138 E.R. C. REYNOLDS AND L.LEYTON 



measure incident precipitation above trees. Further work along these lines 

 may suggest the type of gauge subject to the smallest instrumental errors. 



INTERCEPTION 



When the water reaching the ground is subtracted from that incident upon 

 the canopy, a figure for interception is obtained. Under our particular 

 conditions, interception was generally about 30%, though occasionally as 

 low as 18% (see Table 6) and is of the same order as figures pubHshed 

 elsewhere for forest canopies (e.g. Delfs, 1955). Wider recognition is now 

 being given to the fact that interception losses rarely, if ever, should be 

 interpreted independently of transpirational losses. It is probable that there 

 is an inverse relationship between transpiration from the plant and simul- 

 taneous evaporation of intercepted water. Burgy and Pomeroy (1958) have 

 shown that for grasses, evaporation of intercepted water proceeds to an 

 equal amount, and instead of, the transpiration which would have taken 

 place if the foHage had not been wetted. On the other hand, other evidence 

 suggests that the intercepted water evaporates rather more rapidly than 

 that which would have been lost by transpiration. Nevertheless, it is ahnost 

 certain that transpiration is reduced while intercepted water is being 

 evaporated, and this is supported by consideration of energy relations. 



Graphs representing the relationship between incident rainfall and 

 throughfall (excluding stem flow) as in Fig. 3^4, have been extrapolated to 

 determine incident precipitation at zero throughfall. This value has been 

 termed 'canopy saturation', and under the present conditions, is in the 

 region of a twentieth of an inch. Consideration of stem flow (cf Fig. 3-B) 

 indicates that appreciably more rain must fall in the summer months before 

 water begins to trickle down the stems. However, the computation of 

 'canopy saturation' is very approximate, partly because of the high errors 

 associated with extrapolation, and partly because the relationship neces- 

 sarily departs from Hnearity with small storms, since the sUghtest showers 

 must penetrate canopy gaps. Inclusion of records for small storms in com- 

 puting the linear regression would therefore slew the line giving a smaller 

 figure for 'canopy saturation'. Curves of this type have also been used to 

 compute mean throughfall for the mean storm size (Wilm, 1943). 



CONCLUSION 



Though much useful information on the water cycle in a forest stand 

 might be gained by studies on isolated phases, it is clear that their inter- 



