Introduction - lecture

Sources

Sources used:

(USGS, 2019) (USGS, 2019) (Margulis, 2017)

How much water is there? Where?

The water cycle

Global water distribution

Water source	Volume (km$^3$)	% of freshwater	% of total water
Oceans, Seas, & Bays	1,338,000,000	–	96.54
Ice caps, Glaciers, & Permanent Snow	24,064,000	68.7	1.74
Groundwater	23,400,000	–	1.69
$\quad$Fresh	10,530,000	30.1	0.76
$\quad$Saline	12,870,000	–	0.93
Soil Moisture	16,500	0.05	0.001
Ground Ice & Permafrost	300,000	0.86	0.022
Lakes	176,400	–	0.013
$\quad$Fresh	91,000	0.26	0.007
$\quad$Saline	85,400	–	0.006
Atmosphere	12,900	0.04	0.001
Swamp Water	11,470	0.03	0.0008
Rivers	2,120	0.006	0.0002
Biological Water	1,120	0.003	0.0001

* (Percents are rounded, so will not add to 100)
https://www.usgs.gov/special-topic/water-science-school/science/fundamentals-water-cycle

Energy drives the hydrologic cycle

A key aspect of the hydrologic cycle is the fact that it is driven by energy inputs (primarily from the sun). At the global scale, the system is essentially closed with respect to water; negligible water is entering or leaving the system. In other words, there is no external forcing in terms of a water flux. Systems with no external forcing will generally eventually come to an equilibrium state. So what makes the hydrologic cycle so dynamic? The solar radiative energy input, which is external to the system, drives the hydrologic cycle. Averaged over the globe, 342 W m$^{-2}$ of solar radiative energy is being continuously input to the system at the top of the atmosphere. This energy input must be dissipated, and this is done, to a large extent, via the hydrologic cycle. Due to this fact, the study of hydrology is not isolated to the study of water storage and movement, but also must often include study of energy storage and movements.

Margulis, 2017, “Introduction to Hydrology”

Components of the water cycle

Water storage in oceans

Evaporation / Sublimation

Evaporation $\longrightarrow$ cooling

Evapotranspiration

Water storage in the atmosphere

Cumulonimbus cloud over Africa

Picture of cumulonimbus taken from the International Space Station, over western Africa near the Senegal-Mali border.

If all of the water in the atmosphere rained down at once, it would only cover the globe to a depth of 2.5 centimeters.

\begin{align} \text{amount of water in the atmosphere} & \qquad V = 12\, 900\, \text{km}^3 \\ \text{surface of Earth} & \qquad S = 4 \pi R^2;\quad R=6371\,\text{km}\\ & \qquad V = S \times h \\ \text{height} & \qquad h = \frac{V}{S} \simeq 2.5\,\text{cm} \end{align}

Try to calculate this yourself, and click on the button below to check how to do it.

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Condensation

Precipitation

	Intensity (cm/h)	Median diameter (mm)	Velocity of fall (m/s)	Drops s$^{-1}$ m$^{-2}$
Fog	0.013	0.01	0.003	67,425,000
Mist	0.005	0.1	0.21	27,000
Drizzle	0.025	0.96	4.1	151
Light rain	0.10	1.24	4.8	280
Moderate rain	0.38	1.60	5.7	495
Heavy rain	1.52	2.05	6.7	495
Excessive rain	4.06	2.40	7.3	818
Cloudburst	10.2	2.85	7.9	1,220