Heat balance over saturated sphagnum moss

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Heat balance over saturated sphagnum moss

Williams, G. P.

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Reprinted from PROCEEDINGS O F T H E F I R S T C A N A D I A N C O N F E R E N C E O N MICROMETEOROLOGY: P A R T

I r

Meteorological Service of Canada

Toronto, Canada, 1967.

HEAT BALANCE OVER S A T U R A T E D SPHAGNUM MOSS

b

Y

G .

P .

W I L L I A M S

ABSTRACT

O b s e r v a t i o n s o f h e a t a n d m o i s t u r e e x c h a n g e w e r e made o v e r s a t u r a t e d Sphagnum m o s s i n a g r a s s - c o v e r e d a r e a i n 1 9 6 0 a n d i n a p e a t b o g n e a r O t t a w a d u r i n g t h e summer a n d f a l l o f 1964. The v a r i o u s com- p o n e n t s i n t h e e n e r g y b a l a n c e e q u a t i o n w e r e m e a s u r e d o r c a l c u l a t e d . A i r t e m p e r a t u r e o b s e r v a t i o n s w e r e made o v e r t h e p e a t bog a n d c o m p a r e d w i t h a i r t e m p e r a t u r e s r e c o r d e d a t a n e a r b y s t a n d a r d m e t e o r o l o g i c a l s t a t i o n . The r e s u l t s f r o m t h e s e m e a s u r e m e n t s a n d o b s e r v a t i o n s a r e a n a l y s e d a n d d i s c u s s e d . The e x p e r i e n c e g a i n e d i n m a k i n g t h e s e h e a t b a l a n c e m e a s u r e m e n t s w i l l b e u s e d i n p l a n n i n g h e a t b a l a n c e s t u d i e s a t m o r e i s o l a t e d l o c a t i o n s .

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HEAT BALANCE QVER S A T U R A T E D SPHAGNUM M O S S

b y

G. P. Williams

Snow and Ice Section, Division of

Building Research, Nat ional Research Counci

1,

Ottawa.

Vast a r e a s o f Canada a r e covered by " o r g a n i c t e r r a i n , " t h e

g e n e r a l term u s e d t o ' d e s c r i b e e x t e n s i v e p e a t d e p o s i t s c o v e r e d by Sphagnum moss and o t h e r v e g e t a t i o n . A r e c e n t r e v i e w o f t h e e n g i n e e r i n g problems a s s o c i a t e d with t h i s t y p e o f t e r r a i n i n d i c a t e d t h a t t h e r o l e o f v e g e t a t i o n and its i n f l u e n c e on t h e h e a t b a l a n c e a t t h e ground s u r f a c e w a r r a n t c o n s i d e r a b l e s t u d y ( 1 ) . I n 1959, Brown (2) i n i - t i a t e d ground s u r f a c e e n e r g y exchange s t u d i e s a t Norman Wells, N. W. T . , i n c o n n e c t i o n with t h e o c c u r r e n c e o f p e r m a f r o s t . I n t h e s e s t u d i e s , d i f f i c u l t i e s were e n c o u n t e r e d i n measuring, w i t h t h e i n s t r u m e n t s a v a i l a b l e , t h e exchange o f h e a t and w a t e r vapour between t h e ground s u r f a c e and t h e atmosphere. Because o f t h e s e d i f f i c u l t i e s , t h e problems o f measurement were i n v e s t i g a t e d a t Ottawa where t h e r e were b e t t e r f a c i l i t i e s f o r maintenance and s u p e r v i s i o n . The o b s e r v a t i o n s made on t h e components o f t h e h e a t b a l - ance and comments c o n c e r n i n g a p p a r a t u s and t e c h n i q u e s a r e p r e s e n t e d i n t h i s p a p e r .

During t h e summer and f a l l o f 1960, h e a t b a l a n c e o b s e r v a t i o n s were made i n t a n k s o f moss a t a g r a s s - c o v e r e d a r e a w i t h i n t h e grounds o f t h e N a t i o n a l Research Council i n Ottawa. I n 1964, o b s e r v a t i o n s were made i n a p e a t bog, c a l l e d t h e Mer Bleue, l o c a t e d a b o u t 6 m i l e s s o u t h e a s t o f t h e 1960 s i t e . ( F i g u r e 1 shows t h e gen- e r a l l o c a t i o n o f t h e two s i t e s r e l a t i v e t o t h e C i t y o f Ottawa.)

O b s e r v a t i o n s were made o v e r Sphagnum moss, t h e p r i n c i p a l veg- e t a t i v e c o v e r o f t h e Mer Bleue. I n 1960, s a m p l e s o f t h e moss, o b t a i n e d from t h e Mer Bleue, were p l a c e d i n two m e t a l t a n k s , 4 f t i n d i a m e t e r and 1 f t i n depth. The t a n B s were i n s t a l l e d i n t h e ground so t h a t t h e s u r f a c e was l e v e l with t h e s u r r o u n d i n g g r a s s cover. I n 1964, two t a n k s , 4 f t i n d i a m e t e r and 2 f t i n d e p t h , were f i l l e d with Sphag- num moss and i n s t a l l e d so t h a t t h e s u r f a c e was l e v e l w i t h t h e s u r r o u n d i n g o r g a n i c t e r - r a i n . I n both c a s e s , t h e moss was t r a n s f e r r e d t o t h e t a n k s i n l a r g e p i e c e s so t h a t it

was d i s t u r b e d a s l i t t l e a s p o s s i b l e .

