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Soybean water use in the shelter of a slat-fence windbreak
Agricultural Meteorology, 11 ( 1973) 405-418 © Elsevier Scientific Publishing Company, Amsterdam-Printed in The Netherlands
SOYBEAN WATER USE IN THE SHELTER OF A SLAT-FENCE WINDBREAK* DAVID R. MILLER**, NORMAN J. ROSENBERG and WALTER T. BAGLEY
Nebraska Agricultural Experiment Station, University of Nebraska, Lincoln, Nebr. (U.S.A.) (Accepted for publication March 12, 1973)
ABSTRACT Miller, D.R., Rosenberg, N. J. and Bagley, W. T., 1973. Soybean water use in the shelter of a slat-fence windbreak. Agric. Meteorol., 11: 405 -418.
Evaporative and photosynthetic flux rates were measured in wind sheltered and exposed irrigated soybeans in an eastern Great Plains (U.S.A.) location. Portable slat-fencing of 50% porosity was used to provide shelter. Evapotranspiration was measured with precision weighing lysimeters. CO 2 flux was estimated from CO 2 gradient measurements and calculated exchange coefficients. Multiplicative effects of the windbreak on the irrigated soybean crop over time were eliminated by frequent moves of the barrier between two sites in the experimental field. The intensity of turbulent exchange was decreased in the shelter. Vapor pressure, air temperature and CO 2 concentration gradients were intensified in shelter. During six days of measurement,shelter caused a mean 20% decrease in evapotranspiration. The water saving was greatest when sensible heat advection was important, CO 2 photosynthetic flux rates estimates suggest no difference caused by shelter effect.
INTRODUCTION Windbreaks and shelterbelts have been suggested as practical means to increase water use efficiency of sheltered crops (Rosenberg, 1967). Objective evaluation o f windbreak effects on crop water use efficiency requires an understanding o f the changes in both the evapotranspiration and photosynthesis caused by shelter-induced microclimate. Long term integrated evapotranspiration rates in the lee o f windbreaks and shelterbelts have been estimated by m e a s u r e m e n t o f soil water depletion. Such data generally show a decrease in evapotranspiration on dryland (Van Eimern et al., 1964). Marshall ( 1 9 6 7 ) n o t e d several exceptions and George (1971) presented data indicating increases in seasonal evapotranspiration by sheltered dryland wheat in N o r t h Dakota. Rosenberg (1966) showed increased soil water use by dry bean plants grown under irrigation in the shelter of a slat-fence. He c o n c l u d e d that, w h e n soil moisture is freely available, the physiological * Published with the approval of the Director as Journal Paper No.3456, Journal Series, Nebraska Agricultural Experiment Station. Research reported was conducted under Nebraska Agricultural Experiment Station Projects No. 20-23 and 20-31. **Present address: Plant Science Department, University of Connecticut, Storrs, Conn. 06268, U.S.A.
406
D. R. MILLER, N. J. ROSENBERG AND W. T. BAGLEY
restriction of stomatal closure on plant transpiration is reduced. Brown and Rosenberg (1972) found no significant effect of shelter on soil water use by irrigated sugar beets. Previous to the study reported here there have been, to our knowledge, no direct precise lysimetric measurements of short term evapotranspiration rates in shelter. Brown and Rosenberg (1971) had, however, estimated short term evapotranspiration rates in sugar beets sheltered by corn in western Nebraska using the Bowen ratio-energy balance method. Their calculations show that shelter decreased evapotranspiration during periods of strong environmental moisture stress and permitted increased evapotranspiration during periods of low moisture stress. Strong moisture demand is typical of the afternoons in the western Plains region and at any time of day when advective transport of sensible heat is considerable. Reports of increased crop growth and yield in shelter (Stoeckeler, 1962; Van Eimern et al., 1964) imply an increase in seasonal net photosynthesis. Net photosynthesis on a short term basis is estimable by measurement of CO2 flux rates in the field. There has been extensive field research on the flux of CO2 in open field situations (Monteith, 1962; Inoue, 1965; Lemon, 1969; Denmead, 1969; Van Hylckama, 1969). However, only Brown and Rosenberg (1972) have estimated CO2 flux rates in shelter. Their observations over sugar beets sheltered by corn windbreaks showed no significant effects on net photosynthesis. Brown and Rosenberg (1971) argue, from Bowen ratio-energy balance calculations, that the effects of a windbreak on microclimate, on the vertical fluxes of sensible and latent heat and on CO2 flux rates can be explained by diminished turbulence in shelter. In this study the effects of a windbreak on short period water use efficiency of a protected soybean crop was established by simultaneous measurement of evapotranspiration and photosynthesis. The precision lysimetric measurements of latent heat flux made during the study also allowed us to test previously proposed hypotheses of the mechanism of wind shelter influence on the protected crop(Rosenberg, 1966; Brown and Rosenberg, 1972). METHODS The study was conducted during the summer of 1969 at the University of Nebraska Field Laboratory at Mead (41°09N 96°30W, altitude 354 m above m.s.1.). Evapotranspiration by irrigated soybeans (Glycine max. L., var. Amsoy) was measured simultaneously in the open and at approximately 4 / / ( H = wind barrier height) in the lee of a slat-fence windbreak. The windbreak was 2 m high and constructed of slats 10.8 cm wide and spaced on 10.8 cm centers. Thus the windbreak porosity was 50%. The windbreaks were set in a semi-circle to the south of one or the other lysimeters on a radius of 9.15 m. The experimental field is approximately 105 m by 170 m in size (1.78 ha) with the longer dimension on a north-south line. The lysimeters are on a line 70 m from the
SOYBEAN WATER USEIN SHELTER
407
northern border of the field and 45 and 60 m in from the western border. The minimal fetch to height ratio for 1 m tall soybeans during this study was, therefore, 45. The field was surrounded by alfalfa extending at least 200 m in every direction but west where a 40 m strip was planted next to pasture grass. Rosenberg (1972) has shown excellent agreement between energy balance and lysimetric estimates of the Bowen ratio over soybeans in this field, indicating that the lysimeters are representative of the open field conditions. A photograph of the windbreak established in the field is given as Fig.1.
