Determination of evapotranspiration and crop
coef®cients of rice and sun¯ower
with lysimeter
N.K. Tyagi (Director), D.K. Sharma (Senior Scientist)
*,
S.K. Luthra (Senior Scientist)
Central Soil Salinity Research Institute, Karnal-132001, India
Accepted 9 August 1999
Abstract
Lysimeter experiments were conducted on rice during rainy season (July±October) and sun¯ower during summer seasons (March±June) in a set of two electronic weighing type lysimeters of 2 m2 m2 m size to measure the hourly evapotranspiration of these crops from 1994 to 1995 at Karnal, India. The average weekly ET of rice varied from <3 mm per day in the early growing period to >6.6 mm per day at milking stage. The peak ETcwas 6.61 mm per day and it occurred 11
weeks after transplanting at reproductive stage when LAI was 3.4. In case of sun¯ower, ETcwas
<1.0 mm per day at the initial stage, achieved a peak value of 14.1 mm per day between 8 and 9 weeks after sowing (WAS) and it declined to 3 mm per day during maturity phase. Precise information on crop coef®cients, which is required for regional scale irrigation planning is lacking in Asian countries. Crop coef®cients (Kc) for rice and sun¯ower from ETc measurements and
weather data have been developed. The estimated values of crop coef®cient for rice at the four crop growth stages (initial, crop development, reproductive and maturity) were 1.15, 1.23, 1.14 and 1.02 and correspondingKcvalues for sun¯ower were 0.52, 1.1, 1.32 and 0.41. The estimatedKcvalues of
sun¯ower is 11.6±74.2% higher than the values suggested by FAO. Relationship between standard FAO±Penman±Monteith with other reference evapotranspiration methods has also been established.
#2000 Elsevier Science B.V. All rights reserved.
Keywords:Rice; Sun¯ower; Crop ET; Crop coef®cients; Lysimeter; Reference ET
*Corresponding author. Tel.:91-184-291218; fax:91-184-290480.
E-mail address: cssri@x400.nicgw.nic.in (D.K. Sharma)
1. Introduction
Rice (Oryza sativaL.) is an important staple food crop and occupying an area of 25.4 million ha in India. It is cultivated throughout the country under upland as well in lowland conditions and consumes 42% of available water supply. In oilseeds, sunflower is the third important crop in term of cultivated area next only to mustard and groundnut and covers about 1.89 million ha in India. Yield and quality of these crops suffer due to insufficient water supply and improper scheduling of irrigation. Available irrigation water has to be utilized in a manner that matches the water needs of these crops. Water requirements of the crops are vary substantially during the growing period mainly due to variation in crop canopy and climatic conditions (Doorenbose and Pruitt, 1977). The knowledge of crop water requirements is an important practical consideration to improve water use efficiency in irrigated agriculture. There is considerable scope for improving water use efficiency of these crops by proper irrigation scheduling which is essentially governed by crop evapotranspiration (ETc). Accurate estimation of crop ET is an
important factor in efficient water management. Therefore, the first objective of this study was to measure daily, seasonal and peak ETcrates of rice and sunflower with weighing
lysimeters for efficient irrigation scheduling.
To extrapolate the measurement of ETcfor irrigation planning in regional scale, crop
coefficient (Kc), which is the ratio of ETcto grass reference evapotranspiration (ET0), is
often used. Allen et al. (1990) have suggested that the crop coefficient values need to be derived empirically for each crop based on lysimeteric data and local climatic conditions. Crop coefficient values for a number of crops grown under different climatic conditions have been suggested by Doorenbose and Pruitt (1977). These values are commonly used in places where the local data are not available and they emphasized the strong need to develop crop coefficients under given climatic conditions. Crop coefficients obtained through lysimeter have not been developed for important crops under semi-arid climatic conditions in India and other Asian countries. The second objective is to drive theKcof
these crops using daily weather and crop ET data for irrigation planning and management in regional scale. According to Smith et al. (1992), FAO Penman±Monteith gives more consistent ET0estimates and has shown to perform better than other ET0methods. The
climate data required to use in Penman±Monteith equation are not always available in developing countries. Therefore, the third object of this paper is to assess the relationship between standard FAO Penman±Monteith with other ET0methods.
