daylength factor (dimensionless) Penman ventilation term (g/crn2/dayl)

Transcription

1 ESTIMATING WATER USE BY SUGARCANE FROM METEOROLOGICAL AND CROP PARAMETERS G. D. Thompson and J. P. BoyceX S.A.S.A. Experiment Station, Mount Edgecornbe, Natal South Africa ABSTRACT Evapotranspiration from sugarcane crops was measured weekly by means of hydraulic lysimeters at Chaka's Kraal and daily by means of weighing lysimeters at Pongola. The meteorological parameters necessary for the estimation of evapotranspiration by 4 methods were measured daily at both sites. The roughness length of sugarcane was estimated from wind profile data on 10 occasions at Pongola. The mean value obtained was t cm, and this was used to calculate the aerodynamic resistance at both sites. The Chaka's Kraal data were used to calculate values for e (the equivalent of a stomatal impedance-daylength factor) and for r, (a surface resistance term). Four models for estimating evapotranspiration (E,) were then tested using the daily Pongola data. These were: 1) the Penman equation; 2) the Penman equation with Businger's proposal for the aerodynamic resistance (r,); 3) the Penman and Schofield equation, including a substitute for their stomatal impedance-daylength factor (E) 4) the equation proposed by Monteith, including a surface resistance term (r,). The results showed that t'he Penman equation provided satisfactory estimates of E, despite an inapplicable wind function. The Businger modification for potential evaporation led to a cross overestimate of E,. When the equivalent of a stomatal impedance-daylength factor was incorporated, the situation was largely retrieved. The best estimates resulted from the use of Monteith's formula. If the Businger equation provides a reasonable estimate of potential evaporation, then the results show clearly that actual evapotranspiration from sugarcane falls well below the potential level, due to a real resistance to the diffusion of water vapour from the crop canopy, even when the soil moisture deficit seldom exceeds 5 cm. In the light of the increasing complexity of models of general applicability for predicting E, it is suggested that the requirements of practicing agriculturists may be met more conveniently by using reliable empirical relationships, such as that between E, and evaporation from a Class A pan. However, the surface resistance parameter may still be used to advantage in exploring soil-plant-atmosphere relationships. SYMBOLS USED D Ea daylength factor (dimensionless) Penman ventilation term (g/crn2/dayl) "Present address: Cranbrook Agricultural Consultants Ltd.: P. 0. Maidstone, Natal.

2 814 IRRIGATION evapotranspiration when surface resistance = rso (mm/day-1) open water evaporation (mm/day-1) potential evaporation (mm/day-l) evapotranspiration (mm/dayl) soil heat flux (cal ~m-~/day-l) bulk Richardson number net radiation (cal ~m-~/day-l) stomata1 impedance factor (dimensionless) specific heat of air (cal gm-l/"c-l), zero plane displacement (cm) saturated water vapour pressure at mean air temperature (mb) water vapour pressure in air 2 m above crop (mb) wind function (g ~m-~/day~/mb-~) crop height (cm) von Icarman constant (0.42) number of observations aerodynamic resistance (sec/cm-l) surface resistance (sec/cm-l) minimum surface resistance (sec/cm-l) run of wind (cm/day-l) (mileslday-1) height above ground level (cm) roughness length (cm) slope of saturated vapour pressure curve at mean air temperature (mb/"c-l) resistance coefficient (= I/SD) (dimensionless) psychrometric constant (mb/"c-1) I density of dry air (1.29 g/~m-~) soil water potential (bars) INTRODUCTION Penman (1948) showed that evapotranspiration could be estimated from a relationship which included a radiative term (R,) and a ventilation term (E,) as follows: The ventilation term included an empirically determined wind function which was intended (Penman, 1963) to allow for the extra roughness of a crop in comparison with an open water surface: Ea = ft (u) (ea - e,) 9 where f, (u) = 0.35 (1 + ~/100) Penman and Schofield (1951) introduced additional factors to account for stomatal impedance (S) and the daily duration of stomatal opening (D) :

