Lab 08: Surface Water Balance

Gillian Krezoski

Where precipitation goes once it reaches the Earth’s surface can be studied using a water budget, or surface water balance. Similar to a bank account, water can be ‘deposited’ and ‘stored’ in the soil profile as soil water storage. Some of the water flows overland to become surface water if the soil water ‘account’ becomes too full. Sometimes additional expenses like environmental use through evapotranspiration can remove some of the water from the ‘bank’, and in some locations where usage is high, could create a deficit. This lab will use a simple bucket model to examine soil surface water budgets.

Learning Objectives

 

After completion of this lab, you will be able to:

  • Calculate a simple soil surface water budget.
  • Understand how water is distributed and utilized at the Earth’s surface.
  • Learn about on-line sources for climate data (Environment and Climate Change Canada).
  • Learn about local evapotranspiration resources for agricultural purposes (Farmwest.com).
  • Begin to hypothesize how water resources could be impacted by climate changes.

Pre-Readings

The Water Cycle

Environment and Climate Change Canada. (2013-09-09) Water basics: the hydrologic cycle. Accessed 2020-06-26.  https://www.canada.ca/en/environment-climate-change/services/water-overview/basics/hydrologic-cycle.html

Water Budgets

Most physical geography introductory textbooks have a segment discussing water budgets. For example, Christopherson et al. (2019) includes a section on Water Budgets and Resource Analysis.

The Bucket Methodology

The monthly water ‘bucket’ methodology was first used in the 1940s and 1950s to improve our understanding of the relationship between climate and agriculture (e.g. to determine irrigation needs). It has been applied in many different ways over the years and although modern models have many more features, a simple ‘bucket’ model still forms the core of many climate modeling concepts (Figure 1).

Figure 8.1. A simplified bucket model. Figure courtest of J. Feddema
  1. Imagine a point on the ground surface of Earth as a bucket (Figure 1). As it rains (precipitation, P), water soaks into the soil becoming soil water storage (ST). This soil water begins to fill the bucket.
  2. The addition of water to the bucket from precipitation is balanced out by evapotranspiration, the name we give to the amount of water that is evaporated (turned from liquid to vapor) or transpired (drawn up by plants and evaporated from their leaves) from all elements of the environment (plants, animals, soil, etc.). This is called utilization.
  3. If there is more precipitation than evapotranspiration, the bucket will fill. Once there is no more room for water to go (i.e. the bucket is full), the bucket “overflows” and any more precipitation becomes surplus (S). This water will run off to streams and flow out of the area.
  4. We can further classify evapotranspiration into two types: actual evapotranspiration (AE), the actual amount of water that is evapotranspired, and potential evapotranspiration (PE), the amount of water that could be evapotranspired at the current climate conditions if there was ample water available in the bucket.

For example, in a dry, hot and windy desert the potential for evapotranspiration is quite high. However, in reality, there is very little water available in the soil, or from precipitation, and therefore the actual evapotranspiration is quite low.

Another example that might be easier to understand is a refrigerator. Potential is related to how hungry you are, but actual is related to how much food you have available in the refrigerator to eat.

  1. The starting point for a water budget is therefore to compare the inputs (P) to the potential demand (PE).  If there is lots of precipitation, or lots of water in storage, then the actual evaporation (AE) might be able to keep up to the potential evaporation.

However, if the precipitation is low, or the soil water storage is low, then there may not be enough water available for the plants to use and AE might be lower than PE.  This can create a water deficit (D) in the environment, where the environment does not have enough water coming in, or contained in storage, to keep up with demand.

Once there is a deficit, plants and the environment are not able to satisfy all of the demands they have for water, which will create stress.  A moisture deficit can be a major limiting factor to most biomes (i.e. plant and animal survival).

  1. The deficit has to be made up with precipitation before the bucket begins to fill up again (soil water recharge).

Example Water Bucket Water Balance

A simplified sample annual water budget of the South Okanagan at elevation (near Penticton and Osoyoos) is included below (Figure 8.2, based on Table 8.1). In this case, the amounts of precipitation, evapotranspiration, surplus and storage are all expressed in mm. The size of the bucket, or the Water Holding Capacity (WHC) has been set to be 150 mm of storage available in the soil column.

The winter months see some rainfall and mostly snowfall (P). During this period, the bucket is filling up during a recharge phase, which is expressed in Table 1 as storage (ST). Surplus (S) water escapes via stream systems rapidly in the late spring, often causing flooding.

One should note the South Okanagan, especially at elevation, is a bit more complex than the table demonstrates due to snowmelt occurring in the late spring. Since that snow already fell as precipitation earlier in the year, it is not accounted for in this simple soil water balance exercise. In reality, the surplus in the spring is much higher due to the snowmelt.

The South Okanagan is a high-latitude desert so evapotranspiration needs (PE) are quite high during summer. As the summer gets warmer, the soil water storage (ST) is gradually depleted, actual evapotranspiration (AE) decreases, and a deficit (D) is created.

Vegetation native to the area experiences a deficit all summer and has strategies to cope to survive the low water conditions until some rain appears in the fall.  However agricultural vegetation has been increasing in the South Okanagan, with fruit trees and grape vines becoming common crops. These plants are not adapted to the naturally dry environment and would be permanently harmed by the natural summer water deficits. Therefore, farmers supply the plants with extra water through irrigation from reservoirs. This supplementary water is used to fulfill agricultural needs and prevent soil water deficits that could harm the crops until the winter recharge can occur.

