Evapotranspiration and Crop Coefficient of Sorghum
Evapotranspiration and Crop Coefficient of Sorghum
Evapotranspiration and Crop Coefficient of Sorghum
research-article2023
ASW0010.1177/11786221231184206Air, Soil and Water ResearchNegash et al.
Evapotranspiration and Crop Coefficient of Sorghum Air, Soil and Water Research
Volume 16: 1–7
ABSTRACT: Sorghum has an enormous role in the economy of sorghum-growing nations. Supplying a precise amount of water to a crop based
on crop needs is the main agenda in implementing water-saving agriculture. Non-weighing type lysimeters were used to determine actual crop
evapotranspiration and crop coefficient of sorghum at the experimental farm of Melkassa Agricultural Research Center situated in the semi-arid
area of Ethiopia. Soil-water balance approaches were applied to obtain actual crop evapotranspiration, while the Penman-Monteith technique
was used to determine reference evapotranspiration. Growth stages-wise crop coefficient was computed as a ratio of actual crop evapotranspi-
ration to reference evapotranspiration. The total seasonal sorghum actual crop evapotranspiration during the 2017 and 2018 experimental years
was 358.6 and 377.54 mm, respectively. The 2 years average sorghum actual crop evapotranspiration was 368.07 mm. The mean locally devel-
oped actual crop coefficient values of 0.55, 1.15, and 0.59 were observed for the initial, mid, and end-season, respectively. The FAO-adjusted
crop coefficient values for mid and end-season were 1.01 and 0.52, respectively. The developed Kc values considerably differed from the FAO-
adjusted Kc values. So, the determination of actual crop evapotranspiration and crop coefficient for crop growth at local climate conditions is
vital for decision-making concerning water management in the area where irrigation is practiced.
Keywords: Crop evapotranspiration, crop coefficient, reference evapotranspiration, soil-water balance, non-weighing lysimeter
RECEIVED: March 20, 2023. ACCEPTED: June 1, 2023. CORRESPONDING AUTHOR: Tatek Wondimu Negash, Ethiopian Institute of Agricultural
Research (EIAR), Melkassa Agricultural Research Center (MARC), P.O. Box 436, Adama,
Type:Original Research Article Ethiopia. Email: tatek.wondimu456@gmail.com
Creative Commons Non Commercial CC BY-NC: This article is distributed under the terms of the Creative Commons Attribution-NonCommercial
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further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/open-access-at-sage).
2 Air, Soil and Water Research
sorghum crop coefficient (Kc) values (Bashir et al., 2008; 13.8°C to 28.7°C. The average annual rainfall of the area over
Howell et al., 2006; Lima et al., 2021; Piccinni et al., 2009; the same periods (1977–2018) was 825 mm, concentrated in
Shenkut et al., 2013; Tyagi et al., 2000). Nevertheless, depend- July, August, and September. The annual mean relative humid-
ing on local weather conditions, soil texture, and duration of ity and solar radiation ranges from 46.17% to 75.33%, and
the crop, the sorghum Kc values obtained from the above stud- 19.14 to 22.02 MJ m2 day−1, respectively during the period 1977
ies significantly vary from place to place. Hence, all the above to 2018. The long-term (1977–2018) mean annual reference
studies suggested the strong need for local calibration of Kc evapotranspiration (ETo), and wind speed of the area were 3.8
under local climate conditions since the climate and their to 5.42 and 0.3 to 2.71 m s−1, respectively. The weather param-
interaction with crops differ in their spatial or temporal scale. eters for each month during the 2017 and 2018 sorghum grow-
Therefore, a lysimeter (non-weighing type) experiment was ing periods are given in Table 1. The soil texture class of the
conducted to develop actual crop evapotranspiration and crop study site is clay loam. The Filed Capacity (FC) and Permanent
coefficient values for sorghum at Melkassa in a semi-arid area Welting Point (PWP) of the soil are 0.346 and 0.176 m3 m−3,
of Ethiopia. Water-balance approaches were employed for this respectively with a bulk density of 1.13 g cm−3.
purpose.
The experimental setup
Materials and methods
To increase the accuracy of the data collected from the experi-
Study area
mental site two non-weighing type lysimeters having different
The study was carried out at Melkassa Agricultural Research internal planting areas of 2 m2 (2 m × 1 m) and 4 m2 (2 m × 2 m)
Center (MARC), a semi-arid part of Ethiopia. Geographically, with the same total depth of 2.6 m located near the metrologi-
the site is situated at 8°24´N latitude and 39°21´E longitude cal station of the research center were used to identify actual
with an elevation of 1550 m a.m.s.l (Figure 1). The climate of crop evapotranspiration and crop coefficient for sorghum. Each
the area is characterized as semi-arid with erratic and uneven lysimeter has contained access chambers and connected under-
distribution of rainfall patterns. The mean minimum and max- ground steel pipes to drain excess water to the collector cham-
imum temperature over the period 1977 to 2018 ranged from ber. To prevent the entry of surface runoff inside the lysimeter
Negash et al. 3
Table 1. Selected Weather Parameters During the 2017 and 2018 Sorghum Growing Periods.