2. Materials and methods
The experiments were conducted in weighing lysimeters at the research farm of Central Soil Salinity Research Institute, Karnal, India, from 1994 to 1995. This place is located at an elevation of 245 m (MSL) with latitude of 298430N and longitude of 768580E. The climate of Karnal is semi-arid. A mean monthly maximum temperature of 39.68C was recorded during May and the lowest value of 6.78C was measured during the month of January. The average monthly relative humidity was the lowest (32%) during May, while the highest value of 87% was observed during September. The sunshine hours
during April, May, September and October ranged between 9.0 and 10.7 h per day, and between 6.8 and 8.9 h per day in remaining months. Average annual rainfall is 667 mm, of which 68% is received during July±September.
2.1. Lysimeters
The dimensions and other details of lysimeters are given in Fig. 1. The height of lysimeter rim was maintained near the ground level to minimize the boundary layer effect in and around the lysimeter. The total suspended weight of the lysimeter including tank, soil and water was about 14,000 kg. This provided a safety factor of 2.85. A high safety factor was provided to take care of extra load in case of replacement of a load cell without the danger of overloading and also to take care of the shock loading.
2.2. Weather station
A weather station was established within 10 m of the lysimeters. The weather sensors included two anemometers, two humidity sensors, one pyrometer, one net radiometer, a tipping bucket rain gauge, a wind direction sensor and the two soil heat flux plates were installed. The sensors were battery operated and the power was supplied either from a 12 V lead acid battery or with a 10 W solar cell. The weather sensors were attached to the datalogger. The operation of the lysimeter and weather station is automatic and hourly and 24 h values of crop ET and weather data were allowed to be stored in the datalogger for a week. Every week, the data from the datalogger was first electronically transferred to a cassette tape and then from the cassette to a personal computer. A programme (Allen, 1991) named TAPCARD was used to analyze the data to prepare hourly and daily summarized of crop ET and weather data. The average of minimum and maximum temperatures and rainfall during the cropping seasons are given in Table 2.
2.3. Crop details
Rice crop (variety Jaya) was transplanted in both lysimeters at row spacing of 20 and 15 cm between plants on 4 July in 1994 and 30 days old seedlings were transplanted in and around the lysimeters. The row spacing and plants within the row of all the crops was adjusted to give equal plant density in and around the lysimeters. The crop in and around the lysimeter was harvested on 30 October 1994. Sunflower (variety hybrid Varun) was sown on 5 and 8 March during 1994 and 1995, respectively, at row-to-row distance of 30 cm. The crop was harvested on 14 and 15 June during respective seasons. The similar height and plant densities of all the crops inside and outside of the lysimeter were maintained by sowing the crop on the same date and following the same recommended agronomic practices. It helped to check the clothesline effect on ET of crops. Traffic was minimized near the lysimeter so that the crop and soil conditions would be representative of the bulk of the surrounding field. Neutron moisture access tubes and tensiometers to measure the soil water content at different depths were installed on both lysimeters. The sunflower crop in the lysimeters was irrigated when there was 25% depletion of the available soil moisture in the crop root zone. Rice crop in and around the lysimeters was
grown under continuous submergence. The deep percolating water collected at the bottom of the tanks was pumped periodically to keep water table below 1.5 m from soil surface most of the time. The water pumped out or added through rainfall/irrigation was accounted for in the crop ET computations. Recommended doses of fertilizers were given
Fig. 1. Lysimeter top (a) and side (b) views.
to crops and similar irrigation and cultural practices were followed in and around the lysimeters to ensure uniform plant growth throughout the 1.5 ha area to check the oasis effect on crop ET.
3. Results and discussion
3.1. Crop evapotranspiration
The crop evapotranspiration (ETc) is, generally, measured with weighing type
lysimeter, as the change in mass of the lysimeter is divided by the evaporating area of lysimeter vegetation. The differences in ETcmay be expected from the development of
crop canopy and difference in energy±absorption characteristics. The duration of crops with respect to stage of growth is given in Table 1. The ETcand other crop characteristics
of rice and sunflower are described below.
