The influence of a windbreak on evaporation

Agricultural Meteorology- Elsevier Publishing Company, Amster0am- Printed in The Netherlands

T H E I N F L U E N C E OF A W I N D B R E A K O N E V A P O R A T I O N J. LOMAS AND E. SCHLESINGER

Division of Agricultural Meteorology, Meteorological Service, Bet-Dagan (Israel) (Received March 9, 1970)

ABSTRACT

LOMAS,J. and SCHLESINOER,E., 1970. The influenceof a windbreak on evaporation. Agr. Meteorol., 8: 107-115. In the present paper data from two previous investigations on windbreaks are reviewed to test their influence on evaporation in areas with and without advective energy contributing to evaporation and also to check how well the actual results could be predicted by the Penman formula. It appears that this formula permits an accurate estimate of the reduction of evaporation by a windbreak and also indicates whether or not significant reductions should be expected. The use of a Piche-type evaporimeter in measuring reduction of evaporation behind windbreaks is criticized. In areas of relatively small advective energy the use of evaporation pans is recommended.

INTRODUCTION Numerous investigations have been carried out on the influence of windbreaks on evaporation, cf. for instance the excellent review in the W.M.O. technical note on windbreaks and shelterbelts, or to mention only one of the more recent publications the paper by SHAH (1962). Most authors report a considerable reduction of evaporation behind the windbreaks, but this reduction was generally less than that of windspeed. The Piche evaporimeter, although used by some authors, seems unsuitable for this type of investigation, since by its very nature (partial shading of the evaporating surface by the instrument itself) it tends to overemphasize the influence of the wind. It has been argued (VAN EIMERN et al., 1964) that the actual influence of a windbreak on evaporation ought to be relatively small, since the radiation balance remained essentially unchanged except for the immediate vicinity of the windbreak. Following the same line of thought, it may be conjectured that this argument holds in the absence of advection, but that under advective conditions evaporation should be noticeably reduced by a windbreak. Advection in this context should be understood as advective energy contributing to evaporation. Data from two previous investigations on windbreaks, one by LOMAS et al. (1966) in the B'sor region, the other by L o ~ s et al. (1971) near Eilat, were reviewed to test this hypothesis, and to check how well the actual results could be

Agr. MeteoroL, 8 (1971) 107-115

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J. LOMAS AND E. SCHLESINGER

predicted by the Penman formula. The latter part of the project met with some difficulties, since both experiments had been designed more generally to study the influence of a windbreak on the microclimate, plant development, and yields of peanuts (B'sor) and eggplants (Eilot), which did not require simultaneous measurements of evaporation and climatological data at the experimental site. However, data from meteorological stations several kilometers distant from the fields gave satisfying results. METHODS

The experiment in the B'sor region (LoMAS et al., 1966) was carried out in Mivtahim (34 °23'N 31 ° 14'E, height 110 m) in the northeastern part of the Negev, some 20 km from the Mediterranean shore. In this region, the winds during the summer months blow almost exclusively from the northwest sector, i.e., from the sea. The layout of the experimental site is shown in Fig. 1. ~ 10m

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Evaporation from the Class A pans at various distances downwind from the windbreak was measured on 15 days from 25.7.1965 to 13.9.1965 and averaged. Cloudiness data for those days were taken from the records of the climatological station at Tseelim. Temperature and relative humidity data for the various distances downwind were averaged over all days on which they were measured; the run of wind was averaged over the entire duration of the experiment. Agr. Meteorol., 8 (1971) 107-115

INFLUENCE OF A WINDBREAKON EVAPORATION

109

The second experiment (LoMAS et al., 1971) was set up in the fields of Kibbutz Eilot (29 °3YN 34°57'E, height 5 m) (cf. Fig.2), approximately 2 km north of Eilat. For the present investigation, 18 days during the months November-March with northerly (i.e., desert) winds were selected on which the evaporation from the [] I

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Fig.2. The experimental field at Eilot. Class A pans at various distances downwind and run of wind in the open were measured simultaneously. On most of these days, no other climatological data were measured at the site, so temperature, relative humidity and cloudiness were taken from the records of the meteorological station in Eilat. Temperatures and relative humidities measured at Eilot on other dates showed very little variation from the protected to the unprotected plot or at various distances downwind, the maximum differences being 0.8 °C and 2 ~o, respectively. N o corrections were made for these variations. Run of wind at the various distances downwind was calculated by multiplying the run of wind in the open with an appropriate factor derived from windspeed measurements on ca. 20 other days in the course of the experiment. The results from the Penman formula as well as the measured evaporation were averaged over all days of measurement. The Penman formula (PENMAN, 1948) was used in the following form: E - - AH,. + ?Ea A+7 where E is evaporation in mm/day, A is the derivative of the saturation vapor pressure as a function of temperature, taken at the air temperature, in mbar/°C, is the psychrometer constant in mbar/°C, H.~ is the daily total net radiation Agr. Meteorol., 8 (1971) 107-115

