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Energy balance at the surface of saturated peat moss
NATIONAL RESEARCH COUNCIL CANADA
D I V I S I O N OF B U I L D I N G RESEARCH
THE
EmERGY BALANCE AT THE SURFACE O F SATURATED PEAT MOSSby G.
P.
Williams Internal R e p o r t No. 223 o f the Division o f Building R e s e a r c h OT'PAWA June 1961PREFACE
S t u d i e s of t h e energy balance a t v a r i o u s n a t u r a l s u r f a c e s may seem f a r removed from b u i l d i n g r e s e a r c h , b u t many of t h e b u i l d i n g problems which a r e p e c u l i a r t o Canada a r i s e from t h e combination of t e r r a i n and c l i m a t e . Ground temperatures, which a r e determined by t h e s u r f a c e energy exchanges, o f t e n e n t e r i n t o problems i n v o l v i n g h e a t l o s s e s from b u r i e d s t r u c t u r e s , depth of annual f r o s t p e n e t r a t i o n under roads and s t r e e t s , changes i n a snow cover, and permafrost occurrence and degradation.
The p r e s e n t r e p o r t d e a l s with measurements of energy exchange a t t h e s u r f a c e of a wet moss cover. This i s of p a r t i c u l a r i n t e r e s t i n view of t h e a p p a r e n t h i g h i n s u l a t i n g value a t t r i b u t e d t o n a t u r a l moss covers
i n permafrost a r e a s . The a u t h o r , a r e s e a r c h o f f i c e r w i t h t h e Snow and I c e S e c t i o n , h a s a s p e c i a l i n t e r e s t
i n evaporative and o t h e r processes involved i n energy exchanges a t n a t u r a l ground, w a t e r , i c e and snow
s u r f a c e s .
Ottawa
THE ENERGY BALANCE AT THE SUlXFACE OF SATURATED PEAT MOSS
G.
P.
WilliamsThis r e p o r t i s concerned with measurement o f t h e d i f f e r e n t components of t h e energy balance equation a t t h e s u r f a c e o f s a t u r a t e d peat. F i e l d experiments were c a r r i e d out a t Ottawa during t h e summer of 1960 a t t h e r e q u e s t o f
t h e Northern Building Section of t h e Division o f Building Research. I t was considered t h a t t h i s s t u d y would provide experience with i n s t r u m e n t a t i o n which would prove valuable i n planning energy exchange s t u d i e s f o r t h e w i n t e r season a t Ottawa, and i n planning s i m i l a r experiments a t o t h e r s i t e s .
I n cooperation w i t h M r , Roger Brown of t h e Northern Building S e c t i o n , p e a t samples were obtained from t h e Mer Bleu p e a t bog l o c a t e d approximately 1 0 miles s o u t h e a s t of Ottawa, These samples were placed i n -hvo metal t a n k s ( 4 f t i n diameter, 1 f t i n d e p t h ) i n s t a l l e d i n t h e ground so t h a t t h e i r s u r f a c e s were l e v e l w i t h t h e surrounding n a t u r a l g r a s s cover. One t a n k was maintained i n a s a t u r a t e d c o n d i t i o n , whereas i n t h e second t a n k t h e water l e v e l was maintained
6 i n , below t h e surface. I n t h i s r e p o r t , Tank No. 1 w i l l r e f e r t o t h e s a t u r a t e d sample and Tank No. 2 t o t h e un-
s a t u r a t e d sample.
Because t h e r e was only one s u i t a b l e n e t radiometer a v a i l a b l e , a l l energy balance s t u d i e s were made w i t h t h e s a t u r a t e d sample. Although some comparisons o f evaporation r a t e s and thermal c o e f f i c i e n t s a r e made between t h e two t a n k s , t h i s r e p o r t i s p r i m a r i l y concerned w i t h t h e energy balance a t t h e surface of Tank No. 1. Some observations on t h e energy balance from a 1 0 - f t water t a n k a r e included i n o r d e r t o e v a l u a t e f u r t h e r t h e technique and i n s t r u m e n t a t i o n used i n t h i s t r i a l p r o j e c t .
The energy balance equation a t t h e evaporating s u r f a c e of t h e s a t u r a t e d p e a t sample, n e g l e c t i n g h e a t flow through t h e s i d e s of t h e t a n k , can be expressed a s :
where
Qn = n e t r a d i a t i o n , considered p o s i t i v e when n e t energy flows downward t o t h e s u r f a c e Qs = r a t e of change of h e a t s t o r a g e i n t h e sample,
considered p o s i t i v e when t h e s t o r e d h e a t i s decreasing and h e a t i s being supplied t o t h e evaporating s u r f a c e
Qb = h e a t flow through t h e bottom of t h e sample, considered p o s i t i v e when h e a t i s flowing upward t o t h e s u r f a c e
Qe = evaporative h e a t , negative when energy i s being used f o r evaporation
Qc = coavective h e a t , p o s i t i v e when h e a t i s
moving 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 t h a n s u r f a c e ) .
I n
t h e s e experiments, t h e components$,
Qs, Q b , Qe were measured. A s it i s m o s t d i f f i c u l t t o measure t h e convective h e a t component, Qc was obtained i n d i r e c t l y . The success of t h i s s t u d y depended on t h e accuracy w i t h which t h e s e components could be estimated and c o n s i d e r a b l e e f f o r t was expended i n c a l i b r a t i n g and checking t h e v a r i o u s instruments used.Figure 1 i s a diagram of t h e i n s t r u m e n t s and peat sample i n p l a c e ; Figure 2 c o n t a i n s two photographs of t h e f i n a l i n s t a l l a t i o n .
&,
-
Net r a d i a t i o n measurementPerhaps t h e most d i f f i c u l t p a r t of t h e program was t o measure t h e n e t r a d i a t i o n . Considerable d i f f i c u l t y had been experienced p r e v i o u s l y i n o b t a i n i n g c a l i b r a t i o n f a c t o r s f o r Schulze and f o r Beckman and Whitley n e t radiometers, t h e two instruments a v a i l a b l e f o r t h i s study.
The Beckman and Vilhitley radiometer was most s u i t a b l e f o r t h e f o l l o w i n g reasons:
(1) The Schulze radiometer c a s t a much l a r g e r shadow on t h e exposed s u r f a c e . This shadow e f f e c t must be consf dered f o r a r e a s t h e ' s i z e of t h e 4 - f t diameter tank.
( 2 ) P a r l e s s computation time i s r e q u i r e d t o o b t a i n t h e n e t r a d i a t i o n w i t h t h e Beckman and Whitley instrument.
