18
2.4 Water Use Efficiency/Water Productivity Under Deficit
Irrigation
Water use efficiency serves as a key variable in the assessment of plant responses to
water stress induced by deficit irrigation (Chai et al. 2016). It describes the intrinsic
trade-off between carbon fixation and water loss, because water evaporates whenever
stomata opens for CO 2 acquisition for photosynthesis (Bramley et al. 2013). In plant
research, water use efficiency is defined as crop yield per unit of water used (Chai
et al. 2014). It can also express the ratio of photosynthesis rate to transpiration rate or
the ratio of photosynthesis rate to stomatal conductance of CO 2 (Bramley et al. 2013).
Deficit irrigation increases water use efficiency through increase in application
efficiency, consumption efficiency and yield efficiency (Hsiao et al. 2007). Increases
in application efficiency occur as a result of lower amount of water applied than full
evapotranspiration, thus most or all the water applied remains in the root zone and
water lost by run-off and deep percolation decreases (Sepaskhah and Ghahraman
2004). The consumption efficiency is defined as the ratio between the amount of
evapotranspired water and the amount of water in the root zone. It may increase due
to crops are forced to extract water from deeper soil (Hsiao et al. 2007). Furthermore,
the yield efficiency is defined as the proportion of biomass in the harvested products,
which may be enhanced due to an excessive vegetative growth of some crop species
under full irrigation (Capra et al. 2008). When farmers have less water than the maximum evapotranspiration needs, they practice deficit irrigation, thus increasing application, consumption and yield efficiency, mainly through applying the available
water in the root zone, forcing crops to extract more water from the soil and improving harvest index by regulating vegetative and reproductive growth (Hsiao et al. 2007).
Other quantifications of the term of water use efficiency are described by different terms and scales. Photosynthetic water use efficiency is the most basic at leaf
level (Fereres and Soriano 2007). Furthermore, other common water use efficiency
parameters are: instantaneous water use efficiency and intrinsic water use efficiency.
The instantaneous water use efficiency is defined as the ratio between photosynthesis rate and transpiration rate and the intrinsic water use efficiency is defined as the
ratio between photosynthesis rate and stomatal conductance of CO 2 (Li et al. 2014).
From an agronomic and genetic viewpoint, it has also been argued that effective
use of water is the important determinant of plant production. Effective use of water
is defined as maximal soil moisture capture for transpiration, and minimal water
loss by soil evaporation under drought stress (Blum 2009). Hsiao et al. (2007) indicated that recycled water in the farmland and canal systems at the farmland, irrigation district, and regional levels, the seepage, water loss represent multiplicative
water use efficiency chain. They developed a systematic and quantitative approach
to clearly improve the multiplicative water use efficiency chain from reservoir to
crop yield to quantify the integrative effects of agricultural water management,
engineering, agronomical, and physiological processes.
Similarly, crop water productivity is an alternative term, has been used for the
expression of water use efficiency by some irrigation managers (Chai et al. 2016).
S. Ouda and T. Noreldin
2.4 Water Use Efficiency/Water Productivity Under Deficit
Irrigation
Water use efficiency serves as a key variable in the assessment of plant responses to
water stress induced by deficit irrigation (Chai et al. 2016). It describes the intrinsic
trade-off between carbon fixation and water loss, because water evaporates whenever
stomata opens for CO 2 acquisition for photosynthesis (Bramley et al. 2013). In plant
research, water use efficiency is defined as crop yield per unit of water used (Chai
et al. 2014). It can also express the ratio of photosynthesis rate to transpiration rate or
the ratio of photosynthesis rate to stomatal conductance of CO 2 (Bramley et al. 2013).
Deficit irrigation increases water use efficiency through increase in application
efficiency, consumption efficiency and yield efficiency (Hsiao et al. 2007). Increases
in application efficiency occur as a result of lower amount of water applied than full
evapotranspiration, thus most or all the water applied remains in the root zone and
water lost by run-off and deep percolation decreases (Sepaskhah and Ghahraman
2004). The consumption efficiency is defined as the ratio between the amount of
evapotranspired water and the amount of water in the root zone. It may increase due
to crops are forced to extract water from deeper soil (Hsiao et al. 2007). Furthermore,
the yield efficiency is defined as the proportion of biomass in the harvested products,
which may be enhanced due to an excessive vegetative growth of some crop species
under full irrigation (Capra et al. 2008). When farmers have less water than the maximum evapotranspiration needs, they practice deficit irrigation, thus increasing application, consumption and yield efficiency, mainly through applying the available
water in the root zone, forcing crops to extract more water from the soil and improving harvest index by regulating vegetative and reproductive growth (Hsiao et al. 2007).
Other quantifications of the term of water use efficiency are described by different terms and scales. Photosynthetic water use efficiency is the most basic at leaf
level (Fereres and Soriano 2007). Furthermore, other common water use efficiency
parameters are: instantaneous water use efficiency and intrinsic water use efficiency.
The instantaneous water use efficiency is defined as the ratio between photosynthesis rate and transpiration rate and the intrinsic water use efficiency is defined as the
ratio between photosynthesis rate and stomatal conductance of CO 2 (Li et al. 2014).
From an agronomic and genetic viewpoint, it has also been argued that effective
use of water is the important determinant of plant production. Effective use of water
is defined as maximal soil moisture capture for transpiration, and minimal water
loss by soil evaporation under drought stress (Blum 2009). Hsiao et al. (2007) indicated that recycled water in the farmland and canal systems at the farmland, irrigation district, and regional levels, the seepage, water loss represent multiplicative
water use efficiency chain. They developed a systematic and quantitative approach
to clearly improve the multiplicative water use efficiency chain from reservoir to
crop yield to quantify the integrative effects of agricultural water management,
engineering, agronomical, and physiological processes.
Similarly, crop water productivity is an alternative term, has been used for the
expression of water use efficiency by some irrigation managers (Chai et al. 2016).
S. Ouda and T. Noreldin