The mathematical procedure of Pearson and Pearson6 is attractive in its simplicity and yet generally compatible in accuracy of depiction with the rather coarse data customarily available for describing the complex physical conditions in estuaries.45. In common with mathematical models generally it has the merit that, once the model is set up, it is a relatively straightforward matter to vary the values assigned to the input parameters and to ascertain the effects. Particularly appropriate to this kind of treatment are environmental problems where, for example, the long-term plans for a region envisage changes in effluent quantities and qualities and it is desired to predict what the general pollutional status will be in, say, the future 25 years. Such an estimate, albeit tentative, has been made recently for Southampton Water, again using salinity as the tracer for mixing.13 46. However, it is important to recognize the limitations of the simplified treatment, especially in respect of the assumption of complete mixing. Although the lower reaches of Southampton Water are fairly well mixed, there being only a small increase of salinity with depth below the surface, the upper reaches are stratified (segments 7 and 8 in particular) and probably more markedly so now as a result of the considerable change in configuration brought about by the new container berths.47. The method is also very dependent on fairly reliable assessments of salinity gradient, which in the present instance is quite small in the lower reaches, as is apparent from the large values of F,* in Table 3. Unfortunately, salinity has not been monitored in Southampton Water and this means that, unlike the case with temperature, the long-term averages are not known, although the values assumed are no doubt, in the light of present knowledge, a fair representation. More recent data have shown that there is significant variation with river flow.14 48. As is evident in Table 3, there is a marked difference between the salinity value of segment 1 and that for the Solent outside, which has been taken as 35 parts per 100Gthe same as that for the open sea. The conditions assumed for the seaward end affect the results and if the initial salinity difference were less, temperature estimates would be higher and conform more to measured values. On the basis of reliable and representative salinity values in this important entry region it would be possible to predict the flushing characteristics of the estuary (i.e. the proportion of new water which enters on each tide): a feature of obvious importance from the point of view of the heat balance. Recent indications are that about 30% is new water, but the proportion varies significantly with tidal range and river flow. M r Jarman and M r de TurvilleWe agree with Mr Webber that stratification and variable river flow could have an important bearing on variations of the temperature distribution in the upper part of
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