( 3 ) During t h e summer months, t h e output of t h e Schulze sometimes exceeded t h e s c a l e of t h e r e c o r d e r used. The c a l i b r a t i o n c o n s t a n t of t h e Beclman and Whitley instrument was checked i n two ways: it was compared w i t h t h e Schulze which was used i n t h i s case a s t h e s t a n d a r d ; a f i e l d
method was developed and i s c a l l e d t h e "aluminum p l a t e technique". Dc-tails of t h i s second. method a r e given i n Appendix A. D e t a i l s
of t h e f i r s t method a r e presented i n a n unpublished r e p o r t by
M r . S. Tavares, a summer s t u d e n t w i t h t h e Snow and I c e S e c t i o n d u r i n g t h e summer of 1960.
For both methods of c a l i b r a t i o n one has t o choose a c l e a r day when t h e n e t r a d i a t i o n i s steady. Over t h e period J u l y t o September, 1960, t h e r e were v e r y few days i d e a l f o r c a l i b r a t i o n t e s t s . A s t h e most favourable c o n d i t i o n s occurred
4
August 1960, c a l i b r a t i o n r e s u l t s obtained on t h a t day a r e considered most r e l i a b l e . C a l i b r a t i o n f a c t o r s obtained a r e a s follows:Method I
-
Comparison w i t h Schulze= 9.4 mv/cal cm-2 min-I Method I1
-
Aluminum P l a t e TechniqueThe c a l i b r a t i o n f a c t o r f o r t h e Beckman and Whitley instrument used i n t h e a n a l y s i s was t h u s assumed t o be 9.5 rnv/cal cm-2 min-l, which a g r e e s w i t h i n 1 0 p e r c e n t with t h e manufacturer1 s c a l i b r a t i o n of 10.3 mv/cal-cm-2-min-1. The radiometer s u r f a c e was painted w i t h o p t i c a l b l a c k p a i n t i n June 1960 and might be one reason f o r t h i s d i f f e r e n c e . Q,
-
Rate of change of h e a t s t o r a g eThe r a t e of change of h e a t s t o r a g e i n t h e p e a t sample over a given period of time was obtained by using t h e following equation:
where
4
= s p e c i f i c h e a t c a l o r i e s / g m " ~p
= d e n s i t y-
-/ccT1 = mean temperature of peat sample a t beginning of period O C
T2 = mean temperature of peat sample a t end of period O C
V = volume of p e a t sample/sq cm ( c c )
The mean temperatures were obtained by averaging h o u r l y values from t h e t h r e e thermocouples i n s t a l l e d i n t h e
p e a t a s shown i n Fig. 1. 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°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 measured a t 1.1 gm/cc. It was e s t i m a t e d t h a t t h e weight of f r e e o r g r a v i t a t i o n a l w a t e r was over 70 p e r c e n t of t h e t o t a l weight of t h e sample. It was considered t h a t Qs could be o b t a i n e d by t h i s method w i t h s u f f i c i e n t accuracy f o r t r i a l experiments.
Q,
-
Measurement of h e a t flow through bottom of t a n k Keat flow through t h e bottom of t h e t a n k s was measured u s i n g Beckman and Whitley h e a t meters f a s t e n e d t o t h e i r i n n e r s u r f a c e s . The h e a t meters were c a l i b r a t e d by comparing t h e i r o u t p u t i n m i l l i v o l t s w i t h t h e hovm h e a t f l o w through a s l a b of r u b b e r under s t e a d y s t a t e c o n d i t i o n s . D e t a i l s of c a l i b r a t i o n and i n s t a l l a t i o n a r e given i n Appendix B.The c a l i b r a t i o n v a l u e s o b t a i n e d a r e compared w i t h
t h e manufacturer's c a l i b r a t i o n i n Pig.
3 .
They a r e c o n s i d e r a b l y h i g h e r and do n o t a p p e a r t o depend on t e m p e r a t u r e , a t l e a s t f o r t h e temperature r a n g e s checked. A f u r t h e r check was made by c a l c u l a t i n g t h e thermal c o n d u c t i v i t y of t h e p e a t samples, u s i n g t h e h e a t meter c a l i b r a t i o n v a l u e s and comparing t h e thermalc o n d u c t i v i t i e s obtained w i t h t h o s e r e p o r t e d by o t h e r i n v e s t i g a t o r s . D e t a i l s of t h e s e c a l c u l a t i o n s , i n c l u d i n g t h e assumptions made,
a r e a t t a c h e d a s p a r t of Appendix B.
The v a l u e s of thermal c o n d u c t i v i t y o b t a i n e d were i n r e a s o n a b l e agreement w i t h v a l u e s r e p o r t e d i n t h e l i t e r a t u r e .
It was considered t h a t t h e hea% meters gave r e s u l t s c o n s i s t e n t enough f o r Q b t o be measured w i t h s u f f i c i e n t
accuracy f o r t h e s e t r i a l experiments.
Qp
-
Evaporation measurementEvaporation was measured by t h e same technique a s i s used w i t h t h e s t a n d a r d C l a s s A e v a p o r a t i o n pan. P l a s t i c s t i l l i n g w e l l s 4 - i n . i n d i a m e t e r were placed i n t h e t a n k s a s shorvn i n Pig. 1, and t h e l e v e l of w a t e r i n them was determined by a p o i n t e r placed i n t h e c e n t r e . A s t h e r e was f r e e passage of w a t e r from t h e p e a t samples i n t o t h e s t i l l i n g w e l l , t h e w a t e r l e v e l s i n t h e s t i l l i n g w e l l and i n t h e samples were assumed equal.
The t a n k was f i l l e d each morning t o t h e t o p of t h e pointed marker. I t was assumed t h a t t h e amount of w a t e r
added e q u a l l e d t h e amount of w a t e r l o s t by e v a p o r a t i o n f o r t h e preceding 24 h o u r s . The volume of w a t e r added was measured w i t h a c o n t a i n e r graduated s o t h a t 1 i n . of w a t e r i n t h e
c o n t a i n e r equalled 1/100 i n . over t h e s u r f a c e of t h e 4 - f t
diameter tank.
If r a i n f e l l d u r i n g t h e 24-hr period preceding measurement, t h e amount of r a i n f a l l measured w i t h a standard r a i n gauge and t h e amount of water added w i t h t h e graduated c o n t a i n e r were t o t a l l e d t o give t h e t o t a l evaporation. I f t h e r a i n f a l l exceeded evaporation ( i f 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 e v a p o r a t i o n ) t h e water l e v e l dropped below t h e l e v e l of t h e p o i n t e r . I n t h i s c a s e , t h e evaporation r a t e obtained was f o r a period l o n g e r t h a n 24 hours, u s u a l l y 48 hours.
