3.1.1.2 - Natural and human changes to water cycling

Storms and seasonal change

A storm can change a basin's balance within hours, whereas seasons alter the background conditions on which that event acts.

A short intense storm may exceed infiltration capacity; a prolonged storm may fill soil and surface stores, increasing saturation-excess runoff. Once stores fill, a larger fraction of additional rain can become rapid river inflow. Wind can redistribute precipitation, and rainfall concentrated over fast-responding tributaries can produce a larger outlet peak than the same total spread elsewhere.

In many temperate basins, winter has lower temperatures, shorter days and less plant activity. Evapotranspiration tends to be lower, so precipitation more readily replenishes soil and groundwater. Summer energy and plant growth often increase evapotranspiration and draw stores down, even where rain continues. These are conditional relationships: a wet summer can recharge stores, and monsoon basins may have their principal recharge during a warm wet season. In cold basins, winter snow stores water; spring melt transfers it into soil, lakes and channels, sometimes coinciding with rainfall or frozen soil that restricts infiltration.

Farming changes stores and routes

Farming affects water cycling through soil structure, vegetation and deliberate drainage or irrigation.

Heavy machinery and livestock can compact wet soil, reducing pore space and infiltration capacity. More rain may pond or flow overland, especially where tracks connect directly to a ditch. Bare ground after harvest has less canopy interception; erosion can remove permeable topsoil. Conversely, continuous cover, organic matter and well-developed roots can improve infiltration and soil-water storage. 'Agriculture increases runoff' is therefore too broad: crop type, management and soil condition determine the mechanism.

Field drains transfer water from the soil to ditches and channels. They can lower a local water table and create storage before a storm, yet also provide efficient routes once water enters the drain network. Their net effect on a river peak depends on capacity, antecedent wetness and timing relative to other tributaries. Irrigation adds water to a field, raising soil moisture and often evapotranspiration. If that water is abstracted from the same basin's river or aquifer, the field gain is an internal redistribution; importing it across the boundary adds an external input. Some irrigation returns by drainage, so abstraction and consumption are different quantities.

A causal account should name the operation, the changed store or pathway, and the consequence. For example, compaction reduces infiltration, which can increase rapid surface delivery; it does not make extra water from nothing.

Land-use change and catchment response

Replacing vegetation with built surfaces changes both storage and the connectivity of water routes.

Removing trees reduces interception storage and transpiration, potentially leaving more water available for runoff. Disturbance may compact soil and speed overland flow, but regrowth can restore some storage and infiltration. Urban roofs and roads seal the surface; gutters and storm drains connect rainfall rapidly to channels. A shorter travel time can reduce lag and raise a peak, even if an event's total runoff is unchanged. Greater runoff volume is a separate possible consequence of reduced evapotranspiration and infiltration.

Sustainable drainage systems interrupt these routes. A swale slows water in a vegetated channel; permeable paving admits it to underlying storage; a detention basin temporarily holds water and releases it later. Infiltration depends on the receiving soil and groundwater level: permeable paving above saturated or poorly permeable ground may mainly provide detention. Overflow routes remain necessary when storage fills. Adding trees alone cannot guarantee protection during every prolonged storm.

Assess a proposed change against its conditions. In an original scenario, new houses cover only 5% of a basin but their drains connect directly to a fast tributary. Their effect may be larger than the area share suggests because timing and connectivity matter. A retained wetland can delay local flows, yet cannot be assumed to control the entire basin's flood peak. Use the hydrograph and water balance to keep these scale distinctions clear.

Abstraction and water availability

Abstraction removes water from a river, lake or aquifer, but its effect on the whole basin depends on where that water subsequently goes.

Groundwater abstraction can lower the water table if removal exceeds recharge over the relevant period. A reduced hydraulic gradient towards a river may weaken groundwater-supported baseflow; in some settings the river can instead lose water to the aquifer. River abstraction directly reduces downstream discharge before any return flow. These effects are especially significant in a dry season, when recharge and river inflow may be low.

Water supplied to homes may return to a river through treated wastewater, often at a different location and time. Irrigation can return some drainage water while losing a larger share to evapotranspiration. Exporting water to a neighbouring basin is an additional boundary output. A reservoir stores and retimes river flow rather than creating a new supply; evaporation from its surface is another loss.

Consider an original annual aquifer budget with recharge 30 million cubic metres, abstraction 24 and natural river discharge 10. With no other exchanges, storage changes by $30-24-10=-4$ million cubic metres. Continued deficits cannot be treated as a permanently balanced supply. A licence based on a wet-year recharge estimate may fail during drought; monitoring water tables, river flows and demand helps assess the changing balance. Sustainable use also considers ecological flow needs and who receives water, not merely whether a pump can continue operating.