164 FOREST INFLUENCES. 



In the department of L'Heranlt, in the Oevennes Mountains in 

 southern France, the following calculation was made of the amount of 

 water retained by the forest cover after a heavy storm. 



The basiu of Lampy, comprising l,60f> acres, of whicli more than 50 per cent are 

 under forest, the rest in grass and field (6 per cent), rests on impermeable granite 

 and quartz roclc with a layer 2 feet deep of rich humus soil. All the water falling 

 must i)ass into a reservoir formed by closing the valley with a dam which stores the 

 water to a heiglit of 51 feet. The reservoir, nearly 100 years old, has never needed 

 to be dug out, which is mentioned as a sign of the absence of soil erosion. When 

 full all the water must pass over the dam in an overflow race, which permits a toler- 

 ably exact calculation of the discharge of water from this reservoir. The discharge 

 through pipes and overflow in one ye.ar (1860) amounted to 4,060,038 cubic meters, 

 while from the record of one rain gauge at the reservoir situated, therefore, at the 

 base of the elevations which rise to 1,000 feet above it, the rainfall was calculated 

 6,837,350 cubic meters. prol)ably on account of the position of the gauge an under- 

 statement, showing, therefore, that not less than one-third and probably as much as 

 cue-half of the rainfall had been retained in the soil or evaporated. 



On two days (July 28 aud 29, 1863) there fell on this area of 1,600 acres, according 

 to the rain gauge. 530,500 cub. m. of rain. Before the rain the reservoir was full to 

 high-water mark, and the overflow, the only means of discharge open, had been 

 delivering for days 3,936 m. or 45 liters per second, which must, therefore, be con- 

 sidered the natural discharge of the basin. 



After the rain the water level rose 8 inches and the discharge was: 



Cubic meters. 

 July 29, moi-ning and evening 30, 504 



30, morning and evening 28, 864 



31, morning and evening 7, 872 



Aug. 1, morning 7, 872 



2, evening 4, 920 



Total in live days 80,032 



At the end of the fifth day the water level had returned to its former height and the 

 discharge the next day was again 3,936 c. m., Avhich it maintained for three mouths. 

 If we deduct from the discharge of 80,032 c. m. the water that would have been dis- 

 charged during these five days Avithout the additional rainfall, namely (5 X 3,936=) 

 19,680, there remain 60,000 c. m. round, which without doubt were furnished by the 

 two storms, and since the total fall had been at least 500,000 c. m. it follows that 

 more than eight-ninths of the rain was absorbed and held by the soil to be delivered 

 gradually. 



In a neighboring basin, that of Salagou, of 10,701 acres extent, with 

 just 10 per cent wooded, also on impermeable rock (permian), bnt the 

 soil otherwise considerably washed and thin, observations could not 

 be carried on with precision. But while the discharge of this much 

 larger basiu in ordinary times Is calculated at not more than 20 to 25 

 liters per second, after a storm the discharge into the river is almost 

 immediate and has been observed to rise to more than 000 c. m. per 

 second. 



Of examples in this country we may give the following as coming 

 from good anthorities and well substantiated. 



Jn a report to the Chief of Engineers of the War Department in 



