Everyone realises that they cannot live very long without water to drink, but less people realise the enormous amounts of water that is needed to grow food.
The quantity required varies depending on the crop type, soil type, temperature etc, but as an example, it takes about 600 tonnes of water to produce 1 tonne of corn. In some fortunate places (such as western Europe) we have enough natural rainfall at the right time to give good crops on most soil types without additional water supplied through irrigation. In many important food producing regions this is not the case though, and much of the food production increases we have seen in the past 60 years has been due to the irrigation of dry land that would otherwise be uncropped or very low yielding.
Unfortunatly, in many areas we will not be able to continue to maintain current production yields. As water supplies lessen there will be huge reductions in yield. The great aquifers that have been such a blessing in the past are being depleted quickly. Huge pumps driven by fossil fuels are bringing fossil (ancient) water from ever deeper levels, but eventually supplies will dwindle and then stop.
Similarly, rivers are drying up as cities and farming compete for available supplies. In China, so much water is extracted from the once mighty Yellow River that now hardly any water reaches the sea. This causes cities and farmers closer to the sea to extract more underground water which is in turn running out.
Climate change is making things very much worse. Not only is rainfall less reliable and there are more droughts and floods, but snowmelt is earlier. Many large coastal plains rely on melting snow flowing down from inland mountain ranges. In the past with some help from storage behind dams, this water flowed at the right time to irrigate the plains. Now some of the snow is replaced by rain, and what snow there is melts earlier. The dams can only hold so much and so valuable water flows away into the oceans before it can be used.
As with most of our natural resources, we have been wasteful with the huge reserves the earth held and will have to manage more on the natural rainfall reaching us each year. This will not be easy because although amounts falling are massive, its not in the right place for the vast areas that now use ancient aquifers.
As water availability per person declines so will food availability per person, adding to the other food production problems.