Review paper: ‘Water productivity’ can mean different things to different people, and may differ between but also within groups of water users. 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 circumstances. 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 under study, 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 amount of crop produced or the value of crops produced in a farm or catchment in relation to the water supply of that farm/catchment. 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 by crop), yield WP (crop grain per unit area, to the transpiration loss from the crop) (Hong-Xing et al., 2007). In essence, WP represents the output of a given activity to the water input. Where inter-cropping or multiple culture (crop + fish) is possible, monetary return per unit of water 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, we have to pay for and in many cases we have 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 (Lui et al., 2007).
In crop production sector, water productivity (WP) is used to define the relationship between crop produced and the amount of water involved in crop production, expressed as crop production per unit volume of water. Crop production may be expressed in terms of total dry-matter yield or seed (or grain) yield (kg.ha-1) or, when dealing with different crops, yield may be transformed into monetary units (i.e. $.ha-1). More options are available to define the volume of water. Different water productivity indicators result from different options:
WP1 = Grain or seed yield / Water applied to the field, (kg. ha-1cm-1) ....……..... (1)
WP2 = Total dry matter yield / Water applied to the field, (kg. ha-1cm-1) .………….(2)
WP3 = Total monetary value / Water applied to the field, ($.m-3) .……….….(3)
If the effectiveness of water use in a single crop is described, the equation (1) or (2) is a good indicator. If regions should be compared or the effectiveness of water use in multiple cultures, or under water-limiting condition (but land is not limiting) (Ali et al., 2007), equation (3) is more appropriate.
For discussion and interpretation in this paper, equation (1) will be considered and is further divided into irrigation water productivity (IWP) and water productivity (WP), as follows:
IWP = Grain or seed yield / irrigation water applied , (kg.ha-1cm-1) .................... (4)
WP = Grain or seed yield / crop evapotranspiration , (kg.ha-1cm-1) ..................(5)