2. Concepts and scale considerations of water productivity ‘Water productivity’ may carry different meanings to different people, and may differ between but also within groups of water users (Kijne et al., 2003; Tuong et al., 2005; Zoebl, 2006; Playan and Mateos, 2006; Dugan et al., 2006; Wesseling and Feddes, 2006). It can be defined with respect to different water using production sectors (e.g. crop production, fishery, forestry, domestic and industrial use) (Igbadun et al., 2006) as the amount of output produced per unit of water involved in the production, or the value added to water in a given circumstance. To a hydrologist, it means the ratio of the volume of water used productively, i.e., transpired and in some cases also evaporated, from the area understudy to the volume of water potentially available for that purpose. In fisheries, it might mean the ratio of fish produced to the volume of water used. To an economist, it might mean the monetary value of output divided by that of the necessary water input. To an irrigation engineer, it might mean the ratio of the amount or the value of crops produced in a farm (or catchment) to the supplied water. Even a crop scientist can use WP in different aspects: leaf WP (leaf photosynthetic rate per transpiration rate), whole plant WP (the ratio of aboveground biomass or dry matter per unit area to water use), yield WP (crop grain per unit area to transpiration) (Hong-Xing et al., 2007). In essence, WP represents the output of a given activity to the water input. Where there is inter-cropping or multiple culture (crop + fish), monetary return per unit of water input is the best option. In agriculture, we are interested to produce more with less water because water is a limiting factor in many parts of the world. Water is an economic good, because we have to pay for it, and in many cases we also have to pay enormous environmental costs. Water productivity is a useful indicator for quantifying the impact of irrigation scheduling decisions with regard to water management. It is also a basis for virtual water through the trade of food both at the international and intra-national level (Liu et al., 2007).
3. Factors affecting WP The factors which affect or influence crop yield (the numerator of the productivity equation), and water applied or need to be applied (denominator of the same equation), obviously influence the water productivity. The factors are 3.1. Crop cultivar-type Plants unavoidably lose a large quantity of water when they open their stomata for CO2 uptake in unsaturated air. Vapor diffuses out from the sub-stomatal cavity into the air while CO2 goes in the opposite way. Mathematical modeling of these two opposing diffusion processes has shown that WP is largely a function of the CO2 and vapor concentration gradients between the inside and outside of the leaf (Jones, 1992). These two opposing fluxes are regulated by stomata. Therefore, the stomatal behavior will determine the WP of a particular species or cultivar. It is well known that C4 plants have higher WP than C3 plants. Within C3 plants, many reports have shown that genotypes can be selected for higher WP according to their carbon isotope discrimination, a function of the CO2 concentration gradient between the inside and outside of the leaf (e.g. Craufurd et al., 1991; Ehdaie et al., 1991).
3.2. Applied water In agriculture, many ways of conserving water have been investigated and techniques such as partial irrigation, deficit irrigation or drip irrigation have shown that WP can be enhanced (Ali et al., 1997, 2007; Jalota et al., 2006; Zhang et al., 2004; Oweis et al., 2000, 1998; Talukder et al., 1987, 1999; Oweis, 1997). In general, these techniques are a trade-off: a lower yield for a higher WP. Then the question arises if it is possible to increase WP without a significant yield reduction. There are many examples where grain yield, a large proportion of the total biomass, shows a negative parabolic relationship with the amount of irrigation. This suggests that when water supply is sufficient, excessive vegetative growth may lead to less root activity, unhealthy canopy structure and a lower harvest index or harvest ratio (ratio of seed yield to straw yield). That means that high biomass production, supported by high water supply, will not lead to high WP if defined as the grain production per unit amount of water irrigated. Therefore, the goal is to increase WP of grain yield by limiting water supply to increase harvest index or harvest ratio. Recent research has shown that in some irrigated situations, grain yield can be improved while reducing the amount of water applied to the crop (Yang et al., 2000, 2001, 2002), mainly via improved harvest index which has been shown as a key component to improve WP (Ehdaie and Waines, 1993).
3.3. Remobilization of pre-stored carbon, the variable fraction in grain filling: Grain filling is the final stage of growth in cereals where fertilized ovaries develop into caryopses. At this stage, about 40–50% of total biomass is deposited into the grains. Delayed whole plant senescence, leading to poorly filled grains and unused carbohydrates in straws, is a new problem increasingly recognized in rice and wheat production in recent years (Zhang et al., 1998). Slow grain filling may often be associated with delayed whole plant senescence. Monocarpic plants such as rice and wheat need the initiation of the whole plant senescence so that stored carbohydrates in stems and leaf sheaths can be remobilized and transferred to developing grains (Zhang and Yang, 2004). Normally when these crops are grown in a high-input system, pre-stored carbohydrates contribute 25–33% to the final weight of grain.
3.4. Soil factor Evaporative loss of water from the soil surface plays a significant role on plant growth during germination and seedling establishment, and also during other growth periods. Soil texture and organic matter content determine the water storage and release properties. Rapid drying of soil does not provide an opportunity for osmotic regulation and adjustment, and thus affects yield and water productivity. The nutritional status of the young crop, especially nitrogen, can markedly affect the rate of development of leaf area and hence evaporation losses from the soil. Organic matter in soil interacts with other nutrients and microbial activities.
3.5. Agronomic factor Agronomic factors which can affect WP are timeliness of sowing, evenness of establishment, use of herbicides, and the role of the previous crop. Water productivity depends not only on how the crop is managed during its life, but also on how it is fitted into the management of a farm, both in space and time (Ali et al., 2005). By better tuning both the vegetative and floriferous development of a crop, optimal timing of harvest can be ensured; and thus, the tactical advantage of variable weather can be taken by sowing the next crop at the right time, and facilitating good establishment. Any husbandry technique that facilitates rapid development and enables the crop to cover soil surface, shade out weeds, and reduce wind speed may, in most circumstances, increase crop competitiveness and WP (Cooper and Gregory, 1987). Practices that particularly contribute to these factors are early sowing, selection of varieties with early growth (under cool conditions), adequate fertilization, adequate plant population and close spacing (Gregory, 1991; Harris et al., 1991). Within the concept of improved WP, water transpired by crops should be increased relative to evaporation from the soil surface. Soil and stubble management may influence the water balance of the soil by affecting infiltration and water storage in the soil, and evaporation losses from the soil surface. These combined effects can also substantially affect the amount of water available to a crop. Runoff during intense rainfall can be greatly reduced by a good mulch cover. Mulch cover and organic matter impact on plant response in terms of increased plant growth or yield, and offer opportunities to improve water productivity. The rapid development of ground cover relies on good seedling establishment. Crusting of the surface of soils with poor structure, uneven sowing depth, and poor quality seed can all lead to large gaps in plant cover. Leaf growth of seedlings is strongly affected by the temperature of both air and soil. So, sowing winter-growing crops early, when soil and air are still warm, leads to good canopy cover during late autumn and winter with less evaporative losses from the soil surface.
3.6. Economic factors: Economic factors may influence the optimum level of WP. Sometimes large additional costs are involved in increasing WP, for example, the investment in sprinkler, drip or hose pipe irrigation systems. These systems include the fixed and operational costs of changing the irrigation system. The returns may include water saved plus the increased crop production (if any). The lining of the irrigation channel also involves a considerable amount of fixed cost, which may not be bearable by the farmers, especially in third world countries. The adaptability of any cropping pattern depends on its profitability. The farmers respond to market rules searching for the highest return per unit of land or water, depending on the relative scarcity of both resources.