Water Resources

Chapter 9

Read these notes and the first and last parts part of Chapter 9 in your text.  For this course you are responsible for 4CE pages 240-256 and 265-270 (3CE pages 232-246 and 254-259).

We do the middle of the chapter,  “Groundwater Resources,” 4CE pp. 256-265 (3CE pp.246-254), in the other course :).

**There is a video version of this lecture here:  https://youtu.be/jNwSMtglNx4

**The exam is based on the content in these notes, so please print them off to study from.

Elihu says to Job:  “Look, God is exalted beyond what we can understand. His years are without number.  He draws up the water vapor and then distills it into rain.   The rain pours down from the clouds, and everyone benefits from it.   Can anyone really understand the spreading of the clouds and the thunder that rolls forth from heaven?   See how he spreads the lightning around him and how it lights up the depths of the sea.

By his mighty acts he governs the people, giving them food in abundance.   He fills his hands with lightning bolts.   He hurls each at its target.   The thunder announces his presence; the storm announces his indignant anger.”

 Job 36:26-33 NLT

Water is important!  After all, life would be impossible without it.  Humans, other mammals, and plants are all about 70% water.   We use water to produce food, bathe, cook, wash clothes, deal with waste, and for all sorts of industrial processes.  In Canada, we tend to take water for granted.  Tragically, at least 40% of the world’s population, or about three billion people, live in countries where it is difficult or impossible to get enough water to satisfy basic needs.  Over 1 billion people lack safe drinking water.  Almost 2.5 billion people lack adequate sanitation facilities (mostly in Africa).  Globally, 2 million people die annually as a direct result of lack of water; 5 million more die, indirectly, due to waterborne infections and diseases, mostly associated with inadequate sanitation.

Water is a renewable but finite resource.  Water is continually cycling from the atmosphere to the earth and back to the atmosphere, so in that sense it is a renewable resource.  However, there is the same amount of water now as when the Earth was formed.   So it is also a finite resource.  Most of Earth’s water originated from comets and hydrogen- and oxygen-laden space debris that coalesced to form the Earth.  A very small amount of water emerges from within the Earth’s crust each year, but an equivalent amount is lost each year as some water molecules dissociate and break down.   Essentially there is the same amount of water now on the Earth’s surface as 2 billion years ago.

Water is a renewable but finite resource.  Water is continually cycling from the atmosphere to the earth and back to the atmosphere, so in that sense it is a renewable resource.  However, there is the same amount of water now as when the Earth was formed.   So it is also a finite resource.  Most of Earth’s water originated from comets and hydrogen- and oxygen-laden space debris that coalesced to form the Earth.

Nearly three quarters of the world’s surface is covered in water (71%).  This is distribute unevenly.  There is much more continental mass (and thus less water) in the northern Hemisphere.  There is much more water in the Southern Hemisphere (Figure 9.2).  To mess with your mind, check out a map of the world with the Southern Hemisphere at the top; it realizes emphasizes how much more water there is in the global south than in the north.

Over 97% of the Earth’s water is contained in oceans.  That means that less than 3% of the world’s water supply is freshwater.  Two-thirds of this fresh water is “trapped” within icecaps and glaciers.  Almost 25% is stored in groundwater.  How much freshwater is actually available for human use in fresh water rivers and lakes (“renewable” water)?  Less than 0.5% of all water on the planet (See Figure 9.3a).

Unfortunately, human use fresh water faster than it is replenished through rainfall and snowmelt.  For direct human consumption, industrial processes, and agriculture, we use more fresh water than returns to the system.  So each year, there is a little less fresh water available.  In addition, population growth, ever-rising demand, and environmental pollutants are increasingly taxing the available water supply.  Water supply issues are critically important political, economic, and security issues in many parts of the world.

9 Water_Cycle_(v1.10)

The hydrologic cycle (footnote 1)

I. The Hydrologic Cycle

Water in the Earth-atmosphere system is continually in motion, transferring from oceans, lakes, streams, groundwater, atmospheric water vapour, precipitation, snow and ice, etc.  The hydrologic cycle refers to the whole system of water movement through the Earth-atmosphere system.

Water goes through all 3 states within the hydrologic cycle – solid, liquid, gas.  Water is the only compound on earth that exists in all 3 states naturally.

The water cycle can be divided into three main components:  atmosphere, surface, and subsurface.  In general, a water molecule spends the shortest amount of time in the atmosphere (about 10 days).  If it ends up in deep-ocean circulations, groundwater, or ice, it can spend 10s of thousands of years in that form.

A.  Inputs into the Global Hydrologic Cycle

1. Precipitation (P or PRECIP)

Precipitation can be measured using gauges.  However it can also be considered in terms of where the water which falls as precipitation goes.

Precipitation, when it reaches the earth’s surface moves into other aspects of the hydrologic cycle including the soil, plants, streams, rivers, and oceans.

