Water Reserves, Underground

the quantity or volume of groundwater contained in a water-bearing horizon.

A distinction is made between static (natural, capacitive, or secular) groundwater reserves, that is, the total amount of water in the water-bearing horizon expressed in volumetric units, and elastic groundwater reserves, or the amount of water released when a water-bearing horizon is breached and its stratal pressure is reduced (by pumping or spontaneous outflowing) owing to the volumetric expansion of the water and the reduction in porosity of the stratum.

In hydrogeological investigations to determine water supply, an estimate is usually made of both natural and available groundwater resources. The term “natural” resources, that is, dynamic reserves, refers (according to B. I. Kudelin) to the discharge of an underground stream supported by recharging. Natural groundwater reserves are continuously being replenished through the hydrologic cycle and in an average multiannual profile are equivalent to the underground flow. They characterize the natural productivity of the water-bearing horizons. Available resources correspond to the amount of water that can be recovered per unit of time from a water-bearing horizon by technically and economically efficient water collection, without progressive lowering of productivity and the dynamic levels and without deterioration of water quality throughout the entire exploitation period. In estimating available resources, such factors as the possibility of using static and elastic reserves and the inflow of water from external sources are taken into account.

In the USSR, the available groundwater resources for specific users (cities, plants) are being determined and estimates are being made of the natural and available groundwater reserves of large areas and of the country as a whole (regional estimates).

Groundwater reserves are classified into categories A, B, Q, and C2, established by the State Commission on Mineral Reserves (GKZ). Category A includes reserves that have been explored and studied in sufficient detail so as to fully describe the geological structure, bedding, and recharging of the water-bearing horizons, pressure heads, filtration properties, the relationship between the used water and water from other water-bearing horizons and surface runoff, and the possibility of replenishing the available reserves. Category B includes reserves explored and studied in enough detail to explain only the main features of the bedding, structure, and recharging of the water-bearing horizons. When category C₁ groundwater reserves are being determined, only the general features of the structure, bedding, and extent of the water-bearing horizon are described. Category C₂ reserves may be established on the basis of general geological and hydro-geological data confirmed by probing water-bearing horizons at various points, or they may be determined by analogy with studied or explored sections.