Changes in canopy resistance to water loss from alfalfa induced by soil water depletion

AgriculturalMeteorology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

C H A N G E S IN CANOPY RESISTANCE TO W A T E R LOSS FROM A L F A L F A I N D U C E D BY SOIL W A T E R D E P L E T I O N 1 C. H. M. VAN BAVEL U. S. Water Conservation Laboratory, Phoenix, Ariz. (U.S.A.)

(Received May 25, 1966)

SUMMARY

A field of alfalfa was irrigated and then allowed to deplete soil water reserves close to the --15 bar percentage during June 1964 in central Arizona. The average potential evaporation rate during the experiment was 9 mm/day. From hourly measurements of the energy balance and ambient conditions over the field, the ratio between actual and potential evaporation was followed, and, where significantly less than unity, interpreted as a canopy resistance to evaporation. This resistance did not become noticeable during the daytime until 20 days after irrigation, and then rose rapidly to an average value of 15 sec/cm on the last day of measurement, 31 days after irrigation. When significant, the canopy resistance typically showed a diurnal course with low values after sunrise, increasing up to 20-fold during early afternoon and then diminishing again. The data imply that the alfalfa stomata exercised a regulatory role by holding the evaporation to a maximum value during the diurnal cycle. The maximum itself diminished from day to day as soil water was depleted. The soil water potential in the root zone was estimated at - - 4 bars when stomatal control of evaporation became first noticeable. Further decrease of the soil water potential brought about sharply rising values of the daytime values of the canopy resistance. Qualitatively, this observation supports other evidence on the relation between soil water availability and water use by crop plants.

INTRODUCTION

The potential evaporation rate from a crop surface can be defined accurately in terms of ambient weather conditions and the aerodynamic nature of the evapo-

1 Contribution from the Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture. This work was supported in part by the Atmospheric Sciences Laboratory, Research Division, U.S.A., ECOM, Fort Huachuca, Ariz. Agr. MeteoroL, 4 (1967) 165-176

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rating surface. This so-called combination model is tile result of early work by PENMAN (1948), FERGUSON(1952) and BUDYKO(1956) and has been refined by others since. A recent publication (VAN BAVEL, 1966) traces these developments and gives data, obtained over wet surfaces, showing that the combination model holds true for periods as short as one hour under exacting ambient conditions. The combination model is expressed by the equation for the potential evaporation rate: Eo

----4

~H/L. . . + g/cm~/sec . . . . . . .peda/pRA ...

(I)

in which Ra, following MONTEITH and SzEICZ (1962) represents an aerodynamic "resistance" defined as: RA .

(ln ZdZo)~ 1 . . . sec/cm kz u,~

(2)

The explanation of symbols is given in the Appendix. Equivalent expressions, using turbulent transfer coefficients and practical units, can be found, for example, in VAN BAVEL(1966). AS defined by eq. (1), the potential rate would obtain if, under given atmospheric conditions, the entire surface were covered with an infinitesimally thin layer of water, similar to the surface of a wet blotting paper. In reality, the preponderant part of the water evaporated from a crop surface originates from within the leaf. Therefore. it must be transported by diffusion through the interstitial leaf spaces and the stomata, requiring a modification of eq. (1) by introduction of a leaf or canopy transport coefficient or, more conveniently, an equivalent resistance. Following earlier work by PENMAN and SCHOFtELD (1951) and by RASCHKE (1962), the combination equation for actual evaporation can be written inthe terms used by MONTEITHet al. (1965) as: E =