  • Some precipitation will be evapotranspired (evaporated from and transpired by plants).
  • Some precipitation will runoff (directly flowing into streams, rivers, and lakes) as a surplus.
  • Some precipitation will soak into the ground as groundwater storage.

Precipitation is not distributed evenly, of course.  About 78% of precipitation occurs over the oceans, so it is not really available for human use.  Of the 22% that falls on land, its distribution is affected by global weather patterns and local topography.  Remember that in the ITCZ, a region of constant low pressure, there is lots of precipitation.  In subtropical high pressure areas, there is very little precipitation.

In our latitudes, with mid-latitude westerly winds, precipitation is strongly affected by those westerly winds and topography (Figure 9.7, “Precipitation in Canada …”).  Notice

  • the highest precipitation is on the west coast, as westerly winds bring moisture laden air off the Pacific Ocean
  • the highest precipitation is on the windward slope of the Coast/Interior/Rocky Mountains due to orographic lifting
  • there is moderate precipitation on the east coast due to low pressure systems from the Atlantic, and (inland) the effect of major lakes.
  • there is least precipitation on the leeward slopes of the Rockies and the continental interior.

2.  Other inputs

Some water also enters a given region as groundwater and runoff (often in the form of streams or rivers) from other regions.  These are often balanced by outputs of groundwater/runoff as well (thus ST or STRG represents the change in soil-water storage).  Normally input from groundwater and runoff is balanced by outputs of runoff and groundwater.

These represent inputs to the cycle.  In order for balance to be maintained, they must equal inputs …

B.  Outputs from the Hydrologic Cycle

  1. Evapotranspiration (AE, PE, D or POTET, ACTET, DEFIC)

The combined water loss from direct evaporation and the transpiration of plants is referred to as evapotranspiration

Evaporation is simply the process whereby water on the surface is evaporated into water vapour by energy (directly or indirectly) from the sun.  Evaporation will continue as long as there is moisture at the surface and the air is not yet saturated.

Since 97% of the Earth’s water is in the oceans, it’s not surprising that most evaporation occurs over the oceans (86%).  The rest of the evaporation occurs from lakes, rivers, groundwater, and plants (through evapotranspiration).

Transpiration occurs when stomata, small pores on plants, open under the influence of sunlight and release water from within the plant into the atmosphere by evaporation.  Plants use solar energy to evaporate water from their leaves.  This results in the leaves “drying out.”  As the leaves dry out, the plant draws up moisture from the soil through its roots and stem, into the leaves.  Water is drawn up through the plant from the roots.  This process transports nutrients throughout the plant.  This, in turn, feeds the plant.  The moisture which is drawn from the soil is rich in dissolved nutrients that the plant needs to survive.  Transpiration is the mechanism plants use to bring dissolved nutrients in the soil into their roots, and then throughout the plant, so they can survive.

Because evaporation and transpiration are intimately connected, they are usually grouped together and referred to as evapotranspiration.  Almost everywhere moisture is present for evaporation, plants are also present, so transpiration occurs.  And whenever plants transpire moisture, it must be evaporated.

The rate of evapotranspiration drops as surface soil water is reduced (there is less water for plants to use).  And the rate of evapotranspiration drops as  plants become moisture stressed (they wilt!) and close their stomata.  This is why you have to keep your plants watered or they die!  Evapotranspiration is also dependent on sunlight.  This is why your plants needs light!  Direct sunlight is not always necessary or desirable (depending on the plant), but every plant needs light to drive the evapotranspiration pump process in order to absorb nutrients from the soil!  If a plant does not have light, it does not evapotranspire … and it dies!

There are two aspects of evapotranspiration:

a. Potential evapotranspiration (PE or POTET) measures how much water the atmosphere could potentially remove from the surface assuming no limitation of water supply.  Given unlimited water supply, this is the potential amount of moisture that could be evapotranspired.  This is a theoretical figure.

PE is primarily related to the amount of insolation a location receives.  It therefore varies with latitude, day-length, cloudiness, and pollution.

  • PE is greatest in clear, hot weather in the tropics.
  • PE is least in cold, cloudy polar regions.
  • PE will be 0 during Arctic winters when there is no insolation.

In a daily basis, PE is minimal at night and greatest shortly after peak insolation in daytime.

Seasonally, PE is greatest in summer in mid-latitudes, and least in winter.

If you think this through a bit, the highest potential evapotranspiration will be in regions that receive a lot of insolation (remember earlier in the course).  For instance along the equator, there is much insolation all year round, therefore there would be much PE all year round.  In our region, we get much insolation in the summer, therefore we would have high PE in the summer.  However we receive little insolation in the winter; therefore lower PE in the winter.