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J. LOMAS AND E. SCHLESINGER

expressed as evaporation equivalent in ram/day, and E a is calculated as follows: Ea = (0.1315 + 1.643.10-3u)(es - e~)mm/day where u is the run of wind in kin/day at a height of 2 m, e~ is the saturated vapor pressure at the daily mean air temperature, in mbar, and e, is the daily mean vapor pressure, also in mbar. The incoming shortwave radiation Hs was calculated according to a regression derived by STANmLL (1962): H~ = H,(0.7985 - 0.0389C - 0.00013C 2) where Ha is the solar radiation arriving above the atmosphere, as listed in the Smithsonian Meteorological Tables, in gcal./cm 2 day, and C is the cloud cover according to the octas scale. The quadratic term in the regression was neglected. The longwave radiation balance H t was obtained from an adaptation of the Brunt equation given by STANHILL (1963): Ht = aT4(0.56 - 0.078x/ea)(1 - 0.ll C) where a is the Stefan-Boltzmann constant and T the air temperature in °K. The net radiation is then given by: Hht = (1 -- r ) . Hs - Hz gcal./cm2day and is converted to mm/day. The albedo r of a screened Class A pan was taken to be 0.19 (G. Stanhill, personal communication, 1969). RESULTS

Class A pan evaporation in the protected and unprotected plots at Mivtahim and Eilot is shown in Fig.3 and 4. In Mivtahim the net radiation equivalent was 6.5 mm/day, so no advected energy was used for evaporation. In Eilot, on the other hand, where the measurements were carried out during the winter, net radiation accounted for only 2.3 mm/day, so advection was responsible for most of the evaporation observed. From a statistical point of view, the difference between evaporation from the protected and the unprotected plot at Mivtahim just barely tends to be significant at the 10 ~ level at 2H; at all other distances, the difference is not significant. In Eilot, the differences between the protected and the unprotected plot is significant at the 1 K-level at 2 H and at 8H, and not significant at 20H. In Table I, measured and calculated values of evaporation at different distances downwind are shown, as well as the corresponding reductions of evaporation and run of wind expressed in percentage of evaporation and run of wind in the open. Agr. Meteorol., 8 (1971) 107-115

111

INFLUENCE OF A WINDBREAK ON EVAPORATION EvQ

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Fig.4. Measured Class A pan evaporation at diff6rent distances from the windbreak. Eilot, November 1967-March 1968. ( H = 2.5 m.)

For Eilot, measured and calculated Class A pan evaporations have been plotted in Fig.5. The regression is reasonably similar to the one obtained by G. Stanhill (personal communication, 1969) using monthly mean values. Fig.6 shows a comparison between evaporation measurements by Piche evaporimeter and the U.S.W.B. Class A pan at Mivtahim. (The Piche evaporimeter data were taken from another experiment carried out by YAKOBI et al. Agr. Meteorol., 8 (1971) 107-115

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J. LOMAS AND E. SCHLESINGER

TABLE I MEASURED AND CALCULATED VALUES OF EVAPORATION

AT DIFFERENT DISTANCES DOWNWIND

FROM THE WINDBREAK, WITH THE CORRESPONDING REDUCTIONS OF EVAPORATION AND WINDSPEED IN ~oo OF THE RESPECTIVE VALUES IN THE OPEN; DISTANCES DOWNWIND ARE MEASURED 1N UNITS OF WINDBREAK HEIGHT H

Class A pan evaporation measured calculated (ram/day) (% reduction) (mm/day)

Windspeed* - ( % reduction) (°//o reduction)

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*, Mivtahim: average r u n o f wind in the open 130 kin/day; for Eilot 370 kin/day. ** Statistically n o t significant.

(1968) at the same site.) There was no significant difference in the evaporation from the U.S.W.B. Class A pan between the protected and unprotected plot, and practically no difference in evaporation downwind. Piche evaporation, on the other hand, showed considerable differences between the protected and unprotected plot. DISCUSSION

It is shown that at Mivtahim, where there is no evidence of advection, the windbreak had no significant influence on Class A pan evaporation, whereas in Eilot, where advection is dominant, evaporation was markedly reduced. The Penman formula permits an accurate estimate of the reductions, which supports the results of ASLVNG (1958), and also indicates whether or not significant reductions should be expected. The similarity of the two regressions in Fig.5 shows

Fig.5. M e a s u r e d a n d calculated Class A p a n evaporation at different distances f r o m the windbreak. Eilot, N o v e m b e r 1 9 6 7 - M a r c h 1968. ( H = 2.5 m.) Fig.6. C o m p a r i s o n o f evaporation f r o m a Piche evaporimeter a n d a Class A p a n at M i v t a h i m (Piche: F e b r u a r y 1966; Class A pan: J u l y - S e p t e m b e r 1965).