~H/L 4- peddp RA ~ ~ i +IRs)-RA .... g/cm~/sec

(3)

defining Rs as a canopy resistance in sec/cm. It is necessary to remark that the validity of equations (1), (2), and (3) is insured only if measurements of ambient conditions are made over and close to the evaporating crop surface. A current question is whether, for a well-watered crop, this additional resistance, Rs, to water vapor transport from the evaporating surfaces to the ambient air results in actual evaporation rates substantially below the potential ones. The studies made by the author with alfalfa, (VAN BAVEL, 1966), imply that, during daylight hours, canopy resistance is of no consequence provided the alfalfa is well-watered. As soil drying proceeds, evaporation from an alfalfa stand may fall suddenly from the potential rate to a quite low level, or this condition may be approached more gradually. Correspondingly, at the incidence of a certain condition in the plant, stomata may close rapidly and completely, or they may exercise a more refined type of control in which gradual closure results in a balance between water gain and loss. In either

Agr. MeteoroL,4 (1967) 165-176

C H A N G E S IN C A N O P Y RESISTANCE T O W A T E R LOSS F R O M A L F A L F A

167

case, a measurement of the value of Rs is a logical method to characterize plant response to soil water depletion. Theoretical studies by GARDNER (1960), and experiments under controlled conditions with individual plants reported by GARDNER and EHLm (1963), suggest a gradual closure of stomata as soil moisture becomes depleted beyond a threshold value which depends upon soil water content and soil water conductivity, as well as plant properties and evaporative demand. Similar observations with corn plants in containers, but in a natural above-ground environment, have been reported by DENMEAD and SHAW (1962). A recent study reported by PALMER et al. (1964), with cotton grown in containers in a greenhouse, confirms this pattern. Illustrative data from these three studies, all based on daily observation, are reproduced here as Fig. l, in which considerable quantitative differences can be seen, though the pattern of response is the same. The differences may have been caused by different hydraulic properties of the soils involved and by different crop responses to water deficits. A general discussion by GARDNER (1960) attempts to analyze the mutual relation between plant and soil factors. A unified theory accounting for atmospheric demand as well has not yet been developed. Our studies were aimed at a quantitative description of the response of alfalfa plants to diminishing soil water supplies under typical field conditions in a hot, arid climate. Further, the present work represents an attempt to study the interaction between soil, plant and weather through a detailed, hourly analysis. Almost all earlier studies report daily values only. Recent work by MONTEITH et al. (1965) gives hourly values for Rs for barley. The crop was always well-watered, however, and, furthermore, Rs was not calculated from expression (3) but from profile data, a procedure that has been questioned (MoNTEITH, 1963).

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Fig.1. The relation between soil water potential and relative transpiration for three different crops. The figure above the authors indicates the potential transpiration rate in mm/day. Agr. Meteorol., 4 (1967) 165-176

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C H A N G E S IN C A N O P Y RESISTANCE TO W A T E R LOSS F R O M A L F A L F A

169

EXPERIMENTAL PROCEDURE

The nature and methods of the observations are described in other references (FRITSCHEN and VAN BAVEL, 1962, 1963; VAN BAVELand REGINATO, 1965). In brief, hourly evaporation rates were measured in triplicate with weighable lysimeters, and net radiation, temperature, humidity and windspeed over the alfalfa field were recorded half-hourly. Soil heat flow was not measured; it is small under a welldeveloped canopy of alfalfa. The field, about 0.8 ha in size, was irrigated by flooding on 28 May 1964 and again on 1 July 1964. The alfalfa was trimmed to about 20 cm height on 4 June, leaving a well-developed canopy. No rain occurred and clear weather prevailed for the entire period, except on 26 June when the sky was partially cloudy. Potential evaporation was computed for the daytime hours of 13 days using equation (1) and (2). In the beginning of the period the record of every fifth day was studied; later, of every other day as the evaporation reduction became apparent. For days on which the recorded evaporation rate fell markedly below the potential rate, the canopy resistance to evaporation was computed for daylight hours with an expression that follows from (1), (2) and (3): Rs

(In za/zo) 2 (~ + 1) (Eo/E - - 1)/k 2 ua

(4)

With the exception of ua, the entire expression (4) is in dimensionless numbers and Rs is obtained in sec/cm, corresponding to RL, the leaf diffusion resistance (VAN BAVELet al., 1965). The water content of the root zone was measured periodically with the neutron method, using 5 cm diameter access tubes in the center of each weighable lysimeter and at three field sites in the vicinity of the lysimeters. Crop growth was recorded twice-weekly as the average height of the stand. The entire period of observation, straddling the summer solstice and typified by the usual high pressure pattern and valley breezes, was one of remarkable climatic uniformity as may be seen from Table I. Air temperatures climbed gradually during the period from maxima around 30°C to 38°C.