At both s i t e s , t h e moss i n o n e o f t h e t a n k s was m a i n t a i n e d i n a s a t u r a t e d c o n d i t i o n . The w a t e r l e v e l i n t h e second t a n k was m a i n t a i n e d about 6 i n . below t h e s u r f a c e i n 1960, and a b o u t 1 2 i n . below t h e s u r f a c e i n 1964, s o t h a t t h e e v a p o r a t i o n l o s s from t h e s a t u r a t e d s u r f a c e c o u l d be compared with t h a t from an un- s a t u r a t e d s u r f a c e . A l l c a l c u l a t i o n s f o r t h e h e a t b a l a n c e were made f o r t h e s a t u r a t e d samples.

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1960

OBSERVAT t ONS

2. 1 N e t R a d i a t i o n

A Beckman and Whitley n e t radiometer was used t o measure n e t r a d i a t i o n i n t h e 1960 experiments. The radiometer was p o s i t i o n e d about 12 i n . above t h e s a t u r a t e d moss and connected t o a r e c o r d e r i n s i d e a h e a t e d building. Figure 2 shows t h e radiometer i n p o s i t i o n over t h e buried tank.

A s p r e v i o u s experiments (3) had i n d i c a t e d t h a t c a l i b r a t i o n c o n s t a n t s f o r n e t radiometers could be i n e r r o r , t h e c a l i b r a t i o n was checked by two d i f f e r e n t methods, d e t a i l s ofwhich can be o b t a i n e d from t h e author. A mean c a l i b r a t i q n c o n s t a n t of 9. 5 mv/cal cm-2 min-' was obtained and used i n t h e a n a l y s i s . T h i s agreed within 10 per c e n t with t h e c o n s t a n t s u p p l i e d by t h e manufacturer ( 1 0 . 3 mv/cal cm _'min).

2. 2 R a t e o f C h a n g e o f H e a t S t o r e d i n Moss

The change i n t h e amount of h e a t s t o r e d i n t h e moss i n t h e tank was estimated f o r a given period of time using t h e following equation:

where a

=

s p e c i f i c h e a t , calories/gm

O C

p

= d e n s i t y , gm/cc

T

=

mean temperature o f p e a t sample a t beginning of p e r i o d ,

O C

T 2

= mean temperature o f p e a t sample a t end o f p e r i o d , p e r i o d ,

OC

V

=

volume o f p e a t sample, cc.

'The mean temperatures were obtained by averaging h o u r l y v a l u e s from t h r e e thermocouples i n s t a l l e d a t 1, 6, and 12 in. below t h e s u r f a c e o f t h e moss. The thermocouples were connected t o an a p p r o p r i a t e r e c o r d e r .

The s p e c i f i c h e a t of s a t u r a t e d p e a t was assumed equal t o 1.0 cal/gm

O C and t h e d e n s i t y of a t y p i c a l sample o f s a t u r a t e d p e a t was found t o be

L 1 gm/cc.

,The change i n t h e h e a t s t o r e d i n t h e ground under t h e tank was astimated by measuring t h e h e a t flow through t h e bottom of t h e t a n k s with h e a t meters cemented t o t h e i n n e r s u r f a c e . The h e a t meters were c a l i b r a t e d i n p l a c e and t h e i r out- p u t s recorded. The h e a t flow through t h e i n s u l a t e d s i d e s o f t h e tank was considered q e g l i g i ble.

2. 3 E v a p o r a t i o n M e a s u r e m e n t

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i s used with t h e s t a n d a r d Class A evaporation pan. P l a s t i c s t i l l i n g wells, 4 i n . i n diameter, were p l a c e d i n t h e moss ( t h e top o f a s t i l l i n g well can be seen i n F i g u r e 2).

'lhe l e v e l o f water i n t h e s t i l l i n g well was determined by a p o i n t e r p l a c e d i n t h e c e n t r e of t h e well. A s t h e r e was f r e e passage of water through h o l e s i n t h e wall of t h e s t i l l i n g well, t h e water l e v e l i n t h i s well was assumed equal t o t h e water l e v e l i n t h e moss. A t r e g u l a r i n t e r v a l s , water was added t o t h e tank t o b r i n g t h e water l e v e l t o t h e top o f t h e p o i n t e r marker. 'lhe amount o f water was assumed equal t o t h e water l o s t by evaporation s i n c e t h e l a s t measurement.

I f t h e r e was r a i n during t h e p e r i o d preceding measurement, t h e t o t a l evaporation was assumed equal t o t h e amount o f r a i n f a l l measured with a standard r a i n gauge p l u s t h e amount of water added. I f t h e r a i n f a l l exceeded evaporation ( t h e water l e v e l was above t h e p o i n t e r ) , evaporation was n o t c a l c u l a t e d u n t i l , because of a d d i t i o n a l evaporation, t h e water l e v e l dropped below t h e l e v e l o f t h e p o i n t e r .

2. 4 The C o n v e c t i v e Component

?he h e a t balance equation can be s t a t e d a s follows:

where

Q,

=

n e t r a d i a t i o n

Q b

=

h e a t flow through bottom o f tank

Qe

=

e v a p o r a t i v e h e a t , n e g a t i v e when h e a t is used f o r evaporation

Q C

=

convective h e a t , considered p o s i t i v e when h e a t i s supplied t o t h e s u r f a c e from t h e a i r ( a i r i s warmer than s u r f a c e )

QS

=

r a t e of change of h e a t s t o r e d i n moss.

Values f o r t h e convective component were obtained by measuring a l l o t h e r components i n Eq. (2) and assuming t h e remainder equaled QC. Values o b t a i n e d by t h i s method were checked by comparing them with v a l u e s obtained by a second method. Ry assuming Bowen's r a t i o v a l i d and c a l c u l a t i n g v a l u e s f o r t h e r a t i o Q C / Q e , from a p p r o p r i a t e temperature and vapour p r e s s u r e measurements, Eq. (2) can be solved and second v a l u e s f o r QC obtained. A s a by-product o f t h i s second method a c a l c u l a t e d v a l u e f o r

Qe

i s o b t a i n e d which can be compared with t h e measured evaporation.

