The fundamental and critical role of storage of water in sustaining life is becoming increasingly relevant. The storage has however to be interpreted holistically. The storage manifests itself in several forms. All of them, individually, as well as collectively, are critical to the productive use of water, especially for agriculture.
ROOT ZONE STORAGE
If we start with our basic need for food and our dependence on plants, either to feed ourselves or our livestock, we can see that the most fundamental form of water storage is in the plant's root zone (and to some extent in the plant itself). This sustains the plant through dry periods that may extend from hours to weeks.
It is a form of storage that we tend to take for granted, but it is critical to our survival, and we need to take good care of it, which means good husbandry to look after the soil structure and organic content, and good drainage. In many parts of the world, especially in humid regions, agricultural water management means just that, good drainage. In the humid tropics, food production is often about controlling drainage so that water draining from one area becomes the supply for the next.
But there are limits to the amount of water that can be stored in the root zone, however carefully we manage it. Most obviously, it is not capable of sustaining crops through a long dry season, and more subtly, it may not be able to bring the crop to maturity reliably enough without our intervention to reduce the deficit. That is, of course the function of irrigation. It is the reason that crops that are irrigated typically have a yield more than double those that are not. It is the reason why 44% of the food that we eat comes from irrigation of 18% of cropped area.
UPSTREAM STORAGE
Dams are one form of storage in an integrated system, but have two outstanding virtues. First, it is a form of storage we can create and it is a form of storage from which we can extract multiple uses. Those who oppose dams should consider how else we are to create storage, and how better we can use storage for multiple uses. There are ways of doing this, but not on the scale that is enabled by dams and is on a scale able to sustain human needs. In arid and semi-arid zones with a sizeable temporal variation in rainfall, dams' backed storages {in some cases, channel storages backed by Barrage(s)} are ideal for agricultural and other water uses. Storage in the root zone is fundamental for food supply and upstream storage in natural and artificial reservoirs support this process. We must not neglect ''downstream'' storage or virtual storage.
VIRTUAL WATER STORAGE
Virtual water, a term coined by Professor Tony Allan, means the water intrinsic to the products of water, in grain that then can be exported from one area to another area. If it takes 500-1000 tonnes of water to produce one tonne of grain, then moving that grain to another basin is like transferring more than 500 times the productive quantity of water.
Going back to fundamentals, even storing the grain from one harvest and using it over the rest of the year, makes water that was only available for part of the year, available to sustain us throughout the year. Building up a surplus to sustain us during years of poor harvest, makes the water in good years available to sustain us through a drought. It is IRRIGATION AND DRAINAGE