This method of evaporation measurement has s e v e r a l shortcomings. It 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 each day; furthermore, i f t o o much was added it was n o t f e a s i b l e t o remove t h e e x c e s s , a s i s possible i n t h e case of t h e Class A evaporation pan. During a period of r a i n t h e c a l c u l a t e d evaporation would be no more a c c u r a t e t h a n t h e recorded r a i n f a l l , which can be i n e r r o r , p a r t i c u l a r l y f o r summer showers accompanied by wind.
A d e t a i l e d a n a l y s i s of t h e evaporation measurements was made f o r t h e period June 24 t o J u l y 29, and t h e r e c o r d s a r e given i n Appendix C. This a n a l y s i s i n d i c a t e d t h a t t h e method of measuring evaporation was s a t i s f a c t o r y f o r 7 t o 4 day averages, but on a d a i l y b a s i s t h e measurements were n o t a c c u r a t e enough t o be used i n energy balance c a l c u l a t i o n s .
Qc
-
t h e convective componentThe d i r e c t measurement of Q t h e convective component, c
'
i s most d i f f i c u l t and was n o t attempted i n t h i s study. Two methods of o b t a i n i n g values f o r Qc were used. I n t h e first method, Q was obtained by measuring a l l o t h e r components i n
C
equation (1) and by assuming t h a t t h e remainder equaled Q c .
A s a l l t h e e r r o r s i n measuring
g,
Q b , Q,, Qe may be accumulated i n Qc, it r e q u i r e s v e r y a c c u r a t e measurements of t h e s e o t h e r components t o o b t a i n a r e l i a b l e value o f Qc.I n t h e second method a r e l a t i o n s h i p known a s t h e Bowens r a t i o i s assumed between Q and Qe. The expression i s :
where
Ts = s u r f a c e temperature O C
Ta = a i r temperature O C 4 f t above s u r f a c e
e = vapour pressure of a i r a t t h e s u r f a c e (mb)
S
= vapour pressure of a i r 4 f t above t h e
ea s u r f a c e (mb)
A s t h e sum of Qc + Qe can be obtained by measuring t h e o t h e r components i n equation ( I ) , t h e r e l a t i v e value of Qc and Qe can be estimated if Ts, T a , e s , ea a r e lmown.
From t h e a n a l y s i s , t w o values of Qc and Qe were t h u s obtained.
Q c l
r e f e r s t o t h e value of Qc c a l c u l a t e d by t h e f i r s t method, and Q r e f e r s t o t h e value of Qe t h a t was
e
,
Iobtained by measurement. and Q r e f e r t o v a l u e s obtained e2
by u s i n g Bowen's r a t i o , B y comparing v a l u e s obtained by t h e s e two methods t h e amount of confidence t h a t could be placed i n t h e r e s u l t s of t h i s energy balance a n a l y s i s was a s s e s s e d ,
ENERGY BALANCE ANALYSIS S a t u r a t e d Peat
Figure 4 i s a g r a p h i c a l p l o t of
3-
t o 4-day average 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 energy balance f o r August 9 t o September 1 9 , 1960, t h e period analyzed i n d e t a i l , These values a r e given a l s o i n Table I . I t may be seen t h a t t h e h e a t from t h e r a d i a t i o n absorbed a t t h e s u r f a c e i s l a r g e l y balanced by t h e h e a t r e q u i r e d f o r t h e observed evaporation. The h e a t l o s t through t h e bottom of t h e t a n k , t h e change i n h e a t content of t h e moss, and t h e convective h e a t t r a n s f e r component a r e small f r a c t i o n s of t h e t o t a l r a d i a t i o n energy received during t h i s period.The accumulated energy l o s s e s o r g a i n s f o r t h e v a r i o u s components of t h e energy balance f o r t h i s period a r e summarized a s follows:
C a l o r i e s Per Cent of T o t a l Radiation
The accumulated v a l u e s of t h e convective and
evaporative components, u s i n g Bowenfs r a t i o , a r e a s f o l l o w s : Qe2 = -9341 c a l (92.3 p e r c e n t of t o t a l r a d i a t i o n ) Qc2 =
-
700 c a l (6.9 p e r c e n t of t o t a l r a d i a t i o n ) Values of t h e f a c t o r s r e q u i r e d t o c a l c u l a t e Bowenfs r a t i o , i n c l u d i n g d e t a i l s of how t h e r e c o r d s were o b t a i n e d , a r e presented i n Table 11.Although t h e accumulated average v a l u e s of Q and el Q c compare reasonably w e l l w i t h Qe and Qc
,
t h e r e s u l t s a r e1 3 C )
I L L
n o t s o s a t i s f a c t o r y i f 3- t o 4-day averages a r e compared. Figure
5
shows t h e 3- t o 4-day average v a l u e s o f Qc and Q1 el
compared t o Qc and Qe
.
I n a d d i t i o n , v a l u e s of Bowen's r a t i oQ c l 2 2 (To
-
T a )R1 =
%
i s compared w i t h R2 = 0.61 es-
ea f o r3-
t o $-dayI
periods. On a 3- t o 4-day b a s i s t h e r e i s c o n s i d e r a b l e d i f f e r e n c e between R1 and R2 and t h u s c o n s i d e r a b l e d i f f e r e n c e between Q,
1
and Qc
,
and Qe,
and Q~.
2 1 2
A s t h e d i f f e r e n c e between Q and Qc i s of t h e
C1 2
same magnitude a s v a l u e s of t h e convective component, n o t much confidence can be placed i n t h e value of Qc obtained by e i t h e r c a l c u l a t i o n f o r 3- t o 4-day periods. Since t h e v a l u e s f o r t h e evaporation component, however, a r e l a r g e i n comparison w i t h t h e d i f f e r e n c e between Qe and Qe
,
more confidence can be1 2
I n i n t e r p r e t i n g t h e s e r e s u l t s , t h e probable
accuracy of t h e measured v a l u e s of t h e energy balance components should be considered. The change i n h e a t c o n t e n t of t h e moss o r t h e h e a t flow through t h e bottom could be i n e r r o r by a l a r g e p e r c e n t without a f f e c t i n g a p p r e c i a b l y t h e balance obtained. I f however, e i t h e r t h e measured evaporation o r measured r a d i a t i o n a r e i n e r r o r , t h e f i n a l balance would be a f f e c t e d considerably.
