The hydrological cycle is the general movement of the water, upwards by evaporation and downwards first because rainfalls and then as surface runoff and groundwater. This is no so simple as the water evaporates in the ocean and precipitates over the continents. There is rain over the oceans and water can be evaporated in the continents as well.

The movement of the groundwater is, by, much more slower than surface runoff. This slowly movement is responsable, for example, that some rivers has flow although the rainfall is over. Groundwaters are just one part of the hydrological cycle and they has no other origin but rain.
As I don't want to extend too much, here is a diagram that shows quite cleary the fluxes and water in storage and I think is very illustrative.


Groundwater which arrived to the saturated zone (this is the area where the water completely fills the pores of the rock) circulates along the regional hydraulic gradients during few months, years, decades etc... This water can be extracted directly by wells and boreholes, or in springs, also it can feed the bed of a river or in coastal areas can run underground to reach the sea. Usually, the most important are the waters which feed a river.

Resources, reserves and overexploitation.

When we use the water that may be renewed, we say that we are exploiting the resources. When we are using more water that can be renewed, we are exploiting the reserves and we are producing overexploitation and the water levels in the drills down.

Clasification of geologic formations according of their hydrological behavior.

Aquifer: geological formation that contains water in appreciable quantities and that allows that
flow through it easily. For example: sand, gravels or compact rocks with important fracturation.
Aquiclude: geological formation that contains water in appreciable quantities but do not allow the flow trough it. For example: silts, clays. A m3 of clays contains more water that the same volume of sand but, the water is trapped so it cannot flows through the soil, nor to a well that is pumping.
Aquitard: geological formation that contains water in appreciable quantities but that flow through it with difficulty. For example: loamy sands, sandstones, compact rock with moderate alteration and/or fracturation.
There are other geological formations that do not contain water because do not allow that the water flows through it. For example: granite, non altered/fractured schist.
It is important to understand the geology of the area to know where we can find water, in what quantities and if is possible to extract it.
The hidrological behaviour of any rocks or geological formation depends of its ability to store the water and its permeability.
Permeability is how easy a fluid moves inside the rocks and it is directly dependant on the porosity of the material and the relationships between the voids in the material (size, shape, connectivity ...).
Porosity is given by the percentage of voids existing in a volume of rock/soil compared to total volume.
P_v = frac{rho_m - rho_a}{rho_m}cdot 100%
P_v = frac{rho_m - rho_a}{rho_m}cdot 100%

is the bulk density and
is the real density of the material.
Some interesting LINKS
Alley, W.M. Et al.- Sustainability of Ground-Water Resources (86 pp. 19 Mb)
Ralph C. Heath, R.C. (1983) Basic Ground-water Hydrology. (88p., 10 Mb)
Winter, T.C. Et al. - Ground Water and Surface Water A Single Resource (87 pp. 12 Mb)

Atmospheric Detail

Evaporation and Water Saturation Pressures (over ice and over water)
Consider a container in which half is full of water and the other half is vacuum or with container pressure (P) equal to 0 (Figure 1a). After some time, a little bit of the water will escape into the vacuum side of the container (known as evaporation) until the pressure stabilizes (Figure 1b). This pressure is known as the saturation pressure (Pwater), and is the point at which the same of amount of water is evaporating (leaving the liquid water) as is condensing (going into the liquid water). If we were to start again, but instead of vacuum we filled the top half of the bucket with 1 atmosphere (1 atm) of dry air (which is the pressure of air at sea level) (Figure 1c), the exact same amount of water vapour would evaporate and would slightly increase the pressure of the top half of the container (Figure 1d).
Figure 1 – (a) water in a vacuum filled container, (b) after some time, the vacuum portion will contain water vapour at the saturation pressure, (c) water in a container with 1 atm of air, (d) an air filled container after some time, note that the pressures are simply added.
The results would be similar if we replaced the water with ice, but instead of evaporation this process of water leaving the ice is known as sublimation, and the saturation pressures above ice would be much less than over water.