Fig. 1. A view of the windbreak in place. The lysimeter is near the large white mast. Crop height and leaf area index (LAD averaged 95 cm and 7.2 during the course of the study. Solar radiation was measured with a sixteen junction Eppley pyranometer; net radiation (Rn) was measured with miniature net radiometers (Fritschen, 1965); and soil heat flux (S) with two sets of five each dime-sized National Instrument Laboratory flux plates wired in series and embedded near the lysimeters. These measured terms applied to the energy balance: -(Rn
+S+LE)=A
(1)
permit estimation of the sensible heat flux.(A) as the residual term. The sign convention used here considers all energy fluxes to the canopy as positive and all away from the canopy as negative. Profiles of vapor pressure, temperature and C02 concentration above the crop were measured in the open and in shelter simultaneously. The vapor pressure and temperature profiles were measured with an assembly of thermocouple psychrometers
408
D.R. MILLER, N. J. ROSENBERG AND W. T. BAGLEY
having a time constant of 30 seconds (Rosenberg et al., 1969). CO2 concentration at a reference level was measured with a Beckman Model 315 Infrared Gas Analyzer. Gradients of CO2 were measured differentially with a Grubb-Parsons Infrared Gas Analyzer (Model SB-2). Lysimeters, COs analyzers and psychrometric data were synchronized so that 15-min time averaged or integrated values were available for computations. Vapor pressure gradients in conjunction with lysimetrically determined values of LE were used to calculate exchange coefficients for the vertical turbulent transport of latent heat (KE):
KE-
LE
p
Axz
(2)
Lp Mw/Ma Ae
where L is the latent heat of vaporization; E is the quantity of water evaporated; p is the air density; p is the atmospheric pressure;Mw/Ma is the ratio of the molecular weights of water and air; 2xe is the vapor pressure difference over the height interval &z. The exchange coefficient for CO2 transport was assumed equal to that for water vapor (K = KE = KCO2). The vertical exchange of COz, presmned proportional to photosynthesis (Ps), was calculated from measured gradients of CO2 concentration [COs] as follows:
AtCOsl es
= f.K
-
-
Az
(3)
where f i s the conversion factor from p.p.m. CO2 to units of mass. During the period of study (July 11-24, 1969) the windbreak was moved from one lysimeter to the other every few days. In this way multiplicative effects of the barrier on the sheltered plants were minimized and the statistical replications needed for a cross-over design (Cochran and Cox, 1968, pp.127-233) were provided. Micrometeorological data used in the analysis presented below were taken during periods when wind direction was such that the windbreak effectively protected one of the instrumented plots. Weather conditions during days when the windbreak was effective are given in Table I. Uneven plant growth in the soybean stand in the experimental field* caused higher evapotranspiration from one of the lysimeters. The cross-over experimental design compensated for this source of error. Therefore, data uncorrected for stand effects was used in the statistical calculations. However, in order to use equations 1-3, lysimeter-2 data were corrected for stand effects (Table 1). To establish the magnitude of the needed correction factors, water use in both lysimeters was measured during periods when the windbreak was either not in use or ineffective because of wind direction. Evapotranspiration rates in lysimeter 2 were reduced by a constant percentage determined on these "non-windbreak" days. *During late 1968 repairs made on the lysimeters required heavy equipment traffic in the field. Some variation in plant growth resulted during the following season.
SOYBEAN WATER USE IN SHELTER
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RESULTS
Energy balance A summary of mean morning and afternoon energy balance in the open and shelter is given in Table II. Because of full canopy development by the time the study was undertaken soil heat flux was negligible. The windbreak decreased LE and A during periods of strong wind speed and sensible heat advection (positive A in open). The effect was minor during periods of low wind speed. The grand mean differences in LE and A for all days presented are significant at the 5% level of probability. An example of the windbreak effect on LE and A on a day with strong wind and sensible heat advection is shown in Fig.2. On this day (July 15) a frontal passage during the morning was accompanied by strong winds. Strong sensible heat advection occurred 8
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SOL.AR TIME IN HOURS dULY 15.1969 Fig.2. Components of the energy balance in open and sheltered soybeans on July 15, 1969 at Mead, Nebraska. RN, S, A, and LE are the net radiation, soil, sensible and latent heat fluxes. U is wind speed at 175 cm above the surface.
412
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from 09h00 to 1 lh00. The effect diminished as wind speed decreased in the afternoon. LE in the shelter was reduced by about 40% during the period of strong wind. Sensible heat delivery to the unsheltered plants (positive A) continued throughout the day. The advection of sensible heat to the transpiring crop was reduced by the windbreak. During much of the day sensible heat was generated (negative A) in the protected area. The ratios of l LE I/RN for this day were 1.62 and 1.23 for unsheltered and sheltered sites. Fig.3 shows that the effect of the windbreak on LE and A was minor on July 22, a day of light winds. A cyclic pattern of sensible heat generation at the surface in the 8
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Fig.3. Sameas Fig.2 for July 22, 1969. morning and return to the surface in the afternoon is apparent on this day. The windbreak damped the amplitude of the cycle slightly. The significant shelter-induced reduction of LE during periods of sensible heat advection is in agreement with Brown and Rosenberg's (1971) results showing that shelter decreased LE by sugar beets in western Nebraska during periods of strong atmospheric evaporative demand. Their finding of increased LE in shelter under low stress is not supported by these results, however.
SOYBEAN WATER USE IN SHELTER
413
Gradients of temperature and vapor pressure Average morning and afternoon gradients of T a n d e in the open and shelter are given in Table II. Vapor pressure and air temperature gradients were altered in shelter. The psychrometers used measure AT and Ae with maximum error estimated at less than 0.2°C TABLE III Mean morning (08h00-1 lh45) and afternoon (12h00-15h45) vertical exchange coefficients (K) and wind speeds in open and shelter Date
Time
K (cm 2 sec-t) open shelter
K(shelter) K(open)
Wind speed (m sec-1 ) open shelter
July 14
a.m. p.m.
2,750 4,580
2,070 2,480
0.75 0.54
2.0 2.0
1.5 1.6
July 15
a.m. p.m.
3,440 3,190
2,440 2,490
0.71 0.78
2.1 1.6
1.7 1.3
July 17
a.m. p.m.
