Our astronauts will be settling on Mars indefinitely. It's not feasible to send water, oxygen and food from Earth to the astronauts: they will produce those on Mars.
On Mars, water can be extracted from the soil. The Rover will select the location for the settlement primarily based on the water content in the soil. We expect this to be at a latitude of between 40 and 45 degrees North. Water extraction will be performed by the life support units. The Rover will deposit soil into a water extractor in the life support units. The water extractor will heat the soil until the water evaporates. The evaporated water will be condensed and stored, the dry soil expelled, and the process repeated to extract more water.
About 1500 liters of reserve water will be stored in each Life Support Unit, which will be consumed primarily at night, and during periods of protracted low power availability, for example during dust storms.
Since Mars has gravity, water can be used in the same way as on Earth. Each astronaut will be able to use about 50 liters of water per day. The water will be recycled, which takes much less energy than extracting it from the Martian soil. Only water that can not be recycled will be replaced by water extracted from the soil.
Oxygen can be produced by splitting water into its constituent parts, hydrogen and oxygen. The oxygen will be used to provide a breathable atmosphere in the Living Units, and a portion will be stored in reserve for conditions when there is less power available, for example at night, and during dust storms.
The second major component of the Living Units' atmosphere, nitrogen, will be extracted directly from the Martian atmosphere by the Life Support Unit.
When the astronauts land, there will be limited rations of food available for them to use. Food from Earth will only serve as emergency rations, the astronauts will eat fresh food that they produce on Mars.
Mars One will make use of high efficiency plant growing methods that require much less space (e.g. PlantLab). Food production will be hydroponic, eliminating the need for soil. Food production will happen indoor, lighted by LED lighting. By providing the plants with only the frequencies of light that they use most efficiently, power consumption is limited. Some of the plants will be grown in multiple levels on top of each other, limiting space requirements.
In total there will be about 50 m2 available for plant growth. A thick layer of Martian soil on top of the inflatable habitat will protect the plants (and the astronauts) from radiation. CO2 for the plants is available from the Mars atmosphere and water is available through recycling and from the soil of Mars.
There will be sufficient plant production capacity to feed about three crews of four. Any plant production surplus will be stored as emergency rations for the second crew, and for other emergencies. Non-edible parts of the plants will be recycled, or will be stored until more advanced recycling equipment is shipped from Earth.