2 . 5 H e a t B a l a n c e

F i g u r e 3 i s a p l o t o f accumulated v a l u e s f o r t h e d i f f e r e n t components of t h e h e a t and moisture exchange f o r t h e p e r i o d 9 August t o 19 September

1960, t h e p e r i o d t h a t was analysed i n d e t a i l . I t i s seen t h a t n e t r a d i a t i o n i s about equal t o t h e h e a t a s s o c i a t e d with evaporation. The h e a t l o s t through t h e bottom o f t h e tank, t h e change i n h e a t content o f t h e moss, and t h e convective h e a t t r a n s f e r were

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small i n comparison with t h e n e t r a d i a t i o n .

The accumulated h e a t l o s s o r gain f o r t h e v a r i o u s components o f t h e energy balance f o r t h i s p e r i o d a r e summarized below.

C a l o r i e s P e r c e n t o f

/

s q cm N e t R a d i a t i o n

1. Net r a d i a t i o n + 10, 120

-

2. Evaporation

-

3. Heat s t o r e d i n moss

4. Heat flow out bottom of tank

5. Convection o r remainder

The average value o f Bowen's r a t i o , c a l c u l a t e d f o r t h e obser- vation p e r i o d from t h e values o f temperature and vapour p r e s s u r e shown i n Table 1, was 0.073. Using t h i s value f o r Bowen' s r a t i o , t h e convective l o s s was found t o be -734 c a l o r i e s / s q cm and t h e evaporative l o s s 9310 c a l o r i e s / s q cm.

2 . 6 Accuracy o f O b s e r v a t i o n s

When i n t e r p r e t i n g t h e r e s u l t s , t h e probable accuracy o f t h e measured values of t h e h e a t balance components need t o be considered. The change

i n

h e a t content o f t h e moss o r t h e h e a t flow through t h e bottom could have a l a r g e per-

centage e r r o r without appreciably a f f e c t i n g t h e balance obtained. The h e a t meters were s a t i s f a c t o r y f o r measuring t h e h e a t flow through t h e bottom o f t h e tank and t h e t h e r - mocouples were adequate f o r recording h e a t s t o r a g e changes i n t h e moss.

A comparison of measured evaporation from t h e s a t u r a t e d moss with evaporation from a nearby C l a s s A evaporation pan and from t h e u n s a t u r a t e d moss sample i n d i c a t e d t h a t t h e measurements were r e l i a b l e i f averaged over a p e r i o d o f two t o t h r e e weeks. The method o f measuring evaporation was n o t s a t i s f a c t o r y , however, f o r s h o r t p e r i o d s o f t h r e e o r f o u r days. Often i t was d i f f i c u l t t o judge a c c u r a t e l y t h e amount of water t o be added. I f too much was added, it was n o t p r a c t i c a l t o remove t h e excess a s i s done i n t h e c a s e o f t h e C l a s s A evaporation pan.

The accuracy o f t h e n e t r a d i a t i o n measurements was d i f f i c u l t t o determine. The Beckman and Whitley n e t radiometer appeared t o g i v e reasonable re- s u l t s , but t h e accuracy probably was n o t good enough t o be a b l e t o c a l c u l a t e t h e con- v e c t i v e component with confidence. For example, i f t h e m a n u f a c t u r e r ' s c a l i b r a t i o n o f 10.3 mv/cal cm-2 had been used f o r t h e radiometer i n s t e a d of 9. 5 mv/cal min, t h e n e t r a d i a t i o n would have equaled 9300 c a l o r i e s / s q cm, r e s u l t i n g i n a convective compo- n e n t o f +482 c a l o r i e s / s q cm, i n s t e a d of t h e n e g a t i v e value o f -338 c a l o r i e s / s q cm t h a t was obtained.

There was no way o f checking t h e accuracy o f t h e a l t e r n a t i v e method o f o b t a i n i n g t h e convective component. 'Ihe problems o f o b t a i n i n g r e l i a b l e measurements o f s u r f a c e temperature and o f vapour p r e s s u r e a t two l e v e l s , and t h e question of whether Bowen's r a t i o i s v a l i d under a l l c o n d i t i o n s , make t h e value obtained o f

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3. T H E

1964

O B S E R V A T I O N S

3. 1 A p p a r a t u s

The r a d i o m e t e r developed by t h e Commonwealth S c i e n t i f i c and I n d u s t r i a l Research O r g a n i z a t i o n i n A u s t r a l i a h a d g i v e n r e l i a b l e r e s u l t s and s o was chosen f o r t h e 1964 o b s e r v a t i o n s . F i g u r e 4a i s a p h o t o g r a p h t a k e n d u r i n g t h e i n s t a l l a - t i o n o f t h e t a n k with t h e Mer Bleue p e a t bog i n t h e background. F i g u r e 4b shows t h e CSIRO n e t r a d i o m e t e r i n p o s i t i o n o v e r t h e t a n k .

The CSIRO r a d i o m e t e r h a s a h o r i z o n t a l b l a c k p l a t e exposed t o incoming and o u t g o i n g r a d i a t i o n , which i s p r o t e c t e d from wind by two p o l y e t h y l e n e hem- i s p h e r e s . During t h e o b s e r v a t i o n s , t h e a i r i n s i d e t h e h e m i s p h e r e s was k e p t dry by c i r - c u l a t i n g a i r , which was p a s s e d o v e r a d r y i n g a g e n t , through t h e hemispheres. A s m a l l v i b r a t o r pump, such z s u s e d i n household aquariums, was u s e d t o pump t h e d r y a i r . By means o f a t i m i n g d e v i c e , t h e pump o p e r a t e d f o r a b o u t two m i n u t e s e v e r y h a l f h o u r . I n f u t u r e e x p e r i m e n t s some m o d i f i c a t i o n o f t h i s pumping r a t e might be d e s i r a b l e . The v i - b r a t o r pump would o p e r a t e f o r s e v e r a l weeks from two $ v o l t d r y - c e l l b a t t e r i e s .