3 G. D. THOMPSON, J. P. BOYCE 815 In discussing this equation, Businger (1956) suggested that l/sd or was approximately equal to Businger (1956) also proposed that the wind function for an adiabatic profile could be represented logically by a relationship having the form: The notation l/r, was used by Monteith (1965) to represent the wind function in equation (4), the term r, being identified as the aerodynamic resistance. He emphasized that the calculation of E, from the combined formula gives the rate of evaporation from a surface thoroughly wetted by rain or dew. This relationship, as pointed out by van Bavel (1966), establishes potential evaporation as a truly micrometeorological concept, and earlier definitions based on water use by a crop "freely supplied with soil moisture" are neither adequate nor any longer necessary. The often-misused concept of potential evapotranspiration was redefined as "water loss from any surface, when it is wet and imposes no restriction on the flow of water vapour," and it is now correctly called potential evaporation. The combination model for potential evaporation is expressed by the equation: If actual evapotranspiration lags behind the potential rate, (E,), an additional restriction or resistance ;o water movement must be operating. Monteith (1965) proposed that such a resistance was real for all surfaces which were not physically wet, and introduced the surface resistance (r,). He showed that transpiration from externally dry leaves in a crop canopy depended upon the ratio r,/r,, and generalized Penman's combination formula for crops with surface resistance as follows: For periods over which the soil heat flux is significant the term R, becomes (R, - G). Lysimetric work wi~h sugarcane was started in South Africa in 1959 with non-weighing lysiineters at 3 sites (Pearson et al.,' 1961), and further developments were reported at the 12th TSSCT Congress (Thompson, 1967). It was shown that evapotranspiration calculatecl from the Penman formula (equation 1) correlated very well with measured amounts of water loss from sugarcane, but the calculated values were consistently lower than the measured ones. Subsequent work carried out with hydraulic lysimeters at Chaka's Kraal (Thompson and Boyce 1967), showed that the experimental data did not fit a linear wind function as proposed by Penman and also that the substitution of Businger's wind function (equation 4), with an estimated roughness length of 9 cm, led to a gross over-estimation of evapotranspiration. This over-estimation of E, using equation (5) constitutes evidence that resistance to water vapour transport may

4 l IRRIGATION have been operative. The conclusion was reached that equations (3) and (6) might be used to estimate actual evapotranspiration of sukarcane, and the results could be compared with measured E, and estimates obtained from equations (1) and (5). The reiitionships between evapotranspiration and meteorological and crop parqmeters were therefore explored further in the 1st ratoon crop of the 1 experimefit at Chaka's Kraal and in the plant crop of a direct-weighing lysimeter experiment which was established at Pongola. The roughness length of sugarcane was also studied on the site at Pongola, which lies in the semi-arid northern area of the South African sugar industry. EXPERIMENTAL 'PROCEDURES rc The layout and design of the experiment at Chaka's Kraal (29" 26'S, 31" 12%) have been described by Thompson and Boyce (1967). Weekly data were obtained for the 1st ratoon crop from four hydraulic lysimeters to estimate actual evapotranspiration from a well-irrigated sugarcane crop. The plant crop was harvested on 12 November, 1965, and the 1st ratoon crop on 20 January, Net radiation was measured by means of a Funk-type net radiometer mounted on a telescopic mast and maintained at a height of 1.5 m above the crop canopy. On days when radiation was not recorded due to power failures or for any other reason, net radiation was estimated from the formula given by Penman (1963), using Campbell-Stokes sunshine-recorder data and a reflection coefficient of Mean daily temperatures were estimated from the average of maximum and minimum temperatures which were obtained from thermometers located in a Stevenson screen maintained 1 m above the crop. The mean daily saturated vapour pressure deficit was calculated on 'the basis of wet and dry bulb temperatures recorded daily at 8 a.m. and 2 p.m. from thermometers also located in the Stevenson screen. Run of wind at 2 m above the crop was measured by means of a cup-type anemometer which was read at 8 a.m. gaily. ' The experiment at Pongola (27" 235, 31" 37'E) was planted on 12 November, 1967 and harvested on 9 October, Descriptions of the site, construction and operation of lysimeters, measurement of precipitation and cultural procedures have been given previously by Thoinpson and Boyce (1971). In addition the following parameters were measured: Net radiation by means of 2 Funk-type net radiometers mounted on telescopic masts and maintained at a height of 1.5 m above the level of the crop. Air temperatures by means of maximum^, minimum, wet and dry bulb thermometers mounted in a Stevenson screen which was raised on metal frames so that the thermometer bulbs were always approximately 2 m above the level of the crop. Run of wind by means of a recording anemometer mounted on a telescopic mast and maintained with cups at a height of 2 m above the level of the crop. The calibrations of the net radiometers (at both sites) were checked against readings of a sub-standard actinometer before the experiments began. Daily potentiometer traces for each radiometer, starting at 8 a.m., were inte~ated planimetrically. The maximum and minimum thermometers were read and

