The “evaporation brush”, an evaporimeter for measuring the potential evaporation of meadow grass

Journal of Hydrology, 41 (1979) 363--369 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

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Technical Note [4] THE "EVAPORATION BRUSH", AN EVAPORIMETER FOR MEASURING THE POTENTIAL EVAPORATION OF MEADOW GRASS

C.L. PALLAND Research Department, N. V. Heidemaatschappij Beheer, Arnhem (The Netherlands)

(Received August 10, 1978; accepted for publication September 15, 1978)

ABSTRACT Palland, C.L., 1979. The "evaporation brush", an evaporimeter for measuring the potential evaporation of meadow grass. J. Hydrol., 41: 363--369. A new type of evaporimeter, called the "evaporation brush" is described. It measures continuously the evaporation rate. The operation of the evaporation brush has been tested in a field of meadow grass, sufficiently supplied with soil moisture. It is found that the evaporation measured by this device in meadow grass during periods of 1 h and 24 h compares well with calculated data for meadow grass from a modified Penman formula. The instrument needs only to be tended once a week.

INTRODUCTION T h e R e s e a r c h D e p a r t m e n t o f N.V. H e i d e m a a t s c h a p p i j B e h e e r has d e v e l o p e d a s e l f r e c o r d i n g device t h a t s i m u l a t e s t h e e v a p o t r a n s p i r a t i o n o f m e a d o w grass w h i c h is s u f f i c i e n t l y s u p p l i e d w i t h soil m o i s t u r e . This i n s t r u m e n t , called t h e " e v a p o r a t i o n b r u s h " , is i n t e n d e d t o serve, in t h e first instance, as a b e t t e r alt e r n a t i v e t o t h e e v a p o r a t i o n p a n used in irrigation areas. Besides, in c o u n t r i e s w i t h h y d r o l o g i c a l c o n d i t i o n s c o m p a r a b l e t o t h o s e o f T h e N e t h e r l a n d s it can also be u s e d t o great a d v a n t a g e in w a t e r b a l a n c e studies. T h e e v a p o r a t i o n b r u s h is able to s u p p l y d a t a o n p o t e n t i a l e v a p o t r a n s p i r a t i o n o f m e a d o w grass even d u r i n g s h o r t p e r i o d s o f 1 h or 24 h, if r e q u i r e d . F o l l o w i n g t h e s h o r t d e s c r i p t i o n o f t h e p r i n c i p l e s o f o p e r a t i o n o f t h e evaporat i o n brush, r e c o r d e d 24-h a n d 1-h d a t a in m e a d o w grass are c o m p a r e d w i t h the d a t a c a l c u l a t e d f o r m e a d o w grass a c c o r d i n g t o a m o d i f i e d P e n m a n f o r m u l a . PRINCIPLE OF ACTION Fig. 1 s h o w s t h e o p e r a t i o n a l d i a g r a m o f t h e e v a p o r a t i o n brush. E v a p o r a t i o n t a k e s place m a i n l y t h r o u g h t h e m e d i u m o f t h e hairs o f t h e brush, see Fig. 2. T h e hairs are p r e p a r e d v e g e t a b l e fibres, identical t o t h o s e used f o r f a b r i c a t i n g

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Fig. 1. Operational diagram of the evaporation brush. domestic brushes. The base of the brush consists of a circular sheet of Pertinax ® , with a diameter of 0.47 m. The brush is placed in a tray. The evaporated water is replenished by capillary rise from a layer of water in the tray. The water is kept at a c ons t ant level at 0.02 m above the b o t t o m of the tray. Th e water supply is obtained f r om a reservoir. The latter is o p er ated by a small electrically-driven pulsating p u m p with a capacity of 20 1 per 24 hours at a tension of 14 V d.c. The excess water drains through the overflow and the dirt catcher back to the reservoir. The reservoir contains sufficient water for one week of registration. F u r t h e r m o r e , a filter has been built in the piping system to remove the suspended dirt. The lowering of the water level in the reservoir is registered electronically on a chart at a 1:5 r e d u ctio n ratio (Fig. 3). T he evaporation from the brush can be f o u n d from the records by application of a fixed factor. COMPARISON WITH PENMAN EVAPORATION DATA Evaporation data registered by means of the evaporation brush in m e a d o w grass are c o m p a r e d with calculated data for m e a d o w grass from a modified Penman formula. Measurements were taken at the experimental site of the Research Departm e n t o f N.V. Heidemaatschappij Beheer near Raalte in Salland, The Netherlands (Jacobs et al., 1977). The measuring period ran from July 1975 to August 1976. O ut of this period several days showing varying evapotranspiration a m o u n t s were selected. These days had sufficiently large time intervals between t h e m in order to guarantee mutually i n d e p e n d e n t data. Several very

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dry and warm periods were n o t taken into account. In connection with the short measuring periods, i.e. 24 h and 1 h, the c o m m o n Penman calculation m e t h o d cannot be applied. The reason for this is that the original Penman formula is meant to be applied for periods of one week or longer. This formula is as follows: C/Xq~.~ / L p w a + .yEa Z w = fEo = f

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where E w represents the potential evapotranspiration of short grass, E o the evaporation of an "open-water surface", and f a factor which Penman established empirically for each season (Penman, 1948; Kramer, 1957).