The r a d i o m e t e r :vas c o n n e c t e d t o a s m a l l p o r t a b l e lrlervyn r e - c o r d e r powered by two % - v o l t d r y - c e l l b a t t e r i e s which would o p e r a t e t h e r e c o r d e r f o r a b o u t 10 d a y s t o 2weelrs. The r e c o r d e r was i n s t a l l e d i n a weather-proof box and wi.apped i n p o l y e t h y l e n e s h e e t i n g t o keep t h e c h a r t p a p e r d r y . T h i s p o r t a b l e r e c o r d e r proved t o be r e a s o n a b l y r e l i a b l e but would n o t b e s a t i s f a c t o r y f o r s t a t i o n s n o t v i s i t e d r e g u l a r l y by maintenance p e r s o n n e l .

E v e p r a ' c i o n was measured on t h e a v e r a g e o f a b o u t 3 t i m e s a

week by t h e s a n e t e c h n i q u e u s e d i n 1960. No s p e c i a l problems were zncount,ered o t h e r t h a n t h e d i f f i c u l t y o f d e t e m i n i n g e x a c t l y how much w a t e r t o add t o b r i n g t h e w a t e r l e v e l up t o t h e top o f t h e p o i n t e r , p a r t i c u l a r l y i n t h e u n s a t u r a t e d t a n k , where t h e w a t e r l e v e l was m a i n t a i n e d 1 2 i n , below t h e s u r f a c e .

C!mrges i n h e a t s t o r a g e i n t h e moss were e s t i m a t e d u s i n g t h c s m e v a l u e s f o r s p e c i f i c h e a t and d e n s i t y a s u s e d i n 1960. Ground t e m p e r a t u r e s ,:!isre measured with a r e c o r d i n g m e r c u r y - i n - s t e e l thermometer b u r i e d 8 t o 10 i n . b e i o a t h e s u r f a c e o f t h e moss. The t e m p e r a t u r e was r e c o r d e d with a clock-wound 7-day c h a r t r e - c o r d e r . Numerous c h e c k s on t h e ground t e m p e r a t u r e were made with maximum and minimum themilometers b u r i e d a t d i f f e r e n t l e v e l s . No s p e c i a l d i f f i c u l t i e s were e n c o u n t e r e d with t h e ground t e m p e r a t u r e measuremeats. I t wouid h a v e been u s e f u l t o have had more tern- p e r a t u r e measurements j u s t below t h e s u r f a c e o f t h e moss. Also, f o r more d e t a i l e d i n - v e s t i g a t i o n s , m e a s u r e a e n t s of d e n s i t y and s p e c i f i c h e a t a t d i f f e r e n t d e p t h s wow-ld h a v e been d e s i r a b l e .

F i v e p r e c i p i t a t i o n g a u g e s were u s e d t o measure r a i n f a l l . Three o f t h e s e were made o f c o p p e r , 12 i n . i n d i a m e t e r a n d w i t h t h e same r e l a t i v e p r o p o r t i o n s a s t h e s m a l l e r 4-in. s t a n d a r d r a i n gauge. Each o f t h e t h r e e l a r g e g a u g e s was i n s e r t e d o v e r t h e top o f a 5 - g a l l o n c o n t a i n e r wfiich was b u r i e d i n t h e moss s o t h a t t h e l i p o f t h e gauge was a b o u t 6 i n . above t h e l e v e l o f t h e s u r r o u n d i n g moss. Two s t a n d a r d p r e - c i p i t a t i o n gauges were i n s t a l l e d a b o u t 1 2 i n . above t h e moss s u r f a c e . The t h r e e l a r g e r

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gauges were i n s t a l l e d i n a row, a b o u t 50 f t a p a r t . The s m a l l e r gauges were i n s t a l l e d a t r i g h t a n g l e s t o t h e row o f l a r g e gauges, a b o u t 25 f t from t h e c e n t r e gauge.

A hygrothermograph was i n s t a l l e d on 2 1 August i n a Stevenson s c r e e n p l a c e d on t h e ground. On 11 September t h e s c r e e n was moved t o a h e i g h t 4 f t above t h e moss. The a c c u r a c y o f t h e t e m p e r a t u r e r e a d i n g s was checked a t i n t e r v a l s u s i n g maximum and minimum thermometers.

A groundwater gauge, c o n s i s t i n g o f a f l o a t and i n d i c a t i n g d i a l , was i n s t a l l e d on 8 September and t h e l e v e l o f t h e n a t u r a l w a t e r t a b l e r e a d a b o u t t h r e e t i m e s a week. T h i s a l l o w e d t h e c h a n g e s i n l e v e l o f t h e w a t e r t a b l e t o b e compared w i t h t h e e v a p o r a t i o n l o s s e s r e c o r d e d a t t h e s a t u r a t e d and u n s a t u r a t e d t a n k s . I h e equipment u s e d t o measure w a t e r t a b l e f l u c t u a t i o n s was q u i t e a d e q u a t e and, i f f i t t e d with a

clockwork r e c o r d e r , would r e c o r d h o u r l y o r d a i l y w a t e r t a b l e f l u c t u a t i o n s .