Remember PE simply refers to potential evapotranspiration, given unlimited water resources.  The reality is that in many regions water is NOT unlimited!  Therefore the amount of water that actually evapotranspires might be much less!

b. Actual evapotranspiration (AE or ACTET) is the amount of water that is actually evaporated.

AE will only equal PE if there is enough water so that the atmosphere can constantly evaporate moisture.

If there is not enough water, the atmosphere will not be able to evaporate all the moisture it could.  PE (the potential amount the atmosphere could evaporate) would exceed AE (the amount actually evaporated).  In this case, some solar energy which could be used to evapotranspire moisture is “wasted.”

In reality, PE often exceeds the actual water supply (AE) during the hottest times of the day and year.  There is either not enough water to achieve maximum potential evapotranspiration.  Or water cannot be evapotranspired fast enough.  For instance … think of the Arizona/Nevada desert in summer.  There is a tremendous PE:  lots of insolation, clear skies, etc.  But there is no water!  Therefore, AE will be very small.  The same is true of summer conditions in the Canadian Prairies:  PE is high, but AE is relatively small because there simply is not enough water.

For instance, see Figure 9.8, “Potential evapotranspiration …”  Compare this with Figure 9.7.

  • The highest PE is in SE California and SW Texas, but this area receives very little rainfall.  PE will really exceed AE.
  • Although PE is not really high in the Canadian Prairies (during the winter, the insolation is modest), there is also very little P, meaning these areas can experience drought easily.
  • SW BC actually has fairly high PE, but also receives lots of P, so it is less drought prone.

Note that AE may be less than or equal to PE.  But AE can never exceed PE (PE is the maximum potential evapotranspiration that could possibly occur).  It is impossible for more actual evapotranspiration (AE) to occur than the amount of evaporation that is possible (PE).

When PE exceeds AE, the difference is called a deficit (D or DEFIC).  The deficit represents the water that could have been evapotranspired by the insolation, but which was not evapotranspired because the water supply was limited.  This can be thought of as “excess” or “wasted” energy.  It is energy that could potentially have been used for evapotranspiration, but a limited water supply prohibited the maximum possible evapotranspiration.

2. Change in Soil Water Storage (+/- ST or STRG)

The amount of soil water in a region can change throughout the year.

a. When the amount of precipitation is large enough so that more rain falls than can be evaporated (it’s really wet!) (P > PE and, thus AE), the amount of water stored in the ground will increase.  This is groundwater recharge.  This means that the water will stored in the soil (+ST).

In our region this typically happens through late fall until mid spring. You may notice the ground seems to get wetter and muddier during this time of year.  The water is absorbing more and more water (both from precipitation and melt water runoff).  The amount of water stored in the ground is increasing.

If the ground is saturated, the extra water will run off on the surface.  Every soil has a maximum amount of water it can hold.  Once it is saturated, the extra water runs off.  Different soils have different moisture storage capacities:  these are determined by the composition of the soil (Chapter 18).  In general, soils with a high clay and low sand content can hold more moisture than soils with a low clay and high sand content.

b. If the amount of water being evaporated from the soil exceeds the amount of incoming precipitation (it’s relatively dry!) (AE > P), the result is that the soil dries out.  The amount of water in the soil decreases.  Soil water is evaporated by the sun faster than it is recharged either by precipitation or by melt water runoff.  In this case soil water storage decreases (-ST).

In our region this typically happens from mid spring through mid fall.  You may notice the ground gets drier and drier during this time of year.  The sun is shining, “sucking out” moisture through evapotranspiration, but there is not enough water in the soil to keep the plants healthy.  In this condition plants may experience “moisture stress.”    Ultimately plants may not be able to continue to evapotranspire at all because they cannot obtain water through their roots.  The result, of course, is wilting, lower plant productivity, and ultimately death.  To prevent this we add groundwater by sprinkling.  Irrigation is one attempt to increase this ST component to prevent soil reaching the wilting point.

Drought is a term that simply refers to a situation where high PE and low P lead to dry conditions over an extended period of time.  (Don’t worry about the four kinds of drought in your text).  Drought is a natural, recurring experience in many parts of the world, like the SW USA and Australia.  However drought-prone regions are experiencing longer periods of, and more intense droughts than ever before, lasting decades rather than simply years, as previously.  The past two decades in the US SW have been the driest or the second driest in the last 1,200 years, posing existential questions about how to secure a livable future in the region.  This has been directed linked to effects of climate change (Chapter 11).

In Canada, the Prairies are susceptible to drought as well, especially due to climate change (see the opening of Chapter 9, “Water Resources and Climate Change in the Prairies”).  Climate change is resulting in less annual precipitation, diminishing glaciers in the Rocky Mountains (a key source of freshwater), and increasing temperatures.  These factors, combined with more water usage for agricultural, industrial, and urban uses is threatening the long-term health of the Prairie water system.  Real-time drought updates for Canada are found here

  •  Canadian Drought Outlook (A monthly drought forecast; the map predicts if drought conditions across Canada will develop, stay the same or improve for the end of the following month)
  •  Canadian Drought Monitor (official source for the monitoring and reporting of drought in Canada)

Soil which is saturated (already holds as much water as it possibly can) or impermeable (no air spaces to allow water in) causes a surplus (S).  When rain falls, the water can not soak into the soil.  Therefore it runs off the surface into streams, rivers, or lakes.