5 G. D. THOMPSON, J. P. BOYCE 817 re-set at 8 a.m. daily, and the wet and dry bulb thermometers were read at 8 a.m. and 2 p.m. daily. Mean daily temperatures were estimated from the maximum and 'minimum values, and mean daily saturation deficits from the 2 wet and dry bulb readings. The run of wind was estimated from the recorder chart, starting daily at 8 a.m. The shape of the wind profile above the crop at Pongola was determined by means of Sheppard anemometers mounted on a mast in a 0.4 ha block of sugarcane adjacent to the lysimeter experiment, as shown in Fig. 1. The anemom- Fig. 1. Sheppard anemometers mounted above sugarcane to study wind profiles and estimate roughlless length. eters were checked by mounting them on a horizontal test bar and recording the counts over repeated 10-minute runs. The agreement between instruments was good in all instances and it was not considered necessary to adjust any of the readings obtained in the profile studies. For the profile woyk, the lowest anemometer was mounted so that the cups rotated only slightly clear of the uppermost crop leaves. Successively higher instruments were usually mounted at increasing intervals above one another. No data were used from anemometers more than 2 m above the level of the uppermost crop leaves, and as the fetch over uninterrupted sugarcane was 110 m in both wind directions, the approximate effective ratio of anemometer height to fetch distance was 1 :55. Run of wind was measured in all instances over 10-minute periods, and a series of 10 successive measurements was usually made. During the 10-minute wind measurement periods, wet and dry bulb temperatures were recorded at 1- minute intervals from Assman ventilated psychrometers mounted 1 m and 2 m above the crop. For successive 10-minute periods the instruments were moved

6 from the lower to the upper position and vice versa. To augment a limited amount of data obtained during the plant crop of the experiment, additional results obtained during the 1st ratoon crop were also included, so that a total of 10 sets of observations were available. Roughness Length, z, RESULTS The roughness length of the crop was estirnated'for each of the 10 occasions in the plant and 1st ratoon crops at Pongola when satisfactory wind profile data were obtained. This was accomplished by the iterative method outlined by Lemon (1963), whereby successive values of d were substituted in the plot of In (z - d) against wind speed, until a minimum value for deviations from linear regression was obtained in the semi-logarithmic relationship. Tile extrapolation of this linear regression line to zero wind speed, then gave a value of (z - d) equal to z,. The 10 occasions providing acceptable data were limited mainly to those when atmospheric conditions approximated neutral stability, i.e., when the Richardson number lay between the limits of and reported by Tanner (1968). The results are shown in Table 1. It will be observed that the estimated Table 1. Estimates of roughness length (2,) and zero plane displacement (d) at Pongola. Wind Zero plane (Z - dl speed at Crop displace- Roughness when 2 m height ment, length z= h +ZOO Richardson Date - (cm/secl) (cm) (cm) (cmj (cm) number 10/1/ /1/ /3/ , /6/ / /1/ /1/ /2/ /2/ /4/ Mean S.E roughness length did not vary widely, and that there was no clear association between z, and wind speed, height of crop or age of crop. It is also apparent that (z - d), when z was a height 2 m above crop level, did not vary widely about a mean value of 266 cm. For the purposes of calculating aerodynamic resistances (r,), standard values of 12.6 cm for z, and 266 cm for (z - d) were, therefore, used. Stomata1 Impedance-daylength The 1st ratoon data from Chaka's Kraal were used in equation (3) to

