I forget to mention "why is it important to improve agricultural water productivity":

There are important reasons to improve agricultural water productivity:

• To meet the rising demand for food from a growing, wealthier, and increasingly urbanized population, in light of water scarcity.
• To respond to pressures to reallocate water from agriculture to cities and to ensure that water is available for environmental uses.
• To contribute to poverty reduction and economic growth. For the rural poor more productive use of water can mean better nutrition for families, more income, productive employment, and greater equity. Targeting high water productivity can reduce investment costs by reducing the amount of water that has to be withdrawn. 
• ...

I forget to mention "why is it important to improve agricultural water productivity":

There are important reasons to improve agricultural water productivity:

• To meet the rising demand for food from a growing, wealthier, and increasingly urbanized population, in light of water scarcity.
• To respond to pressures to reallocate water from agriculture to cities and to ensure that water is available for environmental uses.
• To contribute to poverty reduction and economic growth. For the rural poor more productive use of water can mean better nutrition for families, more income, productive employment, and greater equity. Targeting high water productivity can reduce investment costs by reducing the amount of water that has to be withdrawn. 
• ...

Farmers in developing countries could raise water productivity by adopting proven agronomic and water management practices because raising land productivity generally leads to increases in water productivity.

Many promising pathways for raising water productivity are available over the continuum from fully rainfed to fully irrigated farming systems. These include supplemental irrigation (some irrigation to supplement rainfall); soil fertility maintenance; deficit irrigation; small-scale affordable management practices for water storage, delivery, and application; modern irrigation technologies (such as pressured systems and drip irrigation); and soilwater conservation through zero or minimum tillage.

http://www.iwmi.cgiar.org/assessment/Water%20for%20Food%20Water%20for%20Life/Chapters/Chapter%207%20Water%20Productivity.pdf

Due to vastly different types and scales of use, communicating about water between professionals and non-water professionals is quite difficult. Policy decisions are often taken without a clear understanding of consequences on all water users. As competition for a limited supply of water increases, it becomes increasingly important to clearly communicate about how water is being used, and how water resource developments will affect present use patterns. As irrigation is a large consumer of water, developments in irrigation have profound impacts on basin-wide water use and availability. Yet, planning and execution of irrigation interventions often take place without consideration of other uses. One of the main reasons for this restricted view of irrigation workers is inadequate means to describe how irrigation water is being used. Irrigation efficiency is the most commonly used term to describe how well water is being used. But increases in irrigation efficiency do not always coincide with increases in overall basin productivity of water.

http://secnet087.un.org/unsd/envaccounting/ceea/PImeetings/AC-116-10.pdf

Use of water in food and agriculture

An overview of the water consumption in food and agriculture:

Product                Quantity           Equivalent water in liters

glass of milk              200 ml                              200

glass of apple juice    200 ml                              190

cup of coffee            125 ml                               140

glass of orange juice 200 ml                               170

cup of tea                250 ml                                 35

chips bag                 200 g                                185

slice of bread             30 g                                   40

egg                           40 g                                135

hamburger               150 g                                2400

potato                     100 g                                    25

tomato                     70 g                                     13

apple                     100 g                                      70

orange                   100 g                                      50

cotton T-shirt         500 g                                   4100

pair of shoes            1                                       8000

sheet A4 paper    80g/m2                                     10

microchip               2 g                                         32

http://www.lenntech.com/water-food-agriculture.htm

Dear Mahmoud,

This is a very circumstance depending situation. It is difficult to guess without knowing the detailed case. Generalities cannot be efficient in each case. As a sober suggestion may be to avoid wasting water, to collect natural precipitation and realising planned water management.   

The following answer was posted by Dear @Fadel Djamel to me. I re-send it via Q&A for the benefit of all.

