MEASUREMENT AND SIGNIFICANCE OF 

 THROUGHFALL IN FOREST STANDS 



E.R.C.P^YNOLDS &L.LEYTON 



Department of Forestry, Oxford University 



INTRODUCTION 



Much work is currently in progress attempting to measure the water 

 balance of vegetative covers, and very frequently, these investigations are 

 made on a comparative basis to obtain information relevant to the problem 

 of land use and water supply. However, to discriminate between vegetative 

 types in this way, it is essential that the various parameters of the water 

 balance should be measured by techniques which give data of adequate 

 and measurable precision. 



Some of these techniques have been examined in a woodland near 

 Oxford. In a small plantation of 1 7-year-old Picea abies (L.) Karst., a square 

 plot (side 140 ft) was marked out, and for sampHng purposes, a one-foot grid 

 superimposed; random positions within the stand were defmed by co- 

 ordinates drawn from tables of random numbers. The average distance of 

 one tree from another was 4| ft and the mean tree height at the beginning 

 of 1959 was 26 ft. Methods of estimating different phases of the water 

 balance (gross precipitation, throughfall, evaporation from the woodland 

 floor, the transpiration of the trees and the soil moisture) are being studied, 

 but only work on the first two have advanced sufficiently to report in this 

 paper. 



PATTERN OF THROUGHFALL 



Throughfall is defmed as the precipitation which reaches the ground under 

 the trees; included in this is stem flow, which is commonly measured 

 separately. 



A defmite pattern of throughfall distribution was detected by using 

 twenty gauges with smaU collecting areas (the British Meteorological 

 Office standard 5 in. diameter rain gauge), distributed at random within 

 the plot. This pattern can be represented by hnear regressions of catch on 

 distance from the stem, as shown in Table i. Evidently, the tree crowns 

 were sufficiently close to each other to allow for such simple Hnear regres- 

 sions despite the presence of a drip zone located near the edge of the 

 crown where somewhat higher throughfall is found than under gaps in 