RESULTS A N D DISCUSSION

Calculation of canopy resistance

A summary of the experimental results is given in Fig.2, where daily totals of actual and potential evaporation are plotted. These totals pertain to the period of daylight only, for reasons to be stated later. It is evident that the irrigation date is followed by a period of about 20 days on which the evaporation approximated the potential rate, after which it declined almost linearly with time from an initial rate of about 9 mm/day to a value of 2 mm/day, 11 days later. The regularity of this pattern Agr. Meteorol., 4 (1967) 165-176

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is brought out more clearly in Fig.3, which shows E/Eo for daily periods, Fig.3 also gives the height of the alfalfa stand, showing that by 15 June most of the growth had ceased. A roughness parameter Of 1 cm, based upon earlier work (VAN BAVEL, 1966), was used in all calculations. This may have been an overestimate from 5 to 8 June and an underestimate after 15 June. Hourly computations of Rs were made starting with 18 June for every other day. It was soon discovered that the results obtained for the hours of darkness were erratic because of generally low values for the critical parameters, ua, E and Eo, Agr. Meteorol., 4 (1967) t65-176

CHANGES IN CANOPY RESISTANCE TO WATER LOSS FROM ALFALFA

171

with correspondingly high relative errors. In view of the instrumental limitations, the computation of nighttime values for Rs was abandoned. Three progressive stages of drying are illustrated in Fig.4, 5 and 6 pertaining to 20, 24, and 28 June, respectively. The figures show the potential evaporation rate, the actual rate, and the canopy resistance in sec/cm as derived from both. On 20 June during the morning hours, one sees the first occurrence of significant values of Rs, increasing to 1.2 sec/cm in the early afternoon. A clearer response of the stomatal mechanism was observed four days later, as in Fig.5, with the maximum value of Rs being around 12 sec/cm. The nature of the response suggests a proportional control system that, by first closing and later opening the stomata, appears to maintain an evaporation rate of about 0.35 mm/h in spite of a highly variable demand. 1.2 z

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Fig.4. Actual (E) and potential (Eo) hourly evaporation from alfalfa in Phoenix (Ariz.) 23 days after irrigation. Also, hourly values of canopy resistance in sec/cm during daylight hours. 1.2 ~ 1.0

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Fig.5. Actual (E) and potential (Eo) hourly evaporation from alfalfa in Phoenix (Ariz.) 27 days after irrigation. Also, hourly values of canopy resistance in sec cm -1 during daylight hours.

Agr. Meteorol., 4 (1967) 165-176

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Fig.6. Actual (E) and potential (E 0) hourly evaporation from alfalfa in Phoenix (Ariz.) 31 days after irrigation. Also, hourly values of canopy resistance in sec/cm during daylight hours:

On 28 June soil drying had progressed further and the regulating stomatal resistance is even better portrayed. Evaporation rate is---during daylight--held at a constant rate of about 0.20 mm/h, requiring Rs values of up to 24 sec/cm. Over the entire range of observation there is no suggestion of a "midday depression,' in the evaporation rate, a phenomenon often alluded to in the literature and sometimes confused with midday wilting. ECKARDT(1960) has suggested that the classical method of measuring transpiration by excision and weighing of a leaf or branch could be responsible for erroneous ideas about the daily course of plant transpiration. A recent study by BRUN (1965) discusses the excision effect in detail. + 600

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Fig.7. Principal components of the daily energy balance of irrigated alfalfa at Phoenix (Ariz,) in 1964 during an irrigation cycle.

