Influence of intercepted water on transpiration and evaporation of Salix

Agricultural Meteorology, 23 (1981) 331--338

331

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

INFLUENCE OF INTERCEPTED WATER ON TRANSPIRATION AND EVAPORATION OF SALIX STIG LARSSON* Energy Forestry Project, The Swedish University o f Agricultural Sciences, S-750 07 Uppsala (Sweden)

(Received August 26, 1980; accepted September 22, 1980) ABSTRACT Larsson, S., 1981. Influence of intercepted water on transpiration and evaporation of Salix. Agric. Meteorol., 23: 331--338. Water sprayed on leaves of Salix caprea var. viminalis and S.viminalis covered about 30% of the leaf surfaces, mostly in form of drops. Leaves from S. caprea vat. viminalis intercepted up to 210g waterm -2, as an average for both sides of the leaves, while S. viminalis intercepted 150g waterm -2. Transpiration rates were reduced by 95% during the first 15 minutes after spraying. Subsequently, transpiration rates increased and evaporation rates decreased as the water on leaves disappeared. Water sprayed on the lower side of the leaves had the greatest influence on the reduction of transpiration. Relationships between transpiration, evaporation and amount of intercepted water were found, which gives support to a formula for calculating evapotranspiration, ET, from partially wet plant surfaces ET =

1-

To +

Eo

where To = transpiration from dry plant surfaces, E 0 -----evaporation from totally wet plant surfaces, C = actual intercepted amount of water, and S = interception capacity. About 50% of the intercepted water was indirectly "saved" through the reduced transpiration during the time that the leaves were wet. INTRODUCTION D u r i n g p r e c i p i t a t i o n , rain a n d m i s t w a t e r are i n t e r c e p t e d b y leaves a n d b r a n c h e s . In y o u n g stands o f c o n i f e r s in England, a b o u t 30% o f t h e a n n u a l p r e c i p i t a t i o n is i n t e r c e p t e d ( R u t t e r , 1 9 6 3 ; F o r d a n d Deans, 1 9 7 8 ) , a n d f o r a s p r u c e s t a n d in S w e d e n e v e n higher values h a v e b e e n r e p o r t e d (St~Ifelt, 1963). I t has b e e n s h o w n t h a t t h e e v a p o t r a n s p i r a t i o n f r o m w e t p l a n t surfaces can be several t i m e s larger t h a n t h e t r a n s p i r a t i o n a l o n e ( L e y t o n et al., 1967; R u t t e r , 1 9 6 7 ; T h o r u d , 1 9 6 7 ; W a g g o n e r et al., 1 9 6 9 ; M c N a u g h t o n a n d Black, 1 9 7 3 ; S t e w a r t a n d T h o r n , 1 9 7 3 ; S t e w a r t , 1977). F o r f o r e s t s a value o f e v a p o t r a n s p i r a t i o n 3 - - 4 t i m e s t h e t r a n s p i r a t i o n has b e e n q u o t e d . In c o n t r a s t t h e r e are r e p o r t s o f e q u a l w a t e r losses f o r b o t h w e t a n d d r y plants, especially f o r grasses (Burgy a n d P o m e r o y , 1958; McMillan a n d Burgy, 1 9 6 0 ; M c I l r o y a n d Angus, 1964). * Present address: Sval~f AB, 8-26800 Sval~v, Sweden. 0002-1571/81/0000---0000/$02.50 © 1981 Elsevier Scientific Publishing Company

