NO. 10 WATER TRANSPARENCY — WILLIAMS, JOHNSON, DYER 



or in terms of natural logarithms: 



Equations (12) and (13), then, express a relationship involving k, 

 the extinction coefficient, D, the Secchi Disc reading, R d , the reflec- 

 tivity of the disc used, and U w , the relative amount of light traveling 

 in an upward direction compared to that traveling downward. Let 

 us look at each one of these variables a little more closely. 

 If we define a term E, sometimes called optical density, as : 



„ 1 100 

 E = los %f 



where %T= percent transmission, we may express k in terms of E 

 by: 



k = 2.3 E 



since k is given in terms of natural logarithms. Since E values and 

 %T values are conveniently tabulated in readily available tables, we 

 may easily obtain a k value for any %T value we may have as given by 

 the hydrophotometer. In this manner we may reduce any hydro- 

 photometer reading to its equivalent Secchi Disc reading or vice versa 

 by substituting the k or D value in equation (12) or (13). 



The D is, of course, the Secchi Disc reading which may be either 

 read directly or calculated from the hydrophotometer reading. For 

 the disc used R d was about 0.8. 



The relative amount of upwelling light, U w , however, was not meas- 

 ured and values were assumed for this quantity, based on other data 

 taken by Williams in Chesapeake Bay and by the calculated values 

 from the large number of stations where both Secchi Disc readings 

 and hydrophotometer readings were taken. 



If equation (12) is rewritten: 



*~ D 

 where 



, Rd 



, = 2.3108-^ 