Agr. Meteorol., 4 (1967) 165-176

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Fig.8. Principal components of the hourly energy balance of irrigated alfalfa at Phoenix (Ariz.) in 1964, 8 and 27 days after irrigation.

Our observations demonstrate that the canopy resistance varies considerably during the daylight period--typically by a factor of 5 10--and, therefore, a day value cannot realistically be defined. For practical purposes, nevertheless, a nominal value can be calculated using eq. (1) with daily average figures for the weather parameters. These values for Rs are portrayed in Fig.3. Energy balance and canopy resistance

By inference, development of an appreciable canopy resistance as a result of soil moisture depletion is accompanied by a drastic change in the surface energy balance. Typically, in the central Arizona climate, well-watered alfalfa extracts sensible heat from its environment on a 24-h basis and does so during the entire day with exception of the early morning hours. As Fig.7 shows, this pattern is reversed as the canopy itself begins to control evaporation. Accordingly, the inversion gives way to a lapse--at least during the daytime. To illustrate this point further, an hourly energy balance is given in Fig.8 for two days, 5 June and 24 June, having almost identical weather conditions. Canopy resistance and soil water

Under the conditions of the experiment, the soil water potential was determined by both the soil water content, measured periodically with the neutron method, and by the salinity of the soil solution. The latter was considerable since the irrigation Agr. Meteorol.,4 (1967)165-176

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Fig.9. Water potential in the root zone in bars × (-1) and canopy resistance in sec/cm of irrigated alfalfa at Phoenix (Ariz.) in 1964 during an irrigation cycle. 1.2 Z

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Fig.t0. Ratio of actual to potential evaporation (from Fig.3) versus water potential in the root zone in bars (from Fig.9) during an irrigation cycle of alfalfa in Phoenix (Ariz.) in 1964. water contained 800-1,000 p.p.m, total salts and practically no leaching took place in the lysimeters over a seven-month period prior to the experiment. What little effluent was collected during June showed a 7,000 p.p.m, total salt content and it was assumed in the absence o f direct measurement that this was the average concentration in the saturated soil during the month of June. From the measured soil water content and the known relation between pressure potential and water content plus the estimated concentration of the soil solution, the total water potential was calculated. In Fig:9 this value is plotted together with the (nominal) day value for Rs, suggesting that significant values o f Rs (say, in excess of 0.1 sec/cm) are not occurring above a soil water potential value of around - - 4 Agr. Meteorol., 4 (1967) 165-] 76

CHANGES IN CANOPY RESISTANCETO WATER LOSS FROM ALFALFA

175

bars, in spite of high values for the e v a p o r a t i o n rate itself. In c o m p a r i n g this figure with the data from GARDNER a n d EHLIG (1963), it should be remembered that our figures for the water potential are n o m i n a l only a n d that higher values occurred in the lowest part of the root zone. In the greenhouse experiments of G a r d n e r a n d Ehlig the root volume was more confined a n d water u n i f o r m l y distributed. Fig.10 summarizes the results in a m a n n e r c o m p a r a b l e to the relationships d r a w n from the literature that were given in Fig.1. Fig. 1 0 - - c o m b i n e d from Fig.3 and F i g . 9 - - a g r e e s with earlier studies in showing a gradual d i m i n u t i o n of the E/Eo ratio with decreasing soil water potential below a threshold value. The general shape of our alfalfa curve is n o t too different from the result obtained by G a r d n e r and Ehlig for birdsfoot trefoil, though the former is based u p o n field data and the latter u p o n a pot experiment.