For example, if t h e manufacturer's c a l i b r a t i o n of 10.3 mv/cal-crn-2 min-I i s used f o r t h e Beckman and Whitley radiometer i n s t e a d of 9.5 mv/cal-cm-* min-l, t h e following balance may be obtained f o r t h e period August 9 t o September 1 9 , 1960:
C a l o r i e s
Under t h e s e circumstances Q i s p o s i t i v e , +482
C
c a l o r i e s compared t o a negative value of -338 c a l o r i e s
obtained i n t h e f i r s t balance. A s t h e mean s u r f a c e tempera- t u r e of t h e p e a t , T,, was g r e a t e r than t h e mean a i r tempera- t u r e , Ta, over t h e period analyzed, h e a t should be t r a n s f e r r e d from t h e surface t o t h e a i r by convective a c t i o n , i . e . , t h e negative value appears t o be t h e most l i k e l y value f o r Qc. I t
i s evident t h a t t h e r a d i a t i o n temn has t o be measured a c c u r a t e l y if t h e convective term i s t o be determined by t h e energy balance method
.
F h e r a ~ Balance of 1 0 - f t Water Tank
On September 30, 1960, t h e radiometer was moved over a shallow 1 0 - f t diameter water t a n k i n s t a l l e d a t t h e Ottawa s i t e . An energy balance was attempted f o r a water s u r f a c e , using t h e same g e n e r a l techniques a s were used t o o b t a i n t h e energy balance over t h e p e a t sample. The r e s u l t s from t h i s study (Table 111) a r e presented i n t h i s r e p o r t because t h e y demonstrate t h e problem of o b t a i n i n g an energy balance when t h e n e t r a d i a t i o n and evaporation v a l u e s a r e comparatively small.
During t h e period October 1 t o 31 t h e c a l c u l a t e d and measured energy balance components gave reasonable values
-
a s t h e measured value of Qe agreed w i t h i n 10 per1
c e n t with t h e value Qe obtained by assuming Bowenfs r a t i o 2
v a l i d .
During t h e period November 1 t o 28 t h e measured evaporation d i d n o t agree with t h e value f o r evaporation
obtained by assuming Bowenfs r a t i o v a l i d . During t h i s p e r i o d , t h e evaporation measurements were n o t considered v e r y r e l i a b l e a s r a i n f a l l was f r e q u e n t during November. It should a l s o be pointed out t h a t a n e r r o r of 500 c a l o r i e s i n t h e n e t r a d i a t i o n would be l a r g e r t h a n t h e convective term, and about one h a l f t h e value o f t h e evaporative term. An e r r o r o f 500 c a l o r i e s i n n e t r a d i a t i o n f o r t h e period August 9 t o September 1 9
would have r e s u l t e d i n only a 5 p e r c e n t e r r o r i n t h e c a l c u l a - t e d evaporation, and would n o t have changed t h e g e n e r a l con- c l u s i o n s reached i n t h a t a n a l y s i s .
Prom December 7 t o
1 4 ,
an energy balance was attempted on t h e 1 0 - f t t a n k during a period of i c e formation. A sevaporation was n o t measured, t h e c a l c u l a t e d evaporation r a t e could n o t be compared with a measured evaporation r a t e . Under t h e s e circumstances n o t much confidence can be placed i n t h e r e s u l t s , a s an e r r o r i n r a d i a t i o n of 500 c a l o r i e s would be twice t h e sum of Qe + Qc and almost h a l f t h e energy used i n i c e growth. I t i s e v i d e n t t h a t when t h e n e t r a d i a t i o n term i s of t h e same magnitude a s t h e o t h e r terms i n t h e energy balance equation, it must be measured a c c u r a t e l y i f t h e o t h e r terms i n t h e equation a r e t o be determined by t h e energy
balance method.
GENERAL
DISCUSSIONThe major conclusions reached i n t h i s t r i a l p r o j e c t a r e summarized a s follows:
Measurement of n e t r a d i a t i o n p r e s e n t s t h e most d i f f i c u l t problem f o r a s t u d y o f t h i s kind. The Beckman and Whitley radiometer appears t o be adequate i f t h e magnitude of
t h e n e t r a d i a t i o n term i s l a r g e compared t o p o s s i b l e e r r o r s and i f measurements a r e averaged over a period of s e v e r a l days. V e n t i l a t e d type n e t radiometers a r e s u b j e c t t o e r r o r s , some of which a r e d i s c u s s e d by Suomi and o t h e r s (1).
( 2 ) Heat meters appear t o be a c c u r a t e enough f o r determining h e a t flow through t h e bottom of a tank. Since t h e y
t o analyze r e s u l t s , however, c o n s i d e r a t i o n should be given t o o t h e r methods o f o b t a i n i n g h e a t flow. Heat flow from t h e bottom need n o t be c a l c u l a t e d a s a s e p a r a t e item. Appropriate thermocouples can be
i n s t a l l e d and v a l u e s assumed f o r t h e t h e r m a l c o e f f i c i e n t s of t h e t e s t sample; from t h e s e o b s e r v a t i o n s t h e t o t a l
rate-of-change of h e a t s t o r a g e may be e s t i m a t e d a c c u r a t e l y enough f o r t h i s type of experiment.
( 3 ) The means of measuring e v a p o r a t i o n a p p e a r t o be adequate
f o r t h i s type of f i e l d study. Since a c c u r a c y of evapora- t i o n measurements depends on t h e a c c u r a c y w i t h which
p r e c i p i t a t i o n can be measured, t h e method. of measuring r a i n f a l l should be improved. I t might be p o s s i b l e t o i n s t a l l a metal t a n k of t h e same a r e a a s t h e t e s t samples t o measure r a i n f a l l , o r a s u i t a b l e network of s t a n d a r d r a i n gauges could be l o c a t e d around t h e t e s t a r e a .
( 4 ) If t h i s type of experiment i s t o be c a r r i e d o u t i n a p e a t bog t h e thermal and r a d i a t i o n a l p r o p e r t i e s of t h e surround-
i n g t e s t a r e a should be a b o u t t h e same a s t h e c o r r e s p o n d i n g p r o p e r t i e s of t h e t e s t sample. For example, it i s impor- t a n t t h a t t h e albedo of t h e t e s t s u r f a c e be a b o u t e q u a l t o t h e albedo of t h e s u r r o u n d i n g a r e a . Otherwise, t h e n e t r a d i a t i o n absorbed w i l l n o t be t h e same and t h e measured e v a p o r a t i o n w i l l . n o t be t y p i c a l of e v a p o r a t i o n
o v e r t h e t e s t a r e a . I n a d d i t i o n , t h e t e s t sample would be surrounded by a n a r e a w i t h perhaps q u i t e d i f f e r e n t e v a p o r a t i v e and convective boundary c o n d i t i o n s . I t may be d i f f i c u l t t o e v a l u a t e t h e e f f e c t of t h e d i s c o n t i n u i t y i n t h e boundary c o n d i t i o n s on t h e numbers which t h e
o b s e r v a t i o n s y i e l d .
