CROP EVAPOTRANSPIRATION -- A TECHNIQUE FOR CALCULATION OF ITS COMPONENTS BY FIELD MEASUREMENTS

Transcription

1 Field Crops Research, 7 (1983) Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands CROP EVAPOTRANSPIRATION -- A TECHNIQUE FOR CALCULATION OF ITS COMPONENTS BY FIELD MEASUREMENTS P.J.M. COOPER, J.D.H. KEATINGE and G. HUGHES* International Center for Agricultural Research in the Dry Areas (ICARDA), P.O. Box 5466, Aleppo (Syria) (Accepted 27 July 1983) ABSTRACT Cooper, P.J.M., Keatinge, J.D.H. and Hughes, G., Crop evapotranspiration -- a technique for calculation of its components by field measurements. Field Crops Res., 7: A simple technique is described whereby standard field measurements of crop evapotranspiration (ET), evaporation from an uncropped soil (Es), green area index (G), the crop extinction coefficient (K) and above ground dry matter production (TDM) are manipulated to compute the seasonal variation in crop transpiration (T), evaporation from the soil beneath the crop (E s C) and transpiration efficiency (TE). The technique is illustrated using data from a trial in which wheat (c.v. Mexipak) and barley (c.v. Beecher) were grown with and without nitrogen and phosphate fertilizer at two locations in Northern Syria, namely Tel Hadya and Breda. T values were very low during the cool winter months, reflecting poor interception of radiant energy, and Esc accounted for nearly 100% of E T during this period. In early spring, as G increased, T values also increased and reached maximum values around anthesis of 3.7 and 3.4 ram/day for fertilized and unfertilized wheat at Tel Hadya, and 2.0 and 1.7 ram/day for fertilized and unfertilized barley at Breda. Esc values dropped correspondingly. In the post anthesis period, due to greater soil moisture depletion during the pre-anthesis period, fertilized crops had lower T values and this was reflected in both plant water status and grain weight measurements. During this period, as moisture stress increased and leaf senescence occurred, T values dropped sharply and Esc values rose. On a seasonal basis, T accounted for 68 and 61% of E T in the fertilized and unfertilized wheat and 50 and 38% of E T for the fertilized and unfertilized barley. The pattern of seasonal variation in TE was influenced, in the preanthesis period, by the absence of root dry matter measurements, but reached values between 65 and 75 kg/ ha/ram around anthesis when root growth ceased. TE values were inversely related to vapour pressure deficit (represented by E0) in the post anthesis period and corresponded well to predicted values. On a seasonal basis, TE values were very similar across crop, treatment and location ranging between 42.8 and 45.7 kg/ha/mm. *Permanent address: Crop Division, Edinburgh School of Agriculture, West Mains Road, Edinburgh EM9 3JG (Great Britain) /83/$ Elsevier Science Publishers B.V.

2 300 INTRODUCTION Rainfed farming systems in Mediterranean type environments of West Asia and North Africa face two major constraints to improved and stable agricultural production, namely erratic and often chronically low rainfall, and widespread deficiencies in major plant nutrients, particularly phosphorus and nitrogen. Recent work at the International Center for Agricultural Research in the Dry Areas (ICARDA) has concentrated on investigating ways of improving the efficiency of use of limited soil water and nutrient resources. Investigations combining fertilizer trials with crop growth and evapotranspiration studies have indicated that there is a large potential for improving the water use efficiency of cereal production in areas receiving less than 500 mm of seasonal precipitation (Cooper et al., 1981). To obtain a greater understanding of the effect of crop management on crop water use, it was felt necessary to design an analytical technique based on field measurements whereby evapotranspiration (ET) could be split into its two components, namely crop transpiration (T) and evaporation from the bare soil under the crop (Esc). Ritchie (1981) states that... "It is possible to separate soil and plant evaporation logically when we know the fraction of the energy intercepted by the plant canopy and the critical soil parameters." Attempts have been made to produce models which predict these parameters (Ritchie, 1972; Tanner and Jury, 1976). Such models have relied largely on empirical relationships which describe the temporal nature of moisture loss from an uncropped soil and the effect of crop canopy development on radiant energy interception and the ratio T/Eo, where E0 is the potential evaporative demand. This predictive model approach, with minor modifications has proved successful (Kanemasu et al., 1976; A1-Khafaf et al., 1978). The technique described in this paper is based on the logic described by Ritchie (1981), but is not a predictive model. Instead it describes how standard field measurements of ET, E S (evaporation from an uncropped soil), crop canopy size and its relationship with interception of radiant energy can be logically and simply used to determine T and ESC. The technique is illustrated utilizing data collected from fertilizer trials conducted with wheat and barley at two contrasting locations in Northern Syria. The implications of fertilizer addition on the seasonal variation in T and ESC and the transpiration efficiency (TE) of the crop are discussed. MATERIALS AND METHODS Description of trial, soil and crop measurements The trial was conducted at two locations in Northern Syria, Tel Hadva (long term seasonal precipitation 342 mm, 35 55'N 36 55'E) and Breda (275 mm, 35 55'N 37 10'E). The soil at Tel Hadya is described as a Vertic

