## 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

### Evaporation / Sublimation

Evaporation $\longrightarrow$ cooling

### 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.

#collapse-hide
# amount of water in the atmosphere
V = 12900 # km^3
R = 6371 # km
# surface of Earth = 4 pi Rˆ2
S = 4 * 3.141592 * R**2
# Volume: V = S * h, therefore
# height
h = V / S # in km
h_cm = h * 1e5 # in cm
print(f"The height would be ~ {h_cm:.1f} cm")

The height would be ~ 2.5 cm


### 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

### Streamflow

The Mississippi river basin is very large

The Amazon river basin is Huge

Lake Malawi

### Groundwater storage

Center Pivot irrigation in Nebraska taps the Ogallala Aquifer.

### Spring

Ein Gedi

Thousand Springs, Idaho