( 5 ) I n a s t u d y of t h i s k i n d , t h e b e s t hope f o r s u c c e s s i s t o use f a i r l y long-term a v e r a g e s . With t h e a v a i l a b l e
i n s t r m e n t a t i o n it was impossible t o g e t c o n s i s t e n t r e s u l t s on a 24-hour b a s i s o r even on a 3- t o 4-day b a s i s . . I t h a r d l y seems j u s t i f i a b l e t h e r e f o r e , t o r e c o r d and analyze h o u r l y v a l u e s of a i r t e m p e r a t u r e , r a d i a t i o n and s o i l temperature. I n s t e a d , a s i m p l i f i e d approach t o t h e problem of measuring t h e d i f f e r e n t terms i n t h e energy balance e q u a t i o n m i g h t be c o n s i d e r e d where- by l o n g e r term a v e r a g e s of p r e c i p i t a t i o n , e v a p o r a t i o n , changes i n h e a t s t o r a g e , and r a d i a t i o n a r e used. For example, when t h e p e r i o d s e t f o r t h e b a l a n c e c a l c u l a - t i o n s i s one week o r more, r a d i a t i o n i n t e g r a t o r s t h a t r e a d t h e t o t a l r a d i a t i o n t o
-
+ 5 p e r c e n t would be extremely u s e f u l .( 6 ) Perhaps t h e most i m p o r t a n t l e s s o n l e a r n e d , and t h i s a p p l i e s e s p e c i a l l y t o t h e c a s e where a number of o b s e r v a t i o n s a r e t o be made and r e c o r d e d , i s t h a t
a n a l y s i s must proceed concurrently w i t h observations. I n t h i s way, observational e r r o r s can be d e t e c t e d and corrected before they s e r i o u s l y a f f e c t t h e purpose of t h e p r o j e c t . Furthermore, t h e experimenter can o b t a i n a r e a l i s t i c a p p r e c i a t i o n of t h e problems while t h e study
i s under way r a t h e r than a f t e r t h e observations have been completed.
The w r i t e r i s indebted t o s e v e r a l members of t h e Division of 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 preparation of t h i s r e p o r t . The a s s i s t a n c e of M r . S.
Tavares, M r . R. G. Brown and M r . R. Amour i n preparing t h e experimental equipment i s g r a t e f u l l y acknowledged.
1. Suomi, V. E. and o t h e r s . An Improved Net-radiation
Instrument. Journal of Meteorology, Vol. 2 , No. 4 ,
Aug. 1954, p. 276
-
282.2. Geiger, R. The Climate Near t h e Ground. Harvard Univer- s i t y Press, 1950, 482 p.
TABLE I
ENERGY BALANCE COMPONENTS SATURATED 4-F'T PEAT MOSS AUGUST
9
TO SEPTEMBER19, 1960
P e r i o dI
Aug. 9-
12
"
12
-
15
"
15
-
19
'
I
19
-
22
It2 2 - 2 6
"
26
-
29
29
- S e p t , 2 S e p t .2
-
6
"
6 -
89 - 1 2
"
12
-
16
16
-
19
Qn Qe Qs Qb Qc Net R a d i a t i o n +332
+314
+
381
E v a p o r a t i o n-
264
-
278
-
296
H e a t S t o r a g e-
7
+9
+ 1 r a d i a t i o n r e c o r d s m i s s i n g +30
-
27
-
9
+19
-
28
-
33
+57
-
23
+
286
+336
+235
+202
+371
+
228
+151
+
188 H e a t Out Bottom-
3
-
6
-
7
-
266
-
318
-
247
-
225
-
382
-
218
-
199
-
150
C o n v e c t i o n-
58
-
39
-
79
-
5
-
4-
5
-
5
-
8 +3
+3
+4
-
45
+13
+
26
+9
+47
+20
-
12
-
19
TABLE I1
CALCULATION OF BOWEN'S R A T I O FOR OBSERVATION PERIODS
Ts = obtained from h o u r l y averages of thermocouple r e a d i n g s
4
i n . under s u r f a c e of p e a tTa = thermocouple
-
i n s m a l l s c r e e n 4 f t above groundPeriod
Aug. 9
-
1 2 12-
1 5
1 5
-
1 9e = obtained by assuming t h e a i r was s a t u r a t e d a t t h e temperature of t h e s u r f a c e ( e s - e "mb 10.6 8.7 9.4 (Is
-
T o + 1.36-
0.50+
1.20e, = obtained from h o u r l y averages f r o m dew-cell r e a d i n g s of dew-point R = 0.61
-
(aT
P e+
0.08-
0.04+
0.08 1 9-
22 22-
26 26-
29 29 -Sept.2 Sept. 2-
6 6 - 8 9-
12 1 2-
16 16-
1 9 n o t used-
r e a d i n g s u n r e l i a b l e + 0.21-
0.10+
0.02+
0.16-
0.05+
0.17 + 0.24+
0.16 + 3-30-
1.50+
0.28+
1.80-
0.72+
1.60+
1.30 + 2.50 9.7 9.0 8.4 6.7 8 . 4 5.7 3.4 9.4TABLE I11
BEST ESTIIVLATE OF
ENERGY
BALANCE 10-FT TANK OF WATER OTTAWA 1960 Qn n e t r a d i a t i o n-
1180 ( w a t e r ) change i n Qs s t o r a g e energy Qb h e a t flow o u t bottom of t a n k 9, energy i c e growth Qe + 'c measured Q e l Q remainder C1 R Bowenfs r a t i o + 7 1 0 0+
2251-
2020-
231 + 113+
28 0 +5 1
-
940 + 9 9 1I
+
.16 + 51+
7
+ 1360-
238-
-
-
-067-
4 +55
Qe2 c a l c u l a t e d 'c2 c a l c u l a t e d+
2.01-
79
-
153-
1890-
361ROUND EVEL
/-
BECKMAN 8 WHITLEY NET RADIOMETEREVAPORATION CONNECTIONS
RECORDERS SS APPROACH
CO NDI TlONS INSULATION
I IN. CRUSHED ROCK AWA SAND
FIGURE
1
FIGURE 2 'PPVo views of p e a t sample w i t h Net
Radiometer i n p o s i t i o n and Evaporation Gauge i n foreground.