2,560 2,940
650 990
0.25 0.33
1.4 1.5
1.1 1.2
July 18
a.m. p.m.
1,930 1,330
980 1,210
0.50 0.90
0.6 1.1
0.5 0.8
July 22*
a.m. p.m.
2,310
1,900 1,850
0.80
1.2 1.3
1.1 1.1
July 24
a.m. p.m.
2,420 1,640
1,190 1,140
0.49 0.69
1.1 0.8
0.8 0.7
Mean:
2,645**
1,590
1.39"*
1.12
*K calculated using vapor pressure gradient between 125-200 cm. (this day only). **Mean difference significant at the 5% level of probability. and 0.3 mbar. During periods of advection, when temperature gradients were inverted in the open, they remained lapse in shelter. This is particularly evident in Fig.4 for July 14. Temperature gradients between 200 and 100 cm were inverted in the open but were lapse in shelter for a large part of the day. On days with lapse conditions in the open the temperature gradient was intensified by shelter. Vapor pressure gradient, always lapse during the daytime, was usually intensified by shelter.
Exchange coefficient Lysimetrically determined K, defined by eq.2, estimates the intensity of vertical water vapor transport. Mean values of K in shelter and in the open, calculated using vapor pressure
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Fig.4. Temperature gradients between 200-100 cm above ground in open and sheltered soybeans. July 14, 1969 at Mead, Nebraska. gradients between 200 and 100 cm above ground surface, are given in Table III for days of widely differing weather conditions. July 14 and 15 were clear with strong sensible heat advection. July 17 was overcast and moderately windy. July 18 was cloudy and calm. July 22 and 24 were clear and calm. Values of K in the open were high on days with strong wind. On calm days under non-advective conditions, K was substantially lower in the open. The windbreak reduced K significantly during periods of strong wind when the barrier caused a significant reduction in wind speed (Table III). When wind speed was low, the windbreak effect on K was small. The average reductions in K of from 10 to 75% are similar to those reported by Brown and Rosenberg (1972). The increase in K with increasing wind speed, for the range of wind speed encountered in the study, is shown in Fig.5. The regression equation fitted is K (cm 2 sec-1) = 387 + 5.81 U(cm secl).
C02 gradients and flux Because of a failure in one CO2 sampling line direct comparisons of above-canopy CO2 gradients could not be made. Table IV reports mean CO2 gradients from a point above to a point into the canopies. Data are variable and when CO2 flux is calculated using the exchange coefficients of Table III, a slight decrease in shelter during windy periods is observed. Variability in crop canopies and plant growth during the experimental period may have been responsible for this effect. DISCUSSION AND CONCLUSIONS
Brown and Rosenberg (1971) found by Bowen ratio-energy balance techniques that exchange coefficients were reduced in shelter. Results of lysimetric measurements reported here confirm this finding.
SOYBEAN WATER USE IN SHELTER
41 5
TABLE IV Mean morning ( 0 8 h 0 0 - 1 1 h 4 5 ) and afternoon ( 1 2 h 0 0 - 1 5 h 4 5 ) [CO 2 ] gradients (175 c m - 7 5 cm) and CO: vertical flux in open and shelter
Date
Time
A CO 2 (p.p.m. m -~ ) open
Flux of CO z (gin cm -~ sec-~ • 10-7) open shelter
shelter
July 14
a.m. p.m.
2,6 2.9
3.2 2.5
1.6 2.3
0.9 0.4
July 15
a,m. p.m.
6.3
4.4
3.4
1.8
July 17
a.m. p.m.
3.7 6.2
7.4 8.9
1.6 3.1
0.8 1.6
July 18
a.m. p.m.
11.3 12,4
16.4 15.7
3.2 3.2
2.7 3.3
July 22
a.m. p.m.
11.0 7,0
10.4 9.0
1.7
3.3 2.9
July 24
a.m. p.m.
9.5 -
11.7 -
3.5 -
2.6 _
Mean:
7.3
9.0
2.6
1.9
Adequate "fetch" is prerequisite to the application of micrometeorological methods in estimating mass transfer. Windbreaks by definition, however, destroy fetch and appropriate assumptions or adjustments must be made in order to apply micrometeorology to the shelter problem. Brown and Rosenberg (1971) were also able to show that a boundary layer of measurable thickness, characteristic of the sheltered surfaces, developed within 2H of the corn windbreak sheltering sugar beets. So long as the surface is approachable the measurements of flux and/or gradient can be meaningful. There is no better way to approach the surface than with a lysimeter. 50i
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416
D. R. MILLER, N. J. ROSENBERG AND W. T. BAGLEY
Brown and Rosenberg (1971) also consider that a number of the microclimatic changes found consistently in shelter can be explained as due to the decrease in turbulent exchange. These m'_'croclimatic effects include increased vapor pressure and temperature gradients caused by slowed dispersion of transpired water vapor and transport of sensible heat. Our findings of reduced exchange coefficient and increased intensity of vapor pressure and temperature gradients are consistent with the above. Similar reasoning can explain an increased [CO2] gradient, but the CO2 observations reported here appear inconclusive with regard to this point. Data on stomatal aperture and diffusion resistance (reported elsewhere by Miller, 1971), however, indicate that the sheltered soybeans maintained wider stomatal aperture and lower resistance to diffusion. A lower CO2 concentration would be expected near the canopy of the sheltered crop. Transport of CO2 downward to replace that used in photosynthesis is, however, slower in shelter, resulting in larger gradients. Experimental errors in calculation of photosynthetic flux using measured exchange coefficients and CO2 gradients has been explored by Brown and Rosenberg (1968) and more recently in greater detail in Sestak et al. (1971). The magnitude of the differences reported here fall within experimental error and the effect must, therefore, be considered unproven. There is much evidence of increased plant growth in wind shelter compiled by researchers and summarized by Van Eimem et al. (1964) and Stoeckeler (1962). These increases must result from greater seasonal net assimilation in shelter. Reports of reduced stomatal resistance in shelter by Brown and Rosenberg (1970) and Miller et al. (1971) indicate increased photosynthetic opportunity. Therefore, the slight reductions in calculated CO2 flux were unexpected. In the calculations of eq.3 the increase in the CO2 gradient was more than offset by the decrease in K. Either the assumption K = KE = KCO~ is in error or the net photosynthesis in shelter is increased by other processes. A nocturnal decrease in respiration in shelter because of lower plant temperatures might explain this effect. Mean nocturnal leaf temperature was 0.35°C lower in shelter during the study. We note an apparent insensitivity of photosynthetic flux rates (Table IV) to changing solar radiation conditions (Table I), regardless of shelter. Rosenberg (1972) reports that soybeans of the Amsoy variety become light saturated, particularly when dense canopy structure is developed at solar radiation intensities of about 1.0-1.2 Ly min -~ . During the experiment reported this intensity occurred at between 08h00 and 10h00 in the morning. This may be the reason that the soybean crop did not show significant photosynthetic response to shelter. Brown and Rosenberg (1970) also found photosynthesis of sugar beet unaffected by shelter. Water use in shelter was reduced most sharply during periods of strong sensible heat advection. This reduced latent heat flux in shelter is evidently due to the physical reduction in quantity of warm air transported to the sheltered crop. Also involved is the reduced intensity of turbulent transport. The apparent lack of a shelter effect on CO2 flux in this study, coupled with the definite decrease in latent heat flux indicates that wind shelter, used over short periods or an entire season, can be effective in increasing water use efficiency by soybeans.
