392 TRANSACTIONS OP SECTION C. ' ' ' 



'run-off,' because the term 'run-off' is used in two senses — either to express 

 the total river-discharge in a catchment area, when it would obviously include 

 practically all percolation within such area; or to express the local surface 

 run-off, which could be utilised for reservoir^torage in the area in question, as 

 distinct from the fraction of the rainfall which percolates into the ground and 

 subsequently emerges at lower levels. 



Another source of error arises from a disregard of the fact that the perco- 

 lating water in any area may be regarded as a storage-reservoir which tends to 

 equalise the surface stream-riow during periods of varying rainfall ; and that in 

 pumping operations on a large scale the natural equilibrium becomes disturbed, 

 notonly water of percolation but also part of the surface run-off in the form of 

 springs, seepages, and streams being drawn upon. 



The conditions are so complex and the controlling factors vary so much 

 in different river-basins that it is impossible to obtain for the whole country 

 anything like an accurate and reliable expression for the relationship between 

 rainfall, percolation, and run-off. The interminable and- costly legal wrangles 

 during the passage of a Water Bill through Parliament bear witness to the 

 truth of this statement. What is needed is a continuous record in the differ- 

 ent catchment areas of the country of observations on river discharge, percola- 

 tion, and so forth, extended over many years. Fortunately, our rainfall obser- 

 vations, thanks to the British Rainfall Organisation, are now, or could be made, 

 ample for this purpose. But except for attempts by local water companies and 

 corporations to obtain the data I have referred to, there exists no public 

 control to deal with the matter. 



In 1906 a Committee of the Royal Geographical Society, with Dr. Strahan 

 as Chairman, and with the aid of a grant from the Royal Society, undertook 

 to investigate river discharge, suspended and dissolved matter, rainfall, area, 

 and geological conditions in some specially selected river-basins. The final 

 report, which has now appeared, dealing with the Severn above Worcester, the 

 Exe, and the Medway, constitutes a most valuable record. 



The mean discharge of the Severn above Worcester from 1882 to 1889 comes 

 out as 46-2 per cent, of the rainfall, and for the Exe 55-9 per cent. The 

 Severn may be taken as an average river for these purposes, and we note that 

 the discharge is distinctly higher than, what we should expect from figures 

 usually given in text-books. 



It will be obvious to all geologists that important theoretical questions, such 

 as the rate of denudation and deposition, and vital engineering matters, such as 

 the position and permanency of harbour works, would be greatly assisted by 

 exact quantitative e.stimates of the material carried down by rivers. 



In 1878 Joseph Lucas urged the importance of a Hydro-geological Survey 

 of England, and the Royal Commission on Canals and Waterways in their final 

 report in 1909 recommended the appointment of some public authority to do for 

 the whole country what this Committee has so admirably done for these three 

 river-basins. 



Organisation of Expert Knowledge. 



We are reminded by the report of a later Royal Commission — that on 

 Coast Erosion in 1911 — that systematic observations and the collation and 

 organisation of geological and engineering knowledge are urgently needed in 

 connection with the protection of our coasts and the reclamation of nevsr lands. 

 For it will be remembered that the Commission found that during the last 

 thirty-five years the gain of land, as shown by Ordnance Survey maps, has been 

 more than seven times the loss by erosion. 



Here, again, the British Association may reflect with pride that it paved the 

 way for this national inquiry. For many years its Committee on Coast Erosion 

 gathered and collated evidence on erosion, and induced the Admiralty to instruct 

 the Coastguard to observe and report upon, changes that take place from time 

 to time. 



After recommending ' that the Board of Trade should be constituted the 

 Central Sea-Defence Authority for the United Kingdom for the purpose of the 

 administration of the coast-line in the interest of sea defence,' the Commis- 

 sioners go on to urge that ' that Department should have the assistance of 

 scientific experts to collate information and to secure systematic observations 