7 G. D. THOMPSON, J. P. BOYCE 819 estimate mean daily values of E (the equivalent of l/sd) for each week from the time the crop was 2 weeks old until it was 63 weeks old. The results are shown in Fig. 2. For the 6-month period following the attainment of full can-,,, - ;, *.,.;.,...,..*:..,, *,.., Mean = 3.40 t Age of crop In weeks N D J F M A M J J A S O N D J F Fig. 2. Weekly estimates of, the coefficient substituted for l/sd, calculated from Chaka's Kraal data. opy when the crop was 12 weeks old, the average value of I/SD was , while over the ensuing 7 months it was t Surface Resistance, r, The 1st ratoon data from Chaka's Kraal were used in equation (6) to estimate mean daily values of r, for each week from the time the crop was 2 weeks old until it was 63 weeks old. The results are shown in Fig. 3, where it can be seen that the value of r, varied mainly over a range from sec/cm after full canopy had formed in mid-january, excepting for a 9-week period in mid-winter when the mean value rose to approximately 1.1 seclcm. Excluding the data obtained before full canopy was formed, and also the high values obtained in mid-winter, the mean value obtained for r, was t seclcm.

8 IRRIGATION Fig l Age of crop in weeks N D J F M A M J J A S O N D J F Weekly estimates of surface resistance, calculated from Chaka's Kraal data. Estimation of Euapotranspiration The daily meteorological data from Pongola were used to estimate E,, from the time the crop was 12 weeks old until it w7s harvested at 47 weeks of age. Four separate estimates were made for each day. These were: 1) E, (Penman) using equation (1) with the wind function given in equation (2) ; 2) E, (Businger) using equation (5) with the wind function given in equation (4) ; 3) E, (Peman and Schofield) using equation (1) with the wind function given in equation (4) and values of E as determined at Chaka's Kraal for the periods following the attainment of full canopy (Fig. 2) ; 4) E, (Monteith) using equation (6) with r, = 0.75 sec/cm for all days except those during weeks inclusive, when a value of 1.1 sec/cm was used. The correlation coefficients between measured E, and each of the calculated values were determined for the 208 days on which comprehensive data were available. Linear regression equations were also calculated and the results are shown in Table 2. The regression lines are'shown in Fig. 4. Table 2. Correlation coefficients and linear regression equations for relationships between measured Et (mm/day) and various estimates of Et (n = 208). Correlation Estimate coefficient Regression equation, - 1. Penman 0.93 Et (measured) = Et (Penman) 2. Businger 0.79 Et (measured) = E, (Businger) 3. Penman and Schofield 0.83 Et (measured) = Et (P & S) 4. Monteith 0.91 Et (measured) = Et (Monteith) -

9 I G. D. THOMPSON, J. P. BOYCE I I d Estimated Et. inmiday Fig. 4. Relationships between measured evapotranspiration at Pongola and values calculated by means of 4 methods. DISCUSSION Estimates of evapotranspiration from meteorological data alone, as proposed by Penman (1963), have always closely approximated measured values; they have, therefore, served a useful practical purpose. The empirical wind function, however, is not applicable to sugarcane, and experimental data have shown that $no simple empirical substitute can be proposed. The apparent accuracy of the Penman estimates for a tall crop such as sugarcane appears to be largely fortuitous, and results from the compensating effects of a general underestimation of the effect of wind, and the absence of any factor to account for surface resistance. When #the more complex, but theoretically based, alternative wind function suggested by Businger (1956) is included in Penman's combination formula, this establishes the potential rate of evaporation from a physically wet crop. It is now generally accepted that estimates of evapotranspiration should include Businger's wind function, with either an empirically determined factor or a theoretically correct model to account for the fact that the crop is not physically wet. The additional information required to predict Et using equations (3), (5) and (6) includes the roughness length of the crop and either a stomata1 impedance-daylength factor as proposed by Penman and Schofield (1951), or a surface resistance parameter for the crop according to Monteith's (1965) model.