SOY BEAN WATER USE IN SHELTER
417
ACKNOWLEDGMENTS S u p p o r t for this p r o j e c t was received f r o m M c l n t i r e - S t e n n i s F o r e s t r y R e s e a r c h F u n d s a n d f r o m the Office o f Water Resources Research D e p a r t m e n t o f the I n t e r i o r , u n d e r t h e Public Law 88-379 p r o g r a m .
REFERENCES Brown, K. W. and Rosenberg, N. J., 1968. Errors in sampling and infrared analysis of CO: in air and their influence in determination of net photosynthetic rate. Agron. J., 6 0 : 3 0 9 - 3 1 1 . Brown, K. W. and Rosenberg, N. J., 1970. Effect of windbreaks and soil water potential on stomatal diffusion resistance and photosynthetic rate of sugar beets (Beta vulgaris). Agron. J., 62: 4 - 8 . Brown, K. W. and Rosenberg, N. J., 1971. Turbulent transport and energy balance as affected by a windbreak in an irrigated sugar beet field. Agron. J., 6 3 : 3 5 1 - 3 5 5 . Brown, K. W. and Rosenberg, N. J., 1972. Shelter effects on microclimate, growth and water use by irrigated sugar beets in the Great Plains. Agric. Meteorol., 9: 241-263. Cochran, W. G. and Cox, G. M., 1968. Experimental Designs. Wiley, New York, N.Y., 2rid ed., 611 pp. Denmead, O. T., 1969. Comparative micrometeorology of a wheat field and a forest ofPinus radiata. Agric. Meteorol., 6: 357-372. Fritschen, L. J., 1965. Miniature net radiometer improvements. J. Appl. MeteoroL, 4: 523-532. George, E. J., 1971. Effect of tree windbreaks and slat barriers on wind velocity and crop yields. U.S.D.A., Agric. Res. Serv. Prod. Res. Rep., 121:23 pp. Inoue, tL, 1965, On the CO 2 concentration profiles within crop canopies. J. Agric. Meteorol. (Japan), 2 0 : 1 3 7 - 1 4 0 (Japanese with English summary). Lemon, E., 1969. Gaseous exchange in crop stands. In: J.E. Eastin, F.A. Haskins, C.Y. Sullivan and C. H.M. van Bavel (Editors), Physiological Aspects o f Crop Yield. Am. Soc. Agron., Madison, Wisc., pp. 117-136 Marshall, J. K., 1967. The effect of shelter on the productivity of grasslands and field crops. Field CropAbstr., 20: 1-14. Miller, D. R., 1971. Lysimetric and Energy Balance Determination of Slatfence and Tree Windbreak Effects on Water Use Efficiency under Irrigated and Dryland Conditions. Ph.D. Thesis, University of Nebraska, Lincoln, Nebraska, 280 pp. (unpublished). Miller, D. R., Bagley, W. T. and Rosenberg, N. J., 1971. Modification of microclimate with tree shelterbelts in the Great Plains. A bstr. Bull. Am. MeteoroL Soc., 51: 206. Monteith, J. L., 1962. Measurement and interpretation of CO 2 fluxes in the field. Neth. J. Agric. Sci., 10: 334-346. Rosenberg, N. J., 1966. Microclimate, air mixing, and physiological regulation of transpiration as influenced by wind shelter in an irrigated bean field. Agric. MeteoroL, 3: 197-224. Rosenberg, N. J., 1967. The influence and implications of windbreaks on agriculture in dry regions. In: R. A. Shaw (Editor), Ground Level Climatology. Am. Assoc. Advan. Science, Washington, D.C.,pp.327-349. Rosenberg, N. J. and Brown, K. W., 1970. Improvements in the Van Bavel-Myers automatic weighing lysimeter. Water Resour. Res., 6: 1227-1229. Rosenberg, N. J. et al., 1969. Research in Evapotranspiration. Report to the Office of Water Resources Research on Project A-001 NEB. Univ. of Nebraska Horticulture Progress Report 73, 153 pp. Rosenberg, N. J. et al., 1972. Simultaneous Determination of Short-Period Photosynthesis and Evapotranspiration. Final Report to NOAA on Grant E-293-68 (G). Univ. of Nebraska Horticulture and Forestry Progress Report 91, 61 pp. Sestak, Z., Catsky, J. and Jarvis, P. G., 1971. Plant Photosynthetic Production - Manual of Methods. Junk, The Hague, 819 pp.
418
D. R. MILLER, N. J. ROSENBERG AND W. T. BAGLE'~
Stoeckeler, J. H., 1962. Shelterbelt influence on Great Plains field environment and crops - a guide for determining design and orientation,Prod. Res. Rep., USDA Forest Serv., 62:25 pp. Van Eimem, J., Karschon, R., Razumova, L.A. and Robertson, G.W., 1964. Windbreaks and shelterbelts. WorldMeteorol. Org., Tech. Note, 59:188 pp. Van Hylckama, T. E. A., 1969. Photosynthesis and water use by salt-cedar. Bull. Int. Assoc. Sci. Hvdrol., 14: 71-83.