332 Intercepted water has often been considered as water lost from the hydrological cycle (Waggoner et al., 1969}, because this water does not reach the soil but evaporates back to the atmosphere. If, however, the intercepted water reduces the transpiration of the plants, the interception indirectly improves the water status of the plant. The influence of intercepted water on transpiration has been discussed extensively in the literature (Goodell, 1963; Penman, 1963; Reynolds and Leyton, 1963; Jeffrey, 1964), but only in a few investigations has this relationship been determined quantitatively. Rakhmanov (1958) placed excised branches of aspen, oak, willow and pear in water bottles, applied water to the foliages, and measured transpiration losses. Compared to the controls, transpiration was reduced by between 12 and 26%. Schindel (1963) studied small potted red oak seedlings and observed a transpiration reduction of between 10 and 30%. Hart (1966), using Colorado blue spruce and Austrian pine, reported an average reduction in transpiration of about 6%. The average reduction of transpiration for Ponderosa pine was 14% (Thorud, 1967), and for white spruce 13% and eastern white pine 12% (Nicolson et al., 1968}. It is difficult from these investigations to see what the relationships are between interception storage and water losses from plants. In the present investigation transpiration and evaporation have been determined simultaneously during the time when the leaf surfaces were drying out. The effects on transpiration are discussed in relation to the covering of the leaves by water and to the stomatal processes. The practical effects of the transpiration reduction are also pointed out. MATERIAL AND METHODS Stem segments, about 7 cm long, were cut from young plants of Salix caprea var. viminalis (Q 666) and Salix viminalis (Q 683) grown in a greenhouse. The segments were taken about 40 cm from the tips of the shoots. Two leaves were left after the cutting and the segments were placed with their lower ends in beakers of water. All experiments were done in a climate chamber at a temperature of 15°C, vapour pressure deficit of 5 mb, radiation of 3 0 W m -2 and wind speed of 0 . 5 m s -1. Water was sprayed on both sides of the leaves or on either the upper or the lower side. The water losses were determined from repeated weighings with a balance placed outside the climate chamber. The beakers were sealed by a plastic foam to prevent evaporation from them. The transpiration rate was determined from weighings of the beaker of water after removal of the cutting, and the total evaporation from weighing both beaker and cutting together. The evaporation rate was calculated as the difference between total evapotranspiration and transpiration rates. The a m o u n t of intercepted water was determined by weighing the sprayed cuttings only. Great care was taken to prevent drops of water falling from the leaves.

333 RESULTS

Water sprayed on the leaves mostly formed drops, which implied that the leaf area was only partially covered. About 30% of the leaf surfaces were covered. There were large drops and wet sections on the upper sides and smaller drops on the lower sides. The leaves of S. caprea var. viminalis intercepted 2 1 0 g water m -2 , with one third of the water on the upper side, while S. viminalis intercepted 150 g water m -2 , with half the a m o u n t on the upper side. The water evaporated faster from S. viminalis than from S. caprea var. viminalis (Fig. 1), but after 3--4 hours almost all the intercepted water had gone from both types of leaves. If both sides of the leaves were sprayed the total evapotranspiration exceeded 100 g m -2 h -1 immediately after spraying, which is about double the rate of the transpiration from dry leaves (Fig. 2). The transpiration rate was reduced by 95% during the first 15 minutes after spraying, but increased subsequently as the surface water disappeared. The ratio between evaporation and transpiration rapidly changed during the time of drying out of the leaves. S. viminalis returned more rapidly to a high transpiration rate, as a result of the faster cycle of drying out. Figure 3 shows t h a t there were approximately linear relationships between transpiration, evaporation and the a m o u n t of intercepted water remaining on the leaves. The estimated a m o u n t of total transpiration reduction during the period leaf surfaces were wet or partly wet was found to be about 100 g m -2 leaf surface for S. caprea var. viminalis and about 7 5 g m -2 for S. viminalis. It means that about 50% of the m a x i m u m intercepted a m o u n t of water was "saved", i.e., n o t transpired, for both types of vegetation. i

i

20O

i 100

o" i

0

a 0

100

200

Time, min

Fig. 1. A m o u n t o f i n t e r c e p t e d water o n leaves o f Salix caprea var. viminalis (squares) a n d S. viminalis (circles) against the t i m e after spraying. W a t e r was s p r a y e d o n b o t h sides o f the leaves, and the c a l c u l a t i o n s w e r e m a d e per total leaf area. M e a n values o f f o u r determ i n a t i o n s w i t h the s t a n d a r d e r r o r o f the m e a n indicated.

