Determination of Wheat (Triticum Aestivum
L) Seasonal Water Demand and Crop
Coefficient for Effective Irrigation Water
Planning and Management in Semi-Arid, Central Rift
Valley of Ethiopia
Ketema Tezera*, Gobena Dirirsa and Tilahun Hordofa
Ethiopian Institute of Agricultural Research (EIAR), Melkassa Agricultural Research Centre, Ethiopia
Submission: July 30, 2019; Published: August 19, 2019
*Corresponding author: Ketema Tezera, Ethiopian Institute of Agricultural Research (EIAR), Melkassa Agricultural Research Centre, Ethiopia
How to cite this article: Ketema Tezera, Gobena Dirirsa, Tilahun Hordofa.. Determination of Wheat (Triticum Aestivum L) Seasonal Water Demand and Crop Coefficient for Effective Irrigation Water Planning and Management in Semi-Arid, Central Rift Valley of Ethiopia. Int J Environ Sci Nat Res. 2019; 21(1): 556054. DOI:10.19080/IJESNR.2019.20.556054
Estimating right amount of crop water requirement is the need for designing, establishing and managing irrigation projects, and scheduling irrigation. The objective of this study was to estimate the seasonal water demand and crop coefficient of wheat for effective irrigation water planning and management. Field experiment was carried out at Melkassa Agricultural Research Center, Ethiopia during wet growing season under lysimeter. Two non-weighing type lysimeters with the dimension of 1m × 2m area and 1m depth were used to determine the daily ETc of wheat crop. Crop coefficient (Kc) was determined for each growth stages as the ratio of ETc to ETo. The ETc was determined by soil water balance equation and ETo was computed by CROPWAT version 8.0 using the FAO Penman-Monteith equation. The seasonal ETc was found to be 52.2mm, 97.1mm, 191.5mm and 73.2mm of water calculated for initial, crop development, mid-season, and late-season stages, respectively. The measured crop coefficient (Kc) values were 0.54, 1.15 and 0.67 for the initial, mid and late stages, respectively. Some of the Kc values found in this experiment differed slightly from the average of FAO estimation. This indicates that there is a need to develop Kc values for given local climate conditions and cultivars. The maximum plant height and spike length obtained were 86.6 and 8.2cm, respectively. The maximum obtained grain yield and the above- ground biomass yield were 4559.1 and 10897.1kg/ha, respectively. Also, the maximum number of tillers per square meter of 559.0 and number of grains per spike of 42.0 were obtained.
Keywords: Crop coefficient; Crop water requirement; Lysimeter; Wheat
Water is a finite resource used in different sectors like agriculture, domestic and industry. The competition for both quality and quantity of water is alarmingly increasing from time to time . The rapid exponential increment of population growth worldwide in general and in developing countries is forcing the environment to produce more food and cash crop to feed and enhance economic development of the people. The main reason for increasing pressure on water resources are human activity like population growth, urbanization, increased living standards, growing competition for water, and pollution. These are aggravated by climate change and variations in natural conditions. However, the environmental resources like land and water are limited and even decreasing due to over exploitation, pollution and climate change .
Water is vital for crop production which its shortage has an influence on crop yields. Therefore, farmers have a tendency to over-irrigate than irrigating with some tolerable stress without due consideration for scarce water resources. These problems need optimization of water allocation based on water use efficiency (WUE) and enhancement of water productivity is essential similar to the aim of increasing crop yield.
In arid and semi-arid areas where moisture stress is the main challenge for crop production, the spatial and temporal variations exacerbate the problem. Different physical and biological measures are adopted to conserve moisture in the farm level. Moreover, design of irrigation schemes does not address the situation of moisture availability for crop and the competition between different sectors for these reason determinations of the right amount of crop water requirement and crop coefficient is needed.
Wheat (Triticum aestivum L.) is one of the important grain
crops produced worldwide with larger area of cultivation than
any other crop covering 217 million hectares with average yield
of 3.00t/ha. Wheat is the largest deficit item in the developing
country food basket. Between 1970 and 2010, more than half of
the increment in wheat consumption was met by increased wheat
imports, and several countries became totally dependent on imports
for wheat .
Wheat is one of the major cereal food crops grown highly in
Ethiopia, which ranks the country in second place from sub-Saharan
Africa in terms of the total wheat area and production. Among
cereals, wheat is the fourth most important crop in area coverage
following teff, maize and sorghum holding 13.25% out of the total
grain crops cultivation area in the country. Moreover, in amount of
production volume, next to maize, teff and sorghum, wheat is the
higher production volume from cereals production in the country
with total production of 3,434,706 tons, which accounts around
14.85% from the total grain production during 2012/2013 production
season . The yield and production of wheat in Ethiopia
also is increasing . It is commonly grown in the highlands at altitudes
ranging from 1500 to 3000masl. Major wheat production
areas in the country are Arsi, Bale, Shewa, Ilubabor, Western Harerghe,
Sidama, Tigray, Northern Gonder, and Gojam regions .
