15 On the Evapotranspiration from Paddy Fields in a Southern Part of Kyushu* S. IWAKIRI (Miyazaki Agricultural Experiment Station) Abstract Results of observations on evapotranspiration in both early seasonal cultivation and ordinary cultivation systems for four years (1960 to 1963) are presented. Mean daily totals of evapotranspiration in both cultivation systems were nearly the same, but the extreme of daily total in evapotranspiration in early cultivation exceeded that in the ordinary cultivation. The difference in extremes arises from the fact that the period with dense plant canopy in the early cultivation coincides with the period of most intensive insolation in year. Evaporation from an ordinary small pan was found to agree fairly well with evapotranspiration from paddy fields and to be used as a rough measure of it. The heat balance and the combination methods were applied to estimating the evapotranspiration. The latter was found to be more promising than the former for operational use. 1. Introduction The real information on irrigation water for paddy fields is valuable in scheduling irrigation and in planning the rational use of water resource. Since the growth and the yield of rice plants, moreover, are closely related to the redistribution of water in the plant ecosystem, the investigation of water balance in paddy fields is considered to be very important from the standpoint of agricultural meteorology. Though measurements of transpiration from potted paddy plants have been made by many agronomists in order to determine the value of transpiration coefficient, relatively few experiments have been dons about this problem under completely natural conditions in Kyushu district except Sato and Funahashi's one (1954). * Read at the Kyushu branch meeting, Fukuoka, on 25th March 1964. The purpose of this paper is to make clear the seasonal change of each component in the water balance of paddy fields and to test two methods in estimating evapotranspiration. Measurements of each water balance item and relevant quantities were made for four years (1960 to 1963). 2. Experimental procedure and outlines of the growth conditions Rice cultivation systems in most common use in Miyazaki district are the ordinary cultivation (transplanting in June and harvesting in Oct.) and early cultivation (transplanting in April and harvesting in August). The later was introduced from Tohoku region into this district for preventing crop damage caused by typhoons and for promoting land utilization, respectively. Experiments were made in Miyazaki Agricultural Experiment Station at Miyazaki city (31 54'N, 131 25'E). Rice plants were cultivated in ironboxes which were set in a paddy field as evapotranspirometer in 1958. The daily water loss from the evapotranspirometers and evaporimeters was determined by reading the change in water level in inclined manometers (inclination is sin ľ=0.1)at 0900 JST every morning. The change in the water level between such times on successive two days was assumed to be equal to the daily water loss from those evaporimeters. The transpiration was determined by subtracting the evaporation from the amount of evapotranspiration in evapotranspirometers. Additional measurements necessary to test the heat balance method and combination method were also made. The water temperature in both the evapotranspirometers and evaporimeters was measured with maximum and minimum thermometers of Ruthaf ord 15
16 On the evapotranspiration from paddy fields type set in water layer. The wind velocity at 2 m height above the water surface was also measured by a Robinson cup anemometer. Other relevant meteorological data such as air temperature, air humidity, sunshine duration and solar radiation were taken from data collected in both the meteorological shelters of this station and Miyazaki weather bureau. Cultivation designs are. shown in Table 1. As can be seen in Table 1, there was a slight annual fluctuation in the date of harvesting, but the transplanting of young rice plants was done on almost the same date every year. Young rice plants were transplanted in east-west rows in the early seasonal cultivation and in check in the ordinary cultivation, respectively. Detailed data on growth condition of rice plants and weather conditions for the four years are practical conditions, it is reasonable to assume that Wt is less than 150mm. By taking those usefulness into account, the values of 30mm/day and 1 mm/day were adopted as the upper and lower limits in determining daily effective precipitation, respectively. Total evapotranspiration during the growing season reached 350 mm in the early seasonal cultivation and 410mm in the ordinary cultivation. Total seepage was found to be comparable to total evapotranspirfation in amount, and these quantities in each cultivation systems were 230mm and 420mm, respectively. Total amounts of evapotranspiration and seepage show that the two balance items play much important role in the water balance in paddy fields. 2) Seasonal change of evapotranspiration (Tenday mean) To make clear the qualitative characteristics of evapotranspiration structure in a paddy field, the changes in each water balance item with the development of the plant canopy are presented in Fig. 1. The values in Fig. l are equivalent to mean daily amounts of each item during a period with fine or nearly fine days. Seasonal changes in each presented in an another paper (IWAKIRI et al. 1965). 3. Results and discussion 1) Water balance The water balance equation in a paddy field without discharge from an outlet is I+re=Ew+P+Wt, (1) where I is the irrigation, re effective precipitation, Ew evaporation from water surface, Et transpiration from rice plants, P percolation into ground and Wt amount of water consumed in transplanting. Detailed data of the water balance in the paddy field are presented in a IWAKIRI'S paper (1965). Though the estimation of Wt is much difficult under Fig.l. Seasonal changes of the evapo-transpiration from paddy field (Ten-day mean).
