8. Water conservation and water saving practices and management
8.2. Water use, consumptive use, water losses, and performances
8.2.1. Water use, consumption, losses, and efficiency
The terms efficiency and, more recently, water use efficiency are often used to express the performance of water supply systems and water use activities. However, there are no widely accepted definitions, and those terms may be used with different meanings. In an UNESCO conference dedicated to this subject, Garduño (1994) reviews a large variety of concepts for water use efficiency but does not propose any common definition. Tate (1994), when analysing the respective principles, assumes that water use efficiency “includes any measures that reduce the amount of water used per unit of any given activity, consistent with the maintenance or enhancement of water quality”. This concept would assume both water conservation and water saving is synonymous with water use efficiency. For a better understanding of terminology utilised in relation to water use performances, a more consistent conceptual approach is required.
The term efficiency is often used in the case of irrigation systems and it is commonly applied to each irrigation sub-system: storage, conveyance, distribution off- and on-farm, and on-farm application sub-systems. It can be defined by the ratio between the water depth delivered by the system under consideration and the water depth supplied to that sub-system, usually being expressed in percentage (Wolters, 1992). In case of on-farm application efficiency, the water depth delivered is replaced by the amount of water added to the root zone storage (Pereira, 1999). When referring to the system efficiency, it corresponds to the product of the efficiencies of the respective sub-systems. Adopting an output/input non-dimensional ratio, the term efficiency could be applied to evaluate the performance of any irrigation and non-irrigation water system.
As mentioned by several authors (e.g. Allen et al., 1997, and Jensen, 1996), the term efficiency often leads to misconceptions and misunderstandings, mainly when increasing irrigation efficiencies is almost synonymous with creating more available water. In fact, there is the need to quantify the proportion of water used, i.e. diverted for a given use, that is consumed and the proportion that is not consumptive use and is available for reuse or becomes degraded after use. For the latter case, improving efficiencies would represent a reduction in water losses and contribute to the conservation of the available resource.
Another term commonly used, mainly in irrigation, is water use efficiency (WUE), but again no common definition has been adopted. Some authors refer to it as a non-dimensional output/input ratio. Others adopt it to express the productivity of the water, as a yield to water used ratio. In crop production, the term WUE is applied with precise meanings, such the yield WUE, which is the ratio of the harvested biomass to the water consumed to achieve that yield (Steduto, 1996). To avoid misunderstandings, the term “water use efficiency” should be only used to measure the performance of plants and crops, irrigated or non-irrigated, and the term
“water productivity” (WP) should be adopted to express the quantity of product or service produced by a given amount of water used. Examples for WP are: kg of grain per m3 of water in case of irrigation; metres of fabric per m3 of water in the textile industry; kWh produced per m3 of water in energy generation; m2 of lawns irrigated per m3 of water in recreational areas; or m2 of area washed per m3 of water in commercial areas. When the economic productivity of the water is considered, the numerator in that ratio would be replaced by the value of the product or service due to the water use.
In urban supply systems consumption data are usually available in terms of litres/person/day, and there needs to be an aim to have these numbers continually falling. In all the above cases, for farms, factories and domestic supply operations there need to be policies and incentives aimed at bringing water consumption to the lowest possible level for each unit of production or activity in all areas, i.e. increasing the water productivity in all uses.
8.2.2. Water systems and water use performances
New concepts to clearly distinguish between consumptive and non-consumptive uses, beneficial and beneficial uses, and reusable and reusable fractions of the non-consumed water diverted into an irrigation system or subsystem were proposed by Allen et al.
(1997) and Burt et al. (1997). These consist of alternative performance indicators that are much more relevant than “irrigation efficiency” when adopted in regional water management for the formulation of water conservation and water savings policies and measures. These concepts and indicators are easy to adapt and extend to non-irrigation water uses.
These indicators can be adapted and extended to any water use or system to identify the respective performances under the perspective of water resources conservation as described in Table 8.3. These are more useful for water resources planning and management under scarcity and should lead to less misinterpretation than the term “efficiency”.