334 - -

L

T

100\ E

~ so

\

~-~--

~

m

- -

0

50

100

Time, min

Fig. 2. Total evapotranspiration (full curves), evaporation (broken curves) and transpiration (dotted curves) from leaves of Salix caprea var. viminalis (squares) and S. viminalis (circles) against the time after spraying. Water was sprayed on both sides of the leaves, and the calculations were made per total leaf area. Mean values of four determinations with the standard error of the mean indicated.

100

E

O

-

50

~-tB

1

0

0

100 200 Intercepted water, gm -2

Fig. 3. Total evapotranspiration, evaporation and transpiration against the amount of intercepted water. Symbols as for Fig. 2. This figure is a combination of data mostly from Figs. 1 and 2.

Table I summarises the results of wetting u p p e r and lower sides separately, as well as b o t h sides. The effect on transpiration was much higher when the lower sides were wetted, than when the upper sides were wetted.

335 TABLE I Transpiration rates from Salix caprea vat. viminalis with and without water on the leaf surfaces. Mean values of six determinations with the standard error of the mean. Calculations of the transpiration for the wetted leaves are made during the first 30 minutes after spraying Transpiration from:

Transpiration rate ( g m - 2 h -1 )

Dry leaves Leaves with the upper side wetted Leaves with the lower side wetted Leaves with both sides wetted

50.5 32.8 10.0 9.9

+6.3 + 3.5 -+1.3 +1.8

DISCUSSION

Earlier investigations into the effect of intercepted water on transpiration rates have given highly variable results -- reductions in transpirations of 6 to 30% having been reported. This variation may be the result of differences in the experiments, e.g., in wetting the leaves and in determining the water losses. For example, in the experiments by Thorud (1967) and Nicolson et al. (1968) the plants were sprayed one to four times during each two-hour period, depending on whether or not the plant surfaces had dried o u t since the last spraying. This means that the a m o u n t of water on the leaves varied from 100 to 0% of the maximum interception during the experimental period. Rakhmanov (1958) sprayed his branches six to eight times a day, and weighed each bottle containing a branch once every 24-hour period to determine the transpiration rates. The values of transpiration reductions obtained from these investigations and others must therefore be average values and can be expected to be highly dependent on the experimental design. Even though the transpiration reduction seems to be directly related to the amount of intercepted water, it is impossible to explain the reduction in terms of the physical covering of the stomates alone, since only a b o u t 30% of the leaf surfaces are covered with water. The reduction must depend on a combination of factors including both increased stomatal resistance o f the leaves due to the covering of some of the stomatal openings by water drops, and a decreased water concentration deficit caused by a lower leaf temperature and a higher humidity adjacent to the leaf surfaces. Since these varieties o f Salix have stomata only on their lower sides, the reduction of 35% in transpiration, which occurred when the upper surfaces only were wetted (Table I), may be primarily due to the lowered temperatures o f the leaves. The humidity adjacent to the lower sides should not be increased very much by the upper side spraying, because the volume o f the climate chamber is large and the conditions are kept constant. Since both transpiration and evaporation seem to be dependent on the a m o u n t of intercepted water, the relative a m o u n t o f interception had to be

336 included as a variable in the formula to calculate evapotranspiration. From Fig. 3 it can be concluded that T =

I --

To

(1)

where T = transpiration from partially wet leaf surfaces, T O = potential transpiration from dry leaf surfaces, C = actual a m o u n t of intercepted water on both sides of the leaf, and S = maximum a m o u n t of intercepted water on both sides of the leaf. C

E -- ~ E o

(2)

where E -- evaporation from partially wet leaf surfaces, and E 0 = potential evaporation from totally wet leaf surfaces. If (1) and (2) are combined, we get ET