Dear All

Improving water productivity - whether rainfed or dryland - first requires higher performance or value of crops, that is to say, the marketable yield of culture for each unit of water transpired. Also necessary, the reduction of all the flows or "losses" unnecessary (for drainage, seepage and percolation) except the transpiration of plants, and more efficient use of precipitation, the stored water and marginally water quality. Loss reduction and water control are considered part of the integrated water resources management at the bassin.Il also necessary that the institutions shall adopt rules to ensure that upstream interventions are not made expense of downstream water users. These three principles apply at all scales, from plant level and scope to that of the agro-ecological system, but options and practices associated with them require approaches and irrigation technologies which vary spatial scale.

I agree with Krishnan Umachandran.

Improving water productivity - whether under rain-fed or irrigated conditions - requires, first, an increase in crop yields or values, i.e. the marketable yield of the crop for each unit of water transpired. Also necessary is a reduction of all outflows or "losses" (e.g. drainage, seepage and percolation) except crop transpiration, and more effective use of rainfall, stored water, and water of marginal quality. Loss reduction and water control are considered parts of basin-wide integrated water resource management (IWRM), which gives an essential role to institutions and policies in ensuring that upstream interventions are not made at the expense of downstream water users. These three principles apply at all scales, from plant to field and agro-ecological levels, but options and practices associated with them require different approaches and technologies at different spatial scales.

Well in my opinion, there are already some traditional methods are available for water management such as drip irrigation, soil wetness with micro controller devices, etc. But this will not be sufficient there is a need to understand the exact water requirement for plant and for maximum yield. Water stress management will be a research area where there is a need to focus on.

Agricultural sector accounts for about 85% of global freshwater consumption. Modern developments in water footprinting have adopted three areas of focus: green, blue, and grey water. A water footprint is measured in terms of the volume of water consumed, evaporated and polluted. The Water Footprint Network, whose research provides data that drive our calculator, splits water footprints into three corresponding categories:

Balanced green-blue water use strategy would be required to address the issue of increasing water demand in a world of limited freshwater resources. Detailed information regarding the water footprint can form an important basis for further assessment of how products and consumers contribute to the global problem...

Reference: Markus Pahlow and Mesfin M. Mekonnen (2012). Using the water footprint as a tool for sustainable appropriation of freshwater resources

http://water.jhu.edu/magazine/using-the-water-footprint-as-a-tool-for-sustainable-appropriation-of-freshw/

Dear @Mahmoud, thanks for sharing the question. Why agricultural water productivity is important for the global water challenge : "The key principles for improving water productivity at field, farm and basin level, which apply regardless of whether the crop is grown under rainfed or irrigated conditions, are:

(i) increase the marketable yield of the crop for each unit of water transpired by it;

(ii) reduce all outflows (e.g. drainage, seepage and percolation), including evaporative outflows other than the crop stomatal transpiration; and

(iii) increase the effective use of rainfall, stored water, and water of marginal quality..."

http://www.fao.org/docrep/006/y4525e/y4525e06.htm

Dear @Lewis Kausel. It is a good idea and even it may be such a small improvement it can add up. I also believe that any technique which can reduce one or more of the unproductive water outflows (e.g. seepage, percolation, and evaporation) and hence increases water productivity should be taken into account. Accordingly, defining crop water productivity using evapotranspiration (rather than transpiration) makes practical sense at field and system level.

Dear @Ljubomir. Thank you for your answer and related link. The key principles for improving water productivity at field, farm and basin level are good to start with.

Iran is an arid and semi-arid country with a mean annual rainfall of about 250 mm. It is about 30% of the mean annual precipitation in the world. The increasing water demand has caused an alarming decrease in annual per capita renewable water resources. The low irrigation fee has caused not only inefficient allocation of water resources in our agricultural sector, but it has also resulted in farmers producing crops which require relatively large amount of water as well as non-essential crops (e.g., watermelons, melons, tomatoes, ...). A  comprehensive  study revealed that the global value for crop water productivity was generally very high for crops like watermelons, yet it is one of the major crops whose production and export contribute to the economy of Iran! Would it be wise to cultivate crops like watermelons in such an arid and semi-arid country?

Last winter, the rainfall in our country was very high but a high percentage of this rain water was lost. A good management of water collection, storage, distribution, and usage is required all over the seasons. In hot summer, the people become more concerned about water & they start to regret not utilizing it well during winter.