10 Xoughnes~ Length, z, IRRIGATION The mean roughness length of sugarcane variety NCo 376 at Pongola was estimated to be cm, and this value did not appear to vary significantly when the crop height increased from 90 cm to 378 cm. This is in contrast to the data presented by Chang (1961), who showed that the roughness length of an unidentified variety in Hawaii varied from 4-8 cm over approximately the same range of crop heights. The maximum roughness lengths for a number of crops, given by Szeicz et al. (1969), were 5.6 cm for grass, 6.5 cm for potatoes, 6.7 cm for lucerne, 10.5 cm for barley and 22.0 cm for maize. These authors also showed that the roughness lengths of potatoes and lucerne decreased by more than 50% as the windspeed increased from 100 to 400 cm/sec. There was no indication in the results from Pongola that the roughness length of sugarcane changed either consistently or appreciably over a similar range of wind speeds. The extent to which roughness length affects the estimate of E, is shown in Fig. 5. Meteorological data from Pongola for 11 March, 1968, were used with Rouqhn~ss length, cm Fig. 5. Relationships showing the effects of varying roughness length on estimates of potential evaporation (Businger) and actual evapotranspiration (Monteith). a range of z, values in both the Businger (5) and Monteith (6) equations. It can be seen that roughness length affects E, (Businger), or potential evaporation, by as much as 100% when it increases from 3 to 27 cm, whereas E, (Monteith), over the same range of z, values, increases only by approximately 20y0. It is therefore apparent that, while a reasonably accurate assessment of z,

11 G. D. THOMPSON, J. P. BOYCE is necessary if potential evaporation is being estimated, the estimation of actual evapotranspiration from Monteith's formula is relatively little affected by changes in z,. ~erodynamic Resistance, r, By substituting Businger's wind function, or the reciprocal of the aerodynamic resistance, l/r,, for the empirical function in the Penman formula, estimates of E, deteriorated considerably, as shown in Fig. 4. Over the important range of daily E, values from 5-10 mm/day, the estimated values increased approximately from 1i to 2 times the measured values. Since the Businger estimate is affected so drastically by the value given to z, (Fig. 5), the implication is that this method for estimating actual E, can only be useful for crops with a short roughness length. Tanner and Pelton (1960) showed that the Businger equation gave an improved estimate of actual E,, compared with the Penman estimate, for an alfalfa-brome crop with roughness lengths between 1 and 4 cm, and over a range from 1.5 to 4.0 mm E,/day. Businger's wind function refers specifically to adiabatic conditions in the atmosphere above the crop. Fuchs et al. (1969), stated that the adiabatic approximation can only be successful when the sensible heat flux is small. They have presented values for a profile diabatic influence function, based on the Richardson number, which can be used to correct for non-neutral conditions, but these are only useful when the time-interval of measuring E, is short, e.g., hourly or less. For daily data, corrections are not possible and daily estimates of r, and hence E, are, therefore, in error depending on the extent of departures of atmospheric conditions from neutral stability during the day. 1 Stomata1 Impedance The model proposed by Penman and Schofield (1951) has been criticized by Tanner and Pelton (1960) on the grounds that the SD correction is not justified for general use. In their calculations the inclusion of the 1/SD term, determined as outlined by Penman (1953), caused E, to be underestimated to a greater extent than it was when the */SD term was omitted. Since the direct assessment of SD is open to question, we chose to estimate a substitute coefficient (E) from the Chaka's Kraal data, as shown in Fig. 2. The results were used with the daily Pongola measurements to give the data shown in Table 2 and the regression line, designated Penman and Schofield, shown in Fig. 4. Businger's (1956) mean value of 0.9d for this coefficient was obtained from data for short grass with a roughness length of 1 cm when measured E, varied from mm/day. Fig. 4 and 5 indicate clearly that E must be mucl~ greater than 1 if E, is to be estimated reasonably well by this method for tall crops. Fig. 4 shows that E, was underestimated at low values of daily evapotranspiration and overestimated at high rates when values of 3.4 and 4.8 were ascribed to E for the Pongola data. Surface Resistance When r, was calculated from the weekly Chaka's Kraal data and the results used to estimate daily E, at Pongola, the regression line relating mea-

