PRECIPITATION IN A SPRUG£ STAND 117 



with the transpiration of the trees, which was measured during the 4 months 

 of May-August. At the site in question, appreciable transpiration occurs 

 only during this time. 



During the 4 experimental years, the mean annual precipitation at this 

 site was 793 mm of which 495 mm fell in September- April, and 298 mm 

 in May- August. The distribution of these quantities on the canopy, the top 

 portion of the soil and the root zone of the trees can be seen from Fig. i, 

 showing the distribution on and below the tree canopies. Fig. 2 shows the 

 distribution in the gaps between the trees. 



The crowns of the trees are effective chiefly through their strong intercep- 

 tion. This took about half of the precipitation, namely, 170 mm (57% of 

 298 mm) in May- August, and 257 mm (52% of 495 mm) in September- 

 April. It must, however, be pointed out that the crowns of the experimental 

 trees were well developed, and reached the ground or close to it. The surface 

 covered by the individual crown was minimally 14 m^ and maximally 56. 

 The quantity of needles belonging to each tree- which was measured and 

 calculated at the end of the experimental period -ranged from 53 to 204 kg. 



Actually, the interception of the crowns is greater than that shown in 

 Fig. I. To these figures should be added the precipitation screened off from 

 the gaps by the crowns; it is denoted by the values /and L in Fig. 2. 



The values given for interception are the total figures for several months. 

 If the interception on individual occasions is compared, varying values are 

 noted, since the quantity of intercepted water is determined by the precipi- 

 tation per time unit, and by the duration of precipitation. The quantity of 

 intercepted water may be shght-for example in a cloudburst. It may, on 

 the contrary, be great -approaching 100%-when the quantity of rain is 

 small. 



Interception in the top portion of the soil was also considerable. During 

 the summer months, almost half the precipitation reaching the gaps was 

 retained, and in September-April about one-fourth. 



The annual precipitation reaching the root zone below the canopy was 

 262 mm; 73 mm during the summer, and 189 mm in September- April. 



The water supply to the root zone in the gaps is somewhat greater. This 

 is because the precipitation is greater in the gaps than below the canopy. 

 Consequently, interception is greater in the gaps, and the water supply to 

 the root zone also greater. Of the 793 mm annual precipitation, altogether 

 3 16 mm reached the root zone in the gaps; 100 mm in May- August, and 

 216 mm during the rest of the year. 



The gaps occupied about the same area of ground as the part covered by 

 the crowns of the trees. The water supply to the root zone can therefore be 