Improving water productivity requires collective cooperative actions on many fronts. Few examples: 1) Replacing the old leaking underground pipes with new ones. 2) Monitoring the big wasters of water & controlling the amount they get. 3) Increasing the awareness of the farmers & the public about the types of crops and how optimization of nutrients' input in them could be done. 4) Drinking water differs from irrigation water in terms of quality & reliability. Water coming to agricultural land ought to be tested to see if it fits the plants there. 5) The method of irrigation is important & some plants require very little water (e.g. our local olives & figs). 6) Crops grown in the mountains ought to be different from those grown in plains because of differences in climates & water requirements.

Water productivity, total (constant 2005 US $ GDP per cu. meter of total freshwater withdrawal) - for all countries

A useful Table of comparision for all countries

http://www.factfish.com/statistic/water%20productivity

I found another useful report on "Status of water use efficiency of main crops" published by FAO.

One of the conclusions drawn there: Improvement in grain yield and water productivity arise from breeding for superior varieties, better agronomic practices and the important, but often overlooked, synergy between breeding and agronomy.

Full report (41 pages) is attached.

By using sprinkler irrigation!

Sprinkler irrigation is a method of applying irrigation water which is similar to natural rainfall. Water is distributed through a system of pipes usually by pumping. It is then sprayed into the air through sprinklers so that it breaks up into small water drops which fall to the ground.

https://en.wikipedia.org/wiki/Irrigation_sprinkler

Dear @Mahmoud, here is the project of GIZ, Germany! Agricultural water productivity as a way of adapting to climate change!

Objective :

"As part of efforts to advance adaptation to climate change, improvements have been made to fertiliser and pesticide use, crop rotation and the planting calendar and are resulting in greater agricultural water productivity."

Approach :

"The programme implements strategies to improve the ability of farmers to adapt to a changing climate. At national level, the project aims to initiate changes in the political framework and in political strategies, to implement instruments and measures to change decision-making processes and to carry out activities to prevent or minimise potential damage from climate change..."

https://www.giz.de/en/worldwide/22237.html

Millions Of 'Shade Balls' Protect LA's Water During Drought

You have to see this to believe it.

The sea of 96 MILLION plastic balls that LA hopes will save it from drought: Reservoir is covered in an ocean of black spheres to stop 300 million gallons of water evaporating.

By deflecting UV rays, the opaque spheres are expected to protect more than 300 million gallons of water from evaporating in Southern California's harsh sunlight every year.

Besides slowing evaporation, the shade balls prevent sunlight from triggering a dangerous reaction between bromite, a chemical occurring naturally in groundwater, and the chlorine used to disinfect the water. When sunlight interacts with those two chemicals, Plastic News explains, the suspected carcinogen bromate is formed.

http://www.huffingtonpost.com/entry/los-angeles-reservoir-shade-balls_55ca5a1ce4b0923c12be743d?6tl9dx6r

About 40% increase is expected in water demand over the period of next two decades. Substantially increasing the productivity of water in agriculture will play a vital role in easing competition for scarce resources, prevention of environmental degradation and provision of food security. It can achieved by using improved crop varieties, applying precision agriculture, using modern irrigation, diversifying cropping patterns, and most importantly, improving water management. Improving water and land productivities is achieved through:

Developing options and tools for improved rainwater productivity in dryland farming systems through improved agro-management, germplasm enhancement and use of supplemental irrigation

Identifying and integrating appropriate water harvesting techniques in the crop-range-livestock system of marginal, drier environments

Developing packages for increased irrigation water productivity, through improved irrigation management, water use efficient germplasm, cropping patterns and scaling out to other areas.

http://www.icarda.org/water-productivity

Techniques of improving Water Productivity in Agriculture

The main pathways for enhancing water productivity are to increase the output per unit of water to reduce losses of water to unproductive sinks, to reduce water degradation and to reallocate water to higher priority uses. Promoting water-saving agriculture will not only increase water productivity, but also facilitate the structural adjustment needed for agriculture. Combining biological water saving measures with soil and agronomic manipulation may solve the problem to a large extent.

