RESTRICTED 



15 



the total lagoon voliime. The reservation is laade, hov/ever, that 

 these values maj' be as much as 20% too high, 



A final check on the validity of the data, particularly 

 with respect to the relative proportion of water' that comes in 

 over the reefs as compared with that derived froni the channels, 

 can be obtained by calculating the salt budget. It has been ob- 

 served that water entering the lagoon over the reefs has a salinity 

 of about 34,80°/oo, along the southern passes and channels it is 

 about 34.50°/oo, and thg average salinity of water flowing out 

 during the ebb is 34.63 /oo. Combining these observations with 

 estimated voluiues of flov/, the results are as shown in Table 6. 

 The error is about 1%, 



Inflow 



Table 6. Salt budget 



Estimated 

 total 



volume 

 ofwate 



Reefs 



Channels, passes 



Total 



3.71x10 



6.85 

 10.56 

 ■10.62 



iv 



Salinity 



34.80 

 34.50 



Total 

 transfer 

 of salt 



12.9xl0llg 



23.6 



36.5 



Outflow 



34.63 



311 



3^ 



Thus the total transport into and out of the lagoon is es- 

 tablished with a fair degree of accuracy. There remains only the 

 question of the dispersal of v;ater inside the lagoon. Taking 

 the figures from Table 6, the water coming in over the reef with 

 a salinity of 34.80°/oo constitutes 35% of the total inflow, 

 while 65% is channel water with a salinity of- 34.50°/oo. If 

 these proportions were mixed completely, -the resultant salinity 

 of the lagoon would be about 34.6l°/oo. Instead, the average 

 salinity of the lagoon is nearly 34.80 /oo. The difference is 

 too large to be accounted for by evaporation. The most logical 

 explanation of the apparent discrepancy is that rapid and relative- 

 ly complete nixing occurs only in the immediate vicinity of the 

 passes and channels, and part of the water entering this area on 

 the flood tide is lost on the ebb without becoming incorporated 

 in the main mass of central lagoon water. Again utilizing observed 

 salinity values for purposes of calculation, the v;ater in the 

 passes on the ebb tide, v/ith an average salinity of 34. 63°/ 00, 

 has a percentage composition of 43% central lagoon water (salinity 

 34.80) and 57% ocean water ( 34.50) which has been in the lagoon 

 only a short time. The results of this estimate are summarized 

 as follows: 



(a) It is assumed that the total inflow and outflow are 

 equal and have a value of 10,6 x lO-'-^cm^ per 12 hours. 



