36 BATHYMETRICAL SURVEY OF 



of these lochs above sea-level is 412 feet, so that the mean height above 

 the sea of the entire catchment is about 1347-129 feet. 



The usual practice among engineers is to add 2J per cent, of rainfall 

 for each 100 feet of height above rain-gauges. Applying this rule to 

 the Loch Vennachar catchment-basin, where we have an observed rain- 

 fall of 75-37 inches at an average height of 528 feet, we must add 12-7 

 per cent, for the additional 508 feet of mean height, making an 

 average annual rainfall over the entire catchment of 84-94 inches. 

 This would give an annual fall of rain on the entire catchment equal 

 to 14,857,214,000 cubic feet. Applying this rule, in like manner, to 

 the Loch Lubnaig catchment-basin, where we have an observed rain- 

 fall of 76-25 inches at an average height of 538 feet, we must add 20 

 per cent, for the additional 809 feet of mean height, making an 

 average annual rainfall over the entire catchment of 91-5 inches. This 

 would give an annual fall of rain on the entire catchment equal to 

 15,600,760,000 cubic feet. 



There is another method of estimating the rainfall, without taking 

 the mean height of the drainage-area into consideration. Supposing the 

 usually accepted increase of 2J per cent, per 100 feet of height, and also 

 the mean annual rainfall at the average height of the rain-gauges, to be 

 approximately correct, it is possible to calculate the rainfall at any given 

 height. For the Loch Vennachar catchment the probable rainfall at 

 the same heights and intervals as the contour-lines on the Ordnance 

 Survey maps has been calculated from the starting-point of the mean of 

 the observing stations 75-37 inches at 528 feet. Thus at the surface of 

 Loch Vennachar the rainfall would be about 70*5 inches ; at 500 feet 

 above the sea, 75-2; at 750 feet, 79-9; at 1000 feet, 84-6 inches; and 

 so on, adding 6J per cent, for each succeeding interval of 250 feet. 

 Multiplying the area between any two consecutive contour-lines by 

 the mean of the two figures calculated for the same two lines should 

 give an approximation to the amount of rain falling on that area. The 

 result as obtained by this method for the entire catchment-basin flowing 

 out of Loch Vennachar is given in the following table : 



Cubic feet. 



Level of lochs to 500 feet, 16 - 53 square miles x 72'8 inches = 2,795,710,000 



500,, 750 10-67 x 77 '5 =1,921,117,000 



750 ,,1000 10-35 ., x 82-2 ,, =1,976,514,000 



1000 ,,1250 9-46 ,, x 86 '9 ,, =1,909,847,000 



1250 ,,1500 10-22 x 91 '6 ,, =2,174,874,000 



1500 ,,1750 7-86 x 96 -3 ,, = 1,758,47<>,<KX) 



1750, ,2000 ,, 5-94 ,, x 101 -0 =1,393,784,000 



2000, ,2250 ,, 3-06 x 1057 ,, 751,422,000 



2250, ,2500 ,, 0-99 x 110-4 ,, 253,917,000 



Over 2500 ,, 0"21 ,, x 115-1 ,, 56,154,000 



Total 14,991,815,000 



This result comes very near to that obtained from the calculation 