E
\ 6 0 C IC wg
5 0f
=I
4 0 C m cf o 3 0 I- 0 2 0z
0F
l o ae
rn-
0 3 0 4 0 5 0 6 0 7 0 80 9 0 100 110a
UHEAT METER TEMPERATURE, O F
I
-
UNSATURATED-
- 0 0- 00-
-
-
0 -I
. - Y i v u F A c T u R E R-
CALIBRATION 1955-
-
1
1
I
1
I
I
I
A1
I
1
-
-
-
-8
8
--
-
----4---.---
CALIBRATION 1955-
-
I
I
I
0HEAT METER TEMPERATURE, OF
F I G U R E
3
CALIBRATION CURVES FOR B E C K M A N AND
W H I T L E Y H E A T M E T E R S
- 3 0 0 9 12 15 19 2 2 26 2 9 2 6 8 12 16 19 A U G U S T S E P T E M B E R E V A P O R A T I O N
(-1
H E A T O U T B O T T O M C O N V E C T I O NFIGURE 4
I
I
I
I
I
I
- --
B O W E N R A T I O-
--
- --
- - --
- II
-
FIGURE 5
COMPARISON OF MEASURED VALUES RI, Q,,
9
%I
*
WITH CALCULATED VALUES R2, Q c 2 *
Qe,
+ 2 0 0 h
I
I
I
1
I
I
I
1
1
+ I 0 0-
CONVECTION-
5l -100-
V)-
e
r
-200 Q - 3 0 0 a I<
0-
-
-
-
-100-
E V A P O R A T I O N-
E3
-200-
a
01 o - 3 0 0 .-
- 4 0 0-
-
-500-
-
- 6 0 0 - II
II
L 9 12 15 19 22 26 2 9 2 6 8 12 16 19 A U G U S T SEPTEMBERAPPENDIX A
CHECK CALIBRATION OF BEC,UIAN AND WHITLEY NET RADIOMETER U S I N G ALUMINLmI PPLATE TECHNIQUE
P r i n c i p l e
Heat i s l o s t o r gained from a p l a t e by .conduction, convection and n e t r a d i a t i o n . If t h e p l a t e i s placed on a n i n s u l a t e d base and exposed t o atmospheric c o n d i t i o n s , t h e amount of h e a t l o s t o r gained by conduction w i l l be small when compared t o t h e h e a t l o s s e s o r g a i n s by r a d i a t i o n and convection. Convective h e a t i s p r o p o r t i o n a l t o t h e d i f f e r e n c e between s u r f a c e temperature and a i r temperature. If t h e p l a t e
i s cooled s e v e r a l degrees below mean a i r temperature, placed on t h e i n s u l a t e d base and exposed t o atmospheric c o n d i t i o n s , t h e temperature of t h e p l a t e w i l l r i s e above mean a i r tempera- t u r e on a sunny day. During t h e f i r s t few minutes it w i l l be above t h e s u r f a c e temperature and t h e r e w i l l be convective h e a t g a i n ; when t h e s u r f a c e temperature exceeds mean a i r
temperature t h e r e w i l l be convective h e a t l o s s . When convective h e a t l o s s and convective h e a t g a i n balance, t h e n e t i n c r e a s e i n h e a t s t o r e d i n t h e p l a t e w i l l be due t o t h e n e t r a d i a t i o n .
If t h e i n c r e a s e i n temperature of t h e p l a t e , i t s
mass and s p e c i f i c h e a t a r e known, t h e h e a t absorbed by t h e p l a t e i n a given time can be c a l c u l a t e d . I f t h e p l a t e , d u r i n g t h i s given time i n t e r v a l , i s exposed t o a n e t radiometer and t h e output of t h e radiometer i s recorded, t h e n t h e r a t e of h e a t s t o r e d can be r e l a t e d t o t h e o u t p u t and a c a l i b r a t i o n f a c t o r obtained f o r t h e radiometer.
Apparatus
Requirements of t h e p l a t e a r e : s p e c i f i c h e a t ,
( 2 ) good conduction, s o t h a t temperature v a r i a t i o n s w i t h i n t h e p l a t e w i l l be a minimum,
( 3 ) s u r f a c e having uniform and reasonably l a r g e p r o p e r t i e s of a b s o r p t i o n ,
( 4 ) s u r f a c e l a r g e enough t o cover most of t h e a r e a "seen" by n e t radiometer,
( 5 )
mass ( o f p l a t e ) such t h a t under maximum atmospheric r a d i a t i o n t h e temperature r i s e would be q u i t e r a p i d ; f o r t h e p r e s e n t o b s e r v a t i o n s t h e maximum r a t e of temperature i n c r e a s e was about 10°C i n5
min.A c i r c u l a r aluminum p l a t e painted b l a c k on one s i d e was chosen. It was i n . t h i c k and 2 8 i n . i n diameter, and was l a i d h o r i z o n t a l l y on a block of i n s u l a t i n g m a t e r i a l .
A thermocouple was placed i n a hole d r i l l e d
1s
i n . i n t o t h e s i d e o f t h e p l a t e ; a n o t h e r thermocouple was f a s t e n e d t o t h e under s i d e of t h e p l a t e . Temperature was measured w i t h a potentiometer.Procedure
Two methods were used t o cool t h e p l a t e below a i r temperature: crushed i c e was placed on t o p o f t h e p l a t e and t h e s u r f a c e wiped d r y when it had cooled s u f f i c i e n t l y ; t h e p l a t e was exposed f o r a n i g h t under c l e a r sky c o n d i t i o n s and t h e temperature taken e a r l y i n t h e morning a t a time when t h e dew-point was s e v e r a l degrees belorrv mean a i r temperature ( i f t h e p l a t e cooled below t h e dew-point, moisture would condense on t h e s u r f a c e o f t h e p l a t e ) .
A period had t o be chosen when t h e output of t h e n e t radiometer was s t e a d y ( c l e a r sky c o n d i t i o n s ) , and was
reasonably h i g h ( a t l e a s t 5 mv). S u i t a b l e c a l i b r a t i o n p e r i o d s did n o t occur v e r y f r e q u e n t l y a t Ottawa d u r i n g August and
September 1960.
When a s u i t a b l e period was a v a i l a b l e , t h e f o l l o w i n g procedure was used. The p l a t e was cooled s e v e r a l degrees below a i r temperature and t h e n placed on t h e i n s u l a t e d s l a b below t h e n e t radiometer. The temperature of t h e p l a t e was measured f o r a c e r t a i n time i n t e r v a l . During t h i s same time i n t e r v a l t h e output of t h e n e t radiometer was measured.
