The hydrosphere

The distribution of water and the hydrologic cycle

From space, the Earth looks blue because water covers more than 70% of its surface. Yet a castaway in the middle of the Pacific would die of thirst: almost all of that water has salt dissolved in it. How is the planet's water distributed, and what keeps it moving?

Let's do the math. If fresh water is 2.5% of the total and ice is about 69% of it, then ice is $0.025 \times 0.69 \approx 1.7\%$ of the planet's water. Groundwater is $0.025 \times 0.30 \approx 0.75\%$. Lakes, rivers, and soil moisture add up to only a few hundredths of a percent, and the atmosphere to about $0.001\%$. What is truly usable (rivers, lakes, and the accessible part of aquifers) is less than 1% of the total.

The engine of the whole cycle is the energy of the Sun. Gravity makes water move downward (rain, runoff, infiltration), but without solar radiation the water would never rise again.

Groundwater and aquifers

In the city of Hermosillo or in the Valley of Mexico, many wells keep producing water even after months without rain. Where is that invisible volume stored, and what determines whether we can use it without depleting it?

An aquifer is not an underground river or a cavern full of water. In almost all cases the water fills the pores and fissures of the rock, like a wet sponge. Only in limestone areas, such as the Yucatán Peninsula, are there galleries and cenotes.

What happens when it is overexploited

An aquifer is overexploited when more water is withdrawn than is recharged. In an aquifer near the sea, fresh water (density $1.000\ \text{g/cm}^3$) floats on top of the denser seawater ($1.025\ \text{g/cm}^3$). A saltwater wedge forms, held in place by the pressure of the fresh water. As long as the fresh water level stays high, the wedge does not advance.

When too much water is pumped, the level drops, the pressure of the fresh water decreases, and the salt wedge advances inland. Wells near the coast begin to produce brackish water. This is what has happened in coastal aquifers in Sonora, such as the Costa de Hermosillo, and in many others around the world.

Ocean stratification: temperature and salinity with depth

When you swim in the Pacific near Puerto Vallarta, the water is warm at the surface, but dive down just a few meters and you can feel it get colder. How does the ocean change with depth, and what consequences does this have for life?

The average salinity of seawater is about $35\ \text{g}$ of salts per liter. It rises where there is a lot of evaporation or ice forms, and falls where rivers, rain, or meltwater arrive.

The ocean is organized into layers according to density:

• Mixed layer (the first 50-200 m): warm water, mixed by wind and waves, with enough light for photosynthesis.

• Thermocline: a zone (from about 100 to 1,000 m) where temperature drops quickly with depth. Similarly, the halocline is the zone where salinity changes quickly.

• Deep ocean: very cold (2-4 °C), dense, and dark water.

Density increases with depth, so light, warm water stays on top and dense, cold water stays below. This stratification is very stable: the water above does not sink because it is lighter, and the water below does not rise. Consequences for marine life:

• Nutrients (nitrates, phosphates) accumulate on the bottom when organic matter falls and decomposes. The sunlit layer runs out of nutrients and phytoplankton grows little. That is why very warm tropical seas, with their deep blue color, have low productivity.

• Where wind or currents break up the stratification and bring deep water to the surface (upwelling), productivity skyrockets. This happens in the Gulf of California and off Baja California.

• If deep water does not mix with the surface, it receives little oxygen and can end up with very little.

Thermohaline circulation

When someone in London or Norway looks at a map, they are surprised: those places lie between 51 and 60 °N, at latitudes similar to southern Alaska or Labrador, yet they have much milder winters. What carries the heat all the way there?

In the Atlantic, the Gulf Stream (which begins in the Gulf of Mexico) carries warm, salty water northward. Along the way it cools and gives off heat to the air; near Greenland and the Norwegian Sea it sinks. This circuit is the AMOC (Atlantic Meridional Overturning Circulation). The heat it gives off to the air is the reason northwestern Europe has a milder climate than other regions at the same latitude.

Warm seas and torrential rains

In September and October, a hurricane that forms over a very warm Pacific can dump in a few hours more rain than falls in months. Why does a hot sea produce such intense rains?

The storm is triggered when there is cold air aloft over a very warm surface. One example is a cold-core low aloft (Mexico's SMN also speaks of an upper-level trough): air at $-28\ ^\circ\text{C}$ at $9{,}000\ \text{m}$ over a sea with a surface temperature of $26\ ^\circ\text{C}$. The gradient is huge: the air below, much warmer than the air above, tends to rise. A mountainous coast works like a ramp: it forces warm, humid air to ascend and triggers convection.

Why are the rains stronger over a warmer sea? Calculate: if the sea and the air above it warm by $2\ ^\circ\text{C}$, the air can hold $1.07^2 \approx 1.14$ times more vapor, that is, about 14% more. All that extra vapor also releases more latent heat when it condenses and feeds stronger storms. The result is fewer moderate rain events but more very intense downpours.

The balance of an aquifer

When we think of groundwater as a resource, the key question is simple: does more flow in than out, or the opposite? That is what happens in the Valley of Mexico and in many aquifers in the north of the country.

In practice, "running out" happens sooner than the theoretical zero: wells dry up as the level drops, the cost of pumping rises, the ground compacts (the land surface sinks, as in Mexico City), and brackish water can enter. That is why an aquifer with a sustained deficit is not sustainable, even if the reserve seems large.

Go back to the beginning: the liter of water from your faucet could have come from the sea, from a cloud, and from an aquifer. Its journey depends on the Sun, which evaporates it; on gravity, which brings it down; and on us, who decide how much we take from the reserve.

Summary