close together, such that the land 

 surface is not able to dispose of one rain before another 

 comes. This fact explains the heavy losses sometimes occur- 

 ring because of undue moisture in a year during which the 

 average rainfall is at or below normal. Comparison of one 

 region with another as to rainfall for agricultural production 

 should be based on the daily rain and the number of succes- 

 sive days during which there is a probability of rain falling. 



Disposal of rainfall. In the disposal of rainfall there 

 are five factors : absorption, percolation, evaporation, plant 

 growth, and run-off. Absorption is limited to the quantity 

 of water that can be held by the soil grains; percolation to 

 the quantity that can flow downward between the soil 

 grains; evaporation by the conditions of wind and temper- 

 ature; water used by plant growth by the growing con- 

 dition of the plant; and run-off depends on the quantity 