3. 2 Heat Balance Analysis

F i g u r e 5 shows a i r t e m p e r a t u r e , ground t e m p e r a t u r e , p r e c i p a t a - t i o n , and w a t e r t a b l e f l u c t u a t i o n s r e c o r d e d i n 1964. During t h e o b s e r v a t i o n p e r i o d . c o n d i t i o n s were q u i t e d r y and t h e d r a i n a g e c h a n n e l s a t t h e e d g e o f t h e bog were a l m o s t c o m p l e t e l y dry f o r s e v e r a l weeks. According t o l o c a l f a r m e r s , t h i s c o n d i t i o n was q u i t e u n u s u a l , s o t h a t t h e h e a t b a l a n c e measurements and c a l c u l a t i o n s a r e n o t r e p r e s e n t a t i v e o f a y e a r with a v e r a g e o r above- a v e r a g e m o i s t u r e c o n d i t i o n s .

F i g u r e 6 shows accumulated e v a p o r a t i o n , n e t r a d i a t i o n , and changes i n t h e h e a t s t o r e d i n t h e s a t u r a t e d moss f o r t h e p e r i o d a n a l y s e d . I h e h e a t u s e d f o r e v a p o r a t i o n of 16,600 c a l o r i e s / s q cm exceeded t h e r e c o r d e d n e t r a d i a t i o n o f 11, 200 by a l m o s t 50 _{p e r c e n t .} _{I h e d e c r e a s e i n t h e h e a t s t o r e d i n t h e moss was a b o u t} -600 c a l o r i e s / s q cm, l e s s t h a n 5 p e r c e n t o f t h e r e c o r d e d r a d i a t i o n . Although t h e a c t u a l a c c u r a c y o f t h e measurement o f t h e h e a t b a l a n c e components c a n n o t be s t a t e d , t h e r e i s no r e a s o n t o s u s p e c t t h e r e s u l t s . A com- p a r i s o n o f e v a p o r a t i o n l o s s e s from t h e s a t u r a t e d t a n k w i t h t h e u n s a t u r a t e d t a n k i n d i - c a t e d c o n s i s t e n t r e s u l t s . I h e p r e c i p i t a t i o n measurements were a l s o q u i t e c o n s i s t e n t . The t o t a l measured p r e c i p i t a t i o n f o r t h e t h r e e l a r g e p r e c i p i t a t i o n gauges, from 1 4 August t o 13 November was 4.94, 4. 91, and 5. 14 i n . and 5. 20 and 5. 26 i n . f o r t h e two s t a n d a r d gauges.

The C S I R O r a d i o m e t e r had p r o v e d r e l i a b l e i n t h e p a s t and a r e c e n t check on t h e c a l i b r a t i o n c o n s t a n t i n d i c a t e d t h a t i t was w i t h i n o n e p e r c e n t o f t h a t s u p p l i e d by t h e m a n u f a c t u r e r . During t h e p e r i o d a n a l y s e d , some radiation r e c o r d s were m i s s e d and it was n e c e s s a r y t o e s t i m a t e n e t r a d i a t i o n . M i s s i n g r e c o r d s f o r c l e a r days o r cloudy d a y s were e s t i m a t e d by u s i n g a v e r a g e v a l u e s o f n e t r a d i a t i o n measured d u r i n g cloudy o r c l e a r d a y s d u r i n g t h e same month. I f t h e weather c o u l d n o t be c l a s s i - f i e d a s c l e a r o r cloudy t h e n r a d i a t i o n was assumed equal t o t h e a v e r a g e o f a l l d a i l y r a d i a t i o n v a l u e s measured d u r i n g t h e month. I t was c o n s i d e r e d t h a t e r r o r s r e s u l t i n g from t h i s assumption would n o t be s u f f i c i e n t t o change a p p r e c i a b l y t h e r e s u l t s shown i n F i g u r e 6. For p e r i o d s when complete r a d i a t i o n r e c o r d s were a v a i l a b l e , t h e h e a t e q u i v a l e n t o f t h e measured evaporation was a l w a y s much g r e a t e r t h a n t h e measured n e t r a d i a t i o n .

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I f t h e measurements a r e assumed c o r r e c t , t h e c o n v e c t i v e component o b t a i n e d by t h e remainder method e m a l s

+

4 , 9 0 0 c a l o r i e s / s q cm o r a b o u t 30 p e r c e n t o f t h e h e a t r e q u i r e d f o r e v a p o r a t i o n . T h i s would i n d i c a t e t h a t on t h e a v e r a g e t h e s a t u r a t e d moss g a i n e d h e a t from t h e a i r and t h a t on t h e a v e r a g e t h e a i r t e m p e r a t u r e was wanner t h a n t h e s u r f a c e t e m p e r a t u r e o f t h e moss.

Ground t e m p e r a t u r e o b s e r v a t i o n s t h a t were a v a i l a b l e t e n d e d t o c o n f i r m t h a t t h e a v e r a g e s u r f a c e t e m p e r a t u r e o f t h e s a t u r a t e d moss was c o l d e r t h a n a v e r a g e a i r t e m p e r a t u r e . Maximum and minimum thermometers i n s e r t e d j u s t u n d e r t h e s u r - f a c e o f t h e s a t u r a t e d moss i n d i c a t e d t h a t d u r i n g t h e day t h e maximum t e m p e r a t u r e o f t h e wet s u r f a c e was n o t a s g r e a t a s t h e maximum a i r t e m p e r a t u r e , and d u r i n g t h e n i g h t minimum s u r f a c e t e m p e r a t u r e s were o f t e n c o l d e r t h a n minimum a i r t e m p e r a t u r e s .

The low ground t e m p e r a t u r e s were even more pronounced o v e r t h e u n s a t u r a t e d s u r f a c e , which was more r e p r e s e n t a t i v e o f t h e s u r r o u n d i n g t e r r a i n . On c l e a r n i g h t s t h e s u r f a c e t e m p e r a t u r e , a s measured with a minimum thermometer l a i d on t h e s u r f a c e , was a s much a s 4 t o 8 F d e g r e e s below t h e minimum a i r t e m p e r a t u r e 4 f t a b v e t h e ground.