item of the water balance may be summarized as follows a) Evaporation : As shown in Fig. 1, the evaporation from water surface under the canopy shows the maximum immediately after the transplanting of young plants in which the solar radiation reaches directly the water surface. The maximum evaporation shows little fluctuation among years due to the change in meteorological conditions. The maximum in the early cultivation system ranged between 4 mm/day and 5mm/day and the value in the ordinary cultivation system 5 mm/day to 7mm/day. The evaporation decreased gradually with the growth of paddy plants. Such a phenomenon is obvious to be due to the interception of solar radiation by the dense canopy. b) Transpiration: In general, the transpiration increases with growing the plant and the maximum in transpiration is observed after the heading of ear. Its extreme was larger in the early cultivation than that in the ordinary cultivation. A main reason for this fact may be the coincidence of the time with most dense canopy and the time appearing most intensive solar radiation in early cultivation system. The seasonal variation with two peaks in transpiration from rice plants (after the young ear formation and after the heading stage, respectively) as reported by several researchers (SATO and FUNA- HASHI 1954; HANYU and ONo 1961; et al.) was not distinctly observed in our experiments. This fact seems to indicate that the transpiration from rice plants is more affected by weather conditions than by phislological conditions of rice plants. c) Evaportranspiration : The maximum evapotranspiration of paddy field was observed in the ripening stage for early cultivation and in the tillering stage for ordinary cultivation, respectively. The difference in the time appearing the maximum. evapotranspiration between two cultivation systems arises from the fact that the potential evapotran spiration from paddy fields is mainly affected by the annual course of solar radiation intensity. However, the mean daily values of evapotranspiration in both the systems were found to be 3.71 mm/day and 3.57 mm/day, respectively, implying that there was no essential difference in evapotranspiration intensity between both the systems. d) Dependence of evapotranspiration on LAI : The leaf density dependence of Ew and Et in the growing season of 1962 is shown in Fig. 2. As reported already in a preceeding paper (IWAKIRI,. 1964), it was approximated by E=Eƒ e-0.44a Et=Eƒ (1-e-0.44A) where A is the leaf area index (LAI). In a range over LAI=4.0, the change of Ew and Et with LAI_ becomes little. This feature of LAI dependence of both items agreeded very well with other experimental results (UCHIJIMA, 1961). 3) The evapotranspiration ratio to small pan evaporation In order to eliminate the influence of yearly variation in weather conditions on the water balance of paddy fields and to make clear the general characteristics in the seasonal variation of the main water balance items, each item was normalized in such a way that divides each item by the evaporation frog a small pan (diameter 20cm) in the enclosure The results so obtained are presented in Fig. 3. As can be seen in Fig. 3, the normalized evaporation (Ew/E) decreases gradually with growing paddy plants. The minimum value in Ew/E appearedl in the middle part of June for the early cultivation system and in the middle part of August for the ordinary cultivation system, respectively. The maximam of EW/E in the transplanting period wass Fig. 2. The dependence of Ew/Eƒ on LAI. found to be slightly larger in the early cultivation
On the evapotranspiration from paddy fields that the application of normalized evapotranspiration to estimating evapotranspiration for a shorter period seems to be not very reliable. The value of Eƒ /E found in our experiments was small compared with those obtained by SATO et al. (1954). Results in Fig. 3 indicate that the seasonal change of E is related closely to the plant growth, 4) Heat balance method and combination method Heat balance and combination methods' Fig. 3. Seasonal changes of the ratios of evapo-transpirations to small pan evaporation. Left: Early cultivation. Right : Ordinary cultivation. system than in the ordinary cultivation system. This is ascribed to the fact that evaporation from a small pan in July exceeds largely that from a larger water surface. After the heading stage, were applied to estimating evapotranspiration of paddy fields. The evapotranspiration from a well moist surface is widely known to be proportional to net radiation given on such a surface. Therefore, if the following relationship, as reported by UCHIJIMA (1962), is established on the basis of careful experiments, it