SOYBEAN WATER USE IN THE SHELTER OF A SLAT-FENCE WINDBREAK* DAVID R. MILLER**, NORMAN J. ROSENBERG and WALTER T. BAGLEY
Nebraska Agricultural Experiment Station, University of Nebraska, Lincoln, Nebr. (U.S.A.) (Accepted for publication March 12, 1973)
ABSTRACT Miller, D.R., Rosenberg, N. J. and Bagley, W. T., 1973. Soybean water use in the shelter of a slat-fence windbreak. Agric. Meteorol., 11: 405 -418.
Evaporative and photosynthetic flux rates were measured in wind sheltered and exposed irrigated soybeans in an eastern Great Plains (U.S.A.) location. Portable slat-fencing of 50% porosity was used to provide shelter. Evapotranspiration was measured with precision weighing lysimeters. CO 2 flux was estimated from CO 2 gradient measurements and calculated exchange coefficients. Multiplicative effects of the windbreak on the irrigated soybean crop over time were eliminated by frequent moves of the barrier between two sites in the experimental field. The intensity of turbulent exchange was decreased in the shelter. Vapor pressure, air temperature and CO 2 concentration gradients were intensified in shelter. During six days of measurement,shelter caused a mean 20% decrease in evapotranspiration. The water saving was greatest when sensible heat advection was important, CO 2 photosynthetic flux rates estimates suggest no difference caused by shelter effect.
INTRODUCTION Windbreaks and shelterbelts have been suggested as practical means to increase water use efficiency of sheltered crops (Rosenberg, 1967). Objective evaluation o f windbreak effects on crop water use efficiency requires an understanding o f the changes in both the evapotranspiration and photosynthesis caused by shelter-induced microclimate. Long term integrated evapotranspiration rates in the lee o f windbreaks and shelterbelts have been estimated by m e a s u r e m e n t o f soil water depletion. Such data generally show a decrease in evapotranspiration on dryland (Van Eimern et al., 1964). Marshall ( 1 9 6 7 ) n o t e d several exceptions and George (1971) presented data indicating increases in seasonal evapotranspiration by sheltered dryland wheat in N o r t h Dakota. Rosenberg (1966) showed increased soil water use by dry bean plants grown under irrigation in the shelter of a slat-fence. He c o n c l u d e d that, w h e n soil moisture is freely available, the physiological * Published with the approval of the Director as Journal Paper No.3456, Journal Series, Nebraska Agricultural Experiment Station. Research reported was conducted under Nebraska Agricultural Experiment Station Projects No. 20-23 and 20-31. **Present address: Plant Science Department, University of Connecticut, Storrs, Conn. 06268, U.S.A.
406
D. R. MILLER, N. J. ROSENBERG AND W. T. BAGLEY
restriction of stomatal closure on plant transpiration is reduced. Brown and Rosenberg (1972) found no significant effect of shelter on soil water use by irrigated sugar beets. Previous to the study reported here there have been, to our knowledge, no direct precise lysimetric measurements of short term evapotranspiration rates in shelter. Brown and Rosenberg (1971) had, however, estimated short term evapotranspiration rates in sugar beets sheltered by corn in western Nebraska using the Bowen ratio-energy balance method. Their calculations show that shelter decreased evapotranspiration during periods of strong environmental moisture stress and permitted increased evapotranspiration during periods of low moisture stress. Strong moisture demand is typical of the afternoons in the western Plains region and at any time of day when advective transport of sensible heat is considerable. Reports of increased crop growth and yield in shelter (Stoeckeler, 1962; Van Eimern et al., 1964) imply an increase in seasonal net photosynthesis. Net photosynthesis on a short term basis is estimable by measurement of CO2 flux rates in the field. There has been extensive field research on the flux of CO2 in open field situations (Monteith, 1962; Inoue, 1965; Lemon, 1969; Denmead, 1969; Van Hylckama, 1969). However, only Brown and Rosenberg (1972) have estimated CO2 flux rates in shelter. Their observations over sugar beets sheltered by corn windbreaks showed no significant effects on net photosynthesis. Brown and Rosenberg (1971) argue, from Bowen ratio-energy balance calculations, that the effects of a windbreak on microclimate, on the vertical fluxes of sensible and latent heat and on CO2 flux rates can be explained by diminished turbulence in shelter. In this study the effects of a windbreak on short period water use efficiency of a protected soybean crop was established by simultaneous measurement of evapotranspiration and photosynthesis. The precision lysimetric measurements of latent heat flux made during the study also allowed us to test previously proposed hypotheses of the mechanism of wind shelter influence on the protected crop(Rosenberg, 1966; Brown and Rosenberg, 1972). METHODS The study was conducted during the summer of 1969 at the University of Nebraska Field Laboratory at Mead (41°09N 96°30W, altitude 354 m above m.s.1.). Evapotranspiration by irrigated soybeans (Glycine max. L., var. Amsoy) was measured simultaneously in the open and at approximately 4 / / ( H = wind barrier height) in the lee of a slat-fence windbreak. The windbreak was 2 m high and constructed of slats 10.8 cm wide and spaced on 10.8 cm centers. Thus the windbreak porosity was 50%. The windbreaks were set in a semi-circle to the south of one or the other lysimeters on a radius of 9.15 m. The experimental field is approximately 105 m by 170 m in size (1.78 ha) with the longer dimension on a north-south line. The lysimeters are on a line 70 m from the
SOYBEAN WATER USEIN SHELTER
407
northern border of the field and 45 and 60 m in from the western border. The minimal fetch to height ratio for 1 m tall soybeans during this study was, therefore, 45. The field was surrounded by alfalfa extending at least 200 m in every direction but west where a 40 m strip was planted next to pasture grass. Rosenberg (1972) has shown excellent agreement between energy balance and lysimetric estimates of the Bowen ratio over soybeans in this field, indicating that the lysimeters are representative of the open field conditions. A photograph of the windbreak established in the field is given as Fig.1.
