INLAND WATER SURVEY 385 
of some standard sections of the river ~ likely points for water-level records 
and on typical reaches. 
These static measurements are the foundation work on which to build 
the continuous records of water level and flow. 
4. RAINFALL, STORAGE, AND FLow IN RELATION TO SURFACE 
WATER AND ITS MEASUREMENT. 
(A) Rainfall—The measurement of rainfall by rain gauges distributed 
over a catchment area provides the primary assessment of potential flow 
in the stream or river. In the absence of flow measurements, the total 
flow during any period is estimated from such assessment of rainfall by 
deducting certain assumed losses. Those losses are, in fact, the estimated 
difference between rainfall and run-off derived from some specimen areas 
where rainfall and run-off have been measured over a long period of years. 
In these specimen cases, the assessment of rainfall on the area depends 
on the method of working out the distribution, and must be uncertain unless 
there are a great number of gauges. In practice, the rainfall assessment is 
generally made for the year, and sometimes for each month. Even if the 
gauges are numerous and read daily, it would hardly be practical to make 
a daily assessment. On the other hand, the run-off measurement is a 
concentrated measurement which, with the proper apparatus, may be of 
great accuracy, and may be tabulated for any desired time interval—half- 
hour or three hours or the day. Owing to the present limitations of the 
rainfall assessment there is only an annual estimate of losses, and it is 
necessary to proportion this loss over the months of the year on the basis 
of the evaporation losses measured in a tank. 
This method of estimating run-off from the rainfall has been developed 
on the foundation of our long rainfall records, and in certain directions it 
has been standardised and accepted as a legal measure of run-off, It is 
recognised that this indirect measurement of surface water run-off is un- 
satisfactory and productive of very erroneous results. 
For accurate measurement of water, for any purpose, it is the actual 
continuous records of flow past a measuring point which are essential, and 
rainfall and other measurements then fall into their correct places for use 
in correlation to the measured flows. 
The point emphasised here is that, in fact, the measurement of surface 
water has been dispensed with in practice, because there exists a cheap 
method of estimating flow from rainfall. 
As one example of the danger of such an approximation one may take 
the example of the assessment of compensation flow (Ministry of Health 
Advisory Committee on Water: Report of Technical Sub-Committee on 
the Assessment of Compensation Water. H.M. Stationery Office, 1930). 
As other examples, one may take the cases of water power schemes, etc., 
where erroneous estimates of flow may make the scheme and wreck valuable 
existing interests or, conversely, may wreck a valuable scheme. 
(B) Storage.—Dealing with the whole of a river catchment area, storage 
may take many forms, but, in its widest sense, storage is represented by 
certain natural and artificial physical characteristics of the area which 
modify the intensity, duration and volume of the flow. The need for 
continuous measurement of storage, in its relationship to flow, becomes 
at once apparent. 
: ae forms of storage affecting surface water may be described as 
ollows :— 
