What are some of the newer and more water efficient agricultural processes that can be used to significantly reduce water usage?

First, a discussion in the efficient practice of anything requires some calculation whereby one or more determining factors results in some productivity. If farmer 'A' can produce a kilo of tomatoes with less water than farmer 'B', farmer 'A' can be said to have the more efficient irrigation system. And farmers today should be accountable for these figures. Second, it's unfair to compare field-based irrigation to that used in greenhouses. And further more, it would be unfair to compare the cost of infrastructure between a 10 acre blueberry farm and 10,000 acres of corn; never mind greenhouse production. Drip irrigation works for one, but obviously not for the other, you be the judge. However, it may be fair to compare how efficient one agriculture practice is to another if we consider the value of product produced and in this case the amount of water used. By this rationalization, drip irrigation isn't cost efficient to implement for corn. Instead one may consider that the amount of water used to grow corn and conclude that it is not an efficient agriculture practice. However, implementing drip irrigation on a polyculture farm organized in such a way that it grows higher value produce for a longer portion of the year may prove to be highly efficient. When it comes to the discussion of efficient agricultural practices and water usage, the greenhouse is where the newest technology is being implemented. Their capital costs are higher, their operating expenses are higher, and as a result the relative value of their investment towards new technology (and infrastructure) tends to be higher. In addition, water used in greenhouses can be recycled instead of drained into the earth. As a result the operations behind water usage in greenhouses are more closely regulated. A greenhouse that requires 200-400 liters per kilo of product requires significantly larger tanks to hold the water and pumps to move it than more efficient means like aeroponics and hydroponics. When it comes to the usage of water, the actual watering system itself is only one piece of the puzzle. True, aeroponic watering systems require significantly less water than traditional practices (as much as 90% less); and this means less cost and space used up by large costly tanks. Other factors play integral parts in water usages. Facilities that can better control their environmental climate (like state of the art Biodome Greenhouses, http://www.nuagri.com/biodome/) can dissipate excess heat more efficiently and reduce water evaporation; and water loss through heavy venting. Artificial lighting technology that operates at lower temperatures (http://www.nuagri.com/programmable-spectrum-grow-lights/) some as much as 75% less, significantly reduce excess heat causing excess water evaporation and heavy venting. Vertical farming cabinets that incorporate aeroponic systems not only use water more efficiently they grow more food in each square foot of greenhouse space. Together, the combination of facility, artificial lighting, growing cabinets, and water system all contribute towards a more efficient use of water. What species growers choose to grow in their respective spaces will ultimately determine whether the operation is cost effective and sustainable, vis-a-vis water usage. No doubt, some system of water accountability is needed to ensure that produce value justifies the amount of water used. Maybe another question we need to start asking is, "what are we using our water to grow and why?" http://www.nuagri.com/

Field-based irrigation is extremely inefficient vis-a-vis water usage. A field-grown tomato plant could use between 200 - 400 liters of water to produce a kilo of tomatoes. Hydropnics is more efficient in that it would only use 70 liters to produce the same amount of tomatoes.  On the other hand, aeroponics, which is the delivery of water to the roots in a fine mist, would use only 20 liters of water.  This is because the water and nutrient solution can be recirculated.  This is possible because the water is oxygenated, which minimizes the presence of anaerobic micro organisms.

I don't have any experience in agriculture, but recent expansion in mulch film production suggests that its a viable product to minimize water consumption per plant. There are two primary ways of conserving water(or any other resource) Firstly by targeted application of water. As suggested in other answers,  drip irrigation or porous pipe irrigation systems help in ensuring water is only applied to the crop and no other place. Secondly you can conserve water by reducing losses after application. This is where mulch film helps. Mulch film has a couple of applications, but its main application is to preserve soil mositure. Mulch film is a colored Polyethene sheet generally of some grade of LLDPE. the sheet is spread across the complete farming land, and holes in the sheet are placed to let the grow grow through them. The colored sheet ensures that moisture is trapped beneath it. The land does not lose water due to evaporation and hence allows farmers to Dave on water as well as grow crops closer increasing yield per sq meter. The other method to reduce loss after application is to build a green house, but cost as well as practical issues have shown limited usage of greenhouses. They are a much bigger one time investment.

First, thanks for the pointer to the McKinsey report - very useful. Less than 10% of all growers measure or monitor moisture to control irrigation - they set timers based on instinct, experience and gut and hope for the best. Research has shown that shifting to more efficient irrigation practices (such as drip) and controlling water use to the needs of the plants saves between 35% and 90% of total water usage, with averages around 45-50%., Although hydroponics & aeroponics are more efficient, the cost of infrastructure is so high that only very high-value crops cat turn a profit. There is much more leverage to improving irrigation in fields - where 10-20% water savings world-wide is possible with technology and training.