Fig. 1. A view of the windbreak in place. The lysimeter is near the large white mast. Crop height and leaf area index (LAD averaged 95 cm and 7.2 during the course of the study. Solar radiation was measured with a sixteen junction Eppley pyranometer; net radiation (Rn) was measured with miniature net radiometers (Fritschen, 1965); and soil heat flux (S) with two sets of five each dime-sized National Instrument Laboratory flux plates wired in series and embedded near the lysimeters. These measured terms applied to the energy balance: -(Rn
+S+LE)=A
(1)
permit estimation of the sensible heat flux.(A) as the residual term. The sign convention used here considers all energy fluxes to the canopy as positive and all away from the canopy as negative. Profiles of vapor pressure, temperature and C02 concentration above the crop were measured in the open and in shelter simultaneously. The vapor pressure and temperature profiles were measured with an assembly of thermocouple psychrometers
408
D.R. MILLER, N. J. ROSENBERG AND W. T. BAGLEY
having a time constant of 30 seconds (Rosenberg et al., 1969). CO2 concentration at a reference level was measured with a Beckman Model 315 Infrared Gas Analyzer. Gradients of CO2 were measured differentially with a Grubb-Parsons Infrared Gas Analyzer (Model SB-2). Lysimeters, COs analyzers and psychrometric data were synchronized so that 15-min time averaged or integrated values were available for computations. Vapor pressure gradients in conjunction with lysimetrically determined values of LE were used to calculate exchange coefficients for the vertical turbulent transport of latent heat (KE):
KE-
LE
p
Axz
(2)
Lp Mw/Ma Ae
where L is the latent heat of vaporization; E is the quantity of water evaporated; p is the air density; p is the atmospheric pressure;Mw/Ma is the ratio of the molecular weights of water and air; 2xe is the vapor pressure difference over the height interval &z. The exchange coefficient for CO2 transport was assumed equal to that for water vapor (K = KE = KCO2). The vertical exchange of COz, presmned proportional to photosynthesis (Ps), was calculated from measured gradients of CO2 concentration [COs] as follows:
AtCOsl es
= f.K
-
-
Az
(3)
where f i s the conversion factor from p.p.m. CO2 to units of mass. During the period of study (July 11-24, 1969) the windbreak was moved from one lysimeter to the other every few days. In this way multiplicative effects of the barrier on the sheltered plants were minimized and the statistical replications needed for a cross-over design (Cochran and Cox, 1968, pp.127-233) were provided. Micrometeorological data used in the analysis presented below were taken during periods when wind direction was such that the windbreak effectively protected one of the instrumented plots. Weather conditions during days when the windbreak was effective are given in Table I. Uneven plant growth in the soybean stand in the experimental field* caused higher evapotranspiration from one of the lysimeters. The cross-over experimental design compensated for this source of error. Therefore, data uncorrected for stand effects was used in the statistical calculations. However, in order to use equations 1-3, lysimeter-2 data were corrected for stand effects (Table 1). To establish the magnitude of the needed correction factors, water use in both lysimeters was measured during periods when the windbreak was either not in use or ineffective because of wind direction. Evapotranspiration rates in lysimeter 2 were reduced by a constant percentage determined on these "non-windbreak" days. *During late 1968 repairs made on the lysimeters required heavy equipment traffic in the field. Some variation in plant growth resulted during the following season.
SOYBEAN WATER USE IN SHELTER
409
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SOYBEAN WATER USE IN SHELTER
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RESULTS
Energy balance A summary of mean morning and afternoon energy balance in the open and shelter is given in Table II. Because of full canopy development by the time the study was undertaken soil heat flux was negligible. The windbreak decreased LE and A during periods of strong wind speed and sensible heat advection (positive A in open). The effect was minor during periods of low wind speed. The grand mean differences in LE and A for all days presented are significant at the 5% level of probability. An example of the windbreak effect on LE and A on a day with strong wind and sensible heat advection is shown in Fig.2. On this day (July 15) a frontal passage during the morning was accompanied by strong winds. Strong sensible heat advection occurred 8
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412
D.R. MILLER,N. J. ROSENBERGAND W. T. BAGLEY
from 09h00 to 1 lh00. The effect diminished as wind speed decreased in the afternoon. LE in the shelter was reduced by about 40% during the period of strong wind. Sensible heat delivery to the unsheltered plants (positive A) continued throughout the day. The advection of sensible heat to the transpiring crop was reduced by the windbreak. During much of the day sensible heat was generated (negative A) in the protected area. The ratios of l LE I/RN for this day were 1.62 and 1.23 for unsheltered and sheltered sites. Fig.3 shows that the effect of the windbreak on LE and A was minor on July 22, a day of light winds. A cyclic pattern of sensible heat generation at the surface in the 8
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Fig.3. Sameas Fig.2 for July 22, 1969. morning and return to the surface in the afternoon is apparent on this day. The windbreak damped the amplitude of the cycle slightly. The significant shelter-induced reduction of LE during periods of sensible heat advection is in agreement with Brown and Rosenberg's (1971) results showing that shelter decreased LE by sugar beets in western Nebraska during periods of strong atmospheric evaporative demand. Their finding of increased LE in shelter under low stress is not supported by these results, however.
SOYBEAN WATER USE IN SHELTER
413
Gradients of temperature and vapor pressure Average morning and afternoon gradients of T a n d e in the open and shelter are given in Table II. Vapor pressure and air temperature gradients were altered in shelter. The psychrometers used measure AT and Ae with maximum error estimated at less than 0.2°C TABLE III Mean morning (08h00-1 lh45) and afternoon (12h00-15h45) vertical exchange coefficients (K) and wind speeds in open and shelter Date
Time
K (cm 2 sec-t) open shelter
K(shelter) K(open)
Wind speed (m sec-1 ) open shelter
July 14
a.m. p.m.
2,750 4,580
2,070 2,480
0.75 0.54
2.0 2.0
1.5 1.6
July 15
a.m. p.m.
3,440 3,190
2,440 2,490
0.71 0.78
2.1 1.6
1.7 1.3
July 17
a.m. p.m.
2,560 2,940
650 990
0.25 0.33
1.4 1.5
1.1 1.2
July 18
a.m. p.m.