12 824 IRRIGATION sured E, to that estimated from Monteith's equation (6) was closer to the 1: 1 line than were the lines obtained from other estimates, as shown in Fig. 4. While this may be construed as evidence in support of the correctness of Monteith's model, there has been a considerable amount of criticism (Philip 1966; Tanner and Fuchs, 1968; Cowan and Milthorpe, 1968) of the theoretical basis for the (1 -I- r,/r,) term in equation (6). This criticism is based primarily on the fact that source and sink strengths for vapour, heat and momentum are demonstrably different within a crop canopy. Monteith's (1965) model is based on an assumption that the canopy behaves as though it were a single large leaf. Tanner and Fuchs (1968) have suggested that the apparent success of Monteith's formula, as illustrated in Fig. 4, may be fortuitous, but that it may prove to have "empirical engineering convenience." Since it was not possible to measure r, directly, it was necessary to calculate values as the only unknown in equation (6). Besides any concern regarding the theoretical validity of Monteith's r,, therefore, the accuracy of the values obtained depends on the accuracy with which each of the other parameters was estimated. While net radiation and evapotranspiration were measured directly, there must be some question regarding the extent to which the mean daily values oc temperature and humidity represent the effective values of these parameters. Most particularly, however, values of r, must be regarded with caution, not only for the reasons concerned with atmospheric stability already mentioned, but also because they include a value for daily run of wind which does not differentiate between daytime and nighttime contributions. Bearing these reservations in mind, the weekly values for r, shown in Fig. 2 appear to be reasonably consistent. The high values obtained for the precanopy period of 12 weeks duration are a logical consequence of soil surface drying and the presence of a trash layer in the ratoon crop. The mean value of 0.75 sec/cm is also consistent with values given by Monteith (1965) for a range of crops. Of major interest is the fact that the crops at both Chaka's Kraal and Pongola were irrigated, so that the soil moisture deficit seldom exceeded 5 cm and was generally much less than this extreme. Under these conditions, it may well have been expected that actual evapotranspiration would at least have approached the potential level. In fact, as illustrated clearly in Fig. 4, for rates of evapotranspiration in excess of 2.5 mm/day, actual E, fell well below the potential level predicted by the Businger equation. The implication, therefore, is that sugarcane does not use water at the potential rate even when irrigation water is applied at relatively low soil moisture tensions, and that a significant resistance to liquid and/or vapour flow does exist unless the crop is physically wet. There was a consistent tendency for r, to be higher during a 9-week winter period than it was during the remainder of the period of full canopy (Fig. 3). If this difference was real, it may have been due to the greater average age of leaves exposed to radiation during winter when the rate of leaf emergence from sugarcane is relatively slow, or to the reduced leaf area index of sugarcane during the winter months. The search for a realistic model for predicting evapotranspiration from well-watered cropped surfaces has progressed considerably over the past decade.

13 G. D. THOMPSON, J. P. BOYCE 825 The accumulated evidence, however, seems to indicate that the prediction of E, for irrigation control purposes in commercial agriculture will not be accomplished through the attainment of an entirely theoretical and generally applicable model. The amount of information needed to describe environmental and crop conditions adequately for the purpose is obviously becoming excessive. While the scientific value of continued research into the mechanics of evapotranspiration may never be doubted, it may be well for practicing agriculturists to limit their horizons deliberately at this stage to the entirely empirical relationships which have served and may continue to serve so. well. This is illustrated in Fig. 6 where the relationship between actual E, and Class A pan for 208 days at Pongola is shown. : E, 0 29 t 093 E, r = 0-92 E, Class A Pan. mrnlday, I In spite of the posdible limitations of the' concept of the surface resistance r,, there appears to be merit in its application to situations where soil moisture is not made freely available to the crop. Szeicz et al. (1969) and van Bavel (1967) have shown that the surface resistance is dependent on soil water potential (q) when a crop is subjected to soil moisture stress. From the relation between surface resistance and soil water potential, the change in relative evapotranspiration rate with decreasing water potential in the root zone can be calculated from an expression derived from equation (6) : A/y (rso/r,) Aly (rslr,) ' (7) I