F i g u r e 7 shows minimum and maximum t e m p e r a t u r e s measured a t 2 i n . and 8 t o 10 i n . below t h e s u r f a c e , compared with maximum and minimum a i r tempera- t u r e measured i n t h e S t e v e n s o n s c r e e n s p l a c e d on t h e moss s u r f a c e , 21 August t o 11 September. These r e s u l t s i n d i c a t e t h a t most o f t h e c o o l i n g o f t h e moss was c o n f i n e d t o a s h a l l o w s u r f a c e l a y e r l e s s than 2 i n , t h i c k .

3 . 3 Air Temperature O b s e r v a t i o n s

-- --

Because moss i s a p o o r t h e r m a l c o n d u c t o r , t h e r e l a t i v e l y s m a l l amount o f h e a t s t o r e d i n t h e moss d u r i n g t h e day c a n n o t be r e a d i l y r e t u r n e d t o t h e a i r d u r i n g n i g h t - t i m e c o o l i n g . The r e s u l t i s t h a t t h e ground s u r f a c e and t h e a d j a c e n t a i r l a y e r s c o o l r a p i d l y . Under s l m i l a r a t m o s p h e r i c c o n d i t i o n s , t h e s u r f a c e t e m p e r a t u r e of g r a s s - o r sand - c o v e r e d t e r r a i n would n o t f a l l a s much. In a d d i t i o n , e v a p o r a t i o n from wet moss would t e n d t o keep t h e moss s u r f a c e t e m p e r a t u r e l o w e r d u r i n g t h e day t h a n t h e t e m p e r a t u r e o f d r i e r s a n d o r g r a s s s u r f a c e s .

To show t h e d i f f e r e n c e s i n t e m p e r a t u r e t h a t can o c c u r , t h e maximum and minimum a i r t e m p e r a t u r e s r e c o r d e d a t Uplands A i r p o r t ( l o c a t e d a b o u t 8 m i l e s s o u t h w e s t o f t h e Mer Bleue) were compared with t h o s e r e c o r d e d a t t h e Mer Bleue. On t h e a v e r a g e , d u r i n g c l e a r c o n d i t i o n s , t h e n i g h t - t i m e minimum a i r t e m p e r a t u r e s were 10 t o 20 F d e g r e e s l o w e r a t t h e Mer Bleue s i t e , and d a y - t i m e maximum a i r t e m p e r a t u r e s were up t o 10 F d e g r e e s lower. During cloudy c o n d i t i o n s t h e d i f f e r e n c e s were l e s s , with d i f f e r e n c e s o f 5 t o 10 P d e g r e e s f o r minimum t e m p e r a t u r e s , and up t o 5 F d e g r e e s f o r maximum a i r t e m p e r a t u r e s . Monthly a v e r a g e a i r t e m p e r a t u r e s d u r i n g September and October were 5 t o 6 F d e g r e e s lower o v e r t h e bog. Some o f t h i s d i f f e r e n c e may b e a t - t r i b u t e d t o topography, a s t h e M e r Bleue i s a l o w - l y i n g a r e a i n comparison with Uplands

( s e e F i g u r e 1 ) . A i r t a n p e r a t u r e measurements t a k e n d u r i n g t h e w i n t e r , when t h e ground and moss were snow c o v e r e d , were s i g n i f i c a n t l y l o w e r o v e r t h e Mer Bleue.

The d i f f e r e n c e i n t h e c o o l i n g p a t t e r n o v e r Mer Bleue and Up- l a n d s i s d e m o n s t r a t e d i n F i g u r e 8, where a i r t e m p e r a t u r e and n e t r a d i a t i o n a r e s h o r n

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f o r a cloudy day ( 2 7 September) and a c l e a r day (28 September). E a r l y i n t h e e v e n i n g on t h e cloudy day t h e r a t e o f change o f a i r t e m p e r a t u r e was t h e same a t both s t a t i o n s . During t h e c l e a r n i g h t , t h e r a t e o f c o o l i n g was much more r a p i d o v e r t h e Mer Bleue.

3. 4 Local V a r i a t i o n s

One problem i n h e a t b a l a n c e s t u d i e s is t o d e t e r m i n e i f measurements o f n e t r a d i a t i o n , e v a p o r a t i o n , a n d h e a t s t o r a g e t a k e n a t a p a r t i c u l a r s i t e a r e r e p r e s e n t a t i v e o f c o n d i t i o n s o v e r t h e v a r i e d v e g e t a t i o n t h a t o c c u r s i n t h e bog. A l i m i t e d assessment o f t h i s problem was made d u r i n g t h e 1964 s t u d i e s .

Albedo v a r i a t i o n s were measured w i t h a p o r t a b l e Kipp r a d i o m e t e r a t 1100 t o 12130 h o u r s on a c l e a r day. The albedo v a r i e d from a low o f 11. 3 t o 12. 4 p e r c e n t f o r dead b l u e b e r r y bushes o v e r Sphagnum moss t o a h i g h o f 15. 3 t o 16. 2 p e r c e n t f o r b a r e Sphagnum moss.

V a r i a t i o n s i n n e t r a d i a t i o n were measured w i t h t h e CSIRO r a d i - o m e t e r on a c l e a r day ( 1 O c t o b e r ) a t 0900 h o u r s . Within a 1 0 - f t - d i a m e t e r c i r c l e , t h e n e t r a d i a t i o n was measured t o be 19. 2 c a l o r i e s / c m / h r o v e r dead b l u e b e r r i e s , 17. 2 o v e r s a t u r a t e d moss, and 14.7 c a l o r i e s / c m / h r where l i v e b l u e b e r r y bushes were t h e dominant v e g e t a t i o n .