Essentially, three water use fractions are considered:
the consumed fraction, consisting of the fraction of diverted water which is evaporated or incorporated in the product, or consumed in drinking and food, which is no longer available after the end use,
the reusable fraction, consisting of the fraction of diverted water which is not consumed when used for a given production process or service but which returns with appropriate quality to non degraded surface waters or ground-water and, therefore, can be used again, and
the non-reusable fraction, consisting of the fraction of diverted water which is not consumed when used for a given production process or service but which returns with poor quality or returns to degraded surface waters or saline ground-water and, therefore, cannot be used again.
Each of the above fractions is then divided into two parts, corresponding respectively to the beneficial and the non-beneficial uses. Therefore, it is then easier to identify how water use could be improved, and how water savings should be oriented.
Adopting the indicators explained in Table 8.3, it can be concluded that water losses are those corresponding to non-consumptive and non-reusable quantities of water used, which define the non-reusable fraction. However, in the case of saline environments, part of the water loss is beneficial to the crop and the soil because it is used for leaching of salts and, therefore this loss cannot be avoided. The non-consumptive but reusable quantities of water are in reality not lost because other users or the same system downstream can use them again, mainly when reuse facilities are available. This reusable fraction, like the non-reusable, may be due to poor or less than optimal management, but may be required by the production or service process under consideration. It is often considered as lost but in fact it is only a temporary loss to the system and cannot be considered a loss from a hydrological perspective or under the overall water resource economy. However, the size of the reusable fraction
influences the cost of the system or sub-system operation and management and, moreover, it represents a non-necessary part of the demand, thus inducing negative impacts on the water allocation process and on the conservation of the resource.
TABLE 8.3. Irrigation water consumption, use and losses (adapted from Allen et al., 1997 and Burt et al., 1997) Consumptive Non-Consumptive and
Non-Reusable Non-Consumptive but Reusable Beneficial uses Irrigation:
ET from irrigated crops
evaporation for climate control
water incorporated in product
Irrigation:
leaching added to saline water
Irrigation:
leaching water added to reusable water
ET from vegetation in recreational and leisure
return flows from power generators
excess soil water and phreatophyte ET
drainage water added to saline water bodies
Irrigation:
deep percolation added to good quality aquifers
Reusable runoff
Reusable canal spills Non-irrigation uses:
ET from non beneficial vegetation
Consumed fraction Non-reusable fraction Reusable fraction
Considering the irrigation sector, which has the highest share in consumption and demand in many water scarcity areas, it can be concluded that, assuming the concepts above, the improvement of irrigation efficiencies is of great importance under water scarcity. This is because higher efficiencies correspond to an increase of the beneficial use of the water for agricultural production. However, this objective has to be complemented by others such as:
controlling the non-beneficial consumptive uses, particularly those associated to soil
evaporation, and evapotranspiration by phreatophyte plants and weeds receiving seepage and excess irrigation water,
minimising the non-reusable fraction of the diverted water, thus avoiding percolation to saline water tables or the disposal of drainage water into saline water bodies which would degrade the water quality and would make it non reusable, and
reducing the non-beneficial but reusable fraction by controlling deep percolation, seepage from canals, runoff return flows and canal excess water spills. These negatively impact operation and management costs and may be the cause for water-logging, competition by weeds, loss of nutrients and agro-chemicals, contamination of water bodies used for human consumption, and yield and income losses.
Box 8.1. presents, in summary form, the main performance terms adopted.
BOX 8.1. Performance indicators for water use and water systems
• the consumed fraction of water use (WCF), consisting of the fraction of diverted water that is evaporated or incorporated in the product, or consumed in drinking and food, and is no longer available after the end use,
• the reusable fraction of water use (WRF), consisting of the fraction of diverted water that is not consumed when used for a given production process or service but which returns with appropriate quality to non degraded surface waters or ground-water and, therefore, can be used again,
• the non-reusable fraction of water use (WNRF), consisting of the fraction of diverted water that is not consumed when used for a given production process or service but which returns with poor quality or to degraded surface waters or saline ground-water and, therefore, can not be used again.
• The water system or sub-system efficiency (SEf), consisting in the output to input ratio (%) relative to the system or sub-system under analysis.
• The water productivity (WP), which could be defined by the ratio of the quantity of product or service produced to the amount of water used, i.e. diverted for that purpose.