is possible to estimate evapotranspiration from measurement of net radiation or solar radiation. normalized evaporation shows again the slight increase in relation to the death of leaves in a lower part of the plant canopy. Transpiration is considerably affected not only by -phisiological and ecological conditions such as leaf density, leaf orientation, and resistance to water transmittance through plant body etc., but also by meteorological conditions. The values of normalized transpiration (Et/E) from pddy plant canopy, thus, should be larger than those in Et/E. Fig. 3 shows that the normalized transpiration increases rapidly with growing rice plants, particularly in the early cultivation system. Smaller values of Et/E after the transplanting period in the ordinary cultivation system of 1960 may reflect the temporary debility of rice plants due to the inflence of a vinyl film used as a rain shelter on only rainy days. As shown in Fig. 3, the seasonal course in normalized evapotranspiration shows somewhat markedly annual difference among years because of considerable variation in annual course of transpiration. The variation in Eƒ /E mentioned above indicates where f is a proportional constant depending upon meteorological conditions, l the latent heat of vaporization, Sw the net radiation, and astricts denote the total amount of each element over cultivating period. UCHIJIMA (1962) has pointed out that mean proportional constant f is approximately 0.88. Measured evapotranspiration was compared with net radiation estimated approximately from the following relation in Fig. 4. S=RS(1-a) - (1-cn2) {ƒðta4(0.39-0.058 ãe)}, (4) where RS is the daily total of short wave radiation, n the cloudiness, a the albedo, and c the coefficient of cloudiness of 0.64. As can been seen in Fig. 4, the scatter of data is relatively large and considerable variation in the net radiation dependence of evapotranspiration is found among years. This fact is considered to result mainly from yearly variation in growth conditions of rice plant (such as density of plant canopy, root vitality and disease
the correction term in the above equation should vanish and the equation should be led to Penman's equation. In the calculation of evapotranspiration from Eq. (5), the value of 2x10-4 -ly/sec Ž was adopted as a reasonably mean value of heat transfer coefficient. The evapotranspiration calculated from Eq. (5) is compared with measured one in Fig. 5. Although fairly good agreement was found between calculated and measured ones in 1962, the agreement between them was not very good in 1961 and 1963, respectively. Such yearly variations in the relationship between measured and calculated evapotranspirations are thought to be due to both the yearly change in growing conditions of rice plants and the Fig. 4. The relationship between measured evapotranspiration and net radiation equivalent Sw/l. etc.). Fig. 4 shows also that heat more than net radiation is consumed as latent heat of vaporization. This means that heat amount corresponding difference between as downward to the them is supplied to plant canopy sensible heat flux. By processing data in Fig. 4, the value of 0.96 was obtained as a mean of the constant, f, and this was 16 per cent as large as that reported by UCHIJIMA (1962). Combination method as proposed by PENMAN (1984) is hardly used in evaluating evapotranspiration in this country. of its usefulness However, this method is noteworthy on account of simple form of the equation. By taking dryness of evaporating surfaces into account, UCHIJIMA (1964) presented the next equation : insufficient accuracy in the determination and estimation of meteorological elements, particularly the net radiation. Fig. 6 shows diurnal changes of evapotranspiration, heat balance terms and net radiant equivalent (S/l) on both 19 th and 20th (two weeks after heading) September in 1963. On those days, rice plants were 100 cm in height, 18.4/hill in number of ears and about 4.5 in LAI. The difference in amount between evapotranspiration calculated from Eq. (5) and measured one was less than 0.4 mm/2hr, and it was found to be larger in the (5) where E is the evapotranspiration, B the heat conducted into soil layer, the slope of the saturation vapour pressure curve at air temperature, ƒ E=2h E e(t0) (ƒê-1)the correction term of evapotranspiration depending upon the dryness of surface, h the sensible heat transfer coefficient, e (T0) saturation vapour pressure at soil surface temperature To and u empirical constant (0.0-1.0). Provided that evaporating surface is well moist, Fig. 5. The comparison of the evapotranspiration calculated from Eq. (5) with the measured evapotranspiration.
