Some rocks allow little or no water to flow through; these are known as impermeable rocks, or aquicludes. Others are permeable and allow considerable storage of water and act as major sources of water supply; these are known as aquifers. Aquifers overlain by an impermeable layer are called confined aquifers; aquifers overlain by an unsaturated, or vadose, zone of permeable materials are called unconfined aquifers. The boundary between the saturated and unsaturated zones is known as the water table. In some confined aquifers, hydraulic potentials may exceed those required to bring the water to the surface. These are artesian aquifers. A well drilled into such an aquifer will cause water to gush to the surface, sometimes with considerable force. Continued use of artesian water, however, will cause potentials to decline until eventually the water may have to be pumped to the surface.

The water found in groundwater bodies is replenished by drainage through the soil, which is often a slow process. This drainage is referred to as groundwater recharge. Rates of groundwater recharge are greatest when rainfall inputs to the soil exceed evapotranspiration losses. When the water table is deep underground, the water of the aquifer may be exceedingly old, possibly resulting from a past climatic regime. A good example is the water of the Nubian sandstone aquifer, which extends through several countries in an area that is now the Sahara desert. The water is being used extensively for water supply and irrigation purposes. Radioisotope dating techniques have shown that this water is many thousands of years old. The use of such water, which is not being recharged under the current climatic regime, is termed groundwater mining.

In many aquifers, groundwater levels have fallen drastically in recent times. Such depletion increases pumping costs, causes wells and rivers to dry up, and, where a coastal aquifer is in hydraulic contact with seawater, can cause the intrusion of saline water. Attempts have been made to augment recharge by the use of waste waters and the ponding of excess river flows. A scheme to pump winter river flows into the Chalk aquifer that underlies London has reversed the downward trend of the water table.

Water table levels in an aquifer are measured by using observation wells. Successive measurements of water levels over time may be plotted as a well hydrograph. The hydraulic characteristics of a particular aquifer around a well can be determined by the response of the water table to controlled pumping. Many aquifers exhibit two types of water storage: primary porosity consisting of the smaller pores and secondary porosity or fractures within the rock mass. The latter may make up only a small proportion of the total pore space but may dominate the flow characteristics of the aquifer. They are of particular importance to the movement of pollutants through the groundwater.

Runoff and stream discharge

Runoff is the downward movement of surface water under gravity in channels ranging from small rills to large rivers. Channel flows of this sort can be perennial, flowing all the time, or they can be ephemeral, flowing intermittently after periods of rainfall or snowmelt. Such surface waters provide the majority of the water utilized by humans. Some rivers, such as the Colorado River in the western United States, are used so intensively that often no water reaches the sea. Others flowing through hot, dry areas, as, for example, the Lower Nile, became smaller downstream as they lose water to evaporation and groundwater storage.

Stream discharge is normally expressed in units of volume per unit time (e.g., cubic metres per second), although this is sometimes converted to an equivalent depth over the upstream catchment area. There are a number of techniques for measuring stream discharge. Measurements of velocities using current meters or ultrasonic sounding can be multiplied by the cross-sectional area of flow. Dilution of a tracer can also be used to estimate the total discharge. Weirs of different types are frequently employed at discharge measurement sites. These are constructed so as to give a unique relationship between upstream water level and stream discharge. Water levels can then be measured continuously, usually with a float recorder, to construct a record of discharge over time—namely, a stream hydrograph. Analysis of the hydrographic response to catchment inputs can reveal much about the nature of the catchment and the hydrologic processes within it.

Stream discharge data are presented in terms of daily, monthly, and annual flow volumes, though for some purposes (e.g., flood routing) shorter time periods may be appropriate. The frequency characteristics of peak discharges and low flows are also of importance to water resource planning. These are analyzed using some assumed probability distribution in a way similar to rainfalls. A time recording of annual maximum flood is usually used in flood-frequency analysis. For design purposes the hydrologist may be asked to estimate the flood with a recurrence interval of 50 or 100 years or longer. There are few discharge records that are longer than 50 years, so such estimates are almost always based on inadequate data.

Knowledge of the discharge characteristics of catchments is essential to water supply planning and management, flood forecasting and routing, and floodplain regulation. Discharges vary over short lengths of time during storm periods, seasonally with the seasonal changes in evapotranspiration losses, and over longer periods of time as the rainfall regime changes from year to year. Discharge characteristics also vary with climate. In some places discharge represents only a minor component of the catchment water balance, the losses being dominated by evapotranspiration.

The discharge hydrograph that results from a rainstorm represents the integrated effects of the surface and subsurface flow processes in the catchment. Traditionally, hydrologists have considered the bulk of a storm hydrograph to consist of storm rainfall that has reached the stream primarily by surface routes. Recent work using naturally occurring isotope tracers such as deuterium has shown, however, that in many humid temperate areas the bulk of the storm hydrograph consists of pre-event water. This water has been stored within the catchment between storms and displaced by the rainfall during the storm. This suggests that subsurface flow processes may play a more important role in the storm response of catchments than has previously been thought possible.