Agriculture Reference
In-Depth Information
development in many regions all over the world are increasing water deficits and decreasing soil
retention capacity in the face of growing water demands. Furthermore, global food production
depends on water not only in the form of precipitation but also, and critically so, in the form of
available water resources for irrigation (Kedziora and Kundzewicz, 2013). Although irrigated land,
representing about 18% of global agricultural land (more than 240 million ha), produces 1 billion
metric tons of grain annually, or about half the world's total supply, this is because irrigated crops
yield on average two to three times more than their rainfed counterparts (Somerville and Briscoe,
2001; Kedziora and Kundzewicz, 2013). In addition, up to 45% of the world agriculture lands are
subject to continuous or frequent drought, wherein 38% of the world human population resides (Bot
et al., 2000). Therefore, it is imperative to improve WUE in crop plants through agronomic practices
as well as cultivar improvement (Hussain et al., 2012).
Agriculture is the principal user of all water resources, accounting for 70% of all withdrawals
(e.g., rainfall, water from rivers, lakes, and aquifers) (Heumesser et al., 2013). In comparison, 10% is
assigned to domestic uses and 20% to industrial uses (FAO, 2003). However, according to Nair et al.
(2013), the agricultural sector uses >80% of the developed freshwater supply of the world. Water is
recognized as an extremely important limiting factor in world food production second only to land
area (Wittwer, 1975; Nair et al., 2013). Because both the arable area and availability of freshwater
are limited and crop yield loss due to water availability exceeds that from all other causes, efficient
use of water is a vital aspect of modern-day agriculture (Gleick, 2003; Fedoroff et al., 2010).
Crops typically give a large response to applied fertilizer in favorable environments and a small,
zero, or negative response in an unfavorable environment (Angus et al., 1993). In this context, the
availability of water in adequate amounts is one of the factors determining crop responses to applied
fertilizers. Crops may not be able to use N efficiently if water is a limiting factor for growth and
production. This may result in increased residual N accumulation in the soil after crop harvest,
which can degrade environmental quality through increased N leaching into the groundwater and
emissions of greenhouse gases, such as N
2
O (Wang et al., 2013).
Water plays a significant role in crop production. It is essential for many physiological and bio-
chemical processes in the plants, which determine yield and quality. WUE is defined as dry matter
or the harvested portion of the crop produced per unit of water consumed (Soil Science Society of
America, 2008). Crop WUE originates in the economic concept of crop productivity and therefore
is now known as crop water productivity (CWP) (Jabro et al., 2012). CWP is defined as the amount
of water required per unit of yield and is a vital parameter to assess the performance of irrigated
and rainfed agriculture (www.fao.org/Landandwater/aglw/cropwater). CWP varies according to the
soil type, crop species, climatic conditions, and crop management practices adopted. In western
Australia, despite the relatively reliable winter rainfall in the region, the low water storage capacity
of the soils allows marked variation in the between-season water balance and so causes large varia-
tion in the crop demand for N (Angus et al., 1993).
Land degradation can exacerbate drought because it affects water availability, quality, and stor-
age (Diouf, 2001; Bossio et al., 2010; Sileshi et al., 2011). Therefore, measures that mitigate land
degradation are important to increase water productivity and reduce the risk of crop failure under
rainfed cropping systems (Sileshi et al., 2012). In addition, WUE can be increased by increasing bio-
mass production with the same amount of water use, the same amount of biomass production with
decreased water use, or a combination of both (Blum, 2005). Xin et al. (2009) evaluated 341 sor-
ghum genotypes for transpiration efficiency (TE, the ratio of biomass produced to water transpired)
based on biomass production in controlled environments and reported that TE had little correlation
with the water transpired and a large correlation with the biomass produced. They concluded that
increased biomass production rather than decreased transpiration accounted for increased TE. This
result is in contrast with the selection of increased TE genotypes using the C isotope discrimina-
tion method that are often associated with decreased transpiration, growth, and biomass production
(Condon et al., 2002; Impa et al., 2005; Blum, 2009). Tanner and Sinclair (1983) reported that WUE
or TE within a species is relatively constant and cannot be manipulated. Xin et al. (2009), however,