14 826 IRRIGATION where Eo is the rate of evapotranspiration with minimum surface resistance rs0, and E, is the rate of evapotranspiration with r, = ($,). By solving equation (7) for r, when all other parameters are known, progress might be achieved in elucidating the effects of soil series, stage of crop development, evaporative demand and genotype on evapotranspiration from crops subjected to soil moisture stress. REFERENCES Businger, J. A Some remarks on Penman's equation for the evapotranspiration. Neth. J. Agric. Sci., 4: Chang, J. H Microclimate of sugarcane. Hawaiian Planters' Record, 56: Cowan, I. R., and F. L. Milthorpe Plant factors influencing the water status of plant tissue. In Water Deficits and Plant Growth. Edited by T. T. Kozlowski, 1: Fuchs, M., C. B. Tanner, G. W. Thurtell, and T. A. Black Evaporation from drying surfaces by the combination method. Agron. J., 61: Lemon, E. R Theoretical considerations of aerodynamic exchange in a turbulent boundary layer. In the energy budget at the earth's surface, Part I, ARS, USDA production research report no. 71, pp Monteith, J. L. Evaporation and environment. In The State and Movement of Water in Living Organisms. Edited by G. E. Pogg. Academic Press, New York, p Pearson, C. H. O., T. G. Cleasby, and G. D. Thompson Attempts to confirm irrigation control factors based on meteorological data in the cane belt of South Africa. Proc. Ann. Congress S. African Sugar Technol. Assoc., 25: Penman, H. L Natural evaporation from open water, bare soil and grass. Proc. Roy. Soc. (London) Ser. A, 193: Penman, H. L The physical basis of irrigation control. Rept. 13th Intern. Hort. Cong., 2: Penman, H. L Vegetation and hydrology. Commonwealth Bur., Farnham Roy. Penman, H. L., and R. I<. Schofield Some physical aspects of assimilation and transpiration. Symposia Soc. Exptl. Biol., 5: Philip, J. R Plant water relations: some physical aspects. Ann. Rev. Plant Physiol., 17: Szeicz, G., G. Enrodi, and S. Tajchman Aerodynamic and surface factors in evaporation. Water Resources Res., 5: Szeicz, G., and I. F. Long Surface resistance of crop canopies. Water Resources Res., 5: Tanner, C. B Evaporation of water from plants and soil. In Water Deficits and Plant Growth, edited by T. T. Kozlowski, 1: Tanner, C. B., and M. Fuchs Evaporation from unsaturated surfaces: a generalized combination method. J. Geophys. Res., 73: Tanner, C. B., and W. L. Pelton Potential evapotranspiration estimates by the approximate energy balance method of Penman. J. Geophys. Res., 65: Thompson, G. D The relationship of potential evapotranspiration of sugarcane to environmental factors. Proc. ISSCT, Thompson, G. D., and J. P. Boyce Daily measurements of potential evapotranspiration from fully canopied sugarcane. Agr. Meteorol., Thompson, G. D., and J. P. Boyce Measurements of evapotranspiration of sugarcane in a large and small lysimeter. Proc. Ann. Congr. S. African Sugar Technol. Assoc., 35 (in press). van Bavel, C. H. M The combination concept and its experimental vertification. Water Resources Res., 2: van Bavel, C. H. M Changes in canopy resistance to water loss i-om alfalfa induced by soil water depletion. Agr. Meteorol., 4: