Water Productivity: Definition and conceptual framework, Mechanics of Tillage and Traction | Mechanics of Tillage and Traction Notes- Agricultural Engg - Agricultural Engineering PDF Download

21.1. Scales of Reference and Water Productivity transformations:

The magnitude and meaning of the term ‘water productivity’ is often changes with its scale of reference. Isolated scales of reference in agricultural domain can be plant/crop scale, field scale, project/basin/command scale, state scale and the country scale. By the same token, the industrial domain, drinking water supply and other usage domains can hold their own scales of reference. An increase in production per unit of water diverted at one scale does not necessarily lead to an increase in productivity of water diverted at a larger scale. The classical irrigation efficiency decreases as the scale of the system increases (Seckler et al., 2003).

21.2. Agricultural Water Productivity

Agricultural water productivity can be expressed either as a physical productivity in terms of yield over unit quantity of water consumed (tonnes per ha.cm of water or kg yield per kg water consumed) in accordance with the scale of reference that includes or excludes the losses of water or an economic productivity replacing the yield term by the gross or net present value of the crop yield for the same water consumption (Rupees per unit volume of water).

21.3. Water productivity- definition

Water productivity is defined as ‘crop production’ per unit ‘amount of water used’ (Molden, 1997).       Concept of water productivity in agricultural production systems is focused on ‘producing more food with the same water resources’ or ‘producing the same amount of food with less water resources’. Initially, irrigation efficiency or water use efficiency was used to describe the performance of irrigation systems. In agronomic terms, ‘water use efficiency’ is defined as the amount of organic matter produced by a plant divided by the amount of water used by the plant in producing it (De Wit, 1958). However, the used terminology ‘water use efficiency’ does not follow the classical concept of ‘efficiency’, which uses the same units for input and output. Therefore, International Water Management Institute (IWMI) has proposed a change of the nomenclature from ‘water use efficiency’ to ‘water productivity’. Water productivity can be further defined in several ways according to the purpose, scale and domain of analysis (Molden et al., 2001; Bastiaanssen et al., 2003).

According to Dang et al. (2001) the water productivity is defined in three different ways. The water productivity per unit of evapotranspiration (WP_ET) is the mass of crop production divided by the total mass of water transpired by the crop and lost from the soil. The water productivity per unit of irrigation (WP_I) is the crop production divided by irrigation flow. The water productivity per unit of gross inflow (WP_G) is the crop production divided by the rain plus irrigation flow. Water productivity with reference to evapotranspiration WP_ET takes into accounts only water evaporated or transpired and is therefore focused on plant behavior whereas WPI   and WPG   include not only ET but also water used in other ways for crop products and water that is wasted. Cabangon et al. (2001) pointed out that water productivity of  rice  with   reference  to  evapotranspiration  WP_ET     was  higher  (0.53  kg m-3 under transplanting as compared to dry seeding (0.48 kg m-3).  Ximing Cai et al. (2003) predicted the increment of global average water productivity of rice and other cereals from 0.39 kg m^-3 to 0.52 kg m^-3 and 0.67 kg m^-3 to 1.01 kg m^-3 respectively from 1995 to 2025. He also reported that water productivity of irrigated crops is higher than that of rainfed crops in developing countries, is lower in developed countries.

21.4. Water productivity versus scale of references

The definition of water productivity is scale-dependent. Increasing water productivity is then the function of several components at different levels viz., plant, field, irrigation system and river-basin. An increase in production per unit of water diverted at one scale does not necessarily lead to an increase in productivity of water diverted at a larger scale. The classical irrigation efficiency decreases as the scale of the system increases (Seckleret al.,2003). In India, the on-farm irrigation efficiency of most canal irrigation systems ranges from 30 to 40% (Navalawala, 1999; Singh, 2000) whereas, the irrigation efficiency at basin level is as high as 70 to 80% (Chaudhary, 1997). Basin water productivity takes into consideration  beneficial  depletion  for  multiple  uses  of  water,  including  not  only  crop production but also uses by the non-agricultural sector, including the environment. Here, the problem lies in allocating the water among its multiple uses and users.

Plant  scale  water  productivity  will  vary  with  plants  according  to  its  photosynthetic efficiency. The C3 plants are the most common crop plants (wheat, barley, soybeans…..). Unfortunately, they are also the least efficient assimilators of carbon dioxide from the atmosphere. Therefore, they must keep their stomata open more than the other plants under the same atmospheric conditions and, hence, they have the lowest transpiration efficiency or water productivity (biomass per unit water transpired). Whereas, the C4 plants  (maize, sorghum, sugar cane…) have an enzyme that has twice the affinity for absorbing carbon dioxide  as  that  in  C3  plants.  C3 plants also have photorespiration which occurs with photosynthesis in light and requires oxygen. This process does not occur in C4 plants. Consequently, C4 plants have 2-3 times higher transpiration efficiency or water productivity than C3 plants. Further, the CAM plants (pineapple, agave …) have the ability to assimilate CO2 during the night and store it in the form of organic acids. During the day the stored CO2 is available for producing carbohydrates by photosynthesis. This enables CAM plants to close their stomata during the daytime, when transpiration is the highest, and open the stomata at night when it is lowest. CAM plants can attain a transpiration efficiency or water productivity as much as 10 times that of C3 plants; however, their biomass production per unit land area is low (Keller and Seckler,2003).

At field level, WPET values under typical low land conditions range from 0.4 to 1.6 g kg^-1 and WPIP values from 0.2 to 1.1 g kg^-1 (Bouman and Tuong, 2001). In wheat, the WPET was 0.65 g kg^-1 and WPIP was 0.8 g kg^-1 in Indian condition (Sharma et al., 1990). Ranvir-Singh (2005)  conducted an experiment on the water productivity analysis from field to regional scale at Sirsa district, Haryana State and concluded that at field scale, the average WPET  (kg m^-3) was 1.39 for wheat, 0.94 for rice and 0.23 for cotton, and represents its average values for the climatic and growing conditions in Northwest  India. Factors responsible for low values of WP include a high share (20 to 40%) of soil evaporation into ET for rice, percolation from fields and seepage losses (34 to 43% of the total canal inflow) from the conveyance system.

Finally, WP of different farm enterprises means getting more profit per drop of water. Producing more crops, livestock, fish and forest products per unit of agricultural water use holds a key to both food and environmental security. But, for society as a whole, concerned with a basin or country’s water resource, this means getting more value per unit of water resource used. However, Molden et al (2003) stated the importance of working out WP within agriculture, water use by fisheries, forests, livestock and field crops and concluded that analyzing each water use  independently often leads to false conclusions because of these interactions.

21.5. Water productivity improvement measures

Water productivity could be improved either by reducing the water losses that occur in various ways during water conveyance and irrigation practices or increasing the economic produce of the crop through efficient water management techniques. Principle factors that are influencing water losses and water productivity of a command area are the design and nature  of  construction  of  the  water  conveyance  system,  type  of  soil,  extent  of  land preparation and  grading, design of  the  field, choice of  irrigation  methods and skill  of irrigators.

21.5.1. Plant/crop level

Identification of traits and genes using conventional and molecular breeding techniques that have the following specific characters to improve the water productivity at individual plant level (Bennett, 2003).

•   Traits that reduce the non-transpirational uses of water in agriculture

•   Traits that reduce the transpiration of water without affecting the productivity

•   Traits that increases production without increasing transpiration

•   Traits that tolerance to water logging, drought and salinity stress.

•    Development of short duration varieties reducing the growing time from 5 months to 3.5 to 4 months will also resulting higher water saving.

21.5.2. Field/farm level

Water productivity at the field level can be improved through increasing the total output per unit cumulative water used, reducing the unproductive water outflows and depletions and making more effective use of rainfall. Its reduction at field /farm level is mainly due to the losses occurring by seepage in the supply channel, deep percolation and occasionally runoff occurring in fields. Improper land leveling and grading, faulty choice of irrigation methods, application of excess water, frequent irrigation and faulty design of fields are the major factors that cause low water productivity. Water productivity of an agricultural ecosystem has been improved either by reducing the water losses that occur in various ways during water conveyance and irrigation practices or increasing the economic produce of the crop through efficient water management techniques

Minimize the water use or water conservation measure

• Proper land leveling and grading is a prerequisite for efficient water application.

• Lining of farm channels will reduce the water losses through seepage

• Reducing unproductive water outflows through the following ways will also be helpful viz.,  minimizing idle periods during land preparation, soil management to increase resistance to water  flow (shallow tillage before land preparation to close cracks,  puddling  and  soil  compaction  etc.) and water  management  to  reduce hydrostatic pressure.

• Pipes may be laid for water conveyance in farms wherever feasible to cut the water conveyance losses.

Maximize the crop productivity measures

• Proper selection of irrigation methods according to crops, soil types, topography, climate and stream sizes are important to secure high water productivity.

• Introduction of water-saving irrigation techniques like drip and sprinkler will enhance the crop productivity. Institutional and Governmental policies will also promote the spread of these technologies, which could result in higher productivity.

• Improved agronomic practices, such as site-specific nutrient management, good weed management and proper land leveling can increase the crop yield significantly without affecting ET and therefore, may result in increased water productivity (Hill et al., 2001).

• Adoption of water –saving irrigation technology in rice namely System of Rice Intensification (SRI), aerobic rice and dry-seeded rice techniques would increase the water productivity of rice.

• Optimum scheduling of irrigation is most important way of improving crop water productivity.

21.5.3. Basin level

• Conveyance loss is the main cause of reduction in water productivity at basin level, it can be reduced by lining canals, waterways and channels with impervious materials like bricks or stone masonry or bitumen clay mixture and so on.

• Repairing  of  cracks,  holes,  burrows,  erosion  damages,  leaks  in  water  control structures should be done as a part of continuous maintenance.

• Weed growth should be checked in unlined canals and water ways etc.

• Integrated water and land management will be much helpful in enhancing land and water productivity.

• Participatory irrigation management approach

• Adoption of proper turn out system