3 301 (calcic) Luvisoland that at Breda as Calcic Xerosol. Full profile descriptions are given in Cooper et al. (1981). Two contrasting fertilizer treatments (see Table I) were selected from part of a larger agronomy trial on wheat (c.v. Mexipak) at Tel Hadya and barley (c.v. Beecher) at Breda. Treatments were replicated four times and were drill planted in early December at a seed rate of 90 kg/ha and a between row spacing of 17.5 cm. Germination occurred on 10/12/80 at both locations. Weeds were controlled with a single spraying at Bromonyl at stem elongation. TABLE I Effect of fertilizer addition on crop productivity of wheat (c.v. Mexipak) and barley (c.v. Beecher) at two locations in Northern Syria, 1980/81 Site Crop Fertilizer a TDM at 1000 grain Seed yield (kg/ha) harvest weight (kg/ha) (kg/ha) (g) PzO~ N Tel Hadya Breda Wheat Wheat LSD (5%) Barley Barley LSD (5%) aall P2Os and 20 kg/ha N drilled with seed. Remaining N top dressed at stem elongation. Moisture accession tubes (eight per treatment) were installed at each location to a depth of 180 cm prior to planting. These were monitored regularly through the season at 15 cm depth intervals using a Mk II Wallingford Neutron Probe. Moisture changes in the cm horizon were determined gravimetrically with a volumetric soil sampler. Precipitation (P) and Class A Pan E0 were recorded on a daily basis and ET for given periods was computed from the equation: E w =P- AM-R-D (1) where AM is the change in total moisture in the cm profile, R is surface run off and D is drainage below 180 cm depth. In these trials, R and D did not occur and thus were ignored in computing ET. Accession tubes (four per location) were similarly installed and monitored in uncropped bare soil plots adjacent to the trial area, and ES computed according to equation (1). Destructive crop samples were taken weekly (8 single meter rows per treatment) from which total above ground dry matter (TDM) and green area index (G) were recorded. On alternate sampling dates the interception of radiant energy by the crop canopy was recorded by use of 1 m tube solarimeters (three per treatment). From these data, an estimate could be made of

4 - e 302 the extinction coefficient K in the relationship between intercepted radiation and G formulated as: = 1 - -KG where a is the proportion of incident radiant energy intercepted by the crop canopy of green area index G. Monteith (1965) noted that for small grain cereal crops, K has a value between 0.3 and 0.5. A value of K = 0.37 was calculated from the present data pooled over crops, sites replicates and times. At harvest, crop productivity was assessed from a larger sample of 96 single meter rows per treatment (see Table I). (2) DATA PROCESSING Stage 1 -- Curve fitting Polynomial curves describing the changes over time (days from germination) of accumulated ET, accumulated ES, dry matter and green area were //I 300 (A) r- r- v 200 // (B) / V E,< 100 / 40 8b ' 1~o ' 1~o " 40 " 8b " 1~o ' 160 Days post germination Fig. 1. Accumulated E s (x--x) and E T of (A) wheat at Tel Hadya and (B) barley at Breda, with (e,) and without (A -) added fertilizer.

5 303 T "~e 4... i ~ "0 ".t, v ' ' o ' ' o (etl/i) uo}l:mpold JO lelll '(J(TI i-,==- < ",,e " '-- E ~ a: -< (u'q/i) uo!~,mp(nd Jaue.m,{~([

6 " 16o ,0 (A) 1.0 / j/,' A ia Days post germination 5.0 0a ~- 1.C ;j// / s~) " ". Days post germination Fig. 3. Change in green area index of (A) wheat at Tel Hadya and (B) barley at Breda with (e,) and without (A--A) added fertilizer.