=

1---~

TO + S

o

(3)

where E T = evapotranspiration from partially wet leaf surfaces. This experiment gives experimental support for an earlier suggested equation, equivalent to eq. 3, for evaporation from partially wet plant surfaces given by Gash et al. (1979). Their equation has been used in a simulation model (SIM5T/12, a Model of Forest Transpiration and Interception), which was based on a model developed by Rutter et al. (1971, 1975), Rutter and Morton (1977). For these two varieties of S a l i x , both of which are used in energy forest plantations in Sweden, about 30% of the summer precipitation is intercepted (an average value from one-year old plantations, H. Grip, personal communication). More of the precipitation is probably intercepted in a mature, denser stand of S a l i x . Gash (1979) showed that in a stand of Scots pine, about 1.5 mm of rainfall was necessary to saturate the canopy. In a t w e n t y year old stand of Scots pine in England, Rutter (1963) showed that for a daffy precipitation of 5 mm, interception was 2--3 mm. Further increase in the daily precipitation was accompanied by only a slight increase in interception. If the leaf area index was 10 in a mature stand of S. c a p r e a var. v i m i n a l i s and both sides of the leaves are wetted, a precipitation of 4 mm would be totally intercepted. Most rainfalls in central Sweden give daffy totals of less than 5 mm (Taesler, 1972), which means that a great part of soil water supply to mature S a l i x stands must come indirectly through a reduction in transpiration. It is important to be able to predict how much of the intercepted precipitation is "saved", especially for those stands which are irrigated. Rutter (1967, 1968) suggested a m e t h o d for estimating the combined water loss from vegetation, which takes into account the separate contribution of

337

evaporated and transpired water. He argued that the additional evaporation consequent on interception, i.e., the net interception loss, CN, can be written as

This equation predicts that if To equals E0 as reported for grasses (Burgy and Pomeroy, 1958, McMillan and Burgy, 1960; McIlroy and Angus, 1964), there will be no net interception loss, because the same amount of water is evaporated as is "saved" by reduction in transpiration. Net interception loss increases as the ratio of evaporation to transpiration becomes greater, which occurs for taller vegetation such as forests and stands of Salix. Stewart (1977) estimated the "saved" water in Thetford Forest throughout 1975, when the total annual interception was 214 mm. The analysis showed that the rate of evaporation of intercepted water was 3.1 times the rate of transpiration under the same conditions. The result indicated a transpiration reduction of 6 9 m m and an interception loss of 1 4 5 m m , which agrees well with the assumptions made by Rutter (1967, 1968) in eq. 4. In the same way the present data agree with eq. 4, because for both varieties of Salix the evaporation from totally wet leaf surfaces doubled the transpiration rates from dry leaf surfaces, and the net interception losses were calculated as 50%. ACKNOWLEDGEMENTS

The author wishes to express his thanks to Drs. J.B. Stewart, W.S. Gaud, P.E. Jansson and C. Bengtson for their comments and helpful suggestions. REFERENCES Burgy, R.H. and Pomeroy, C.R., 1958. Interception losses in grassy vegetation. Trans. Am. Geophys. Union, 39: 1095--1100. Ford, E.D. and Deans, J.D., 1978. The effects of canopy structure of stemflow, throughfall and interception loss in a young Sitka spruce plantation. J. Appl. Ecol., 15: 905-917. Gash, J.H.C., 1979. An analytical model of rainfall i n t e r c e p t i o n by forests. Q.J.R. Meteorol. Soc., 105: 43--55. Gash, J.H.C., Lloyd, C.R. and Stewart, J.B., 1979. SIM5T/12 - - a model of forest transpiration and interception, using data from an automatic weather station. In: S. Halldin (Editor), Comparison of Forest "Water and Energy Exchange Models. International Society for Ecological Modelling, Copenhagen, pp. 173--184. Goodell, B.C., 1963. A reappraisal of precipitation interception by plants and attendant water loss. J. Soil Water Conserv., 18: 231--234. Harr, R.D., 1966. Influence of Intercepted Water on Evapotranspiration Losses from Small Potted Trees. Ph.D. thesis, Colorado State University, F o r t Collins. Jeffrey, W.W., 1964. Vegetation, water and climate: needs and problems in wildland hydrology and watershed research. Proc. Water Studies Inst. Symp., University of Saskatchewan, pp. 121--150.

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