Hydrologic (water) cycle
The hydrologic cycle (a.k.a. the water cycle) describes the movement of water within the Earth system. Water can be stored in a reservoir (atmosphere, oceans, lakes, rivers, soils, glaciers, snowfields, groundwater), but each molecule will eventually move on. Whilst replacement of water in large lakes, glaciers, ocean bodies and groundwater can take from hundreds to thousands of years, water in the atmosphere is completely replaced about every eight days. Let’s take a look at various parts of the hydrologic cycle.
We could start anywhere. It is a cycle, after all. But let’s start with getting the water into the atmosphere in the first place. The processes here are evaporation and transpiration. Evaporation is the process where liquid water is transformed into a gaseous state. It requires the humidity of the atmosphere to be less than the evaporating surface (at 100 % relative humidity there is no more evaporation). Because the latent heat of vaporisation of water (the energy required to turn it from a liquid phase to a gas phase) is high (about 2,260 kJ/kg, or 40.8 kJ/mol), the evaporation process requires large amounts of energy. Transpiration is the movement of water from plants (through stomata, small openings found on the underside of leaves) into the atmosphere. Together, evaporation and transpiration are the two processes that move water from the Earth’s surface to its atmosphere. The term evapotranspiration is sometimes used to describe both processes.
Right, so we now have water molecules in the atmosphere. As water vapour, i.e. in gas form. Condensation is the changing of the gaseous molecules back to liquid. We now have clouds. Then we have advection which is the movement of a fluid (liquid or gas). In this case it’s the movement of the clouds.
Coming back down to Earth, we have precipitation. Think, rain, but it’s actually so much more. Rain is just one type of precipitation. Precipitation is any aqueous (watery) deposit, in liquid or solid form, that develops in a saturated atmospheric environment and falls from clouds to the ground surface. In fact most clouds don’t produce precipitation. In many clouds, water droplets and ice crystals are too small to overcome the natural updrafts found in the lower atmosphere. As a result, the tiny water droplets and ice crystals remain suspended in the atmosphere until they are converted back into vapour. Water droplets and ice crystals can only fall to the Earth's surface if they grow to a size that can overcome updrafts. Turbulent atmospheric mixing can cause droplets to grow through the processes of collision and coalescence, until they are heavy enough to fall to the Earth's surface. There are a number of types of precipitation:
Rain: Any liquid that falls from the atmosphere to the ground surface and has a diameter greater than 0.5 mm. The maximum size of a raindrop is about 5 mm. Beyond this size intermolecular cohesive forces become too weak to hold the mass of water together as a single drop.
Freezing Rain: Takes place when falling liquid water droplets encounter a surface with a temperature below 0 °C. Upon contact with this surface, the rain quickly turns into ice.
Ice Pellets (a.k.a. sleet): Transparent or translucent spheres of frozen water. They have a diameter smaller than 5 mm. They develop first as raindrops in a relatively warm atmosphere where the temperature is above freezing. These raindrops then descend into a colder lower layer of the atmosphere where freezing temperatures occur. In this layer, the cold temperatures cause the raindrops to freeze into ice pellets during their journey to the ground.
Snow: Develops when water vapour deposits itself (becomes solid but skips the liquid phase) directly on a deposition nuclei (particle of dust, smoke, etc.) as solid crystals, at temperatures below freezing.
Snow Pellets: Spherical white bits of ice that have a diameter less than 5 mm. Snow pellets develop when supercooled droplets freeze onto the surface of falling snowflakes.
Hail: A type of frozen precipitation that is more than 5 mm in diameter. Hailstones often have successive shells of ice alternating between a white cloudy appearance to being colourless and clear. The cloudy white shells contain partially melted snowflakes that freeze onto the surface of the growing hailstone. The clear shells develop when liquid water freezes to the hailstone surface. Descending hailstones can lose a significant amount of their mass because of melting as they encounter the warm air found in between the cloud base and the Earth's surface. Small hailstones often melt completely before they reach the ground.
Goodness, precipitation took awhile. Let’s move on to infiltration. This refers to the movement of water into the soil layer. The rate of this movement is called the infiltration rate. If rainfall intensity is greater than the infiltration rate, water will accumulate on the surface and runoff will begin.
Runoff refers to the water leaving an area by flowing across the land surface to points of lower elevation. It is not the water flowing beneath the surface of the ground. Runoff involves the following events:
rainfall intensity exceeds the soil’s infiltration rate
a thin water layer forms that begins to move because of the slope and gravity
flowing water accumulates in depressions
depressions overflow and form small rills
rills merge to form larger streams and rivers
streams and rivers then flow into lakes or oceans
Note that there is an imbalance between evaporation and precipitation over the Earth’s land and ocean surfaces. 86 % of the Earth’s evaporation occurs over the oceans, with 14 % occurring over land. However, only 78 % of all precipitation falls back to the oceans, with 22 % falling over land. Surface runoff sends the difference (8 %) back to the ocean to balance the processes of evaporation and precipitation. If we didn’t have this balance the ocean would evaporate away!!! Which gives me an idea for another post, about the Mediterranean drying up. Which happened, by the way...
Anyway, back to the hydrologic cycle.
Throughflow (a.k.a. subsurface runoff) is the horizontal subsurface movement of water on continents. Rates of throughflow vary with soil type, slope gradient, and the concentration of water in the soil.
Precipitation that succeeds in moving from the soil layer down into the underlying bedrock (through percolation) will at some point reach groundwater. Groundwater is the zone in the ground that is permanently saturated with water. The top of groundwater is known as the water table. Gravity causes groundwater to flow to surface basins of water (oceans) located at lower elevations.
So there you have it, the -. Wait, I missed one. Sublimation (shown at the top of the mountain in the image) is where a solid changes straight into a gas, without first melting to a liquid. In this case, the snow that deposited (straight from gas to solid) from water vapour can sublime back to water vapour in the atmosphere.
Aaaand, that’s the hydrologic cycle :)