3.1.1. Rice
Measured leaf area index exceeded 1.0 four weeks after transplanting (WAT) and it reached a maximum of 3.86 during 11 WAT and thereafter decreased due to leaf senescence in both the seasons (Table 3). The net solar radiation (Rn) during the growing
seasons was 13741 W mÿ2
(Table 2). Net radiation is about 67% of total radiation during crop growing season. The average weekly ET (mm per day) of rice rose from 3.54 to 6.55 mm per day during 1±7 WAT and thereafter fell to 5.39 and this may be due to fluctuation in net radiation (Table 2). Fynn et al. (1993) have also reported that ETc
during unshaded or sunny conditions was primarily related to solar radiation. The ETcof
rice reached the peak value of 6.61 mm per day in the 11th WAT (Table 3). Crop ET increased as LAI exceeded 3, presumably because most of radiation was intercepted by crop canopy. In the Indian subcontinent, maximum ETcof 6.5 mm per day was recorded
at 6 WAT under sub-humid climatic condition at Dehra Dun obtained by Bhardwaj (1983) and 7.2 mm per day under semi-arid conditions at Ludhiana during the 9th WAT (Sandhu et al., 1982). There was gradual reduction in crop from 4.92 to 2.88 mm per day during the 15±17th WAT. It was mainly due to lower net solar radiations during the 15±17 WAT and also the LAI was drastically reduced due to leaf senescence. The seasonal ETcduring
the cropping season was 587 mm. Sandhu et al. (1982) who measured crop ET by water balance in the field experimental plots, reported that total seasonal ETc of rice under
semi-arid conditions at Ludhiana was 701 mm and this value was 19.4% higher than the
Table 1
Length of growing stages (days) of rice and sun¯ower in the semi-arid climate of India
Crops Growth cycle Crop growth stages
Initial (I) Crop development (II) Reproductive (III) Late season
Rice 119a 21 35 42 21
Sunflower 105 20 35 30 20
values measured by the weighing lysimeter in this study. The seasonal ETcof 499 mm for
rice was reported by Bhardwaj (1983) under sub-humid climatic condition at Dehra Dun, which is 15% lower than the value reported using weighing lysimeter at Karnal in this study.
3.1.2. Sun¯ower
The LAI of sunflower increased slowly up to 3 weeks after sowing (WAS), reaching the maximum of 4.38 and 4.28 in 1994 and 1995, respectively, at 8 and 9 WAS in each season (Table 2). The net solar radiation during entire season was 12,441 W mÿ2
during 1994 (Table 2). These values during the 1995 was 12,159 W mÿ2
. Net radiation is about 57.1% and 56.4% of the total solar radiation in respective seasons. The average weekly ETc of sunflower during initial growth stage gradually increased mainly because of
incomplete ground cover by the crop. The peak value of 12.86 and 14.11 mm per day were recorded during 9 and 8 WAS in 1994 and 1995, respectively during the crop development growth stage. The maximum value of ETc might have been due to LAI,
which was greater than 4 and because of higher net radiation (141.3 and 121.6 W mÿ2
in 1994 and 1995, respectively). In similar experiments elsewhere it was found that ETcof
sunflower increased as LAI exceeded 3, presumably because most of the radiation was intercepted by the crop canopy (Villalorios and Fereres, 1990). The ET of sunflower suddenly dipped from 4.47 to 3.67 mm in 1994 and 5.38 to 3.78 mm per day during 1995 Table 2
Net solar radiation (Rn, Wm
ÿ2), average temperature (T,
8C) and rainfall (R, mm) during the cropping seasons
WASa Rice Sunflower
10 153.1 28.7 13.2 139.5 31.7 7.2 118.6 28.8 3.3
11 163.8 28.4 ± 135.3 31.1 4.6 133.6 29.8 0.7
Seasonal total 13,741 ± 705.8 12,441 ± 38.6 12,159 ± 54.4
aWASweeks after sowing/transplanting.
from 4 to 5 WAS (Table 3). It could be due to difference in net radiation which varied between 109.3 and 89.5 W mÿ2
in 1994 and from 135.3 to 82.9 W mÿ2
in 1995. The ETc
of sunflower reduced drastically to 3.05 and 1.13 mm per day in respective seasons during the 14th WAS due to leaf senescence. Seasonal ETc amounts were 664 and
646 mm from sowing to harvest in respective years.