Agr. Meteorol., 8 (1971) 107-115

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Agr. Meteorol., 8 (1971) 107-115

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J. LOMASAND E. SCHLESINGER

that the results obtained with the Penman formula are meaningful, even though most of the climatological data were measured at Eilat and not at the site o f the experiment. Research work reported in literature--GOLUBEVA (1941), VAN DER LINDE and WOUDENBERG (1950), WILUSZ (1958), KONSTANTINOV and STRUZER (1965), ZOHAR (1967), LOMAS and YAKOaI (1968), YAKOal et al. (1968)--suggests a considerable reduction in evaporation due to windbreaks and shelterbelts. It is felt, however, that those results are inaccurate in view of the method o f measuring evaporation. In most cases, evaporation was measured by a Piche-type evaporimeter, which being susceptible to wind, showed fair differences in evaporative values between the protected and unprotected plots as well as downwind. Considering that most o f those experiments were carried out in areas where advective energy is relatively small, the use o f evaporation pans might have provided quite different results. In deciding whether or not a windbreak will noticeably affect evaporation, the importance o f advection in the region concerned should be given due consideration. U n d e r non-advective conditions, the windbreak may not diminish evaporation, although the windspeed m a y be considerably reduced. On the other hand, more efficient use o f soil moisture will take place under conditions where advective energy is a relatively large c o m p o n e n t of the total energy available for evaporation and evapotranspiration. ACKNOWLEDGEMENTS This paper is part o f a long-term research project on the effect o f windbreaks on the micro-climate and plant production in an arid and semi-arid climate. The authors are much obliged to the research team which made available the data for this paper. Our special thanks are due to Mr. Uzy L u b o v s k y who helped with the analysis o f the data.

REFERENCES

ASLYNG,H. C., 1958. Shelter and its effect on climate and water balance. Oikos, 9: 282-310. GOLOBEVA,L. A., 1941. The influence of forest shelterbelts of different design on microclimate and snow accumulation. VNIAIMI, 24 pp. (in Russian). KONSTANaaNOV,A. R. and STRUZER,L. R., 1965. Lesnye Polosy i Urozhai. Gidrometeorologidheskoe. Izdatel'stvo, Leningrad, 138 pp. LOMAS,J. and YAKOm, D., 1968. The influence of a windbreak on the microclimate and yields of eggplants in Eilat. lsraelMeteoroL Serv., Bet-Dagan, Set. C, 9 pp. LOMAS,J., YAKOBI,D., I-IETrt, D., BITAN-BUTENVIZER,A. and ZOHAR,J., 1966. The effect of an artificial windbreak on the microclimate, plant development and yields of peanuts in the B'sor region. Israel MeteoroL Serv., Bet-Dagan, Ser. C, 15: 1-57. LOMAS,J., YAKOaI,D. and LuaovsKv, U., 1971. The effect of a windbreak on the microclimate and yields of eggplants in Eilat. In press. PENMAN, H. L., 1948. Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc. London, Set. ,4, 193: 120-145. Agr. MeteoroL, 8 (1971) 107-115

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SHAH,S. R. H., 1962. Studies on Wind Protection. Institute for Biological Field Research, Arnhem, 113 pp. STANHILL, G., 1962. Solar radiation in Israel. Bull. Res. Council Israel, Sect. G, 11: 34-41. STANmLL, G., 1963. Evaporation in Israel. Bull. Res. Council Israel, Sect. G, 11: 160-172. VAN DER Ln~DE, R. J. and WOUDENBERG,J. P. M., 1950. On the microclimatic properties of sheltered areas. The oak-coppice sheltered area. Koninkl. Ned. MeteoroL Inst. Mededel. Verhandel., Ser. A, 56" 1-55. VAN EIr,~RN, J., KARSCHON, R., RAZUMOVA,L. A. and ROBERTSON, G. W., 1964. Windbreaks and shelterbelts. W.M.O. Tech. Note, 59:188 pp. WILUSZ, Z., 1958. The influence of shelterbelts on the water economy and the yield of crops on the neighbouring terrain. Polska Akad. Nauk, Ekologia Polska, Ser. A, 6(1): 1-26. YAKOBI,D., LOMAS,J. and ZOHAR, J., 1968. Microclimate, plant development and crop yields of tomato as affected by an artificial windbreak. Israel Meteorol. Serv. Bet-Dagan, Ser. C, 18: 1-36. ZOHAR, Y., 1967. The Effects of a Cypress Windbreak on the Development of a Grapefruit Orchard in the B'sor Region. Thesis, TelVoAviv~Univ.,Dept. of Botany, Tel Aviv, 197 pp.

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