The measurement o f s u r f a c e t e m p e r a t u r e o v e r o r g a n i c t e r r a i n p r e s e n t s a f o r m i d a b l e sampling problem. The t e r r a i n i s q u i t e h u m o c k y , with t h e crowns o f t h e h u m o c k s o f t e n b a r e and t h e h o l l o w s shaded by low-growing s h r u b s . S u r f a c e tem- p e r a t u r e measured with a thermocouple p r o b e , from 1100 t o 1200 h o u r s on 18 August, v a r i e d hetween 6 1 and 74O F.

4 CONCLUSIONS

O b s e r v a t i o n s o f h e a t and m o i s t u r e exchange o v e r s a t u r a t e d Sphagnum moss a t a g r a s s - c o v e r e d s i t e i n 198) showed t h a t t h e n e t h e a t g a i n e d a t t h e s u r f a c e by r a d i a t i o n was l a r g e l y l o s t by e v a p o r a t i o n . The change i n t h e amount o f h e a t s t o r e d i n t h e moss and c o n v e c t i v e h e a t t r a n s f e r were s m a l l f r a c t i o n s o f t h e t o t a l n e t r a d i a t i o n . S i m i l a r o b s e r v a t i o n s made i n 1964 o v e r Sphagnum moss a t a p e a t bog, l o c a t e d a b o u t 6 m i l e s from t h e 1960 e x p e r i m e n t s , i n d i c a t e d t h a t t h e h e a t r e q u i r e d f o r evapor- a t i o n exceeded t h e measured n e t r a d i a t i o n by almost 50 p e r c e n t . The change i n t h e h e a t s t o r e d i n t h e moss was l e s s t h a n 5 p e r c e n t o f t h e r e c o r d e d n e t r a d i a t i o n . The r e s u l t s i n d i c a t e t h a t on t h e a v e r a g e about 30 p e r c e n t o f t h e h e a t l o s t by e v a p o r a t i o n was g a i n e d from t h e a i r by c o n v e c t i o n d u r i n g t h e o b s e r v a t i o n p e r i o d . I t i s c o n s i d e r e d t h a t t h e e x p e r i e n c e g a i n e d i n making t h e s e h e a t b a l a n c e measurements w i l l prove v a l u a b l e i n p l a n n i n g s t u d i e s a t more i s o l a t e d l o c a t i o n s .

A comparison o f a i r t e m p e r a t u r e s measured a t t h e p e a t bog with t h o s e r e c o r d e d a t a s t a n d a r d m e t e o r o l o g i c a l s t a t i o n , l o c a t e d a b o u t 8 m i l e s from t h e bog, showed t h a t d a i l y maximum and minimum t e m p e r a t u r e s were u s u a l l y s e v e r a l d e g r e e s

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lower over t h e bog s u r f a c e . Brown (2) a l s o noted t h a t a i r temperature measured a t s e v e r a l d i f f e r e n t s i t e s over o r g a n i c t e r r a i n was s i g n i f i c a n t l y lower than measurements made a t t h e standard meteorological s t a t i o n atNorman Wells. These l i m i t e d o b s e r v a t i o n s thus suggest t h a t o v e r l a r g e a r e a s o f Canada (one e s t i m a t e gives t h e a r e a o f o r g m i c t e r r a i n i n Canada a s 500,000 square m i l e s ( 1 ) ) a i r temperatures a r e probably s i g n i f - i c a n t l y lower than t h o s e recorded a t standard meteorological s t a t i o n s u s u a l l y l o c a t e d a t a i r p o r t s o r urban c e n t r e s .

ACKNOWL EDGEMENTS

The w r i t e r i s indebted t o s e v e r a l of h i s colleagues in t h e Division o f Building Research f o r h e l p f u l d i s c u s s i o n during t h e p r e p a r a t i o n o f t h i s paper. The a s s i s t a n c e o f Mr. R. Amour i n p r e p a r i n g t h e experimental equipment and i n t a k i n g o b s e r v a t i o n s i s g r a t e f u l l y acknowledged.

This i s a c o n t r i b u t i o n from t h e Division o f h i l d i n g Research, National Research Council, Chnada, and i s published with t h e approval o f t h e D i r e c t o r of t h e Division.

R E F E R E N C E S

Pihlainen John A. 1963: A Review o f Muskeg and I t s Associated Engineering Problems. U. S. Army Materiel Command, Cold Regions Research and h g i - neering Laboratory, Technical Report 97.

Brown, R. J . E. 1965: Some Observations on t h e Influence of Climatic andTerrain F e a t u r e s on Permafrost a t Norman Wells, N. W. T.. Canada. Can. J of Earth S c i . . Vol,

a,

p . 15-31 (Reprinted a s NRC 8213).

Boyd, D. W.,, Gold, L. W . , and Williams. G. P. Radiation Balance During t h e Snow Melt Period a t Ottawa, Canada. Proceedings of t h e 1961 and 1962 Annual Meeting of t h e Eastern Snow Conference. (Reprinted a s NRC 7152).