3.2. Comparison of reference evapotranspiration (ET0)
Mohan (1991) compared ET0 values obtained by using the four methods of FAO
together with Hargreaves method and concluded that modified Penman method could be adopted for tropical conditions in India. Jensen et al. (1990) reviewed the methods for estimating ET0and recommended that Penman±Monteith equation as presented by Allen
et al. (1989) as the preferred method for daily reference ET. From the daily values of weather parameters monitored at weather station, grass reference evapotranspiration (ET0) was computed by seven weather-data-based (ET0) methods. These included
Penman±Monteith (PMon), (Smith et al., 1992), FAO-ID-24 corrected Penman, FcPn (Doorenbose and Pruitt, 1977), 63 version of original Penman, Pn63 (Penman, 1963), FAO-ID-24 Radiation, FRad (Doorenbose and Pruitt, 1977), FAO-ID-24 Blaney and Criddle, FB-C (Doorenbose and Pruitt, 1977), US Class A Pan Evaporation. The linear regression was used to describe the association between Penman±Monteith (PMon) and other methods during both the seasons (Table 4). From July 7 to October 31 Table 3
Leaf area index (LAI) and average weekly evapotranspiration (ETc, mm per day) of rice and sun¯ower
Weeks after Rice Sunflower
10 3.67 5.73 3.84 12.05 3.95 11.1
11 3.21 6.61 3.11 12.05 3.28 6.06
12 2.47 6.17 2.10 11.02 2.18 6.54
13 2.11 6.26 1.27 4.16 1.55 3.41
14 1.88 5.75 0.88 4.96 0.79 0.99
15 1.55 4.92 0.55 2.17 0.62 0.80
16 1.32 3.80 ± ± ± ±
17 0.45 2.88 ± ± ± ±
Seasonal total ET (mm) 587.6 664.2 646.4
(monsoon season), better agreement was observed between Penman±Monteith and FAO±Blaney and Criddle (FB±C) methods and also between Penman±Monteith and FAO-ID-24 corrected Penman followed by other methods (Table 4). The ET0 estimated by
different methods are given in Fig. 2 indicated that maximum ET0value of 710 mm was
estimated by FAO-corrected Penman and minimum with PanE (422.3 mm) during monsoon season.
During the summer seasons (3 March±15 June), there is good agreement between the Penman±Monteith and FAO-corrected Penman methods although, on average, the PanE, FAO-corrected Penman, FAO±Blaney and Criddle, Hargreaves methods of ET0estimates
were 3.7%, 10.1%, 23.2%, 23.1% higher, respectively, than the Penman±Monteith estimates (Fig. 2). The values ofR2andt-test in Table 4 suggested that FAO-corrected Penman, PanE, FAO±radiation and FAO±Blaney and Criddle methods for estimation of daily ETc are similar to the Penman±Monteith in semi-arid regions during summer
season.
3.3. Crop coef®cient (Kc)
To extrapolate the measurement of ETc for irrigation planning, crop coefficient (Kc)
which is the ratio of crop ET to grass reference ET is often used. Crop coefficient (Kc)
values of rice and sunflower were obtained from crop evapotranspiration measured by lysimeter divided by reference ET calculated by different methods and values of these crops are discussed below.
3.3.1. Rice
During the first growth stage which covered the period from transplanting to the end of the 3rd week after transplanting (WAT), crop coefficients increased from 0.99 to 1.27 and 1.01 to 1.24 based on Penman±Monteith and FAO±Blaney and Criddle methods, respectively (Fig. 3). Crop coefficient of rice during initial stage is about 1.0, could be due to the fact that rice was transplanted in flooded water and more water was available for surface evaporation. During crop development stage (4±8th WAT), Kc values
Table 4
Regression statistics between Penman±Monteith and different methods of ET0during summer (sun¯ower) and monsoon seasons (rice)
Variables PanE FcPn FB±C Harg Frad Kpen
Summer season
Regression line intercept (a) 1.786 1.344 2.286 3.915 2.488 0.195 Regression line slop (b) 0.822 0.881 0.767 0.564 0.677 1.084 Coef®cient of determination (R2) 0.937 0.944 0.907 0.865 0.928 0.428
t-test value (p0.05) 14.76 15.08 11.39 9.11 13.17 3.071
Monsoon season(rice)
Regression line intercept (a) 3.150 0.374 0.457 0.99 0.651 0.171 Regression slop (b) 0.034 0.006 0.055 0.009 0.007 0.009 Coef®cient of determination (R2) 0.120 0.879 0.981 0.397 0.684 0.748
t-test value (p0.05) 1.828 17.5 42.8 1.430 8.468 9.7
Fig. 2. Reference evapotranspiration (ET0) during monsoon (July±October) (a) and summer (March±June) (b) seasons.
calculated by these two methods further increased upto 1.39 and 1.34, respectively, during 7th WAT. Crop coefficient reached a value of more than 1.3 during 7 WAT by these methods and could mainly be due to soil heat flux which contributed about 4 W mÿ2
per day of energy during this period. Apart from net solar radiation, soil heat flux also contributed energy for Crop ET during rice crop season (from 3rd to 11th WAT), which raised theKcvalue to more than 1.1 during 3rd to 11th WAT. Hem et al. (1991)
showed that soil heat flux accounted for 8±10% of total energy and it was slightly larger at early stage when the canopy was small. They also observed that high sub-soil surface temperature caused sensible heat transfer from soil to crop canopy foliage and thus the canopy was absorbing sensible heat from the soil, to maintain the equilibrium within the crop canopy.