The amount of water vapour that will evaporate, i.e. how large the saturation pressure will be, will depend on the temperature of the system as follows:


Figure 2 – Saturation pressure of water at different temperatures (note Torr is another unit of pressure, 760 Torr = 1 atm. Note that 760 Torr (1 atm) occurs at 100OC, which is the temperature at which water boils at sea level)

One final definition is relative humidity. Going back to the examples of Figure 1, if all of the water were to evaporate before reaching the saturation pressure, the pressure inside the container would not increase any further. A simple measure of how close the pressure of water in the container is to the saturation pressure is the relative humidity, given simply as the ratio of water vapour in the container to the saturation pressure multiplied by 100. A relative humidity of 100% means that the pressure is at the saturation pressure, 50% means that it is half and so forth.

Earth’s atmosphere - density, pressure and temperature versus altitude
As we learned from the GRACE presentation, as we move upwards in altitude (height above Earth’s surface) the effect of gravity decreases. This leads to a situation in which the amount of air in Earth’s atmosphere per unit volume (density) decreases as you go up in altitude (Figure 3b), and as it turns out, it decreases at an exponential rate. The pressure is very closely linked to the density (Figure 3c) and also decreases exponentially with altitude (Note there will be slight differences in pressure which cause local weather, but they are small compared with the total pressure). The temperature is slightly more complicated, but in the lowest 10 km, where the majority of water vapour exists, the temperature decreases at approximately 10OC per km.


Figure 3 – (a) Temperature as a function of altitude, (b) atmospheric density as a function of altitude, (c) pressure as a function of altitude

Simple cloud formation – Example of a parcel of air moving up through the atmosphere
Let us define a 1m x 1m x 1m “box” of air, which we will call a parcel, in which no air can come into or out of this box. Let’s assume that our parcel is at the ground and that the relative humidity inside is 50%. Now say we are able to move this parcel upwards within the atmosphere, what happens? First off the parcel of air will start to cool (following the line in figure 3a). Because the water vapour inside the parcel is not allowed to leave, as the temperature decreases the pressure at which saturatin occurs (saturation pressure) will also decrease (following the curve in Figure 2) and thus the relative humidity will increase. Eventually the temperature will reach a point where the pressure of the water vapour inside the parcel will equal the saturation pressure, and at that point the water vapour will turn into a liquid – or in simple terms – it will form a cloud. If we were to continue increasing the altitude of the parcel, the temperature will continue to decrease, but now the relative humidity will not change. Here the water vapour inside the parcel will be maintained at the saturation pressure and any excess water vapour is turned into cloud – the relative humidity is always 100% within a cloud. Eventually the temperature will get cold enough that the water will freeze and ice crystals will form instead of water droplets, but this is all still considered cloud.
Actual Cloud Formation
The mechanisms by which clouds actually form are not entirely understood, but some aspects are given here. The first is that it is the local weather that determines the overall conditions for cloud formation, and is inherently complex. Second, water vapour typically forms into water droplets if there is already a small particle present for it to stick to. This can be another water/ice particle, aerosols (soot, ash, dust, pollution) or salt (farmers sometimes resort to cloud seeding where particles of silver iodide are injected into moisture rich air in the hope of forming clouds). The size of aerosols or salt is also important, as water forms preferentially on some sizes over others. A second point is that the cloud does not form at 100% relative humidity, typically it requires values over saturation, such as 120% in order to form the cloud. Once the cloud is formed, however, the surrounding air quickly goes to 100%.
As the water droplets form, they will continue to grow in size if there is sufficient water vapour present. They will also “bump” into each other and merge forming a single larger droplet. As they continue to grow, gravity will have an increasing effect on them, and they will eventually fall towards the Earth. As they descend, they may pass through other “parcels” of air that are dryer (i.e. have a lower relative humidity) then the one they were formed in. This will lead to some of the droplet evaporating, and may mean that all of the droplet has completely evaporated before it hits the ground (this effect can be observed during rain storms where “fingers” extending from the base of a cloud towards the ground are seen). If the droplet does reach the ground then it is effectively removed from the atmosphere.

Thus in order to really understand atmospheric water, you need to know:
  • the amount of water vapour (pressure or density)
  • the temperature
  • the amount of water/ice clouds
at all altitudes. In order to understand when and how clouds form you will also need to know:
  • weather, winds, constituents of the atmosphere (aerosols, salt)

Global Water Table