The Azimuth Project
Water cycle



The hydrological cycle or water cycle is according to Wikipedia defined as:

describes the movement of water on, above and below the surface of the Earth. In this cycle, it is important that temperatures and pressures on Earth are close to water’s triple point, so water is commonly found in three forms: solid, liquid and vapor. As water moves through the water cycle, there are large transfers of heat energy to and from the environment, especially because water has a remarkably high specific heat.

The water cycle is very important to life on Earth, including all human life: this is easily seen by comparing the population density of the Sahara Desert? to that of coastal regions throughout the world. Many human populations are currently facing water scarcity, not because of an overall lack of water, but because of a lack of drinkable fresh water in certain regions.


According to The Earth’s Biosphere by Vaclav Smil, which is part of our recommended reading list, water is distributed as follows:

This suggests that there will be a need for efficient desalination methods. The major fluxes in the water cycle are:

according to Vaclav Smil

Evaporation of sea water is also affected by the ocean’s overturning circulation, a vertical heat exchange whereby cold, dense water sinks near the poles and is replaced by warmer poleward flow from low latitudes.

For more on this circulation, see the Azimuth article on Thermohaline circulation.

Water scarcity

Oki et al. write:

Detailed knowledge of global water resource certainly has been enriched over the 40 year that have passed since the International Hydrological Decade. Water cycles on Earth can now be measured and simulated on finer temporal and spatial scales with detailed models of each hydrological process, and the current and future status of the global water system can be illustrated. In contrast to these achievements in studies of the natural hydrological cycles, data about the social aspects of water use are not easily available.

Finally, the future development of hydrology requires improved communication between scientists and policy-makers to ensure that hydrological expertise is translated into action that address water challenges and to make sure that scientists understand what kinds of knowledge are required by policy-makers and by society at large.

Here is an image from Oki et al. that shows water scarcity, defined as follows:

R ws=WUnknown characterUnknown characterUnknown characterSQR_{ws}=\frac{W–S}{Q}

where WW, SS, and QQ are the annual water withdrawal by all the sectors, the water use from desalinated water, and the annual Renewable Fresh Water Resources (RFWR), respectively. A region is usually considered highly water stressed if R wsR_{ws} is higher

than 0.4. It is considered to be a reasonable, although not definitive, threshold value because not all the RFWR can be used by human society. Data with shorter time scales will enable more detailed assessments considering the effects of temporal variability in the hydrological cycles.


Modelling and Tools

Here is some snapshots from the World Ocean Circulation Experiment which has produced a lot of hydrological cycle data and also software, such as ewoce, hydrobase2 and ncBrowse.


Abstract: Water is a naturally circulating resource that is constantly recharged. Therefore, even though the stocks of water in natural and artificial reservoirs are helpful to increase the available water resources for human society, the flow of water should be the main focus in water resources assessments.

The climate system puts an upper limit on the circulation rate of available renewable freshwater resources (RFWR). Although current global withdrawals are well below the upper limit, more than two billion people live in highly water-stressed areas because of the uneven distribution of RFWR in time and space. Climate change is expected to accelerate water cycles and thereby increase the available RFWR. This would slow down the increase of people living under water stress; however, changes in seasonal patterns and increasing probability of extreme events may offset this effect. Reducing current vulnerability will be the first step to prepare for such anticipated changes.

Abstract: The seasonal and annual climatological behavior of selected components of the hydrological cycle are presented from coupled and uncoupled configurations of the atmospheric component of the Community Climate System Model (CCSM) Community Atmosphere Model version 3 (CAM3). The formulations of processes that play a role in the hydrological cycle are significantly more complex when compared with earlier versions of the atmospheric model. Major features of the simulated hydrological cycle are compared against available observational data, and the strengths and weaknesses are discussed in the context of specified sea surface temperature and fully coupled model simulations.

The magnitude of the CAM3 hydrological cycle is weaker than in earlier versions of the model, and is more consistent with observational estimates. Major features of the exchange of water with the surface, and the vertically integrated storage of water in the atmosphere, are generally well captured on seasonal and longer time scales. The water cycle response to ENSO events is also very realistic. The simulation, however, continues to exhibit a number of long-standing biases, such as a tendency to produce double ITCZ-like structures in the deep Tropics, and to overestimate precipitation rates poleward of the extratropical storm tracks. The lower-tropospheric dry bias, associated with the parameterized treatment of convection, also remains a simulation deficiency. Several of these biases are exacerbated when the atmosphere is coupled to fully interactive surface models, although the larger-scale behavior of the hydrological cycle remains nearly identical to simulations with prescribed distributions of sea surface temperature and sea ice.