2 Water at the Land Surface
Explains how precipitation is intercepted, infiltrates soil, contributes to evapotranspiration and runoff, and is accounted for in a drainage-basin water balance.
How water moves across land
Water arriving at the land surface may be held temporarily by vegetation, enter the soil, return to the atmosphere, or flow toward streams. The proportions following these pathways vary with storm conditions, soil, vegetation, topography, and land use. Together, the pathways connect to , evapotranspiration, and .
and its arrival
supplies water to land and surface waters. It falls from the atmosphere as rain, snow, sleet, or hail. How quickly and how much it arrives affects what happens next: a short, intense storm can produce more immediate than gentle rain of the same total amount. Frozen may remain stored as snow or ice before melting and entering other pathways.
by vegetation
occurs when leaves, branches, and other surfaces catch before it reaches the ground. Some intercepted water evaporates back into the atmosphere; water that drips from leaves or runs down stems eventually reaches the soil. delays and can reduce the amount of water reaching the ground during a storm. Its effect depends on factors such as vegetation cover and how much water the surfaces can hold.
into soil
is the entry of water from the ground surface into the soil. Its rate depends on soil texture and structure, the condition of the surface, and how wet the soil already is. Water usually enters coarse, well-structured soil more readily than compacted or sealed ground. If rainfall arrives faster than soil can absorb it, water may collect or flow over the surface. Saturated soil also has limited capacity to take up additional water.
as a store
is water retained in soil pores. Plants draw on it through their roots, and some water may move downward through the soil and eventually recharge groundwater. changes as water infiltrates, drains, is taken up by plants, evaporates, or arrives in further . It is both a temporary store and a source of water for later processes.
Evapotranspiration to the atmosphere
combines evaporation and transpiration. Evaporation transfers water from surfaces such as soil and water bodies into the atmosphere. Transpiration is the release of water vapor from plants, mainly through their leaves, after they take up water through their roots. Sunlight supplies energy for these transfers. Actual ET also depends on available water: when soils dry, evaporation and plant water use may decline. ET returns water to the atmosphere rather than sending it directly to streams.
toward streams
is water that flows from a drainage area toward streams, lakes, or other outlets. Surface flows over the land; infiltrated water can also travel sideways through shallow soil or emerge later as groundwater discharge. is more likely when rainfall is intense, soil is saturated or has low capacity, slopes are steep, or surfaces are paved.
For example, heavy rain on a paved parking lot produces rapid surface flow, while rain on permeable, vegetated ground is more likely to infiltrate and replenish .
in a drainage basin
For a drainage basin over a chosen period, the simplified is:
Storage includes water held in soil, snow, surface depressions, and groundwater. The relationship emphasizes that water is redistributed, not simply lost: the change in storage accounts for water remaining in or being released from these stores. The balance among , , , evapotranspiration, and affects how much water is stored on land and how quickly it reaches rivers.