REFERENCES

BRUN, W. A., 1965. Rapid changes in transpiration in banana leaves. Plant Physiol., 40 : 797-802. BVDVKO, M. I., 1956. The Heat Balance of the Earth's Surface. Engl. transl.: U.S. Dept. Comm, Office Tech. Serv., Washington, D.C., 1958, 259 pp. DENMEAD,O. T. and SHAW,R. H., 1962. Availabilityof soil water to plants as affected by soil moisture content and meteorological conditions. Agron. J., 45 : 385-390. ECKARDT,F. E., 1960. Eeo-physiological measuring techniques applied to research on water relations of plants in arid and semi-arid conditions. In: Plant-Water Relationships in Arid and Semi-arid Conditions--UNESCO Arid Zone Res., 15 : 139-171. FER~USON, J., 1952. The rate of natural evaporation from shallow ponds. Australian J. Sci. Res., Ser. A, 5 : 315-330. FR1TSCHEN,L. J. and VAN BAVEL,C. H. M., 1962. Energy balance components of evaporating surfaces in arid lands. J. Geophys. Res., 67 : 5179-5185. FmTSCHEN, L. J. and VAN BAVEL, C. H. M., 1963. Micrometeorological data handling system. J. Appl. Meteorol., 2 : 151-155. GARDNER,W. R., 1960. Dynamic aspects of water availability to plants. Soil Sci., 89 : 63-73. GARDNER, W. R. and EHLIG, C. P., 1963. The influence of soil water on transpiration by plants. J. Geophys. Res., 68 : 5719-5724. MONTEITH,J. L., 1963. Gas exchange in plant communities. In: L. T. EVANS(Editor), Environmental Control of Plant Growth. Academic Press, New York, N.Y., pp. 95-112. MONTEITH,J. L. and SZEICZ,G., 1962. Radiative temperature in the heat balance of natural surfaces. Quart. J. Roy. Meteorol. Soc., 88 : 496-507. MONTEITH, J. L., SzEICZ,G., and WAGGONER,P. E., 1965. The measurement and control of stomatal resistance in the field. J. Appl. Ecol., 2 : 345-355. PALMER,J. H., TRICgET'r,E. S. and LINACRE,E. T., 1964. Transpiration response of Atriph,x nummularia and upland cotton vegetation to soil moisture stress. Agr. Meteorol., 1 : 282-293. PENMAN, H. L., 1948. Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc. (London), Ser. A, 193 : 120-145. PENMAN,H. L. and SCHOFIELD,R. K., 1951. Some physical aspects of assimilation and transpiration. Symp. Soc. Exptl. Biol., 5 : 115-129. RASCHKE, K., 1962. Heat transfer between the plant and the environment.lAnn. Rev. Plant Physiol., 13 : 111-126. VAN BAVEL, C. H. M., 1966. Potential evaporation: the combination concept and its experimental verification. Water Resources Res., 2 : 455-467. VAN BAVEL,C. H. M. and REGINATO,R. J., 1965. Precision lysimetry for direct measurement of evaporative flux. In: Methodology of Plant Eco-physiology-- UNESCO Arid Zone Res., 25 : 129-135. VAN BAVEL,C. H. M., NAKAYAMA,F. S. and EHRLER,W. L., 1965. Measuring transpiration resistance of leaves. Plant Physiol., 40 : 535-540. Agr. Meteorol., 4 (1967) 165-176

176

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APPENDIX

List of symbols da E Eo e H L k p RA RS Ris Rn ta ua za zo e. O

saturation vapor pressure deficit of air (mbar) evaporation rate (g/cmZ/sec) potential evaporation rate (g/cm2/sec) water/air molecular ratio (0.622) sum of energy inputs at surface exclusive of sensible and latent heat (cal/cm2/sec) latent heat of vaporization (cal/g) Von Karman coefficient (0.41) ambient pressure (mbar) turbulent diffusion "resistance" (sec/cm) canopy diffusion resistance (sec/cm) short wave incoming radiation (cal/cm2/day) net radiation (cal/cm2/day) temperature of air ('C) windspeed at level Za (cm/sec or m/sec) elevation above crop surface (cm) roughness parameter (cm) dimensionless parameter (A/y) density of air (g/cm a)

Agr. Meteorol., 4 (1967) 165-1176