 III. Soil Water Budgets

All this information can be used to create a soil water budget for a particular location.  A soil-water balance equation can be created for any local area to indicate this (Figure 9.10):

P = AE + S +/- ST

P: is precipitation (how much water comes into the area from the atmosphere)

AE: is actual evapotranspiration (water which returns directly to the atmosphere through evaporation or transpiration by plants within the area)

S: is water surplus (water which runs off into streams/rivers and leaves the area)

+/- ST: is change in soil-water storage (water which is added to or removed from the soil)

So … if 100 mm of rain falls on your farm, you can track what happens to it. 

  • Some (let’s say 30 mm) is evapotranspired directly back into the atmosphere. 
  • Some (let’s say 30 mm) soaks into the ground and recharges the groundwater in your drought parched fields. 
  • Some (say 40 mm) runs off into the local stream, and runs off to the river. 
  • Thus all 100 mm are accounted for!

        P (100 mm) = AE (30 mm) + ST (30 mm) + S (40 mm)

Notice the equation balances (100 mm on each side).  Thus we can account for all the moisture on your farm!

AE (actual evapotranspiration) can be further broken down into PE (potential evapotranspiration) and D (a moisture deficit) … see Figure 9.10, and below:

AE = PE – D

PE is potential evapotranspiration (how much water could be evapotranspired by the incoming solar radiation, if there were an unlimited  supply of water)

D is soil moisture deficit (often there is not enough water in the area to allow for maximum evapotranspiration (PE).  Therefore, there is a “deficit” … the amount of incoming solar radiation that is “wasted” because there is not enough water (either P or groundwater storage (ST)) in the area to be evapotranspired.

So … back to your farm.  30 mm is actually evapotranspired (AE) directly into the atmosphere.  This comes from water evaporated from puddles, ponds, etc. and transpired by plants.  (During the summer, however, the hot sun has the potential to evapotranspire more than 30 mm!  It could potentially evapotranspire 40 mm (my hypothetical number): PE.  However there is not enough water to provide all 40 mm.  This leaves a deficit of 10 mm that could potentially be evapotranspired but is not, because there is not enough moisture in the system:  D.

         AE (30 mm) = PE (40 mm) – D (10 mm)

Notice the equation balances (30 mm on both sides)

  • Study the Sample water budgets in your text:  4CE figure 9.11 / 3CE figure 9.9, “Sample water budget,” and 4CE figure 9.12 / 3CE figure 9.10, “Sample water budgets for selected locations.”

Here’s a more realistic example (try to follow along.  I will NOT ask you anything so compicated – pr mathematical – on the exam):

Consider this hypothetical location, we’ll call Sillyville:

  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
P 50 40 30 20 20 10 10 20 20 30 40 50
PE 10 10 20 30 40 50 50 40 30 20 10 10
AE 10 10 20 30 40 40 30 20 10 20 10 10


  • Sillyville has more precipitation in the winter than summer.
  • PE is highest during the summer.  This is what we would expect in the Northern Hemisphere. During the summer the Northern Hemisphere is tilted toward the sun.  The days are longer than in the winter; the sun is more intense than in the winter.  More insolation is received in the summer than in the winter.  Thus, more potential evapotranspiration could occur in summer than winter.
  • AE is the same as PE from January until May, and October until December.  During January, for instance, the sun could potentially evaporate 10 mm of water, and actually does evaporate 10 mm. There is enough moisture from precipitation to allow maximum potential evapotranspiration to actually occur:  PE = AE.
  • From June through September, more energy is available to evapotranspire moisture (PE) than moisture is available to be evapotranspired (AE).  During June, for instance, the sun could potentially evaporate 50 mm of water, but it only able to actually evaporate 40 mm.  There is not enough precipitation and groundwater to provide all 50 mm to the atmosphere.

This difference between PE and AE during these months is the deficit (D):  energy that is essentially “wasted” – it could have been used for evapotranspiration, but there was not enough moisture.

  1. Calculating D (PE – AE)

The next step in creating a soil moisture budget is to calculate the deficit (D).  The deficit refers to the “wasted” solar energy – the amount of water that potentially could have been evaporated but was NOT actually evaporated, because there was not enough water in the system.