In Ethiopia due to improvements in seed supply, greater fertilizer
applications and increase in extension support, wheat
production is slightly increasing. However, only during 2012/13
marketing year, 984,000 metric tons of wheat imported from India,
USA and Italy. Moreover, during 2013/14 marketing year, the
Ethiopia Grain Trade Enterprise (EGTE), the government owned
enterprise that controls all commercial wheat imports, planned to
import 400,000 metric tons of wheat . For these reason, expansion
of irrigated wheat production area needed especially in arid
and semiarid. Ethiopian wheat production for 2019/20 will reach
a projected 4.6 million metric tons. This increase is in part due to
a new governmental initiative to make the country wheat self-sufficient
through supplying required inputs, development of irrigation
schemes, promoting mechanization and extension support in
semi-arid areas of the country. The wheat production estimate for
2018/19 reached 4.5 million metric tons, which is like the official
USDA estimate. This is mainly due to good weather conditions, improved
input supply, few pests and lower disease pressure. Most
of the farmers in wheat growing belts started using mechanized
farming systems, especially during harvest. In the past the country
has usually imported 30 to 35 percent of the domestic wheat
demand with no significant volumes of grain exports due to official
export restriction on grains.
Therefore, determination of the crop water requirement and
crop coefficient of wheat is important to utilize the limited water
resources for agricultural water management, design of scheme
and planning of different crop production with available water.
The purpose of this study is to explore the amount of wheat seasonal
water demand and crop coefficient in the rift valley area.
The study was conducted at Melkassa Agricultural Research
Center, Central Rift Valley of Ethiopia. It is geographically located
between latitude of 8024’ to 8026’ N, longitude of 39019’ to 39019’
E and the mean altitude of the area is 1550m.a.s.l. The climate of
the area is characterized as semi-arid with uni-modal low and
erratic rainfall pattern with annual average of 824.9 mm. About
67.4% of the total rainfall of the area occurs from June to September.
The mean maximum temperature varies from 26.3 to 31.0 0C
while the mean minimum temperature varies from 10.4 to 16.4 0C.
Two non-weighing lysimeters were located 100m away from
the Meteorological Station of the Melkassa Agricultural Research
Center was used for the study. The lysimeters used was rectangular
in shape with 1m x 2m dimension. Its effective soil depth was
100cm with additional 100cm layers, 20cm rock, 20cm gravel and
20cm sand pack beneath which excess water from the upper soil
was collected and discharged into the drainage collector placed
in the working chamber through a drainage pipe. The lysimeters
have chambers for aeration and drainage pipes connected to a water
collecting tank and were placed in the working area beneath
the lysimeter. The heights of the lysimeter rims were maintained
near the ground level to minimize the boundary layer effect in and
around it. However, the rims of lysimeters protruded 20cm above
the soil surface so that no surface runoff water entered the lysimeters.
One access tube for each lysimeter was installed at the center
down to 100cm depth.
Kekeba, a bread wheat variety is a semi-dwarf, early maturing
and widely cultivated in various parts of the country. It can grow
in wide agro-ecology, ranging from altitude of 1500 to 2200masl.
It was released by KARC of EIAR in 2010 and it is among the few
wheat varieties grown in lowland area, as it is drought tolerant
. The seed of wheat Kekeba variety was sown by drilling manually
in row after land is prepared well and pre irrigated, with seeding
rate of 125kg/ha at mid-June of the three consecutive years
(2016, 2017 and 2018) in and out of the lysimeters in all directions
to have identical and uniform environment as in normal
fields and decrease adverse effects. Plots size of 1m x 2m with
ridges spaced at 60cm. Seed was sown in double row on both side
of the ridge with spacing of 20cm. The plots were fertilized with
rate of 100kg/ha for both DAP and UREA but UREA was applied in
split. The crop was harvested at the end of October.
Soil samples were collected at interval of 15cm up to 100cm
depth for determination of some soil physical properties like field
capacity, permanent wilting point, bulk density and texture. Particle size distribution was determined using the Bouyoucos hydrometer
method. The water content at FC and PWP were determined
by the pressure plate apparatus technique, whereas total
available water (TAW) was obtained by subtracting PWP from FC.