Fibwre A - 1 g i v e s a sample c a l c u l a t i o n f o r one t e s t p e r i o d ,
-
chosen s o t h a t t h e shaded a r e a s were e q u a l , i . e . convective h e a t l o s s-
convective h e a t gain.Shortcomings of Technique
(1) I t r e q u i r e s i d e a l weather c o n d i t i o n s f o r c a l i b r a t i o n .
( 2 ) The c a l i b r a t i o n u s u a l l y cannot be checked a t s e v e r a l wind speeds; and t h e c a l i b r a t i o n f a c t o r may v a r y w i t h wind speed.
( 3 ) The s p e c i f i c h e a t o f p l a t e must be assumed equal t o standard.
( 4 )
The shadon e f f e c t of n e t radiometer i s n o t b o r n . ( 5 ) The a r e a "seen" by t h e radiometer i s l a r g e r t h a nt h e a r e a of t h e p l a t e .
6 The temperature measurements a r e c r i t i c a l .
( 7
) Non-unif orm p l a t e temperature d u r i n g t h e c a l i b r a t i o n could r e s u l t i n a change i n a i r s t a b i l i t y which would a f f e c t t h e c a l i b r a t i o n .Conclusion
On the b a s i s of experience so f a r , it appears t h a t t h i s method can be used t o check n e t radiometers under f i e l d conditions. Before r e a l confidence can be placed i n t h i s type of c a l i b r a t i o n however, it w i l l have t o be checked under a v a r i e t y of conditions with a radiometer of h o r n c a l i b r a t i o n .
TEMPERATURE
B
INSIDE DRILLEDBOTTOM OF PLATE
0906 09 11 0916 09 2 1 0 9 2 6 0 9 3 1
TIME OF EXPERIMENT, E.S.T.
PERIOD 0906.30 TO 0 9 1 7 * 3 0 1 1 MINUTES RISE IN TEMPERATURE = 21
-
13 = 8 OCH E A T ABSORBED BY P L A T E
= 8 ( 1 3 , 5 5 0 ) ( 0 . 2 1 4 ) MASS OF PLATE
=
13,550 GRAMS= 2 3 , 2 0 0 CALORIES SPECIFIC HEAT OF A L
= 0 . 2 1 4
CALS/GM AT 20°C
-
-
2 3 , 2 0 0 CALS/SQ CM/MIN EFFECTIVE AREA OF PLATE(11 X 3,670) =3,670 CM2
OUTPUT OF B t W CONSTANT DURING PERIOD
AT 5.5 MV
MANUFACTURER'S CALIBRATION = 10.3 MYCAL- CM-'-M IN- I
F I G U R E A - l
CALIBRATION OF HEAT METERS
Heat meters were placed on t h e bottom of two 4 - f t
diameter metal t a n k s . They were l o c a t e d i n t h e c e n t r e of t h e t a n k s and b a k e l i t e s h e e t i n g , of approximately t h e same t h i c k n e s s a s t h e h e a t m e t e r s , was placed around t h e h e a t
meters and glued t o t h e bottom i n s i d e f a c e of t h e t a n k s . A l l
j o i n t s i n t h e b a k e l i t e , connections t o t h e h e a t meter, and t h e h e a t meters themselves were covered w i t h wax. The t a n k s were t h e n placed i n h o l e s i n t h e ground s o t h a t t h e i r t o p s were l e v e l w i t h t h e surrounding s o i l s u r f a c e .
A s l a b of h a r d r u b b e r , 1 2 i n . square and 2 i n .
t h i c k was placed d i r e c t l y over t h e h e a t meter. Thermocouples were f i x e d t o t h e upper and lower f a c e s of t h e r u b b e r s l a b .
A small metal t a n k , 2 f t i n d i a m e t e r , 6 i n . deep and f i l l e d w i t h w a t e r , was placed on t o p of t h e r u b b e r pad. A 200-watt h e a t e r and a n e l e c t r i c s t i r r e r were placed i n t h e water.
The h e a t e r was turned on, t h e thermocouples and o u t p u t from t h e h e a t meter were connected t o a p p r o p r i a t e r e c o r d e r s , and t h e system l e f t u n t i l s t e a d y - s t a t e h e a t flow e x i s t e d , i , e . t h e t e m p e r a t u r e s a t t h e t o p and lower f a c e of t h e r u b b e r were c o n s t a n t f o r s e v e r a l hours. Then, knowing t h e thermal c o n d u c t i v i t y of t h e r u b b e r , it was p o s s i b l e t o c a l c u l a t e t h e h e a t f l o w through t h e r u b b e r which was assumed e q u a l t o t h e h e a t flow through t h e h e a t meter.
I n o r d e r t o check t h e c a l i b r a t i o n f a c t o r a t d i f f e r e n t t e m p e r a t u r e s , t h e h e a t e r was removed and s e v e r a l pounds of
crushed i c e were placed i n t h e water, The procedure was r e p e a t e d , i . e . t h e system was l e f t f o r s e v e r a l h o u r s u n t i l s t e a d y - s t a t e c o n d i t i o n s were obtained. Again, by e q u a t i n g t h e h e a t flow through t h e r u b b e r t o t h e h e a t flow through t h e meter, a c a l i b r a t i o n f o r t h e h e a t meter was obtained. Check on Heat Meter C a l i b r a t i o n
Both 4 - f t t a n k s , f i l l e d w i t h p e a t and s a t u r a t e d w i t h w a t e r , were exposed t o t h e same atmospheric c o n d i t i o n s . Thermocouples were embedded i n t h e p e a t a t d i f f e r e n t depths. The h e a t f l o w through t h e bottom of t h e t a n k was determined
f r o m t h e h e a t meter r e a d i n g s . If t h i s h e a t f l o w i s assumed e q u a l t o t h e h e a t flow through t h e bottom l a y e r of s a t u r a t e d p e a t , t h e thermal c o n d u c t i v i t y can be c a l c u l a t e d .
For t h e p e r i o d June 23 t o 26, t h e average v a l u e f o r t h e thermal c o n d u c t i v i t y of t h e bottom 5 - i n , l a y e r of s a t u r a t e d
n O m
p e a t was c a l c u l a t e d t o be 0.0011 cal/cmL - G s e c , compared t o
O C
- s e c f o r wet, marshy s o i l given by Geiger ( 2 ) . I t should
c m
b e noted t h a t t h i s value f o r t h e lower l a y e r s included about 1 i n . of sand and g r a v e l which might e x p l a i n why it i s lower t h a n those r e p o r t e d by Geiger. The sand and g r a v e l l a y e r had been added t o ensure f r e e h o r i z o n t a l drainage i n t h e lower l a y e r of t h e p e a t sample.