On the evapotranspiration from paddy fields balance of paddy fields. In contrast to the prediction widely adopted, there wass no appreciable transpiration difference in evapo between early seasonal_ Fig. 6. Diurnal changes of the evapotranspiration and the heat balance terms. S/l=net radiant equivalent ; M=measured evapotranspiration ; C=evapotranspiration computed from Eq. 5;E= evaporation from large evaporimeter (90x91cm) ; S=net radiation; lea=latent heat of evaporation due to the water vapour deficit; BS, Bw=heat conducted into the soil and water layers; U=wind velocity. forenoon than in the afternoon. Bihourly values of net radiant equivalent on the rice field were larger than measured evapotranspiration until about 1600 JST, but after that the former became less than the latter. Similar tendency in diurnal changes of net radiant equivalent and evapotranspiration was also reported by MCILROY and ANGUS (1964). This seems to be mainly due to the effect of the storage term (net soil and water heat flux densities). The sum total of calculated evapotranspiration during the daytime was 20-40 per cent as large as the same total of measured one. Though it is difficult to point out main reasons for such a discrepancy in amount between measured and calculated ones, the main error in estimating evapotranspiration with the two methods in this report seems to arise either from the rough measure of short-wave radiation or uncorrect evaluation of effective radiation, the net radiation. 4. Summary from which are calculated The results described in the foregoing section may be summarized as follows : Evapotranspiration was found to play an important role in the water cultivation system and ordinary cultivation system. The mean value of daily evapotranspiration was 3.71 mm/day and 3.57 mm/day, respectively. In relation to the length of growing period, the mean value of totall evapotranspiration was 350 mm. inn the early cultivation system and. 410 mm in the ordinary cultivationn terms system. Bimodal seasonall change in transpiration to be not very distinct was foundd as reportedd by SATO and FUNAHASHI (1954), HANYU and ONO(1961) and et al. The comparison of measured evapotranspiration with evaporation from an ordinary small pan (diameter 20cm) indicated that pan evaporation could_ be used as a rough measure of evapotranspirationn from paddy fields. The mean value of the ratio (Eƒ /E) was 1.05 inn the early cultivation and 0.96 in the ordinary cultivation, respectively. The leaf density de pendences of evaporation under the canopy andd transpiration from the canopy were found to be approximated by exponential functions of LAI. The applicability of heat balance method and combination method to evaluating evapotranspiration was tested by using relevant data. It is difficult to say from the results presented in Figs. 5 and 6 thatt the both method are not very applicable to estimate evapotranspiration of paddy fields, chiefly because of rough estimation of net radiation. Acknowledgement-The author would like to express his hearty thanks to Dr. Z. UCHIJIMA, Div. of Meteorology, Nat. Inst. Agr. Sci. and Mr.. S. KANEGAWA, Miyazaki Agr. Expt. Sta., for their guidance throughout the course of this study and to Mr. R. TOMIYAMA, Miyazaki Agr. Expt. Sta. who gaved to the author the valued help throughout the observation.
References 1) HANYU T. and ONO S. (1960), J. Agr. Met. Japan, 16 111-118 (in Japanese with English Summary). 2) ISHIKAWA E. and NISHINO T. (1959), Agr. and Hort. 34 53-54 (in Japanese). 3) IWAKIRI S. (1964), J. Agr. Met. Japan, 19, 89-95 (in Japanese with English Summary). 4) IWAKIRI S. and TOMIYAMA K. (1965), Bull. Miyazaki Agr. Expt. Sta. No. 4 (in Japanese with English Summary). 5) MCILROY I. C. and ANGUS D. E. (1964), Agricultural meteorology (An international journal) 1, 201-224. 6) NISHIO T. (1961), Proc. Crop. Sci. Soc. of Japan. vol. xxix, No. 2 210-212 (in Japanese with English Summary). 7) SATO S. and FUNAHASHI Y. (1954), Bull. Kyushu Agr. Expt. Sta. 2, 161-177 (in Japanese with English Summary). 8) SATO S. (1960), Bull. Kyushu Agr. Expt. Sta. 6, 292-300. (in Japanese) 9) UCHIJIMA Z. (1961), Bull. N. I. Agr. Sci. Series A, No. 8, 243-265 10) UCHIJIMA Z. (1962), J. Agr. Met. Japan, 17, 85-94 (in Japanese with English Summary). 11) UCHIJIMA Z. (1964), Nogyo Gijutsu 19, 290-295 (in Japanese). Effects of temperature and water deficit on leaf photosynthetic rates of different species. EL-SHARKAWY, M. A., J. D. HESKETH, Crop Sci-