1,930 1,330
980 1,210
0.50 0.90
0.6 1.1
0.5 0.8
July 22*
a.m. p.m.
2,310
1,900 1,850
0.80
1.2 1.3
1.1 1.1
July 24
a.m. p.m.
2,420 1,640
1,190 1,140
0.49 0.69
1.1 0.8
0.8 0.7
Mean:
2,645**
1,590
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1.12
*K calculated using vapor pressure gradient between 125-200 cm. (this day only). **Mean difference significant at the 5% level of probability. and 0.3 mbar. During periods of advection, when temperature gradients were inverted in the open, they remained lapse in shelter. This is particularly evident in Fig.4 for July 14. Temperature gradients between 200 and 100 cm were inverted in the open but were lapse in shelter for a large part of the day. On days with lapse conditions in the open the temperature gradient was intensified by shelter. Vapor pressure gradient, always lapse during the daytime, was usually intensified by shelter.
Exchange coefficient Lysimetrically determined K, defined by eq.2, estimates the intensity of vertical water vapor transport. Mean values of K in shelter and in the open, calculated using vapor pressure
414
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Fig.4. Temperature gradients between 200-100 cm above ground in open and sheltered soybeans. July 14, 1969 at Mead, Nebraska. gradients between 200 and 100 cm above ground surface, are given in Table III for days of widely differing weather conditions. July 14 and 15 were clear with strong sensible heat advection. July 17 was overcast and moderately windy. July 18 was cloudy and calm. July 22 and 24 were clear and calm. Values of K in the open were high on days with strong wind. On calm days under non-advective conditions, K was substantially lower in the open. The windbreak reduced K significantly during periods of strong wind when the barrier caused a significant reduction in wind speed (Table III). When wind speed was low, the windbreak effect on K was small. The average reductions in K of from 10 to 75% are similar to those reported by Brown and Rosenberg (1972). The increase in K with increasing wind speed, for the range of wind speed encountered in the study, is shown in Fig.5. The regression equation fitted is K (cm 2 sec-1) = 387 + 5.81 U(cm secl).
C02 gradients and flux Because of a failure in one CO2 sampling line direct comparisons of above-canopy CO2 gradients could not be made. Table IV reports mean CO2 gradients from a point above to a point into the canopies. Data are variable and when CO2 flux is calculated using the exchange coefficients of Table III, a slight decrease in shelter during windy periods is observed. Variability in crop canopies and plant growth during the experimental period may have been responsible for this effect. DISCUSSION AND CONCLUSIONS
Brown and Rosenberg (1971) found by Bowen ratio-energy balance techniques that exchange coefficients were reduced in shelter. Results of lysimetric measurements reported here confirm this finding.
SOYBEAN WATER USE IN SHELTER
41 5
TABLE IV Mean morning ( 0 8 h 0 0 - 1 1 h 4 5 ) and afternoon ( 1 2 h 0 0 - 1 5 h 4 5 ) [CO 2 ] gradients (175 c m - 7 5 cm) and CO: vertical flux in open and shelter
Date
Time
A CO 2 (p.p.m. m -~ ) open
Flux of CO z (gin cm -~ sec-~ • 10-7) open shelter
shelter
July 14
a.m. p.m.
2,6 2.9
3.2 2.5
1.6 2.3
0.9 0.4
July 15
a,m. p.m.
6.3
4.4
3.4
1.8
July 17
a.m. p.m.
3.7 6.2
7.4 8.9
1.6 3.1
0.8 1.6
July 18
a.m. p.m.
11.3 12,4
16.4 15.7
3.2 3.2
2.7 3.3
July 22
a.m. p.m.
11.0 7,0
10.4 9.0
1.7
3.3 2.9
July 24
a.m. p.m.
9.5 -
11.7 -
3.5 -
2.6 _
Mean:
7.3
9.0
2.6
1.9
Adequate "fetch" is prerequisite to the application of micrometeorological methods in estimating mass transfer. Windbreaks by definition, however, destroy fetch and appropriate assumptions or adjustments must be made in order to apply micrometeorology to the shelter problem. Brown and Rosenberg (1971) were also able to show that a boundary layer of measurable thickness, characteristic of the sheltered surfaces, developed within 2H of the corn windbreak sheltering sugar beets. So long as the surface is approachable the measurements of flux and/or gradient can be meaningful. There is no better way to approach the surface than with a lysimeter. 50i
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416
D. R. MILLER, N. J. ROSENBERG AND W. T. BAGLEY
Brown and Rosenberg (1971) also consider that a number of the microclimatic changes found consistently in shelter can be explained as due to the decrease in turbulent exchange. These m'_'croclimatic effects include increased vapor pressure and temperature gradients caused by slowed dispersion of transpired water vapor and transport of sensible heat. Our findings of reduced exchange coefficient and increased intensity of vapor pressure and temperature gradients are consistent with the above. Similar reasoning can explain an increased [CO2] gradient, but the CO2 observations reported here appear inconclusive with regard to this point. Data on stomatal aperture and diffusion resistance (reported elsewhere by Miller, 1971), however, indicate that the sheltered soybeans maintained wider stomatal aperture and lower resistance to diffusion. A lower CO2 concentration would be expected near the canopy of the sheltered crop. Transport of CO2 downward to replace that used in photosynthesis is, however, slower in shelter, resulting in larger gradients. Experimental errors in calculation of photosynthetic flux using measured exchange coefficients and CO2 gradients has been explored by Brown and Rosenberg (1968) and more recently in greater detail in Sestak et al. (1971). The magnitude of the differences reported here fall within experimental error and the effect must, therefore, be considered unproven. There is much evidence of increased plant growth in wind shelter compiled by researchers and summarized by Van Eimem et al. (1964) and Stoeckeler (1962). These increases must result from greater seasonal net assimilation in shelter. Reports of reduced stomatal resistance in shelter by Brown and Rosenberg (1970) and Miller et al. (1971) indicate increased photosynthetic opportunity. Therefore, the slight reductions in calculated CO2 flux were unexpected. In the calculations of eq.3 the increase in the CO2 gradient was more than offset by the decrease in K. Either the assumption K = KE = KCO~ is in error or the net photosynthesis in shelter is increased by other processes. A nocturnal decrease in respiration in shelter because of lower plant temperatures might explain this effect. Mean nocturnal leaf temperature was 0.35°C lower in shelter during the study. We note an apparent insensitivity of photosynthetic flux rates (Table IV) to changing solar radiation conditions (Table I), regardless of shelter. Rosenberg (1972) reports that soybeans of the Amsoy variety become light saturated, particularly when dense canopy structure is developed at solar radiation intensities of about 1.0-1.2 Ly min -~ . During the experiment reported this intensity occurred at between 08h00 and 10h00 in the morning. This may be the reason that the soybean crop did not show significant photosynthetic response to shelter. Brown and Rosenberg (1970) also found photosynthesis of sugar beet unaffected by shelter. Water use in shelter was reduced most sharply during periods of strong sensible heat advection. This reduced latent heat flux in shelter is evidently due to the physical reduction in quantity of warm air transported to the sheltered crop. Also involved is the reduced intensity of turbulent transport. The apparent lack of a shelter effect on CO2 flux in this study, coupled with the definite decrease in latent heat flux indicates that wind shelter, used over short periods or an entire season, can be effective in increasing water use efficiency by soybeans.