7 305 fitted by multiple regression analysis (see Figs 1, 2a, b and 3a, b). In the cases of the dry matter and green area data, a logarithmic transformation was applied in order to obtain homogeneity of variance. In the cases of ET and E S data, where such a transformation was unnecessary, the fitted curve was constrained to pass through the origin. In fitting polynomial curves to plant growth data, we have followed the guidelines of Hunt (1978}. In some cases (e.g., Figs. 2a, b) we have used more than one curve to describe the data since it appeared that this approach could better emulate the general trend than a single high order polynomial regression line. Stage 2 -- Computation of evaporation from the soil under the crop (Esc) Ritchie (1972} and others recognize two phases of moisture loss from a bare soil surface. Phase one is the constant rate phase during which the soil is sufficiently wet for the water to be transported to the surface at a rate equal to the evaporative demand. During this phase, the rate of evaporation is determined by the supply of energy reaching the soil surface. In phase two, the falling rate phase, the soil surface water content has decreased to a value such that the rate of evaporation becomes more dependent on soil hydraulic properties, and is much less dependent on the evaporative demand. Recognition of these two phases is of particular importance when considering conditions where the soil surface is infrequently wetted, as under irrigation. However, in rainfed systems, where the soil is frequently re-wetted at variable intervals, no such clear distinction exists. Experience has shown that under such conditions, moisture loss from a bare soil can be roughly estimated by the simple expression: 1 ES = E0 -- (3) t where t is the number of days since it last rained. During the cool wet months of December, January and February, when E0 values are low and rainfall is frequent, the ES/Eo ratio approaches unity; and during such periods, E s is well described by Ritchie's phase one model. During the months of March, April and May, the soil surface continues to be wetted, albeit less frequently, but E0 values rise sharply and the Es/Eo ratio decreases. In spite of this, the actual rate of E s rises during this period as predicted by expression (3) (see Fig. 1). It is only in late May/early June, after crop maturity when precipitation ceases and E0 values become very high that Es clearly behaves according to the phase two model, and expression (3) becomes much less reliable. We therefore assume that during the crop growth cycle E s will be largely determined by E0 values and frequency of rainfall. Since radiant energy is the principal component of E0 and frequency of wetting is identical to both

8 306 bare and cropped plots, we further assume that Esc/E s ratio will depend on the proportion of radiant energy intercepted by the crop and ESC can be computed from the equation: ESC =E s (1 - ~) (4) Thus from the curves fitted to accumulated Es vs. time and G vs. time, daily values of E s (mm/day) and G are computed for the life of the crop. Using the daffy value of G and the estimated value for K, a is calculated on a daffy basis from equation (2). Utilizing this value of a, the daily rate of ESC is computed according to equation (4). Stage 3 -- Computation of transpiration (T) Daffy rates of ET are computed from the curves fitted to accumulated ET vs. time (see Fig. 1), and daily values of T are obtained from the expression: T = E w - Esc (5) It can be seen from Fig. 1 that during early growth (up to day 60) when leaf areas, and hence T, are very small there is no measurable difference between accumulated ET and ES at a given location. This has been observed for a variety of crops in other locations (Cooper et al., 1981). However, curves fitted to accumulated E T and E s spanning the whole growing season do not coincide during this early growth period, and thus calculation of T TABLE II Computation of T during the early growth phase for wheat with fertilizer at Tel Hadya Period a ET b ES b Es c b E T -- ESC E s -- ESC Computed T (days) (mm per 5-day period) I.I aday 1 is 10/12/80 bvalues computed from fitted curves.

9 307 by equation (5) can produce nonsensical results. We therefore assume that, during the early growth period, the curve fitted to accumulated ES vs. time adequately describes accumulated ET, and during this period T is calculated from the expression: T = ES - ESC (6) An example of this procedure is given in Table II. It can be seen that during this period, accumulated T values are very small compared with the seasonal total, and thus any error associated with this procedure will be negligible. Accumulated ET, ESC and T data are presented in Fig. 4, and the seasonal variation in daily rates of ESC and T are expressed as 5-day mean values in Fig. 5a, b. (A} / ET 300- T 200" E E? 100- sc ~c v < E 20(> (C) # ET {D} / ET do 1~0 Days post germination Fig. 4. Accumulated ET, T and Esc of wheat at Tel Hadya (A) with and (B) without fertilizer, and of barley at Breda (C) with and (D) without fertilizer. Stage 4 --Computation of transpiration efficiency Daffy rates of increase in dry matter are computed from the fitted curves in Fig. 2 and these are divided by the concurrent daily T values. The seasonal variation in transpiration efficiency (TE) is thus obtained and is epxressed in units of kgha -1 mm -1. These data are presented in Fig. 6. It must be emphas-

