## F V

and similarly.

These equations give us a means of calculating the rates of flow across A j and A2. if the mean salinity at each section and the rates of evaporation, precipitation and run-off are known. Also, the average volume of water flowing through the sections per unit time is in each case equal to the cross-sectional area xthe mean current velocity, i.e. VX=A \ ¡7, and V2 = A2ii2. Thus, if areas A, and A2 are known. ¿7, and U2, the average velocities of the currents flowing through the sections, may be estimated.

In deriving Equations 6.3 to 6.5, we assumed that salt is carried into and out of the channel only by advection of the mean current; the effect of, say, an eddy of exceptionally high or low salinity could not be taken into account. Also, in estimating average values of the different parameters, any variations resulting from tidal flow would have to be allowed for. Nevertheless, this application of the principles of continuity and of conservation of salt provides an extremely useful tool in the study of semi-enclosed bodies of water. Here, we will demonstrate how the principles may be applied to flow into and out of the Mediterranean (Figure 6.13).

At the Straits of Gibraltar, Atlantic water of relatively low salinity flows into the Mediterranean Sea, while high salinity Mediterranean water flows out at depth.

According to Figure 6.13(bi. what are the values of'Sj, the mean salinity of water flowing into the Mediterranean, and 5:. the mean salinity of water flowing out?

Figure 6.13(b) suggests that Sj is between 36.25 and 36.5. and that S2 is between 37.0 and 38.0. The average values are about 36.3 for and 37.8 for S2.