SOY BEAN WATER USE IN SHELTER
417
ACKNOWLEDGMENTS S u p p o r t for this p r o j e c t was received f r o m M c l n t i r e - S t e n n i s F o r e s t r y R e s e a r c h F u n d s a n d f r o m the Office o f Water Resources Research D e p a r t m e n t o f the I n t e r i o r , u n d e r t h e Public Law 88-379 p r o g r a m .
REFERENCES Brown, K. W. and Rosenberg, N. J., 1968. Errors in sampling and infrared analysis of CO: in air and their influence in determination of net photosynthetic rate. Agron. J., 6 0 : 3 0 9 - 3 1 1 . Brown, K. W. and Rosenberg, N. J., 1970. Effect of windbreaks and soil water potential on stomatal diffusion resistance and photosynthetic rate of sugar beets (Beta vulgaris). Agron. J., 62: 4 - 8 . Brown, K. W. and Rosenberg, N. J., 1971. Turbulent transport and energy balance as affected by a windbreak in an irrigated sugar beet field. Agron. J., 6 3 : 3 5 1 - 3 5 5 . Brown, K. W. and Rosenberg, N. J., 1972. Shelter effects on microclimate, growth and water use by irrigated sugar beets in the Great Plains. Agric. Meteorol., 9: 241-263. Cochran, W. G. and Cox, G. M., 1968. Experimental Designs. Wiley, New York, N.Y., 2rid ed., 611 pp. Denmead, O. T., 1969. Comparative micrometeorology of a wheat field and a forest ofPinus radiata. Agric. Meteorol., 6: 357-372. Fritschen, L. J., 1965. Miniature net radiometer improvements. J. Appl. MeteoroL, 4: 523-532. George, E. J., 1971. Effect of tree windbreaks and slat barriers on wind velocity and crop yields. U.S.D.A., Agric. Res. Serv. Prod. Res. Rep., 121:23 pp. Inoue, tL, 1965, On the CO 2 concentration profiles within crop canopies. J. Agric. Meteorol. (Japan), 2 0 : 1 3 7 - 1 4 0 (Japanese with English summary). Lemon, E., 1969. Gaseous exchange in crop stands. In: J.E. Eastin, F.A. Haskins, C.Y. Sullivan and C. H.M. van Bavel (Editors), Physiological Aspects o f Crop Yield. Am. Soc. Agron., Madison, Wisc., pp. 117-136 Marshall, J. K., 1967. The effect of shelter on the productivity of grasslands and field crops. Field CropAbstr., 20: 1-14. Miller, D. R., 1971. Lysimetric and Energy Balance Determination of Slatfence and Tree Windbreak Effects on Water Use Efficiency under Irrigated and Dryland Conditions. Ph.D. Thesis, University of Nebraska, Lincoln, Nebraska, 280 pp. (unpublished). Miller, D. R., Bagley, W. T. and Rosenberg, N. J., 1971. Modification of microclimate with tree shelterbelts in the Great Plains. A bstr. Bull. Am. MeteoroL Soc., 51: 206. Monteith, J. L., 1962. Measurement and interpretation of CO 2 fluxes in the field. Neth. J. Agric. Sci., 10: 334-346. Rosenberg, N. J., 1966. Microclimate, air mixing, and physiological regulation of transpiration as influenced by wind shelter in an irrigated bean field. Agric. MeteoroL, 3: 197-224. Rosenberg, N. J., 1967. The influence and implications of windbreaks on agriculture in dry regions. In: R. A. Shaw (Editor), Ground Level Climatology. Am. Assoc. Advan. Science, Washington, D.C.,pp.327-349. Rosenberg, N. J. and Brown, K. W., 1970. Improvements in the Van Bavel-Myers automatic weighing lysimeter. Water Resour. Res., 6: 1227-1229. Rosenberg, N. J. et al., 1969. Research in Evapotranspiration. Report to the Office of Water Resources Research on Project A-001 NEB. Univ. of Nebraska Horticulture Progress Report 73, 153 pp. Rosenberg, N. J. et al., 1972. Simultaneous Determination of Short-Period Photosynthesis and Evapotranspiration. Final Report to NOAA on Grant E-293-68 (G). Univ. of Nebraska Horticulture and Forestry Progress Report 91, 61 pp. Sestak, Z., Catsky, J. and Jarvis, P. G., 1971. Plant Photosynthetic Production - Manual of Methods. Junk, The Hague, 819 pp.
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Stoeckeler, J. H., 1962. Shelterbelt influence on Great Plains field environment and crops - a guide for determining design and orientation,Prod. Res. Rep., USDA Forest Serv., 62:25 pp. Van Eimem, J., Karschon, R., Razumova, L.A. and Robertson, G.W., 1964. Windbreaks and shelterbelts. WorldMeteorol. Org., Tech. Note, 59:188 pp. Van Hylckama, T. E. A., 1969. Photosynthesis and water use by salt-cedar. Bull. Int. Assoc. Sci. Hvdrol., 14: 71-83.