Month Tmean (°C) RH mean (%) Ave. u2 (m s−1) Ave. Rs (MJ m2 day−1) Eff. rainfall (mm) Ave. ETo (mm day−1)
2017
Total 445.68
2018
Tota1 337.8
during rainy days, the rim of the lysimeters protruded 10 cm the soil particle size distribution. The soil bulk density of the
above the soil surface. To monitor the soil moisture level inside experiment field was determined by taking undisturbed soil
the lysimeters access tubes were installed at the center of each samples using the core method. The soil water content at Filed
lysimeter up to an effective root depth of 100 cm. Then, both Capacity (FC) and Permanent Welting Point (PWP) was
lysimeter’s results were used to compute the sorghum crop determined by the pressure plate apparatus technique, while
coefficient (Kc). the total available soil water (TASW) was obtained by sub-
tracting PWP from FC. The irrigation water was applied when
Crop management 55% of the total available soil water was depleted in the main
rooting layer. A known volume of irrigation was applied using
Sorghum variety of Teshale was sown in mid-July inside and a watering can inside and outside the lysimeter by converting
outside of the lysimeter to keep a similar environment. Before the 55% depletion into the volume of water. Irrigation water
sowing, the two lysimeters were made to have similar moisture was terminated following the maturity sign of the crop. The
content. The row and plant spacings were 75 and 15 cm respec- applied irrigation water in each lysimeter was used to compute
tively. Planting and all other field management such as ferti- the actual crop evapotranspiration (ETa). The amount of
lizer, weeds, and pest control were uniformly applied inside and applied irrigation water was computed from the following rela-
outside of the two lysimeters area. Fertilized rates of 100 kg ha−1 tionship (equation (1); Brouwer et al., 1985):
for diammonium phosphate (DAP) and 50 kg ha−1 for urea
were applied to the plot but urea was applied in a split. Seedling V = A*D (1)
density was controlled by 20 plants per m2. The crop was har-
vested in end-October during the two consecutive experimen- Where: V is the amount of water to be added (m3); A is the
tal seasons (2017 and 2018). surface area of lysimeter (m2); and D is the depth of application
(m).
Soil moisture monitoring and irrigation application
Soil moisture contents were monitored from inside the lysim- Crop evapotranspiration and reference
evapotranspiration
eter at an interval of 15 to 100 cm depth before and after every
irrigation. A CPN503 neutron moisture meter was used to The daily actual crop evapotranspiration was computed using the
monitor soil moisture content at the lower depth (15–100 cm) soil water balance approach (equation (2); Jensen et al., 1990).
in the lysimeter. Soil moisture content in the top 0 to 15 cm soil
layer was monitored by the gravimetric (oven method). The I + P − D + ∆S
ETa = (2)
Bouyoucos hydrometer method was employed to determine ∆t
4 Air, Soil and Water Research
Where: ETa is actual daily crop evapotranspiration (mm), I is conditions and plant height using the following (equation (5);
applied irrigation water (mm), P is effective rainfall (mm), D Allen et al., 1998):
is drainage water (mm), is the change in soil moisture storage
(mm), and Δt is the time interval between two consecutive Kc mid − FAO = Kc mid ( Tab )
measurements in days. The drainage water (D) was measured (5)
in the underground room from drain tubes that were con- + 0.04 ( U 2 − 2 ) − 0.004 ( RHmin − 45 )
nected to the lysimeters with the help of a graduated cylinder. *( h / 3)0.3
Change in soil moisture storage (ΔS) is the difference in mois- Where, Kc mid − FAO is the FAO-adjusted Kc for the mid-sea-
ture content of each consecutive day and it was computed by son, Kc mid ( Tab ) is the tabulated Kc for the mid-season
deducting the moisture content obtained today from the pre- gained from Table 12 of FAO-56, U 2 is the average wind
vious day. The net applied irrigation water to the crop depends speed a 2 m height during the mid-season (m s−1), RHmin is the
on the magnitude of moisture deficit in the soil, leaching average relative humidity during mid-season (%), and h is the
requirement, and expectancy of rainfall. When no rainfall is average plant height during the mid-season stage (m). Some
likely to be received and the soil is not saline, the net quantity approach was employed to compute Kc-end-FAO. The compari-
of irrigation water to be applied is equal to the moisture deficit son was done between locally measured Kc and FAO-adjusted
in the soil, that is, the quantity required to fill the root zone to Kc values.
field capacity. The moisture deficit (d) in the effective root
zone was computed using the following (equation (3); Mishra Results and discussion
& Ahmed, 1990).
