Long-distance trek: water, food and resupply planning

For hiking, the Nomad score counts 3 L of drinking and cooking water per person per day, 3,200 kcal, 300 Wh of electricity per day for the group and about 20 km a day. In summer (June to August), water is multiplied by 1.2 and electricity by 0.85; in winter, by 0.8 and 1.3.

Two rules of the model bear on resupply. It assumes a resupply point every 4 days on foot — on a GR-type trail, a village, a hut or a road crossed every three to five days: an assumption, not a reading of your route. And because a hiker carries everything, the food pillar drops with duration: multiplied by 0.95 beyond 7 days of trekking, by 0.85 beyond 14.

EFSA sets the adequate water intake at 2.0 L a day for women and 2.5 L for men, drinks and water from food included, at moderate temperature and activity. A mountain stage is neither: the score's reference, 3 L and 3.6 L in summer, is a starting point to check on your first stages.

You do not carry the day's water, you carry the stretch's: daily need × share of the day until the next source, plus a reserve if that source is unconfirmed. A whole day without water in July is 3.6 L, about 3.8 kg with containers (1.05 kg per litre in the model).

The score's water pillar starts from the place's annual rainfall — 85 points from 800 mm a year, 65 from 600 mm, 45 from 400 mm, 25 below — divided by the season multiplier. The dashboard turns it into an interval between two refills: 1 day above 70, 2 days above 40, 3 days below. The route planner, for its part, looks for OpenStreetMap drinking-water points within 1 km of the track: crowd-sourced data, to be confirmed before you rely on it.

The US CDC ranks boiling first: 1 minute at a rolling boil, 3 minutes above about 2,000 m (6,500 ft). On the trail, you filter more often than you boil:

• • The filter — an absolute pore size of 1 µm or smaller removes Giardia and Cryptosporidium; 0.3 µm or smaller also removes bacteria. None of these filters removes viruses.
• • Chlorine or iodine tablets — they kill bacteria and viruses but work poorly on parasites such as Cryptosporidium. Filtering first and disinfecting second covers both.
• • The weight — the Sawyer Squeeze (0.1 µm absolute hollow fibre) weighs about 85 g according to the maker, the Mini about 57 g; usable from 0 to 60 °C: keep it from freezing at night.

France's food agency Anses computes the energy of foods in its Ciqual table with 9 kcal per gram of fat and 4 kcal per gram of protein or carbohydrate. Fatty, dry foods (nuts, oil, chocolate) therefore lighten the pack; foods full of water weigh it down. The model uses 4,000 kcal per kilo for what a hiker really carries, dried food and tins mixed.

Food weight = kcal per day × people × days to the next resupply ÷ density. With the score's reference values:

Each extra day of food weighs 0.8 kg per person (3,200 ÷ 4,000): that figure settles whether to carry or to resupply on the way.

On the GR 20 in Corsica, the huts of the Regional Natural Park are staffed for only part of the year, with compulsory booking — 2026 staffed season: May 16 to October 4; check the dates of the following season before you go. During the staffed season they are supplied by helicopter with gas and food, which lets you resupply on the way. Off season they stay open but unstaffed, and there is neither food nor gas to buy: everything must be carried. The Park's route guide lists the Carozzu – Ascu Stagnu stage as having no water, and the French hiking federation (FFRandonnée) advises filtering river water (filter bottle or tablets).

Elsewhere in France, poste restante helps: La Poste keeps letters and parcels in poste restante for 15 days at most at the post office, collected on presentation of an ID, for a collection fee. Check the opening days and hours of the destination office before planning a stage around it.

The score's reference — 300 Wh a day for the group, 255 Wh in summer and 390 Wh in winter — covers the whole camp: it is cautious. Measure your own devices instead. A power bank's capacity in Wh is mAh × nominal voltage ÷ 1,000: 20,000 mAh at 3.7 V make 74 Wh. The score's 100 W reference portable panel yields, at 46° N, about 410 Wh a day in July and 189 Wh in January.

The satellite messenger often sets the charging pace: Garmin rates the inReach Mini 2 at up to 14 days with a point every 10 minutes under open sky, but about 4 days under moderate tree cover, and charging only between 0 and 45 °C.