D = PE – AE:

  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
P 50 40 30 20 20 10 10 20 20 30 40 50
PE 10 10 20 30 40 50 50 40 30 20 10 10
AE 10 10 20 30 40 40 30 20 10 10 10 10
D 0 0 0 0 0 10 20 20 20 10 0 0
  • Note that from October through May, D is 0: as much moisture as could potentially be evaporated is actually evaporated (PE = AE).
  • From June to September, PE exceeds AE.  There is a deficit.  The deficit is calculated simply by subtracting AE from PE.

2. Calculating Soil Moisture Utilization (-ST)

We can also determine soil moisture withrawal (-ST).  In months where the amount of actual evapotranspiration (AE) exceeds the amount of precipitation (P) – more moisture is evapotranspired than falls as precipitation – the additional moisture that is evaporated must come from somewhere.  If more water is being evaporated than falling as rain, moisture must be coming from a source other than rainfall.  It is, in fact, coming from the ground.

This extra moisture represents a reduction in groundwater storage (-ST).  The ground is drying out.

For instance, in April, AE is 30 mm. P is only 20 mm.  Therefore 10 mm must come from groundwater (-ST).  There is nowhere else for it to come from.

So, in months where AE is greater than P … -ST = AE – P

  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
P 50 40 30 20 20 10 10 20 20 30 40 50
PE 10 10 20 30 40 50 50 40 30 20 10 10
AE 10 10 20 30 40 40 30 20 10 10 10 10
D 0 0 0 0 0 10 20 20 20 10 0 0
-ST 0 0 0 10 20 30 20 0 0 0 0 0
  • Note that there is only –ST during the months where AE > P.  This means that groundwater storage is only reduced when more water actually evapotranspires than falls as precipitation; the extra moisture evapotranspired comes from the ground.

For instance, in May, 40 mm is evapotranspire, but only 20 mm falls as precipitation.  The other 20 mm has to come from the ground.

  • Months were P is greater than or equal to AE, there is no –ST.  There is plenty of precipitation to evapotranspire, so no moisture needs to be drawn out of the ground.

For instance, in January, 50 mm of precipitation falls, but only 10 mm is actually evapotranspired.  No moisture needs to be drawn out of the ground; there is plenty of precipitation to provide the 10 mm to be evaporated.

  • Note that the total –ST for the year is 80 mm.  (10 + 20 + 30 + 20)

3. Calculating Soil Moisture Recharge (+ST)

We just calculated that, over the course of the year, 80 mm of water is removed from the soil by evapotranspiration.

Annual  –ST  = 80 mm.

Over the course of a year, the same amount of water has to be added back into the soil, in order for the system to be stable.  Otherwise the soil would become drier and drier, year after year.  Of course, this is happening in some areas – that’s very problematic for the long-term health of a region.

So, for a region to remain balanced … +ST (increase in groundwater storage) must also equal 80 mm for the year.  This will balance the equation for the year.

For the year,  +ST must equal –ST.

Otherwise the system gets out of balance and the soil dries out.  Crisis.

Look at September.  In September, more rain falls as precipitation (P) than is actually evapotranspired (AE):  P > AE.  20 mm falls, only 10 mm is evapotranspired.  Where does this extra rain go?  Into the soil, which has been drying out all summer, as groundwater storage.  Through the summer the soil moisture had been removed, because more and more water was evapotranspired and very little fell as rain.  In September, more is falling as rain than is being evapotranspired.  The extra can begin to recharge the groundwater storage.

The extra rainfall for the next few months will recharge the groundwater storage, up to a total of 80 mm – the amount withdrawn during the summer as –ST.  Once 80 mm has fallen and soaked into the soil, the ground is saturated, and no more water can go into it as +ST.

So, from September on we’ll calculate +ST as the excess rainfall over and above what evaporates as evapotranspiration.

+ST = P – AE

However we need to keep a running total of this +ST, because once it equals 80 mm (the total removed the previous summer), the ground will be saturated.  No more can be stored in the soil.

  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
P 50 40 30 20 20 10 10 20 20 30 30 40
PE 10 10 20 30 40 50 50 40 30 20 10 10
AE 10 10 20 30 40 40 30 20 10 10 10 10
D 0 0 0 0 0 10 20 20 20 10 0 0
-ST 0 0 0 10 20 30 20 0 0 0 0 0
+ST                 10 20 20 30
  • Note that by December we have reached a total of 80 mm (10 + 20 + 20 + 30).  Thus all the excess rainfall from September through December goes to recharge groundwater storage.
  • At the end of December, however, the groundwater is completely recharged.  The amount put back in (+ST) is now equal to the amount removed the previous summer (-ST).  The ground is now saturated.  No further moisture can be added to it.
  • Therefore all the other months will have a +ST of 0.
  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
P 50 40 30 20 20 10 10 20 20 30 30 40
PE 10 10 20 30 40 50 50 40 30 20 10 10
AE 10 10 20 30 40 40 30 20 10 10 10 10
D 0 0 0 0 0 10 20 20 20 10 0 0
-ST 0 0 0 10 20 30 20 0 0 0 0 0
+ST 0 0 0 0 0 0 0 0 10 20 20 30
  • Note that more rain falls in January (P = 50) than evaporates (AE = 10), but the ground is already saturated.  This extra moisture cannot go into the ground.  The ground is holding as much as it possibly can.  For January, +ST = 0.  The same is true for February and March.

