DROUGHTS

What makes droughts different

Droughts are slow-onset and cumulative. There is no event to model and no damage function to apply: the hazard is a deficit that builds over months or years, the impacts are spread across agriculture, water supply, energy, navigation and ecosystems, and they are frequently not recorded as drought losses at all because they appear as reduced yields, higher costs or foregone production.

That makes the assessment harder in three ways. The baseline is not a normal year but a distribution of them, so the counterfactual has to be a sequence rather than a state. The damages are economic and dispersed rather than physical and concentrated, so the damage functions have to be constructed from production relationships rather than from depth-damage curves. And the beneficiaries are often not the people who would pay, because the water that is saved upstream is consumed downstream.

How it is assessed

Water balance and allocation modelling is the core instrument: establishing where water goes under a range of conditions and how an intervention changes the allocation between uses. Groundwater models are needed wherever the resource is an aquifer, because the response of a groundwater body to a change in recharge or abstraction occurs over years, and an intervention that looks ineffective in a single dry year can be substantial over a decade.

For the ecosystem service side the physical assessment of groundwater-dependent ecosystems establishes what the water sustains, which is the quantity that changes when the water balance changes. The valuation then has to address benefits that are largely non-market — habitat, base flow, water quality — together with the market benefits in agriculture and supply that are easier to count and usually dominate the arithmetic.

The insurance value argument is more difficult for droughts than for floods, because drought losses are less comprehensively insured. It is made instead through the cost of the alternatives: the value of an intervention that maintains supply in a dry year is at least the cost of the supply it replaces, and that cost is often far easier to establish than a damage function.

Where the methods are described

The hazards and risk modelling approaches page covers the modelling of slow-onset hazards alongside rapid ones. The regional groundwater flow and physical assessment pages describe the aquifer instruments. The Medina del Campo case is the project's principal drought and water-scarcity demonstration.