Fig. 3. Crop coef®cients (Kc) of rice.
The maximum crop coefficient of 1.39 and 1.34 by Penman±Monteith and FAO± Blaney and Criddle, respectively were calculated during the 7th WAT when LAI was more than 4, close to the panicle initiation stage of rice and soil heat flux also contributed about 3±4 W mÿ2
per day energy for crop ET during this period. Bhardwaj (1983) conducted experiments in gravimetric lysimeter in North India and observed that ratio of crop ET of rice and class A pan became unity just after transplanting and increased to a maximum value of 1.56 in 6 WAT. Singh (1988) and Midmore et al. (1984) reported the ETc/Epan ratio more than 1 during the initial stage of rice may be due to higher LAI (>3).
During the reproductive phase starting from 9 to 14th week after transplanting,Kcof rice
slightly decreased from 1.26 to 1.01 and 1.17 to 1.1 with Penman±Monteith and FAO± Blaney and Criddle methods, respectively because the LAI reduced to less than 1.3 during this stage. Crop coefficient declined rapidly to 0.85 and 0.88 by respective methods during last stage covering the period from 15 to 17 WAT.
The computed Kc values by Penman±Monteith method during initial, crop
development, reproductive and last stages were 1.15, 1.23, 1.14 and 1.02, respectively and these values estimated by and FAO±Blaney and Criddle and FAO±radiation methods were 1.11, 1.19, 1.09 and 0.97 and 1.1, 1.29, 1.11 and 0.92 in respective stages (Table 5). The estimatedKcvalues calculated by Penman±Monteith and FAO±Blaney and Criddle
during all the stages are closer to the values reported by FAO.
3.3.2. Sun¯ower
In the initial stage covering the period from sowing to end of the 3rd week after sowing (WAS), crop coefficients increased from 0.36 to 0.96, 0.30 to 0.88 and 0.42 to 0.99 by Penman±Monteith, FAO-ID-24 corrected Penman and PanE methods, respectively (Fig. 4). The Kc values during this stage increased very slowly because LAI was less
Table 5
Values of crop coef®cient derived from different methods for rice (1994) and sun¯ower (average values of 1994 and 1995)
Methods Crop stages
I II III IV Average
Rice
Penman±Monteith 1.15 1.23 1.14 1.02 1.14
FAO-corrected Penman 1.05 0.88 0.87 0.82 0.90
Kimberly Penman 1.18 1.12 1.13 0.99 1.10
FAO±Blaney and Criddle 1.11 1.19 1.09 0.97 1.09
FAO±radiation 1.10 1.29 1.11 0.92 1.11
than 0.5 during this period. Crop coefficient increased rapidly from 1.06 to 1.39, 1.05 to 1.28 and 1.1 to 1.38 by respective methods in crop development stage starting from fourth to ninth week after sowing (Fig. 4). The maximum values of crop coefficients were also estimated during the ninth week after sowing mainly because of the LAI was more than 4.0 during this week. Sin (1989) reported thatKcvalues was curvilinearly related to the
LAI. The reproductive phase starting from the 10 to 13th weeks after sowing, crop coefficient decreased slowly upto 0.62, 0.53, and 0.56 by Penman±Monteith,
FAO-Fig. 4. Crop coef®cients (Kc) of sun¯ower.
corrected Penman and PanE methods, respectively, could be due to LAI during this week decreased to 2.16.
Table 5 summarizes the growth stages wise computed Kc values for sunflower.
The estimated Kc values by Penman±Monteith method in the first, second and
third stages were 80.0%, 45.3% and 15.1%, respectively, higher than the FAOKcvalues.
The estimated Kcvalue was lower than the FAO Kc value by 42.8% in the last stage.
On the other hand, observed seasonal Kc value was slightly higher than the FAO Kc
value.
4. Conclusions
The estimatedKcvalues for this region during the first, second, third and fourth growth
stages for rice are 1.15, 1.23, 1.14 and 1.02, respectively, and the corresponding values for sunflower are 0.63, 1.09, 1.29, and 0.40. The estimated values of crop coefficients for sunflower differ considerably at all the stages from those suggested by FAO, but in case of rice calculated values are very close to the values given by FAO. Local calibration of crop coefficients is therefore an essential.
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