Hydrological cycle

At the global scale, evaporation from the oceans transfers more water to the atmosphere than precipitation returns directly to ocean surfaces. Atmospheric circulation carries much of this vapour over land, where it falls as precipitation. The surplus land water then returns to the ocean through rivers and surface runoff, while a smaller but important amount moves through subsurface flow and groundwater discharge. This ocean-to-land evaporation and land-to-ocean return maintains the long-term balance. Option A is correct because it identifies the compensating return pathways. Salt does not fly away as water, glacier growth is not permanent, and stopping rainfall would worsen rather than correct the imbalance.

A drainage-basin water balance accounts for the main inputs, outputs, transfers, and changes in stored water. Precipitation is the principal atmospheric input. Evapotranspiration represents water returned from vegetation, soil, and open surfaces to the atmosphere; runoff transfers water to channels and the basin outlet; and storage change records water accumulated or released by soil, groundwater, lakes, snow, or reservoirs. In simplified form, change in storage equals precipitation minus evapotranspiration, runoff, and other withdrawals, with units and time period kept consistent. Option A contains the meaningful budget components. The other groups mix unrelated physical, biological, astronomical, or descriptive properties.

Water availability depends on the balance between inputs, losses, and storage. If precipitation stays constant while evapotranspiration increases, more water is transferred from soil, vegetation, and surface stores to the atmosphere. Unless another input or a compensating reduction in runoff or withdrawal occurs, less water remains for infiltration, groundwater recharge, stream flow, ecosystems, and human use. Therefore option A is correct, using the careful word “may” because the actual outcome also depends on soil moisture, season, groundwater reserves, and management. Groundwater will not always increase, runoff does not automatically double, and rainfall does not become saline merely because evapotranspiration rises.

Residence time is the average length of time that water remains in a particular reservoir or store before leaving it. It is a time-scale concept, not a measure of colour, ocean depth, or sound. In a simple steady-state approximation, residence time can be estimated as the amount of water stored divided by the rate at which water leaves that store. A large store with slow outflow, such as deep groundwater or an ice sheet, generally has a long residence time, whereas atmospheric moisture usually has a much shorter one. Option A is correct because it states the definition without suggesting that every individual water molecule remains for exactly the same duration.

A deep aquifer is a subsurface geological formation that can store groundwater for a long period because movement through pores and fractures is often slow and recharge may be separated from discharge by considerable time. Its residence time varies with geology, pressure, recharge, pumping, and groundwater flow, but it is commonly much longer than the time water spends on a leaf, in a passing cloud, or running from a roof. Option A is therefore the best example of long-term storage. The question does not mean that every aquifer is permanent or non-renewable; excessive pumping can deplete some aquifers faster than natural recharge replaces them.

Urban growth commonly replaces permeable soil and vegetation with impervious surfaces such as concrete, asphalt, and roofs. These surfaces reduce the area and time available for infiltration, so less rainfall enters the soil and groundwater. Water instead travels quickly across the surface into drains and channels, producing a sharper and earlier runoff peak. This can increase local flood risk, especially when drainage capacity is exceeded, although the exact result depends on slope, rainfall, drainage design, and green infrastructure. Option A is correct. Urbanisation does not guarantee greater infiltration, zero flood risk, permanently stopped evaporation, or conversion of all precipitation into snow.

Infiltration capacity is the maximum rate at which soil can absorb water under the prevailing conditions. When rainfall intensity is greater than that capacity, the soil cannot accept all the incoming water at once. The excess first collects on the surface and then moves downslope as overland or surface runoff; it may later enter a stream and contribute to a flood peak. The exact amount depends on soil texture, antecedent moisture, vegetation, slope, compaction, and surface storage. Option A is correct. Rain does not turn into salt, clouds do not sink into the sea, and river flow is not forced to stop by high rainfall intensity.