Bulk density was determined by taking undisturbed soil sample
from the site using the core method . The average FC and PWP
of the soil depth profile were 31.8% and 15.3%, respectively. Thus,
TAW at wheat root depth was 111.9mm. The obtained bulk density
was 1.13g/cm3. Neutron probe was used to monitor the soil
moisture content. The probe was calibrated following standard
procedure for neutron probe calibration by plotting the results
of neutron probe reading and gravimetric sampling around the
access tube. The moisture content was monitored at intervals of
15cm up to 60m soil depth of wheat root depth at different times
during the growing season. Irrigation water was applied to the
crop when there was 55% depletion of the available soil moisture
within the crop root zone . Similar irrigation amount at this
depletion level was given to the crop in and outside the lysimeter
to ensure uniform plant growth. The application of irrigation was
carried out in known volume of water cans by converting the 55%
depletion in terms of volume. Irrigation was terminated at crop
Crop water requirement (ETc) and crop coefficient (Kc) for
wheat was accurately determined using the lysimeters. Drainage
(non-weighing) type lysimeters were installed near to meteorological
station in the center. They were constructed of reinforced
concrete and the inside was lined with a plastic sheet to avoid
leakage or lateral inflow and outflow of water. The actual wheat
ETc was determined using the water balance from lysimeters for
the growth stage of the crop. The ETo was calculated using the
daily weather data of the study area using CROPWATT version
8.0. However, the alternative procedure is to determine ETo from
climatic data using the FAO Penman-monteith method once the
necessary variables specific to the location are determined. In
this study, daily ETo was calculated using FAO Penman-Monteith
Equation  based on the actual daily climatic data collected at
MARC, Agro-meteorological Service Department. The CROPWAT
model calculates ETo based on the formula of FAO Penman-Monteith
using equation 1.
Where: ETo -reference evapotranspiration (mm/day), Rn -net
radiation at the crop surface (MJ/m2/day), G - soil heat flux density
(MJ/m2/day), T -mean daily air temperature at 2m height (0C),
u2 -wind speed at 2m height (m/s), es -saturation vapour pressure
(kPa), ea -actual vapour pressure (kPa), es - ea -saturation vapour
pressure deficit (kPa), Δ -slope vapour pressure curve (kPa/0C)
and γ -psychrometric constant (kPa/0C).
The crop evapotranspiration for each growth stage of the crop
was calculated by using water balance equation 2
Where ETc: crop evapotranspiration (mm), I: irrigation (mm),
R: rain fall (mm), D: drainage collected (mm),
and ΔS: change in storage of soil moisture (mm). The crop coefficient
value over a given period, such as physiological growth
stage or whole season, was then calculated using equation 3
Where Kc: crop coefficient; ETc: crop evapotranspiration, and
ETo: reference crop evapotranspiration.
Water is necessary for proper crop nourishment, when this
water is not naturally available; irrigation makes it possible to
compensate for water deficits. Even when the amount of rainfall
is sufficient, its spatial and temporal distribution may not be as
required. Both under watering and as well as over-watering leads
to soil problems of root and turf diseases, nutritional deficiencies
and reduced plant yields. To ensure success of the wheat production
and the irrigation system monitoring, managing and maintaining
the study was conducted.
The detailed crop water requirement (ETc) of wheat from water
balance components obtained from lysimeter, values are presented
in Table 1. Crop evapotranspiration (ETc) of wheat showed
an increasing from the 20 day after sowing (DAS) to the 45 DAS
and started to decline from 80 DAS to 100 DAS period (Figure 1).
This implies that there was lesser and similar ET of the crop at
the initial stage. In the development stage, there was an increase
in ETc. During mid-season stage the ETc was almost constant as
compared to the other stages. Finally, at the late-season stage
the crop ET showed a decreasing trend, which resulted from leaf
senescence and to the completion of grain formation and filling
thereby limiting transpiration (Figure 1). The result in line with
Allen et al. , that reported the crop water use declined in the
late season stage, which was due to the cessation of leaf growth.
The maximum water use was 191.5mm at the mid-season
stage followed by development stage was 97.1mm. The minimum
water use obtained from initial stage of 52.2mm. Late-season of
73.2mm hold the third in water use (Table 1). The ETc for the
whole growth period of wheat varied from a minimum value of
390.8mm to a maximum of 427.3mm. The overall average was
413.7mm. Similarly, Charles  obtained that 427mm for wheat
in central Arizona, but the result is significantly different from value
obtained by FOA, which is reported as 550mm ETc.
Naturally, crops need sufficient moisture for different physiological
activities like metabolism of food. Over-watering severely
limits (or even cuts off) the supply of oxygen that roots depends
on to function properly, meaning that plants do not get adequate
oxygen to survive. Furthermore, too much water can also lead to
root rotting and the irreversible decay of roots. Though under
watering leads to moisture stress in which the amount of water
applied is not sufficient for potential grain yield production
since food synthesis is reduced. Different crops display a variety
of physiological and biochemical responses to existing drought
stress making complex phenomenon like reduced CO2 assimilation
mainly by stomata closure, membrane damage and disturbed
activity of various enzymes, especially those of CO2 fixation and
adenosine triphosphate synthesis .
