706 SOILS: PROPERTIES AND MANAGEMENT 



efficient, and the bottom of the furrow should extend 

 well below its base. This will allow the water to diffuse 

 laterally rapidly, and the deep dry mulch reduces the 

 extent to which the surface becomes moist, thereby con- 

 serving moisture and reducing the accumulation of alkali 

 at the surface. 



The application of water to the soil in irrigation must 

 be guided by the principles elucidated in the discussion of 

 the physical properties of the soil, and its relation to 

 moisture and its control. 



605. Units of measurement. — The measurement of 

 water in irrigation practice involves the use of units of 

 volume and pressure. By the head is understood the 

 volume of water supplied in the unit of time. The flow 

 of water in canals is usually stated in units of flow per 

 unit of time, that is, the number of cubic feet per second, 

 called the second-foot. Frequently the term second-foot 

 is applied to the volume of water that would result from 

 a flow of that rate throughout the season. A smaller 

 unit is the miner's inch, a term derived from mining 

 practice, which refers to the quantity of w r ater that will 

 flow out of an orifice one inch square under a constant 

 pressure which varies in different states from a four to 

 an eight inch head above the top of the orifice. Like the 

 second-foot, the flow is frequently rated by the season. 

 The pressure is proportional to the depth, or head. It is 

 commonly stated in pounds per square inch. A column 

 of water ten feet in height exerts a pressure of approxi- 

 mately 4.34 pounds to a square inch. 



In the field, water is commonly measured in terms of 

 depth over an acre. An acre-foot is the quantity of water 

 that will cover an acre one foot in depth. An acre-inch is 

 one-twelfth of an acre-foot. These are very convenient 