4. Calculating Moisture Oversupply or Surplus (S)

Once the groundwater storage has been completely recharged (+ST = -ST), the excess rain that falls must go somewhere.  It runs off as a surplus (S).  It will run off into a river, a lake, or the ocean.  That is the only possibility.

In January, for instance, when P = 50 and AE = 10, the extra 40 mm cannot go into the ground because the ground is saturated.  That 40 mm simply runs off as a surplus (S).

  Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
P 50 40 30 20 20 10 10 20 20 30 30 40
PE 10 10 20 30 40 50 50 40 30 20 10 10
AE 10 10 20 30 40 40 30 20 10 10 10 10
D 0 0 0 0 0 10 20 20 20 10 0 0
-ST 0 0 0 10 20 30 20 0 0 0 0 0
+ST 0 0 0 0 0 0 0 0 10 20 20 30
S 40 30 10 0 0 0 0 0 0 0 0 0
  • Note that S only occurs after the annual groundwater storage (+ST) has become equal to annual groundwater loss (-ST).
  • As soon as AE is equal to or greater than P (all the rain is evaporated), there is no longer and surplus (From April on, all the rain is evapotranspired).

We can now put this information for Sillyville onto a graph … See my Water Budget Graph page (PDF)

III.   “Real” Water Budgets

Why did we walk through this whole exercise???  So that, hopefully, “real” water budgets make more sense.

Consider Figure 9.11, “Sample water budget for Hamilton”

  • Precipitation (P) is pretty constant all year.
  • PE and AE are pretty similar.  PE is a little bit larger than AE only in June through September.  This means there is a deficit (D) in these months. 
  • From June to September, AE exceeds P, resulting in removal of soil moisture (-ST).
  • In October-November there is soil moisture recharge (+ST) to balance the -ST through the summer.
  • From December through May, the extra precipitation (P) is surplus (the ground in saturated).  This is either temporarily stored as snow/ice (and will runoff when it melts), or runs off immediately.

Consider Figure 9.12a, “Sample water budget for Vancouver”

  • Precipitation (P) is highest through the winter and least through the summer.
  • PE and AE are pretty similar.  PE is a little bit larger than AE only in June through September.  This means there is a deficit (D) in these months. 
  • From June to September, AE exceeds P, resulting in removal of soil moisture (-ST).
  • In September-November there is soil moisture recharge (+ST) to balance the -ST through the summer.
  • From November through May, the extra precipitation (P) is surplus (the ground in saturated).  With Vancouver’s milder climate, this runs off immediately.  There is a LOT of run off through the winter/spring months!

Consider Figure 9.12b, “Sample water budget for Phoenix”

  • Precipitation (P) is low all year – almost non-existent in May/June.
  • PE is REALLY high in the summer (why would this be?  think of the weather in Arizona in the summer …).  AE is really low all year.  There is a little bit in spring, as a little bit of soil moisture was added through the winter.  There is a HUGE deficit (D) from February through November. 
  • From February through April, AE exceeds P, resulting in removal of what little soil moisture there is (-ST).
  • In December and January there is soil moisture recharge (+ST) to balance the -ST through the summer.
  • There is NEVER any surplus or runoff (S) in Phoenix!

IV. Surface Water

Review Figure 9.3a.  The fresh water on the Earth’s surface is found primarily in snow and ice (99.4%), the balance (0.6%) being in lakes, streams, and wetlands.  Figure 9.13 maps the world’s major fresh water lakes, rivers, and wetlands.  Note they are not evenly distributed.  Some regions have a lot (including Canada).  Some have very little (North Africa, the Middle East, Central Asia, Australia).

A. Snow and Ice

As mentioned, 99.4% of the surface fresh water is store as snow and ice, especially in the Arctic and Antarctic.  In more temperate areas, like southern Canada, water stored by annual snowfall and glaciers are important water sources.  The Rocky Mountain annual snowpack, combined with semi-permanent glaciers, are essential water reservoirs for all four western provinces.  The Himalaya snowpack and glaciers supply water to almost 50% of the Earth’s population (in China, India, Bangladesh, Pakistan, etc.  Again, climate change, which is dramatically reducing the glacial mass, may have dramatic effects on fresh water supplies.

B. Rivers and Lakes

Lakes, fed by precipitation, rivers, and groundwater, are important reservoirs of fresh water.  Combined, lakes store the most non-frozen fresh water on the surface.   Over 50% of this is stored in only 7 lakes (Figure 9.13).  The largest – by far – is Lake Baikal (Russia), larger than all five Great Lakes combined.  Of course, climate change is an issue (read the text for some of the challenges – you don’t need to know specifics for the exam).