Due to absence of actual data on wheat seasonal water requirement
in the study area; FAO result was frequently used as
representatives in the design and planning of irrigation systems.
This leads to erroneous application of water use. Efficient use of
water for irrigated agriculture is fundamental for agricultural production
in arid and semi-arid areas that improves crop water productivity.
In general, as water is scarce and becomes a critical resource
for agriculture, supplying the right amount is essential for
healthy plants and optimum productivity  and also important
for effective irrigation water planning and management.
The meteorological data temperature, relative humidity,
sunshine and wind speed were recorded from weather stations
during growing seasons and using FAO Penman-Monteith equation
1, the reference evapotranspiration values calculated during
the growing seasons of wheat for studied area and is shown in
Figure 1. The result of ETo during initial stage was higher than
ETc and lower at mid stage, this implies that the ground cover of
leaf shadow has a role in reducing the amount of water that evaporates
from a bare soil.
Similarly, Allen et al.  has also indicated that at initial stage
nearly 100% of ET comes from evaporation, while crop full cover
at the mid-season stage more than 90% of ET comes from transpiration.
Seasonal crop coefficient (Kc) for wheat
After determining ETo and ETc, Kc can be calculated using the
appropriate crop coefficient formula by equation (3). The results
of three-year data show that there was a high variation in Kc values
among growth stages. The Kc values changed from one stage
to the other stage rapidly with the changes in crop development
(Table 2). The Kc value ranged from 0.54 at the initial growth stage
to 1.15 at the mid-season stage (Table 1). The curve presented in
Figure 2 represents the changes in the Kc values over the length
of the growing season of wheat. The shape of the curve represents
the changes in the vegetation and ground cover during plant development
and maturation that affect the ratio of ETc to ETo.
The Kc value increased from the initial to the development
stages while it reached its highest and relatively remained constant
at the mid-season stage (Figure 2). The Kc declined rapidly
during the late season stage. A higher Kc value was recorded from
45-80 days after planting as compared to the values at the begin- ning and end of the crop life cycle. The overall average Kc of wheat
values for the initial, mid-season and late season growth stages
were 0.54, 1.15 and 0.67 respectively. The initial value of Kc started
to increase after 10% cover of the ground, reached a maximum
during the mid-season stage and thereafter gradually declined.
This could be explained by foliage senescence that restricted transpiration
and caused a reduction in the crop coefficient.
Crop coefficient was collected from FAO Irrigation and Drainage
Paper 56  for wheat (spring wheat). The crop coefficient
values for respective growth stages are 0.3, 1.15 and 0.4 for initial,
mid and end, respectively. This result is different from the estimated
Kc values of study area. Determining ETc and Kc of agricultural
crops for at the location helps to have wise management of irrigation
water, designing and managing irrigated projects and also
helps researchers to appropriate irrigation scheduling, by using
meteorological data ETo and local crop Kc value of the area.
As indicated in table different agronomic and yield data were
observed from the study. The maximum plant and spike length obtained
were 86.6 and 8.2cm, respectively. The maximum obtained
grain yield and above ground biomass yield from the study were
4559.1 and 10897.1kg/ha, respectively. Also, the maximum number
of tillers per square meter of 559.0 and number of grains per
spike of 42.0 were obtained. Therefore, it could be concluded that
supplying the right amount of crop water requirement is essential
for healthy plants and potential production of wheat.
The ETc and Kc of wheat kekeba variety was evaluated for
each growth stage under climatic condition of Melkassa and areas
which have similar climatic conditions and soil characteristics.
The finding showed that estimates of crop water requirement
made with locally determined crop coefficients differ from estimates
of FAO publications and others. This emphasizes a strong
need for local calibration of Kc for each crop variety. The results of
ETc and Kc reflected on crop variety, climate, location, and growing
seasons. The Kc values obtained at the study site (Melkassa)
could be applicable to areas with similar soil type, climate, growing
seasons, and other agro-ecological conditions.
The author is grateful to Ethiopian Institute of Agricultural
Research, Natural Resource Research National Irrigation and
Drainage Research Project, for providing funds for the experiment
and technical support. He does thankful to Ato Gebeyehu Ashemi, Abere Tesfaye and Tatek Wondimu for their technical support
in the field of experimentation. He highly acknowledges all staff
members of Melkassa Agricultural Research Center Natural Resource
Research Process team for their kind cooperation during
field experimentation and data collection.
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