Weather characteristics
Weather parameters such as; mean air temperature, relative
n
( Fci − Pwi ) *ASi*Di
d= ∑ 100
(3) humidity (RH %), wind speed (u2), solar radiation, rainfall, and
i =1 ETo for the 2017 and 2018 study seasons are presented in
Table 1. These weather variables were similar to some extent in
Where: Fci = field capacity of the ith layer on oven dry weight both growing seasons except for some differences in rainfall
basis; Pwi = actual soil moisture contents of the ith layer on distribution. For instance, the average temperature was 21.66°C
oven dry weight basis; ASi = apparent specific gravity of the in 2017 and 21.5°C in the 2018 sorghum growing season. The
ith layer; Di = depth of ith layer; and n = number of layers in mean value of relative humidity (RH%), wind speed, solar radi-
the root zone ation, and ETo are also comparable in both growing periods
The FAO Penman-Monteith method was applied to calcu- Table 1. Total accumulated effective rainfall amounts of
late daily reference evapotranspiration (ETo) using the cropw- 445.68 mm in 2017 and 337.8 mm in 2018 were observed.
pat8.0 model. Weather data such as daily air temperature Higher rainfall amounts were observed in July (201.12 mm),
(minimum and maximum), sunshine hours, wind speed at 2 m and September (91.04 mm) in 2017, while 150.9 mm of rainfall
height, and relative humidity were used as model input to cal- amount was received in August 2018 (Table 1). The total
culate daily ETo. amount of rainfall recorded in 2018 was less than 24.2% of the
rainfall amount recorded in 2017. Average ETo values over the
Crop coefficient (Kc) sorghum growing season were 4.61 mm day−1 in 2017 and
4.71 mm day−1 in 2018 (Table 1).
The actual sorghum crop coefficient values for each growth
stage of the crop were computed from the following relation-
ship (equation (4)). Sorghum actual crop evapotranspiration
ETa Sorghum Teshale variety could take about 92 days to mature
Kc = (4) under Melkassa climate conditions as presented in Table 2. At
ETo
the study site, the division of sorghum growing stages was
Where Kc is the actual crop coefficient (dimensionless); ETa is depending on the occurrence of plant leaves numbers. The ini-
the actual crop evapotranspiration (mm day−1); and ETo is the tial stage (Kcini) is when the plant develops up to three leaves,
reference evapotranspiration (mm day−1). the crop development stage (Kcdev) from three leaf numbers to
Sorghum crop coefficient values developed under a standard heading, the mid-season stage (Kcmid) from heading to black
climate condition (RHmin = 45% and u2 = 2 m s−1) are listed in layer formation, and the late-season stage (Kclate) from black
FAO-56 Table 12, as 0.3, 1.10, and 0.55 for the initial, mid, and layer formation to harvest. The seasonal sorghum crop evapo-
end-season. These values must be adjusted with local weather transpiration (ETa) during the 2017 and 2018 experimental
conditions, where RHmin and wind speed differ from 45% and years were 358.6 and 377.54 mm, respectively with an average
2 m s−1, respectively. The typical Kc values (>0.45) for the mid- of 368.07 mm (Table 2). The average sorghum daily actual crop
season and end-season stages were adjusted with climate evapotranspiration ranged between 2.27 to 5.41 mm day−1 in
Negash et al. 5
(a)
8
368.07
104.3
93.8
134.4
38.4
2 ETo, mm day-1
11.2
17.0
8.6
21.0
57.7
ΔS
ETa, mm day-1
0
0 20 40 60 80 100
Day After Sowing (DAS)
127.2
41.9
36.7
41.7
(b)
–
D
77.9
183.4
63.1
4
I
Average
2 ETo, mm day-1
ETa, mm day-1
0
103.2
257.2
93.5
4.8
55.6
0 20 40 60 80 100
P
109.9
125.4
37.7
ETa
Note. P = effective rainfall; I = irrigation; D = drainage; ΔS = change in soil moisture storage; ETa = actual crop evapotranspiration; No Irri = number of irrigation.
8.3
8.4
38.4
82.7
25
–
D
3
2
10
from the initial, reaching the peak at the midseason and started
decline toward the end of the season. The variation in ETa is
35.2
205.2
53.3
105.0
11.7
105.6
3.7
92
25
28
24
15
358.6
77.7
39.1
104.1
25.5
21.1
17.1
107
ΔS
174.0
61.7
–
64
D
11
1
5
4
72.9
50.7
10.1
toward the end of the season. This result was alike to the trend
described in FAO-56. The seasonal Kc values during the initial
60.2
100.9
5.9
97.0
264.1
2017
in 2017, while Kc values of 0.55, 1.16, and 0.61 for initial, mid,
Days
28
15
Late-Season
Mid-Season
Growth
Initial
ogy on ETa. As the crop develops and shades the ground to the
6 Air, Soil and Water Research
0.8
was higher than with a Kc-mid-FAO-adjusted value of 1.01 (Table 4).
