The Basics

I’ve started a new habit lately, which is waking up on Saturday mornings and turning on Cesar Millan’s show The Dog Whisperer. And I don’t mean for 30 minutes. I’ll sit there for the entire ‘marathon’ and watch the show like a dog stares at a treat. I don’t know what it is, but to see how a dog’s behavior changes when their owner understands what they need is inspirational. Once humans get over treating dogs like humans, a mutually respectful relationship can be established.

In this same way, we as humans need to be aware of how we use and what we expect of it. This may seem obvious, but I don’t think that many people sit down and think about what it is they want or need water for each and every day. If we understand where it comes from and what we want it to do for us, only then we can progress towards using water in a sustainable and just way.

The term hydrology (a term I will use a lot) is the scientific study of water movement, distribution, and quality on the Earth. It is divided into surface water, groundwater, and marine hydrology. In this post I will introduce the very basics of hydrology. Subsequent posts will discuss hydrology in terms of different disciplines and how they relate and interact with each other.

The Hydrologic Cycle
This is such an important concept and absolutely vital to know if you care about water on this earth. It is simple in terms of processes (or at least I’ll make it here) so don’t get too overwhelmed. Figure 1 shows the basic (and I mean very basic!) components of this cycle. For many of you this will be more of a review than anything, for others, this may be the first time you think of water in this way. It is important I mention that this cycle assumes a natural system (ie. no human intervention!).

The hydrologic cycle transports water around the Earth while exchanging energy and moving materials as part of the process.

Perpetual Flow - The Basic Hydrologic Cycle

Figure 1. The basic hydrologic cycle.


Water is considered a ‘basic need’ for physical well-being. Not just for humans, but for many other organisms on the earth. Just stand in any forest and look around you. Trees are breathing just as you are. Plants are as necessary to your life as water and they rely on water just as much as you do.

The flow of water in the hydrologic cycle involves the transfer of water from the land and sea to the atmosphere and back again. Precipitation falls to the earth (this may be rain, snow, hail) and at this point may follow a number of different paths. It may fall onto leaves of plants (called interception, because the water is intercepted) or, it may reach the ground. If it reaches the ground it may move downwards into the soil (called infiltration, because – you got it, it infiltrates!), or it may accumulate on the soil surface and contribute to overland flow. During the processes of interception or accumulation water may evaporate (the transfer from a liquid state to a gas) back into the air, or be taken up by plant roots and transpired (called transpiration).

The water that moves over the soil surface contributes to overland flow and may eventually end up collecting in surface water bodies such as ditches, streams, or rivers, which usually terminate at the ocean.

However, the precipitation that infiltrated into the soil moves down due to gravity into the soil and contributes to groundwater. Groundwater moves significantly more slowly than surface water, but will at some point flow to the surface naturally. Groundwater flows out of the ground at springs and seeps and can also form very sensitive ecosystems like wetlands. During times of drought, the deep groundwater that contributes to the continued flow in streams or rivers is called baseflow.

Non-evaporated water almost always ends up in the ocean. Evaporation of water over the oceans is a very significant (413 x 103 km3/year [1] – this is big!) portion of the water budget. However, once evaporation takes place, the cycle is complete.

The Basic Water Balance

The principle idea behind the water balance is that of conservation. Meaning, what goes in some controlled volume must either come out or the amount in the volume will change. For example, you sip the one glass of wine or pint of beer you planned to drink in an evening, but every time you turn around the liquid is at the same level in your glass. What’s going on? Well, your friend keeps topping you up when you’re not looking, so they are the input, the drink into your body is the output, and the change in wine or beer in the glass is the change in storage.

This idea becomes a little bit more complicated when discussed water within the framework of the hydrologic cycle.

Perpetual Flow - Storage Components

When we consider this principle over a geographic region, such as a watershed, the inputs and outputs can be specified.

Inputs can be precipitation (P) or groundwater flow in (Gin).

Outputs can be evaporation or transpiration (together known as evapotranspiration) (ET), groundwater flow out (Gout), or river discharge (Q).

The change in storage (ΔS) can be that of changes in groundwater, rivers, or lakes.

The equation now looks like this:


As I mentioned above, groundwater moves very slowly so we usually assume that Gin and Gout are negligible (meaning they are small in relation to the other terms so we can basically ignore them). The equation now becomes even more simple:

P = ET + Q + ΔS

And there you have it! The basic building blocks of hydrology and the water balance that we will continue to use throughout this journey. Feel free to refer back to this post at any point if you feel confused or need a refresher. As I mentioned, I’ll be discussing both of these principles in more detail in later posts.

Works Cited

  1. Trenberth, K. E. (2006). The role of the oceans in climate. Flotsam and Jetsam , 35 (1), 5-7.

Great Reference Material or for more details:

Environment Canada. (2013, Sep 9). The Hydrologic Cycle. Retrieved Mar 14, 2015, from Environment Canada:

National Aeronautics and Space Administration (NASA). (n.d.). A Multi-Phased Journey. Retrieved Mar 14, 2015, from NASA Earth Observatory:

U.S. Geological Survey (USGS). (2014, Mar 18). The Water Cycle. Retrieved Mar 14, 2015, from USGS science for a changing world:

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