Excerpt from: 

http://www.seedbuzz.com/knowledge-center/article/techniques-of-improving-water-productivity-in-agriculture

This desalination facility is coupled to the Madras Atomic Power Station (MAPS), and deploys both multi-stage flash (MSF) evaporation and reverse osmosis (RO) membrane separation technologies. The total capacity of NDDP is 6.3 million litres per day (MLD).

http://www.thehindu.com/sci-tech/science/hybrid-desalination-plant-at-kalpakkam/article4167670.ece

The FAO is developing a publicly accessible near real time database using satellite data that will allow monitoring of agricultural water productivity. This database can be used to propose solutions to reduce water productivity gaps while taking into account ecosystems services and the equitable use of water resources. Eventually, the proposed solutions should lead to an overall reduction of water stress.

http://programme.worldwaterweek.org/event/5148

Irrigation consumes more water than any other human activity, and thus the challenges of water sustainability and food security are closely linked.

Specific management strategies to increase crop water productivity must be tailored to local contexts; studies in Nigeria, Sudan, and Niger, for example, have shown that nitrogen and phosphorus limitation restrict millet and sorghum yields. Water productivity could be increased by increasing yields via improved soil nutrient conditions and reduced wind-driven erosion. Examples of other interventions that have demonstrated improvements in water productivity include: rainwater harvesting and local water storage, applying drip or deficit irrigation, adjusting planting dates, and modifying tillage practices to reduce evaporation. In many cases, techniques for increasing water productivity, especially when it is low, require little or no additional water.

From: Kate A Brauman, Stefan Siebert and Jonathan A Foley.Improvements in crop water productivity increase water sustainability and food security—a global analysis. Published 29 May 2013 • 2013 IOP Publishing Ltd , Environmental Research Letters, Volume 8, Number 2

Figure 1 . Annual water consumption across climatic zones. Rainwater (left of centre line) and irrigation water (right of centre line) consumption by all crops, including those (in greys) that are not included in this analysis. For the 16 food crops analysed, the majority (83%) of water originates as rainfall on cropland. Climatic zones, most arid at the bottom and most humid at the top, are based on P/PET and are evenly distributed in climate space. Cropland in climatic zones in which P/PET > 1.9 are compressed into a single zone.

Figure 2. Annual water consumption by country for 16 food crops in precipitation-limited areas with low water productivity. Countries are ranked by (a) irrigation water consumption as well as by (b) irrigation intensity (consumption per hectare). We limited analysis of high-irrigation intensity countries to those where irrigation consumption on precipitation-limited, low water productivity cropland was at least 0.5 km3 yr−1.

Chapter 8 (Page no: 104): Increasing water productivity in agriculture.

Increasing water productivity is an important element in improved water management for sustainable agriculture, food security and healthy ecosystem functioning. Water productivity is defined as the amount of agricultural output per unit of water depleted, and can be assessed for crops, trees, livestock and fish. This chapter reviews challenges in and opportunities for improving water productivity in socially equitable and sustainable ways by thinking beyond technologies, and fostering enabling institutions and policies. Both in irrigated and rainfed cropping systems, water productivity can be improved by choosing well-adapted crop types, reducing unproductive water losses and maintaining healthy, vigorously growing crops through optimized water, nutrient and agronomic management. Livestock water productivity can be increased through improved feed management and animal husbandry, reduced animal mortality, appropriate livestock watering and sustainable grazing management. In agroforestry systems, the key to success is choosing the right combination of trees and crops to exploit spatial and temporal complementarities in resource use. In aquaculture systems, most water is depleted indirectly for feed production, via seepage and evaporation from water bodies, and through polluted water discharge, and efforts to improve water productivity should be directed at minimizing those losses. Identifying the most promising options is complex and has to take into account environmental, financial, social and health-related considerations. In general, improving agricultural water productivity, thus freeing up water for ecosystem functions, can be achieved by creating synergies across scales and between various agricultural sectors and the environment, and by enabling multiple uses of water and equitable access to water resources for different groups in society.

From: https://www.cabi.org/cabebooks/ebook/20133290919