10 308 ized that these data express the transpiration efficiency of above ground dry matter production only, and do not include dry matter production in the root system. This is discussed in more detail in the next section. DISCUSSION ET, ESC and T Seasonal variation in dally rates of ESC, T and E0 are presented in Figs. 5a, b. During the first 50 days post germination, T values were very low for both wheat and barley, and ESC accounted for almost 100% of ET. This reflects the slow development of green area during the cool winter months and the subsequent poor interception of radiant energy. As green area developed, T became a more significant component of ET, and ESC values dropped. At Tel Hadya, where the fertilized and unfertilized wheat crops reached maximum recorded G values of 6.7 and 4.7, almost complete interception of radiant energy occurred, and maximum T values of 3.7 and 3.4 mm/day were obtained whilst ESC values dropped to a minimum of 0.15 and 0.06 mm/day. At Breda however, where the fertilized and unfertilized barley crop only reached G values of 3.8 and 1.8, the maximum T values obtained were 2.0 and 1.7 mm/day, respectively, and ESC values decreased to a lesser extent. At both locations, as leaf senescence occurred in the post anthesis period, and moisture stress increased, T values dropped sharply and ESC increased as was suggested by Doyle and Fischer (1979). This rise in ESC during senescence was associated with atypically late rains close to crop maturity at both locations, namely 55 mm between day at Tel Hadya and 25 mm between day at Breda, which re-wetted the soil surface under the crop. In more normal years such a pronounced increase in ESC during this period would not be expected. It is interesting to note that whereas fertilizer addition increased T values prior to anthesis, the opposite effect was observed in the post anthesis period. This was due to greater depletion of soil moisture reserves by the fertilized crop during the vegetative growth period. This observation is supported by plant water status measurements which indicated that greater internal moisture stress occurred in the fertilized crop during grain filling, water potentials being up to 1.5 MPa lower than in the unfertilized crop. It is also reflected in the grain weights (Table I) which were significantly reduced in the fertilized treatment at both locations, a well documented result of greater moisture stress during grain filling (e.g., Hochman, 1982). Accumulated ET, ESC and T data are plotted in Fig. 4, and seasonal totals are given in Table III. Bearing in mind the inevitable loss of moisture from the soil prior to canopy closure, wheat at Tel Hadya actively utilised a relatively high proportion of the total ET as T, namely 68 and 61% for the fertilized and unfertilized crop. However, corresponding values of 50% and 38% for the barley crop at

11 309 (A) = E Days post germination E 0 (B} E ~ ~---$22~':... \? Esc Days post germination Fig. 5. Seasonal variation in rates of E_, T and ESC for (A) wheat at Tel Hadya and (B) barley at Breda with (--) and without 1...) added fertilizer (A, Anthesis.)

12 310 TABLE III ET, ESC, T, WUE and TE of wheat and barley at two locations in Northern Syria TDM at E T ESC T WUE TE harvest (kg/ha) (mm) (kgha-1 mm -1) Tel Hadya Wheat (+ fertilizer) Wheat (- fertilizer) Breda Barley (+fertilizer) Barley (- fertilizer) Breda indicate that there is great potential for improvement of water use efficiencies in dryland barley growing regions. This is reflected in the water use efficiency (WUE) data presented in Table III. It should be emphasized that these WUE's for barley were achieved by crops yielding t/ha, whereas typical yields obtained by local farmers in Northern Syria range between 0.4 and 1.2 t/ha. By rough extrapolation, such crops could only be expected to actively utilize between 15 and 25% of the seasonal E T as T. Whereas there is, as would be expected, considerable variation in WUE values across treatments and sites, it is encouraging to observe that the TE values computed by this technique in Table III are very similar across treatment, crop and location. This is in agreement with the observation that TE values for C3 cereal cultivars such as wheat and barley do not differ greatly within a given environment (Fischer, 1981). It is of interest that the addition of fertilizer has little or no effect on the TE of wheat and barley. This has been discussed in detail by Cooper (1983) who showed that: TE WUE = (7) 1 + Esc/T It thus appears that the pronounced increase in WUE resulting from correct fertilizer use as reported here and elsewhere (see Cooper, 1983) is largely due to increased G and its effect on the Esc/T ratio rather than any specific effect on TE per se. Seasonal variation in TE Seasonal variation of TE's are presented in Fig. 6. TE values are inversely related to vapour pressure deficit, which can be represented by E0 and from an empirical relationship derived in Australia (Fischer, 1981), an almost constant TE value of kgha -1 mm -1 for shoot plus root dry matter production would be predicted for the cool winter months in Syria when E0

