THE IRRIGATION AGE. 



11 



The "miner's inch" is the rate of discharge of water 

 that passes through an orifice 1 inch square under a head 

 which varies locally. It is commonly used by miners and 

 irrigators throughout the West, and is defined by statute 

 in each state in which it is used. 



"Second-feet per square mile" is the average number 

 of cubic feet of water flowing per second from each square 

 mile of area drained, on the assumption that the run-off is 

 distributed uniformly both as regards time and area. 



"Run-off, depth in inches on drainage area," is the 

 depth to which the drainage area would be covered if all 

 the water flowing from it in a given period were conserved 

 and uniformly distributed on the surface. It is used for 

 comparing run-off with rainfall, which is usually expressed 

 in depth in inches. 



"Acre-foot" is equivalent to 43,560 cubic feet, and is 

 the quantity required to cover an acre to the depth of 1 

 foot. It is commonly used in connection with storage for 

 irrigation work. 



Explanation of Tables. 



For each drainage basin there is given a brief descrip- 

 tion of general conditions covering such features as area, 

 source, tributaries, topography, geology, conditions of 

 forestation, rainfall, ice conditions, irrigation, storage, 

 power possibilities, and other special features of im- 

 portance or interest. 



For each regular current-meter gaging station are 

 given in general, and so far as available, the following 

 data: Description of station, list of discharge measure- 

 ments, table of daily gage heights, table of daily dis- 

 charges, table of monthly and yearly discharges and run- 

 off. For stations located at weirs or dams the gage- 

 height table is omitted. 



In addition to statements regarding the location and 

 installation of current-meter stations the descriptions give 

 information in regard to any conditions which may affect 

 the constancy of the relation of gage height to discharge, 

 covering such points as ice, logging, shifting conditions of 

 fiow, the backwater; also information regarding diversions 

 which decrease the total flow at the measuring section. 

 Statements are also made regarding the accuracy and re- 

 liability of the data. 



The discharge-measurement table gives the results of 

 the discharge measurements made during the year, includ- 

 ing the date, name of hydrographer, width and area of 

 cross section, gage height, and discharge in second-feet. 



The table of daily gage heights gives the daily fluc- 

 tuations of the surface of the river as found from the 

 mean of the gage readings taken each day. At most sta- 

 tions the gage is lead in the morning and in the evening. 

 The gage height given in the table represents the eleva- 

 tion of the surface of the water above the zero of the 

 gage. All gage heights during ice conditions, backwater 

 from obstructions, etc., are published as recorded, with 

 suitable footnotes. The rating is not applicable for such 

 periods unless the proper corrections to the gage heights 

 are known and applied. Attention is called to the fact 

 that the zero of the gage is placed at an arbitrary datum 

 and has no relation to zero flow or the bottom of the 

 river. In general, the zero is located somewhat below the 

 lowest known flow, so that negative readings shall not 

 occur. 



The discharge measurements and gage heights are 

 the base data from which rating tables, daily discharge 

 tables, and monthly discharge tables are computed. 

 Field Methods of Measuring Stream Flow. 



There are three distinct methods of determining the 

 flow of open-channel streams: (1) By measurements of 

 slope and cross section and the use of Chezy's and Kut- 

 ter's formulas; (2) by means of a weir or dam; (3) by 

 measurements of the velocity of the current and of the 

 area of the cross section. The method chosen depends on 

 the local physical conditions, the degree of accuracy de- 

 sired, the funds available, and the length of time that the 

 record is to be continued. 



Slope Method. Much information has been collected 

 relative_to the coefficients to be used in the Chezy formula, 

 v=C'\/Rs. This has been utilized by Kutter, both in de- 

 veloping his formula for c and in determining the values 

 of the coefficient n which appear therein. The results ob- 

 tained by the slope method are in general only roughly 

 approximate, owing to the difficulty in obtaining accurate 



data and the uncertainty of the value for n to be used in 

 Kutter's formula. The most common use of this method 

 is in estimating the flood discharge of a stream when the 

 only data available are the cross section, the slope as 

 shown by marks along the bank, and a knowledge of the 

 general conditions. It is seldom used by the United States 

 Geological Survey. 



Weir Method. Relatively few stations are maintained 

 at weirs or dams by the United States Geological Survey. 

 Standard types of sharp-crested and broad-crested weirs 

 within the limits for which accurate coefficients have been 

 experimentally obtained give very accurate records of dis- 

 charge if properly maintained. At practically all broad- 

 crested weirs, however, there is a diversion of water either 

 through or around the dam, usually for the purpose of 

 development of water power. The flow is often com- 

 plicated and the records are subject to errors from such 

 sources as leakage through the dam, backwater at high 

 stages, uncertainty regarding coefficient, irregularity of 

 crest, obstructions from logs or ice, use of flashboards, old 

 turbines with imperfect ratings, and many others depend- 

 ing on the type of development and the uses of the di- 

 verted water. 



In general records of discharge at dams are usually 

 accurate enough for practical use if no others are avail- 

 able. It has been the general experience of the United 

 States Geological Survey, however, that records at cur- 

 rent-meter gaging stations under unobstructed channel 

 conditions are more accurate than those collected at dams, 

 and where the conditions are reasonably favorable are 

 practically as good as those obtained at sharp-crested 

 weirs. 



Velocity Method. Streams in general present 

 throughout their courses to a greater or less extent all 

 conditions of permanent, semi-permanent, and varying 

 conditions of flow. In accordance with the location of the 

 measuring section with respect to these physical condi- 

 tions, current-meter gaging stations may in general be di- 

 vided into four classes (1) those with permanent condi- 

 tions of flow; (2) those with beds which change only dur- 

 ing extreme high water; (3) those with beds which change 

 frequently, but which do not cause a variation of more 

 than about 5 per cent of the discharge curves from year 

 to year; and (4) those with constantly shifting beds. In 

 determining the daily flow different office methods are 

 necessary for each class. The field data on which the de- 

 terminations are based and the methods of collecting them 

 are, however, in general the same. 



Great care is taken in the selection and equipment of 

 gaging stations from determining discharge by velocity 

 measurements, in order that the data may have the re- 

 quired degree of accuracy. They are located, as far as 

 possible, at such points that the relation between gage 

 height and discharge will always remain constant for any 

 given stage. The experience of engineers of the Geologic- 

 al Survey has been that permanency of conditions of flow 

 ii the prime requisite of any current-meter gaging station 

 when maintained for several years unless funds are avail- 

 able to cover all changes in conditions of flow. A straight, 

 smooth section without cross currents, backwater, boils, 

 etc., at any stage is highly desirable, but on most streams 

 is not attainable except at the cost of a cable equipment. 

 Rough, permanent sections, if measurements are properly 

 made by experienced engineers, taking measuring points 

 at a distance apart of 5 per cent or less of the total 

 width, will, within reasonable limits, yield better results 

 for a given outlay of money than semi-permanent or shift- 

 ing sections with smooth, uniform current. So far as pos- 

 sible stations are located where the banks are high and 

 not subject to overflow at high stages and out of the 

 influence of tributary streams, dams, or other artificial 

 obstructions which might affect the relation between gage 

 height and discharge. 



A gaging station consists essentially of a gage for de- 

 termining the daily fluctuations of stage of the river and 

 some structure or apparatus from which discharge meas- 

 urements are made, usually a bridge or cable. 



The two factors required to determine the discharge of 

 a stream past a section perpendicular to the mean direc- 

 tion of the current are the area of the cross section and 1 

 the mean velocity of flow normal to that section. 



In making a measurement with a current mete'r a 