Humans create lakes, through dams.  Dams are used both to store freshwater and to generate hydroelectric power.  Hydroelectric power supplies almost 20% of the world’s electricity needs and is the most widely used source of renewable energy.  However, because it depends on precipitation, hydro power only is possible in areas with adequate water supply.  And it can vary, annually, based on precipitation.  Dams, of course, are not benign.  Damming rivers floods massive amounts of territory, disrupts aquatic systems, and (especially in earthquake-prone regions), can produce risk hazards.

V. Groundwater Resources

For this course you are not responsible for the middle of the chapter, “Groundwater Resources” [4CE pp. 256-265 / 3CE pp.246-254]. We deal with groundwater in the other course.  You are responsible for the section “Our Water Supply,” 4CE pp. 265-270/3CE pp. 254-259.

However, be aware that a substantial amount of fresh water is stored as, and moves through the natural environment as groundwater.  For us, in this course, it is good to remember that groundwater is directly linked to precipitation.  Groundwater is recharged when P exceeds AE.

Groundwater provides about 80% of the world’s irrigation water and nearly 50% of the Earth’s drinking water.  In Canada it is a key source of fresh water (providing 100% of PEI’s fresh water supply, for instance).

Of course, if more groundwater is used (for both evaporation and human consumption) than is recharged, annually, the long-term health of the groundwater system is not sustainable.  More of that in the other course …

VI. Our Water Supply

Humans use water in all sorts of ways!

Currently humans use 30% of all the runoff (water flowing in streams and rivers) that is accessible.  Some runoff is too remote to access practically.  For instance, most of the Amazon river — the largest river in the world in terms of runoff — is too remote to be useful.  It is not near the major population centers of the world (which are located in China and India).

50% of all runoff occurs in the form of floods, which is largely unusable.

Therefore, only about 20% of runoff is still available for human use before runoff is 100% accounted for.  Of that 20%, we cannot practically or possibly use all of it.  If we were to use all of this 20%, rivers and streams would essentially cease to flow.

It is easy to assume that Canada has an almost endless supply of clean, fresh water.  Canada has 20% of the world’s total freshwater resources.  However, less than half of this water – about 9% of the global supply – is “renewable” (that is, usable).  13% is “fossil” (non-renewable) water retained in lakes, underground aquifers, and glaciers.  For Canada’s 30 million people – about half a percent of the world’s population – this is still a generous endowment.  But, more than half of this water drains northward into the Arctic Ocean and Hudson Bay.  As a result, it is unavailable to the 90% of the Canadian population who live within 300 kilometers of the country’s southern border.  That means the remaining supply, while still abundant, is heavily used and often overly stressed (see Figure 9.18).

In some river systems (for instance the Bow-Oldman-South Saskatchewan River system in southern Alberta) as much as 80% of the runoff is used for human and agricultural (irrigation) use.  That means that less than 20% is unused.

Some water use is instream use – water is not removed from the river at all, but used for human purposes:  hydroelectric generation, waste treatment and removal, transportation, recreation.

Some water use is nonconsumptive use – water is removed, some is consumed, but some is also returned:  for instance agriculture, industrial uses, domestic uses (your shower and toilet).

Some water use is consumptive use – water is removed and is consumed, not returned:  water vaporized in steam electric plants, bottled, or exported.

Even though water may be still in the system through instream and noncunsumptive uses, the amount of water may be reduced.  And the quality of the water may be dramatically altered by human uses.  For instance, only some water used for agriculture will be returned to the stream system (some will evaporate, some be consumed by plants, some consumed by animals).  Of the water that does return to stream, it may be contaminated by chemicals from fertilizers or animal waste.  Downstream communities inherit less water than upstream cities … and water of much poorer quality.

If you have a choice, live as far upstream (close to the source) as possible.  And ensure your community has good water quality testing and treatment!  If your water is supplied from a well, have the water quality tested regularly.  Groundwater is easily contaminated by fertilizers, animal waste, and other noxious chemicals.  As you may learn (or already have learned) in the other course, groundwater travels.  Pollutants from one area can easily travel to your well.

