Water Resource Index

Variable WRI of nine · scored for every 1 km site

Water is the one input that cannot be substituted, manufactured, or imported at the scale modern agriculture and dense settlement require.

Water is the defining constraint of twenty-first-century land value. As the hydrological cycle accelerates and groundwater depletion continues across the world's major aquifer systems, the difference in value between water-secure and water-stressed land is widening — and doing so in ways that current land prices do not yet fully reflect.

Arcanium measures water availability as a physical fact rather than an inference. The definition below is the whole of it.

What it measures

Fresh water a site can reach, on the surface and beneath it: flowing water — rivers and streams — standing water — lakes and reservoirs — and the groundwater a well on the site could draw from the aquifer under it. Every 1 km site is scored against the nearest river and lake, and against the rock it sits on.

The two questions

How far is the nearest water, and how much moves through it. Distance alone would be misleading — a kilometer from a major river and a kilometer from a seasonal creek are not the same proposition — so flow and proximity are combined into one availability score.

How the number is built

Flow, and what a lake holds. A river's long-term average discharge, or a lake's long-term average outflow, in cubic meters per second — the same quantity, how much water passes through, so a lake is scored as a river of the same flow would be. A lake is also credited with the water it stores, spread over the same hundred and fifty years the aquifer's stock uses, because a body holding cubic kilometers of fresh water is a long-term supply whether or not it refills quickly. Outflow alone measured the creek leaving a lake and called that the supply.
Scale. Flow is placed on one fixed logarithmic scale from a trickle (0.01 m³/s) to the largest rivers on Earth (100,000 m³/s), giving a figure from 0 to 100. The scale is the same everywhere and never re-fitted to a region: a country of small streams is allowed to score low rather than have its biggest creek stretched to 100.
Distance. That figure decays with distance from the water's edge, halving roughly every 5.5 km. For a lake, the edge is the shoreline, so a site on the shore is at the water regardless of how wide the lake is.
The better of the two on the surface. A site takes the higher of its nearest river and its nearest lake — never the sum of those two. The rivers that feed a lake are the lake's own flow arriving, and counting both would count the same water twice.
Groundwater, added as water. The water a well on the site could obtain is worked out as a flow in the same unit — from how much water re-enters the aquifer each year, how much the rock holds, whether it yields at a useful rate, and how deep the water table is — and added to the surface flow before the scale is applied. The stock beneath a site counts, spread over a hundred and fifty years, because it is shared with every other well on the basin; where a basin's measured drawdown gives it a shorter life, a site gets proportionately less of the stock. The surface figure is turned back into the flow it stands for, the well's flow is added, and the sum is scored once, so ground with no usable aquifer scores exactly what it did.

What scores zero

Ground with no mapped fresh water within reach. A terminal or saline lake with no outflow — a dead sea, a salt lake — contributes nothing: standing water that nothing leaves is not a supply, and its stored volume is not counted either. Seawater never counts, for the same reason in the extreme: a coastal desert sits beside limitless seawater and has no water availability at all, and a measure that counted the ocean would invert the answer exactly where it matters most.

What is not measured

Groundwater is mapped at two resolutions. In the conterminous United States the aquifer, the recharge and the depth to water come from national products at one kilometre, and the site's own tile is what is read. Everywhere else they come from global products on a fifty-kilometre grid, which sets the level for an area rather than separating one tile from the next — and every dossier line outside the United States says so. The rule, the units and the place in the score are the same on both sides.
Water in storage that is being mined. The stock beneath a site counts through the horizon above, never as a lump. How fast the basin is being drawn down enters only as that basin's remaining life, measured from the trend of the monitoring wells standing on it — a falling water table across enough wells, over enough years, to be a trend and not a drought. Where the well network cannot show one, the stock keeps the full horizon and the dossier says the life is not measured. The life is one value across a basin, so it is a statement about the aquifer and not about the site.
Water quality. Volume and reach, not potability.
Season. Flow is a long-term average; a river that floods and dries is scored at its mean.
Small water. Lakes and ponds under ten hectares are not in the source and do not count.
The legal right to draw. Whether land beside water carries any right to use it varies by country and by state — in most of the world water is public and licensed; in parts of the United States it attaches to the land, in others it is held and traded separately. The index measures the water, not the right, and nothing on the map is a legal opinion.

How to read the score

100	A site on one of the largest rivers on Earth, or the shore of a great lake with the outflow to match.
~50	A river or lake passing about 30 m³/s at the site; or a 500 m³/s river about 5 km away.
~20	A small creek at hand, or a good river well out of reach; ground with no river over a productive aquifer lands here on its wells alone.
0	No fresh surface water within reach.

Every site's own water line — which water it was scored against, how far, how much flows, and which aquifer lies beneath and what a well could obtain from it — is shown in its dossier on the map and in the ground report.

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