BASIN

What a basin setting means

A basin is the unit in which water actually moves, which is why it is the natural scale for a hydrological assessment and an awkward one for everything else. Land use, property, taxation and political authority are organised in other units, and the body responsible for the basin often has powers of coordination rather than of decision.

Working at basin scale therefore means working with a mismatch. The intervention will be justified by an effect that occurs somewhere in the basin, decided by bodies whose jurisdictions cover parts of it, and paid for by beneficiaries who may be in a different part again. Making that work is as much an institutional problem as a hydrological one.

How it is assessed

Hydrological and hydraulic modelling is the core, applied to establish the effect of the intervention on the flow regime and on flood or drought conditions at the points where the damage occurs. Where an aquifer underlies the basin, groundwater modelling is required in parallel, because the surface and subsurface responses operate on different timescales and one can offset the other.

The characterization step is more demanding at basin scale than at any other, because it has to cover a heterogeneous area on a consistent basis: the geology, land cover, water uses and climate of an entire catchment, together with the socio-economic frame and the institutional layers within it. The nine NAIAD catchment characterizations are the reference for how this is done and what it costs in effort.

The institutional analysis is not optional here. Since the basin rarely coincides with a single authority, the analysis establishes which bodies hold the relevant powers, what instruments they have, and where a responsibility exists without the budget to discharge it. That finding usually determines the form the proposal has to take.

Where it is used at NAIAD

Four of the demonstration areas are basins: the Thames, the La Brague, the Lez and the Lower Danube. Together they cover a heavily regulated national basin, small Mediterranean catchments with strong seasonal variation, and a large multi-country agricultural basin, which is the range over which the basin-scale methods were tested.