13 311.~= =_ loo (A) (B) A "-~, 50 J A Days post gcrmillalioh Fig. 6. Seasonal variation in TE of (A) wheat at Tel Hadya and (B) barley at Breda with (--) and without (... ) added fertilizer. (A, Anthesis.) values range between 1 and 2 mm/day. It is clear from Fig. 6 that prior to anthesis, a very different pattern is represented by our data. This would appear to be largely due to the fact that root dry matter production, which occurs prior to anthesis (Gregory et al., 1978), is not included in our analyses. This can be illustrated in more detail for the barley crop at Breda. If the plateau TE value of 60 kgha -1 mm -I, achieved around anthesis, when root growth is ceasing, is assumed to have been constant throughout the preceding period, we can calculate root plus shoot dry matter production per 5-day period from the computed T values (see Fig. 5b). If the TDM per 5-day period (see Fig. 2b) is subtracted from these values, an estimate of root dry matter production is obtained. Summation of these 5-day estimates of root dry matter production gives a total of 1560 and 1430 kg/ha of root dry matter at anthesis for fertilized and unfertilized barley. These values fall well within the range of kg/ha for root dry matter of barley at anthesis as cited by Gregory et al. (1978). We do not suggest that this procedure can be used to predict root dry matter production with any great accuracy at this stage, but include the above example to illustrate the likely cause for the pre-anthesis pattern of TE variation. Reference to Fig. 6 indicates that between day 125 and 135 there was a sharp decline in TE for wheat at Tel Hadya. This coincided with a sudden rise in E0 values from about 5 to 8.5 mm/day. It has already been observed that TE is inversely related to E0, and the empirical relationship derived by Fischer (1981) in Australia predicts a TE value of 30 kgha -1 mm -1 for an E0 value of 8.5 mm. This value corresponds closely to the values observed for wheat in Fig. 6. A similar sharp increase in E0 also occurred at Breda between day 125 and 130, and yet there was no corresponding decrease in TE. However, the crop was close to maturity at this stage and leaf area, dry

14 312 matter production and transpiration levels were all very low. (Figs. 2b, 3b, 5). Doyle and Fischer (1979) suggest from field studies on wheat in Australia, that water limitations may increase TE, but that was not found to be the case in this study. Examination of both extractable soil moisture levels and plant water status indicated that the unfertilized wheat and barley crops suffered less water limitation in the post anthesis period, and yet in both cases the unfertilized crop had a higher TE value during this period. CONCLUSION The data presented describing the seasonal variation in T, ESC and TE correspond well with results reported by other workers in similar environments. We believe that the apportioning of ET into its two components, as described in this paper, adds significantly to the interpretation of the results obtained from crop water use studies at the cost of very little extra workload in the field. Apart from the monitoring of ES at the trial site, no additional measurements are required beyond those normally undertaken in such studies. REFERENCES A1-Khafaf, S., Wierenga, P.J. and Williams, B.C., Evaporative flux from irrigated cotton as related to leaf area index, soil water, and evaporative demand. Agron. J., 70: Cooper, P.J.M., Crop management in rainfed agriculture with special reference to water use efficiency. In: Nutrient Balances and the Need for Fertilizers in Semi-Arid and Arid Regions, 17th IPI Colloquium, May, Rabat, Morocco. IPI, Bern, pp Cooper, P.J.M., Allan, A.Y., Harmsen, K., Keatinge, J.D.H., Nygaard, D., Saxena, N. and Islam, R., Soil water and nutrient research ICARDA Proj. Rep. 3, International Center for Agricultural Research in the Dry Areas, Aleppo, Syria, 191 PP. Doyle, A.D. and Fischer, R.A., Dry matter accumulation and water use relationships in wheat crops. Aust. J. Agric. Res., 30: Fischer, R.A., Optimizing the use of water and nitrogen through breeding of crops. Plant Soil, 58: Gregory, P.J., McGowan, M., Biscoe, P.V. and Hunter, B., Water relations of winter wheat. I. Growth of the root system. J. Agric. Sci., Camb., 91: Hochman, Z., Effect of water stress with phasic development on yield of wheat grown in a semi-arid environment. Field Crops Res., 5: Hunt, R., Plant Growth Analysis. Studies in Biology 96. Edward Arnold, London, 67 pp. Kanemasu, E.T., Stone, L.R. and Powers, W.L., Evapotranspiration model tested for soybean and sorghum. Agron. J., 68: Monteith, J.L., Light and crop production, Field Crops Abstr., 131: Ritchie, J.T., Model for predicting evaporation from a row crop with incomplete cover. Water Resour. Res., 8: Ritchie, J.T., Water dynamics in the soil-plant atmosphere system. Plant Soil, 58: Tanner, C.B. and Jury, W.A., Estimating evaporation and transpiration from a row crop during incomplete cover. Agron. J., 68: