42 



NATURE 



[March ii, 1920 



Rainfall and Land Drainage.^ 



By Dr. Brysson Cunningham. 



''PHE problem of the economical disposal of 

 ■■■ surplus rainfall in cultivated districts is one 

 which naturally engages the attention of the agri- 

 culturist and, as a consequence of his needs and 

 interests, of the meteorologist, the engineer, and 

 the lawyer. All three aspects of the matter have 

 been dealt with recently in an article in Engineering 

 and in two papers read before the Surveyors' 

 Institution. 



The precipitation of atmospheric moisture is 

 counterbalanced in part by the processes of (i) 

 evaporation, (2) transpiration, and (3) percolation, 

 the residue forming the run-oft which collects on 

 the surface of the ground and ultimately finds 

 its way to sea by watercourses, either natural or 

 artificial. In cultivated areas it is essential that 

 the soil should be drained promptly and effec- 

 tively, and left in a "moist," as distinguished 

 from a "wet," or saturated, condition. Ill- 

 drained land is incapable of experiencing the full 

 benefit of those seasonal physical and chemical 

 changes which promote the growth and develop- 

 ment of crops. 



The article by Lt.-Col. Craster discusses the 

 proportion of run-off to rainfall, and the author 

 finds that it varies in this country, as also in 

 America, roughly between the limits of 33 and 67 

 per cent. It has been found that 0065 in. of 

 water is required to wet a crop of rough grass 

 about 5 in. in height, the aftermath in a hayfield, 

 up to the point at which it commences to drip on to 

 the soil. It may therefore be assumed that the 

 amount of water required to wet vegetation and 

 the surface of ploughed land is not less than 004 in. 

 or I mm. The whole of this amount is lost by 

 direct evaporation after every fall of»rain. If the 

 number of days with a rainfall of 004 in. or more 

 be 127 (as in the North-East of England in 191S) 

 and the number of days with less rainfall be 67, 

 the direct evaporation for this area will be 

 o'04 X 127 + o'02 X 67 = 6*42 in. As regards tran- 

 spiration [i.e. absorption by vegetation), figures 

 from German sources show that a beech wood 

 transpires 14*2 in. of water per year; a crop of 

 oats, 8*98 in. ; and a crop of barley, 4*88 in. For 

 an average of 9 in. per year this would be divided 

 as follows: July, 25 per cent.; June, 18; August, 

 15; ; May, 12 ; April and September, 8 each ; March 

 and October, 5 each; and the remaining months, 

 I per cent. each. Percolation is more difficult to 

 estimate, owing to variable geological conditions, 

 but, as a rough rule, may be taken at not less than 

 10 per cent. Summarising these figures for the 

 North-East coast of England, there would be 

 a residue, or run-off, of 8'7 in. out of an. annual 

 rainfall of 26*8 in., i.e. 32*5 per cent., and for 

 Fort William, Inverness, a run-off of 5267 in. out 

 of an annual rainfall of 78*7 in., i.e. 67 per cent. 



I " Estimating River Flow from Rainfall Records." By Lt.-Col. J. E. E. 

 Craster. Engineering, January 2. 



'•Land Drainage from the Engineering Point of View." By C. H. J. 

 Clayton. 



" Land Drainage from the Admini'^trative Point of View." By E. M 

 Konstam. The last two are papers read before the Surveyors' Institution on 

 January 12. 



NO. 2628, VOL. lO<] 



From a survey of the flood discharges in Eng- 

 land and Wales, Mr. Clayton arrives at the con- 

 clusion that, while no general rule can be laid down, 

 it is permissible to assume that in average areas the 

 run-off to the sea is from 50 to 60 per cent, of the 

 total rainfall. As the general average rainfall 

 is about 32 in. per annum, this means that, roughly, 

 1800 tons of water per acre finds its way 

 annually into rills, brooks, strearrs, and rivers. 

 Taking into consideration the fact that about 60 

 per' cent, of the whole rainfall occurs in the six 

 months October to March, the general proposition 

 is established that 36 per cent, of the total rainfall 

 has to be received by watercourses during a period 

 of 182 days, whence it follows that an average wet 

 period run-off to sea is 00633 in. per day. In 

 designing drainage channels and in order to cover 

 reasonable cases of abnormal rainfall, Mr. Clayton 

 advises that this figure should be multiplied by 

 5, and the result so nearly equals i per cent, of 

 the total annual rainfall that he recommends the 

 adoption of this standard. 



The calculation is pursued further to the deter- 

 mination of the flow in tidal rivers necessary to 

 discharge this accumulation of land water. Apply- 

 ing the rule to a catchment area of half a million 

 acres, the total volume to be discharged within an 

 ebb-tide period of fourteen hours per day is 

 576,000,000 cu. ft., or, say, 11,430 cu. ft. per sec.,, 

 which for a distance to sea of twenty miles would 

 necessitate a channel with a theoretical mean area 

 of 2721 sq. ft. 



The maintenance and deepening of these outlet 

 channels are important considerations, but, unfor- 

 tunately, the jurisdiction and supervision exercised 

 over them are casual and unsystematic in the ex- 

 treme. Before the railway era, river and canal 

 navigation brought in revenues from tolls which 

 enabled due regard to be paid to the drainage needs 

 of the districts through which they passed, but the 

 decay of inland navigation has resulted in the los.s 

 of these financial resources, and drainage condi- 

 tions have, in many cases, become deplorable. This 

 view is endorsed in Mr. Konstam 's paper, which 

 deals with the legal and administrative point of 

 view. The startling assertion is made that it is 

 doubtful whether there is a single river in England 

 which is at present in a satisfactory condition as a 

 means of draining water from agricultural land. 

 Whether strictly or approximately true, the situa- 

 tion calls for earnest attention. Drainage authori- 

 ties — known as Commissioners of Sewers in many 

 parts of the country — date back to medieval times, 

 and their powers and functions have, in many 

 cases, become ineffective and obsolete. The Land 

 Drainage Act of 1918, however, does something 

 towards alleviating the situation by enabling the 

 Board of Agriculture and the Ministry of Transport 

 to sanction the transfer of a navigation undertak- 

 ing to drainage functions. No doubt in process of 

 time some degree of co-ordination and system will 

 be established. 