A d d i t i o n a l v a l u e s of thermal c o n d u c t i v i t y were c a l c u l a t e d f o r t h e period J u l y
1 5
t o 18 a f t e r t h e p o s i t i o n s of t h e thermocouples were changed s o t h a t t h e y were above t h e g r a v e l l a y e r . Values c a l c u l a t e d f o r t h e lower l a y e r of s a t u r a t e d p e a t were :Tank No. 1 = 0.0014 cal/cm2
2
s e c Bank No. 2 = 0.0019 cal/cmcm
O C s e cEstimate of Thermal Cond.uctivit.v f o r U ~ a e r Layer
An e s t i m a t e of t h e thermal c o n d u c t i v i t y f o r t h e upper l a y e r s was a l s o obtained by f i r s t c a l c u l a t i n g t h e thermal d i f f u s i v i t y . I f h e a t i s t r a n s f e r r e d i n t h e s o i l i n accordance w i t h t h e t h e o r y of conduction, it i s p o s s i b l e t o compute thermal d i f f u s i v i t y by comparing t h e temperature range a t two depths according t o t h e f o l l o w i n g formula:
( a 2
-
Kh
= H1) 2
P ( l o g e
-
R, )
where R1 and R2 a r e t h e d a i l y temperature amplitudes a t depths Z1 and
a2
r e s p e c t i v e l y ,P i s t h e period of t h e temperature o s c i l l a t i o n
5
= thermal d i f f u s i v i t y .From v a l u e s t h e thermal c o n d u c t i v i t y can be c a l c u l a t e d by assuming v a l u e s f o r d e n s i t y and s p e c i f i c h e a t of t h e p e a t sample. The v a l u e s of thermal c o n d u c t i v i t y c a l c u l a t e d f o r t h e period J u l y
1 5
t o 18 a r e a s f o l l o v ~ s :Tank No. 1, upper s a t u r a t e d l a y e r 0 t o 6 i n . 2 C O = 0.0011 cal/cm s e c Tank No. 2 , upper u n s a t u r a t e d l a y e r 0 t o 6 i n .
s e c = 0.0007 cal/cm
These values f o r K a r e i n reasonable agreement w i t h v a l u e s i n t h e l i t e r a t u r e .
Considerations f o r Future C a l i b r a t i o n
(1) I t was important t o have c o n s t a n t temperature c o n d i t i o n s during c a l i b r a t i o n . T r i a l s during t h e n i g h t ,
where t h e r e was s t e a d y c o o l i n g by long wave r a d i a t i o n , appeared t o be most s a t i s f a c t o r y .
(2) The m a t e r i a l surrounding t h e h e a t meter, i n t h i s case t h e b a k e l i t e , should be t h e same thiclcness a s t h e meter and have t h e same thermal c o n d u c t i v i t y s o t h a t h e a t flow through t h e bottom of t h e t a n k w i l l be uniform and v a l u e s obtained from t h e h e a t meter w i l l be a t r u e measure of t h i s flow.
Considerations f o r Measurement of Thermal Conductivi-bv
(1) Thermocouples should be f a s t e n e d r i g i d l y a f i x e d d i s t a n c e a p a r t ; those used i n t h i s experiment were n o t r i g i d enough and some s e t t l e m e n t may have occurred.
(2) The sample depth should be deeper s o t h a t t h e
un-
s a t u r a t e d depth can be g r e a t e r than 6 i n . The e f f e c t i v ethermal r e s i s t a n c e of t h e u n s a t u r a t e d moss was n o t a p p r e c i a b l y d i f f e r e n t from t h a t o f s a t u r a t e d moss, and it would be d e s i r a b l e t o have a g r e a t e r depth of u n s a t u r a t e d m a t e r i a l .
( 3 ) The most s a t i s f a c t o r y v a l u e s of thermal c o n d u c t i v i t y were obtained when t h e r e was maximum h e a t flow and, c o r r e s - pondingly, l a r g e s t temperature g r a d i e n t s .
( 4 ) Values obtained f o r thermal c o n d u c t i v i t y a r e r e p r e
-
s e n t a t i v e only; under f i e l d c o n d i t i o n s t h e v a r i a b i l i t y of t h e peat moss w i t h i n s h o r t d i s t a n c e s may be l a r g e .APPENDIX C
CHECK ON ACCURACY OF EVAPORATION M.EASURE3IENTS
For t h e period June 24 t o 30 t h e p e a t i n t h e two t a n k s was s a t u r a t e d and t h e evaporation measured. A s both t a n k s were exposed t o t h e same atmospheric c o n d i t i o n s , t h e evaporation r a t e should have been n e a r l y t h e same. The only d i f f e r e n c e between t h e two t a n k s was t h a t i n Tank No. 1 t h e
s u r f a c e of t h e peat was about 1/4 i n . from t h e r i m of t h e t a n k , whereas i n Tank No. 2 t h e s u r f a c e of t h e p e a t was about
3/4 i n . from t h e t a n k r i m .
The f ollovring r e c o r d s were obtained:
Date of
Observation Tank No. 1 Tank No. 2
( i n . of e v a p o r a t i o n ) ( i n . of e v a p o r a t i o n ) June 23 0.11 0.11
*
I t 24 t o 27 (0.47) n o t r e l i a b l e - r a i n 0.38 " 27 0.25 0.23 I' 28 0.30 0.37 " 29 0.10 0.10 " 30 0.08-
-
0.04 T o t a l 0.84 0.85 A f u r t h e r a n a l y s i s of t h e d a i l y evaporation r e c o r d s was made f o r t h e period J u l y 4 t o 29. Figure 6 shows acomparison of d a i l y evaporation; Tank No. 1 i s compared t o Tank No. 2, and Tanks No. 1 and No. 2 a r e compared w i t h a
standard Class A evaporation pan l o c a t e d w i t h i n 50 f t of t h e s i t e . Figure C - 1 i n d i c a t e s t h e s c a t t e r a s s o c i a t e d w i t h d a i l y observations.
3 June 24th observations n o t included i n t o t a l because it
was noted t h a t if t h e r e was more t h a n 0.25 i n . of r a i n , t h e water i n Tank No. 1 would overflow and t h e evaporation measurement would n o t be r e l i a b l e .
- 6 EVAPORATION, INCHES ( T A N K NO.
I )
.6 EVAPORATION, INCHES
( PEAT TANKS)