0.6
The average locally developed Kc-mid-local for this study was
0.4
lower than the sorghum Kc-mid values of 1.24 and 1.18 obtained
0.2
Kc, 2017
Kc, 2018
by Tyagi et al. (2000) and Shenkut et al. (2013), respectively.
Sorghum average locally measured Kc values (Kc-mid-local) were
0
0 20 40 60 80 100 higher than the Kc-mid values of 0.99, 1.0, 1.05, and 1.1 reported
Day After Sowing (DAS) by Lima et al. (2021), Piccinni et al. (2009), Howell et al.
Figure 3. Crop coefficient (Kc) values as a function of days after sowing (2006), and Allen et al. (1998), respectively. However, a similar
for 2017, and 2018 sorghum growing seasons. sorghum Kc-mid-local value of 1.15 was obtained by Bashir et al.
(2008) in Sudan, Gezira, using a satellite-based energy balance
Table 3. Locally Developed Kc Values for Sorghum at Melkassa model.
During 2017 and 2018. Two years’ average locally measured Kc-end-local value of 0.59
was higher than with a Kc-end-FAO-adjusted value of 0.52 (Table 4).
Locally developed Kc
It exceeded that of the Kc-end-FAO-adjusted value by 11.86%. The
Trail Kc-ini-Local Kc-mid-Local Kc-end-Local average locally developed Kc-end-local values of sorghum were
year greater than the Kc values of; (Kc-mid = 0.4 and 0.48) reported
Year I 0.56 1.13 0.57 by Howell et al. (2006) and Bashir et al. (2008), respectively.
(2017) The average Kc-ini-local value of (0.55) measured in this study
Year II 0.54 1.16 0.61 was equal to that of the Kc-ini values of 0.55 reported by Bashir
(2018) et al. (2008). However, the average Kc-ini-local value obtained
Average 0.55 1.15 0.59 from this study was higher than the Kc values developed by
Kc Tyagi et al. (2000) and Lima et al. (2021). In general, this vari-
ation of Kc values between the locally developed, the FAO
adjusted and other studies could be attributable to the differ-
Table 4. Locally Obtained and FAO-adjusted Kc Values for Sorghum ence in local climatic conditions, growing window, soil texture,
at Melkassa During 2017 and 2018.
and management practice.
Trail Kc-mid-(Local) (Kc-mid(adj)) Kc-end-(Local) (Kc-end(adj))
Year
Implication for irrigation and crop management
Year I 1.13 (1.02) 0.55 (0.48)
(2017) Determining actual crop evapotranspiration and developing
regionally based Kc-value helps to provide reference informa-
Year II 1.16 (1.00) 0.61 (0.55)
(2018) tion for irrigation water management for sorghum in semi-arid
areas of Africa in particular the semi-arid part of Ethiopia. It is
Average 1.15 (1.01) 0.59 (0.52)
Kc
key to optimizing irrigation events, determining the timing
and quantity of irrigation events, providing real-time irriga-
Note. Kc-mid-Local and Kc-end-Local are the locally obtained Kc values for mid and tion recommendations, and for irrigation scheduling, plan-
end-season, respectively; (Kc-mid(adj)) and Kc-end(adj) is the FAO adjusted Kc values
for mid and end-season, respectively where RHmin differ from 45% and wind ning, and development. Moreover, it provides precise water
speed differ from 2 m s−1.
application to crops in areas where irrigation practices take
place. Consequently, it can help private consultants and grow-
effective full cover and reaches full size, the amount of water ers to avoid water overuse and to more precisely meet the crop
abstraction increases which in turn increased the ETa. The water demand to produce greater yields, crop quality, and
maximum Kc value of 1.13 and 1.16 was obtained during the enhanced water use efficiency.
mid-season of 2017 and 2018, respectively when ETa reached
its highest demand. The Kc-mid value recorded in 2017 was Conclusions
lower than the Kc-mid value recorded in 2018. Sorghum has an enormous role in the economy of sorghum-
growing states. However, under-estimation of its actual crop
evapotranspiration can cause yield penalty attributable to water
Comparison with FAO adjusted and other studies
stress, whilst over-estimation can result in excessive applied
Table 4 showed a locally obtained and FAO-adjusted Kc value water, consequently reducing available water for other pur-
for mid, and end-season growth stages. It can be observed that poses. Kc is a significant parameter influencing the estimation
the two seasons’ average locally developed Kc-values were of ETa of any crop, and knowledge of stagewise Kc of sorghum
Negash et al. 7