A4 Hydrological Droughts

Topic

Hydrological drought is the manifestation—across the various components of the hydrological cycle—of a precipitation deficit that has accumulated over time; it affects river flow, aquifer levels, and wetland volumes. Furthermore, it represents the final link in a cascade that begins in the atmosphere and ends within the hydrological cycle itself: it first appears as meteorological drought, then as agricultural drought, and only ultimately as this hydrological drought, which impacts the system's deepest water reserves.

Within this context, aquifers respond with far greater inertia than rivers; it is precisely the hydraulic resistance of the aquifers' porous media that delays their response to precipitation deficits—sometimes by months or years—compared to the much faster response already exhibited by rivers.

The Budyko balance serves as the most direct conceptual model for describing this type of drought: it relates the ratio of evapotranspiration to precipitation against a region-specific aridity index. This index determines what proportion of rainfall actually replenishes rivers and aquifers rather than being lost to the atmosphere.

The depletion of river baseflow reflects, at the surface, the same slow draining of aquifers described earlier: as an aquifer empties, the flow it contributes to the river declines along an exponential recession curve—a process that slows progressively as fewer reserves remain to be drained.

The severity of such hydrological drought is formally characterized using a standardized precipitation index, which compares a region's accumulated precipitation against its historical distribution; this allows for the comparison to be calculated across multiple simultaneous time scales—one, three, six, and twelve months. It is precisely this comparison with each region's history that allows the same volume of rainfall to be classified as a severe drought in one region and a normal season in another.

Thus, spanning everything from the precipitation deficit that initiates this cascade to the so-called "memory effect" that concludes it—and encompassing aquifer inertia, the Budyko balance, and the baseflow recession curve—hydrological drought means that the system's full recovery after the drought ends may require several full years of recharge before rivers, aquifers, and wetlands return to their usual levels.

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