The Processes of the Water Cycle Flashcards
What is the water balance equation?
The water balance equation summarises the flows of water in a drainage basin over time. It states that precipitation is equal to evapotranspiration and streamflow, plus or minus water entering or leaving storage: Precipitation (P) = Evapotranspiration (E) + Streamflow (Q) ‡ Storage
What are the principal flows in the water cycle that link the various stores?
The principal flows in the water cycle that link the various stores are: precipitation, evaporation, transpiration, run-off, infiltration, percolation and throughflow.
What is precipitation and how does it form?
Precipitation is water and ice that falls from clouds towards the ground. It takes several forms: most commonly rain and snow, but also hail, sleet and drizzle. Precipitation forms when vapour in the atmosphere cools to its dew point and condenses into tiny water droplets or ice particles to form clouds. Eventually these droplets or ice particles aggregate, reach a critical size and leave the cloud as precipitation.
How does precipitation vary in character and what impact does this have on the water cycle at the drainage basin scale?
Precipitation also varies in character and this impacts the water cycle at the drainage basin scale. Most rain on reaching the ground flows quickly into streams and rivers. But in high latitudes and mountainous catchments, precipitation often falls as snow and may remain on the ground for several months. Thus there may be a considerable time lag between snowfall and run-off. Intensity is the amount of precipitation falling in a given time. High-intensity precipitation (e.g. 10-15 mm/hour) moves rapidly overland into streams and rivers. Duration is the length of time that a precipitation event lasts. Prolonged events, linked to depressions and frontal systems, may deposit exceptional amounts of precipitation and cause river flooding. In some parts of the world (e.g. East Africa, Mediterranean) precipitation is concentrated in a rainy season. During this season river discharge is high and flooding is common. In the dry season rivers may cease to flow altogether.
What is transpiration and what influences it?
Transpiration is the diffusion of water vapour to the atmosphere from the leaf pores (stomata) of plants. It is responsible for around 10 per cent of moisture in the atmosphere. Like evaporation, transpiration is influenced by temperature and wind speed. It is also influenced by water availability to plants. For example, deciduous trees shed their leaves in climates with either dry or cold seasons to reduce moisture loss through transpiration.
What is condensation and how does it occur?
Condensation is the phase change of vapour to liquid water. It occurs when air is cooled to its dew point. At this critical temperature air becomes saturated with vapour resulting in condensation. Clouds form through condensation in the atmosphere.
What are the physical systems that contribute to cloud formation?
Cumuliform clouds, with flat bases and considerable vertical development most often form when air is heated locally through contact with the Earth’s surface. This causes heated air parcels to rise freely through the atmosphere (convection), expand (due to the fall in pressure with altitude) and cool. As cooling reaches the dew point, condensation begins and clouds form. Stratiform or layer clouds develop where an air mass moves horizontally across a cooler surface (often the ocean). This process, together with some mixing and turbulence, is known as advection. Wispy, cirrus clouds, which form at high altitude, consist of tiny ice crystals. Unlike cumuliform and stratiform clouds they do not produce precipitation
What is interception and how does it relate to vegetation?
Interception is the process by which precipitation is intercepted by vegetation and other surfaces before it reaches the ground. Vegetation can intercept precipitation on leaves, stems and branches, and the intercepted water can then evaporate, drip down to the ground as stemflow, or be released to the atmosphere through throughfall.
What is stemflow and throughfall?
Stemflow is the portion of intercepted precipitation that runs down plant stems or trunks, while throughfall is the portion that reaches the ground directly through gaps in vegetation or falls from the edges of leaves.
What are the different pathways that water can take after it reaches the ground?
After precipitation reaches the ground, it can follow different pathways: it can evaporate, run off over the surface, infiltrate into the soil, percolate downwards through soil and rock to become groundwater, or be taken up by plants through transpiration.
What is evaporation and how does it occur?
Evaporation is the process by which liquid water is transformed into water vapour, which enters the atmosphere. Evaporation occurs when energy (in the form of heat) is transferred to the water, causing it to change state from liquid to gas.
What is overland flow and how does it contribute to floods?
Overland flow is the movement of water across the surface of the ground when rainfall intensity exceeds the infiltration capacity of the soil. Overland flow can contribute to floods when it accumulates in streams or rivers.
What is throughflow and how does it relate to channel storage?
Throughflow is the lateral movement of water downslope through the soil layer, before it reaches a stream or river. It can contribute to channel storage, which is the amount of water held in a river channel or floodplain.
What is infiltration and how does it relate to soil moisture?
Infiltration is the process by which water enters the soil surface. It can contribute to soil moisture, which is the amount of water held in the soil layer.
What is percolation and how does it relate to groundwater?
Percolation is the movement of water downwards through soil and rock layers, until it reaches an impermeable layer or the water table. It can contribute to groundwater, which is the water held in permeable rocks or sediments below the water table.