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s c r e e n a i r t e m p e r a t u r e 4 f t a b o v e g r o u n d a e o b t a i n e d by a s s u m i n g a i r s a t u r a t e d a t t e m p e r a t u r e T s s e z h o u r l y a v e r a g e s f r o m d e w - c e l l r e a d i n g s o f dew p o i n t o f a i r a ( 4 f t a b o v e g r o u n d )

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F i g u r e 4a

Installing tank, Mer Bleue in background

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R e c o r d e r :

E. J . Truhl a r

Dr. J. R, Clements: I , t o o , had t a k e n t e m p e r a t u r e measurements a few i n c h e s above t h e s u r f a c e i n a bog - o n e t h a t was much s m a l l e r t h a n y o u r s , b e i n g about 213- ₃₀ a c r e s i n s i z e . There were l a r g e d i f f e r e n c e s i n t e m p e r a t u r e between t h e s e measurements and sim-

i l a r o n e s t a k e n i n nearby f o r e s t o p e n i n g s . However, when I t r i e d t o r e p e a t t h e work t h e n e x t y e a r , I had e n t i r e l y d i f f e r e n t r e s u l t s . The f i r s t y e a r was v e r y wet and t h e n e x t y e a r much d r i e r , markedly a f f e c t i n g t h e w a t e r l e v e l i n t h e bog. Do you have any o b s e r v a t i o n s o r i n f o r m a t i o n t h a t w i l l i n d i c a t e whether t h e r e l a t i v e i m p o r t a n c e o f t h e

" Q " t e r m s i n y o u r h e a t budget e q u a t i o n a r e a f f e c t e d by such d i f f e r e n c e s i n w e a t h e r from y e a r t o y e a r ?

G o P. Williams: We would e x p e c t t h a t d i f f e r e n c e s i n a i r t e m p e r a t u r e between t h e Mer Bleue and t h e m e t e o r o l o g i c a l s t a t i o n a t Uplands w i l l v a r y from y e a r t o y e a r , depending on w e a t h e r and ground s u r f a c e c o n d i t i o n s t h a t p r e v a i l . The 1964 measurements were made d u r i n g a d r y p e r i o d and t h e bog was t h u s abnormally dry. Normally t h e w a t e r t a b l e would be much h i g h e r and t h i s would a f f e c t e v a p o r a t i o n r a t e s . And t h u s t h e r e l a t i v e impor- t a n c e o f t h e c o n v e c t i v e t e r m mlght a l s o be d i f f e r e n t b e c a u s e t h e r e would n o t be such l a r g e d i f f e r e n c e s i n e v a p o r a t i o n between t h e s a t u r a t e d s u r f a c e s o v e r t h e t a n k and t h e s u r r o u n d i n g bog a r e a , and p o s s i b l y t h e a d v e c t i o n o f h e a t i n t o t h e a r e a o v e r t h e s a t - u r a t e d t a n k would be l e s s . However, t h e o n l y way such d i f f e r e n c e s can be d e t e r m i n e d with c o n f i d e n c e is t o make h e a t b a l a n c e measurements a t t h e s i t e o v e r a p e r i o d o f s e v e r a l y e a r s .

L.

IL

M a c t l a t t i e : How do you a c c o u n t f o r t h e l a r g e c o n v e c t i v e t e r n ? M e r e d i d t h e h e a t come from?

G . P. U'illiams: The h e a t b a l a n c e measurements were made o v e r a s a t u r a t e d t a n k o f sphagnum moss, with t h e w a t e r l e v e l a b o u t 2 i n c h e s below t h e s u r f a c e o f t h e moss. The l e v e l o f t h e w a t e r t a b l e i n t h e s u r r o u n d i n g t e r r a i n was a b o u t 12-15 i n c h e s below t h e s u r f a c e and t h e r a t e o f e v a p o r a t i o n was much l e s s t h a n o v e r t h e s a t u r a t e d t a n k . Thus t h e c o n v e c t i v e h e a t p r o b a b l y c a n e by a d v e c t i o n from t h e s u r r o u n d i n g t e r r a i n .

P r o f . k Brewer: What i s t h e g e n e r a l topography o f t h e p e a t bog and s u r r o u n d i n g t e r r a i n ?

G . P. Williams: We haven' t a t o p o g r a p h i c a l map o f Mer Bleue b u t i t i s r e p o r t e d t o b e a l m o s t l e v e l b u t s l i g h t l y h i g h e r i n t h e c e n t r e , l i k e an i n v e r t e d s a u c e r . The e l e v a t i o n o f t h e Mer Bleue i s j u s t s l i g h t l y lower than t h e e l e v a t i o n o f t h e w e a t h e r s t a t i o n a t Uplands ( s e e F i g u r e 1).

E. I. Mukammal: How d i d you measure t h e s u r f a c e t e m p e r a t u r e ?

G. P. Williams: We d i d n o t measure s u r f a c e t e m p e r a t u r e b u t o n l y t h e t e m p e r a t u r e a b o u t 2 " below t h e s u r f a c e . We u s e d a r e c o r d i n g m e r c u r y - i n - b u l b thermograph and checked

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. >

t h e s e measurements by u s i n g s t a n d a r d maximum and minimum thermometers b u r i e d i n t h e moss. One o f t h e r e a l problems o v e r t h i s t y p e o f t e r r a i n i s how do you measure a rep- p r e s e n t a t i v e s u r f a c e t e m p e r a t u r e i n a bog c o n t a i n i n g many i r r e g u l a r hummocks.

J. P. Bruce: How d i d t h e r a t e o f e v a p o r a t i o n from t h e s a t u r a t e d moss compare with t h e r a t e o f e v a p o r a t i o n from a water s u r f a c e ?

G . P. W i l l i a m s : For t h e p e r i o d we s t u d i e d , namely d u r i n g Aug.

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Nov. 1964, t h e r a t e of e v a p o r a t i o n from t h e s a t u r a t e d t a n k o f sphagnum moss was j u s t a b o u t t h e same a s t h e r a t e o f e v a p o r a t i o n from a S t a n d a r d C l a s s A t a n k l o c a t e d on t h e N a t i o n a l Research Council grounds a t Ottawa.

J. P. Bruce: Were C l a s s A pan measurements c o r r e c t e d f o r h e a t t r a n s f e r through t h e pan w a l l s ? a