See Water pollution

Water supply issues are becoming big news items — consider the problems and issues in recent years:

  • Droughts in Canada Page.  Climate models project future increases to temperature and in general, small increases to precipitation over southern Canada. This translates into future increases of summer continental interior drying and associated risk of droughts. The increased drought risk is attributed to a combination of increased temperature and potential evapotranspiration not being balanced by precipitation.
  • The western US has been in a 20+ year drought … or simply a new, much drier climate reality. See Drought.gov (US Integrated Drought Information System) and Drought is here to stay in the Western U.S. How will states adapt?
  • Nexen pipeline spills 5 million litres of emulsion near Fort McMurray | CTV News
  • Harbin, China 2005 (benzene spill suspended water supply for millions in China and Russia)
  • the remote northern Ontario reserve of Kashechewan (e coli contaminated water supply led to evacuation of town in 2005)
  • Walkerton, ON (2000), 7 people died due to e coli contamination in the town water supply.
  • In developing countries, over 80% of industrial waste is released, untreated, into fresh water systems, contaminating the water supply, leading to a multitude of problems …
  • Canadians use the second most amount of water per capita in the world for domestic (home, not industrial) use (only Americans use more):  we use 1,650 cubic metres of water per person per year (picture that — that’s a line of 1m X 1m X 1m cubes, lined up for 1.65 kilometres, for EACH person in Canada per year!!!).  This is more than 2 times as much water as the average person from France, 3 times as much as the average German, almost 4 times as much as the average Swede, and more than 8 times as much as the average Dane.
  • In terms of total water consumption (not per capita, but total overall water use) Canada is fourth in the industrialized world:  only the United States, Japan and Mexico use more water, in total, than Canada.
  • Since 1980, overall water use in Canada has increased by 25.7%. This is five times higher than the overall average increase of 4.5% among industrialized nations.  In contrast, nine industrial nations were able to decrease their overall water use since 1980 (Sweden, the Netherlands, the United States, the United Kingdom, the Czech Republic, Luxembourg, Poland, Finland and Denmark).

As you read this section, reflect on how you can:

  •  reduce your water consumption
  •  reduce your water pollution

There are three golden rules of water conservation – reduce, repair, and retrofit:.

  • Reduce:  It’s surprising how much water gets wasted. We just let it run down the drain. Become conscious of the amount of water you’re using and look for ways to use less whenever you can.
  • Repair:  A leak of one drop per second wastes 10 000 litres of water a year. Most leaks are simple to find and easily fixed, at low or no cost.
  • Retrofit:  Retrofit means adapting or replacing an older, less water efficient fixture or appliance with one of the many water saving devices now on the market.

Some practical ideas:

At work/school

  • Turn off all taps when not in use
  • Report leaking taps or toilets
  • Encourage the us of water-efficient devices
  • Place rubbish in bins and not down the sink or toilet
  • Ask questions about things like landscaping choices, utility use, etc.

At home

  • Fix leaking taps and toilets
  • Install a water-efficient showerhead and an aerator for taps
  • Take shorter showers – most use 10-20 litres of water per minute (take showers, NOT baths!)
  • Replace your toilet with a low flow/dual flush toilet
  • Turn the tap off when brushing your teeth, shaving, cleaning dishes or preparing food except when you need the water running
  • Do not disposing of solvents, cleaners or chemicals down drains.
  • Keep a jug of water in the fridge rather than letting the tap run to get cold water for each drink.
  • Make sure your dishwasher and washing machine are completely full before starting them – dishwashers use about 35 litres of water each run, whilst washing machines consume about 120 litres of water for a full load of laundry
  • Sweep paths and driveways with a broom, not with a hose
  • Use mulch to retain water in the soil
  • Plant native grasses for lawns – they require much less water and mowing
  • Don’t over-water your lawn. Use an automatic timer to avoid puddles and runoff
  • Water your garden during the early morning, to prevent evaporation
  • Wash the car on the lawn (not on the street or driveway) using a bucket, not a hose (use environmentally friendly detergent, don’t use harsh chemicals!
  • Shovel the snow from your driveway/sidewalk onto your lawn/garden … it will recharge the soil moisture!

Some helpful sites:

Water is a precious resource:  people in long- and short-term relief and development work know that water is often one of the most critical factors in community development.

To reflect on …

Taking care of the environment is one of the most important things we can do, says astronomer Jennifer Wiseman (director of NASA’s Hubble Space Telescope programme), and is something that Christians in particular should be concerned about.  Where do you science and your faith lead you in terms of stewardship of the environment? – YouTube

A brief video, “Science involves faith” with Michael, PhD in enhanced oil recovery.

To review …

Remember you are not responsible for the groundwater sections

Check out the resources at www.masteringgeography.com

This page is the intellectual property of the author, Bruce Martin, and is copyrighted by Bruce Martin.  This page may be copied or printed only for educational purposes by students registered in courses taught by Dr. Bruce Martin.  Any other use constitutes a criminal offence.

Scripture quotations marked (NLT) are taken from the Holy Bible, New Living Translation, copyright © 1996. Used by permission of Tyndale House Publishers, Inc., Wheaton, Illinois 60189. All rights reserved

  1. Water Cycle: By Ehud Tal (Own work) [CC BY-SA 4.0 (http://creativecommons.org/licenses/by-sa/4.0)], via Wikimedia Commons