Fig. 2. Brush and tray.

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Fig. 3. Registration unit.

F o r the p r e s e n t p u r p o s e the f o l l o w i n g m o d i f i e d f o r m u l a has been used for the c a l c u l a t i o n o f the p o t e n t i a l e v a p o r a t i o n o f the m e a d o w grass:

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A + 7/e

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In relation to the notation of Monteith: 1/e = 1 + rc/ra, with: r c = crop resistance (s m -1) r a = aerodynamic resistance (s m -1) (Slatyer and Mcllroy, 1961; Rijtema, 1965; Monteith, 1973). From measurements by Szeicz and Long (1969) the e-value of 0.7 is derived. (kg m -3) Pwa = density o f w a t e r (J kg -1) L = latent heat of vaporization of water qrnae = net-radiation flux density (W m-:) (W m -2) qco = heat flux density in the soil at the soil surface Ea = evaporation rate of an open-water surface according to the Dalton formula, where Penman equates the water vapour pressure at the surface to the m a x i m u m vapour pressure at air temperature (Penman, 1948; Kramer, 1957) (mm day -1 or mm h -1) F = 2.5, a factor accounting for the greater aerodynamic roughness length of meadow grass as compared with the short grass in Penman's experiments (Thom and Oliver, 1977). It is to be noted that the modified calculation m e t h o d incorporates the stomatal resistance by introduction of the factor e instead of f. Further it takes into account the heat conduction in the soil, which is necessary when short periods are considered. In our experiments this quantity and q ~ were V.B-I scale units

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Fig. 4. Comparison of daily readings of two brushes with different fibre materials.

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measured directly, respectively in and over the meadow grass. The results are shown in Figs. 4--6 and in Table I. First the applicability of two kinds of natural fibres was investigated. The fibres of the evaporation brush, indicated in Fig. 4 as V.B.-I, originate from the leaf of the Agave type tampica (Mexico). They are flexible fibres of a white-yellow colour, however, under moist condition they are coloured light-brown. Brush V.B.-II consists of fibres originating from the shell of a coconut. These fibres are more rigid and have a dark-brown colour. The evaporation quantities, expressed in scale units and registered by both brushes, have been plotted for a great number of days. The regression line was determined for a number of dry days and a number of days having some precipitation, respectively 4a and 4b in Table I. In Fig. 5 the data registered by V.B.-I on a number of dry days are compared to the data calculated according to the modified Penman formula. The E.. ram.day "1

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TABLE I Linear regression established by m e a n s o f the least-squares m e t h o d Linear relation

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Fig. 5. C o m p a r i s o n o f 24-h readings w i t h calculated p o t e n t i a l evapotranspiration data. Fig. 6. C o m p a r i s o n o f 1-h readings w i t h calculated p o t e n t i a l evapotranspiration data.

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regression line is given in Table I. In order to compare the 1-h amounts, the registrations of June 7, 1976, a dry and mainly clear day, were used (Fig. 6). DISCUSSION AND CONCLUSIONS

It is concluded that there is no difference in behaviour of the two types of brush fibres employed. It was established that the brush-hairs after being continuously in operation in the open air for a period of at least one year remain entirely unaffected by the weather. The scatter of the points on the graph Fig. 4 is somewhat greater on days with some precipitation compared to completely dry or wet days. This can partly be understood since on partly dry days additional readings before and after each rainfall are required. Based on the band width of the spread around the regression line a measuring inaccuracy of up to -+ 1 scale unit = + 0.4 mm day -1 must be taken into account. Fig. 5 shows that the measured daily evaporation data agree well with the data calculated for meadow grass according to the modified Penman formula. Only a qualitative judgement can be attached to the calculated linear regression owing to the limited number of available observations, especially on days with a high evaporation rate. It is to be noted that the scatter is not greater than the measuring inaccuracy of the evaporation brush for 24-h amounts. Fig. 6 shows very clearly a linear regression between the measured 1-h data of the evaporation brush and the calculated data set with a regression coefficient of about 1. The mean deviation around the regression line amounts to about 0.05 mm h -1.

REFERENCES Jacobs, A.F.G., Nieuwvelt, C., Wartena, L. and de Vries, D.A., 1977. Eddy -- Correlation measurements of fluxes heat and m o m e n t u m over grassland in The Netherlands. Arch. Meteorol. Geophys. Bioklimatol., Ser. A, 26: 51--71. Kramer, C., 1957. Berekening van de gemiddelde grootte van de verdamping voor verschillende delen van Nederland volgens de methode van Penman. Staatsdrukkerij-Uitgeverij, 's-Gravenhage, 85 pp. Monteith, J.L., 1973. Principles of Environmental Physics. Edward Arnold, London, 241 pp. Penman, H.L., 1948. Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc. London, Ser. A, 193: 120--145. Rijtema, P.E., 1965. An analysis of actual evapotranspiration. Pudoc, Wageningen, 107 pp. Slatyer, R.O. and Mcllroy, I.C., 1961. Practical Microclimatology, Unesco, Paris. Szeicz, G. and Long, I.T., 1969. Surface resistance of crop canopies. Water Resour. Res. 5(3): 622--633. Thom, A.S. and Oliver, H.R., 1977. On Penman's equation for estimating regional evaporation. Q.J.R. Meteorol. Soc., 103: 345--357.