Figure 8.2. Simplified modeled water balance of the South Okanagan at elevation based on Table 1. 119.5W, 49.5N, curve C. Bucket capacity (water-holding capacity or WHC) is 150mm. PE- Potential Evapotranspiration, P – Precipitation, AE – Actual Evapotranspiration. Figure courtesy of G. Krezoski
Table 8.1: Modeled water balance of the South Okanagan. All units in mm unless otherwise indicated. WHC = 150mm. *Note: snowmelt not included in this table. See supplemental materials for more information on variable definition and how to  calculate values. Monthly mean Temp, PE, and P values sourced from webWIMP. Table courtesy of G. Krezoski
Month Temp (°C) PE P P – PE (DIFF) ST ST AE D S
Jan -10.4 0 36 36 109 36 0 0 0
Feb -6.5 0 28 28 137 28 0 0 0
Mar -2.2 0 23 23 150 13 0 0 10*
Apr 2.7 25 27 2 150 0 25 0 2*
May 7.3 64 36 -28 122 -28 64 0 0
Jun 11.1 92 46 -46 76 -46 92 0 0
Jul 14.3 113 30 -83 0 -76 106 7 0
Aug 13.7 99 27 -72 0 0 27 72 0
Sept 9.0 59 20 -39 0 0 20 39 0
Oct 3.0 22 22 0 0 0 22 0 0
Nov -4.3 0 33 33 33 33 0 0 0
Dec -8.6 0 40 40 73 40 0 0 0
Total 474 368 356 118 12

Lab Exercises

  1. You will look at Environment Canada Climate Normals data (30-year datasets) to determine average local annual ‘inputs’ into your account (precipitation).
  2. You will examine local evapotranspiration (an ‘output’ from your water budget) using Farmwest.ca, an agricultural resource used by farmers to plan for irrigation needs in times of deficits.
  3. You will complete a modeled water budget local to your (or your school’s) area to determine your ‘outputs’, or where your water goes once it reaches the Earth’s surface:
    • in the ‘bucket’ as soil water storage,
    • as surplus (to become or surface water flows),
    • ‘utilized’ by the environment via evapotranspiration.
  4. Finally, you will create your own water budget graph, similar to Figure 8.2, above.

EX1: Determining Climate Normals

Step 1: Visit the Environment and Climate Change Canada ‘Canadian Climate Normals’ webpage (1981-2010 data).

Step 2: Your instructor will assign you a station name local to the area you are studying. Navigate to the station name.

Station Name: ________________________________________________________

Step 3: Scroll down and you will observe a ‘climograph’ where average monthly temperatures (lines) and average monthly precipitation (bars) are plotted together over the course of a year using climate normal data (30-year data-set from 1981-2010).

Step 4: Take a screen capture of the climograph for use in your reflection questions, below.

Step 5: Click on the ‘View Data Table’ tab in the lower portion of your screen. The data that is used to plot the climograph is included here. You might want to refer to this data in your answers if you cannot determine numbers from the climograph on your screen.

Once complete, answer the following questions:

  1. Briefly describe quarterly (Jan-Mar, Apr-Jun, Jul-Sept, Oct-Dec) precipitation patterns in your study location. Be sure to mention predominant precipitation forms (snow vs rain). Also include the following:
    1. Which is the driest month.
    2. Which is the wettest?
  2. Briefly describe quarterly (Jan-Mar, Apr-Jun, Jul-Sept, Oct-Dec) temperature patterns in your study location.
    1. Which is the hottest month?
    2. Which is the coolest month
    3. Approximately during which month(s) do temperatures start warming enough to melt snow?

EX2: Examining Evapotranspiration

Now that we are familiar with seasonal precipitation and temperature patterns in your study area, we will examine environmental evapotranspiration needs. Evaporation indicates movement of water way from a wet surface into a dry atmosphere. Transpiration is where plants release water from their leaves into the atmosphere that has made it through their root/trunk/branch system. On a hot day, trees transpire more than cold days (tree sweat!). Once the growing season starts (typically after freezing temperatures cease), plant ‘needs’ in the environment increase and evapotranspiration increases. Growing seasons can vary throughout the province based on temperatures and precipitation.If the summer months experience dry periods, and plants begin using water from the soil water storage (‘bucket’), eventually the soil water storage might empty, and irrigation will be needed to reduce the local water deficit. Farmers often use a website called Farmwest.com to view local precipitation inputs, and view evapotranspiration (ET) ‘outputs’ so they know how much to water their plants in order to keep water in soil water storage for their crops to use.

The site calculates the potential evapotranspiration assuming there is always sufficient soil moisture in storage, so that irrigators can determine how much water they need to add to the soil each day to keep the soil moisture storage “topped up”. Farmwest calculates evapotranspiration for a standard grass crop (like a typical garden lawn).  Farmers can calculate ET for their particular crop based on standard calculations and may have to use more or less water than a typical grass lawn. How, when, and for how long, farmers irrigate is a complex science.

Step 1: Navigate to the Farmwest website and look at Evapotranspiration.

Step 2: Enter the station information provided by your instructor for your local study area. Select Date range from January 1 – December 31 of the previous full year. Click GO. Note that we are using a different date range than our Climate Normals (1981-2010) because 1991-2020 Climate Normals data are not available yet. This is because not all current station data is available for 2010 or earlier on farmwest.com.  

Station information: __________________________________________________

Step 3: Scroll down to the two graphs at the bottom of the screen. The upper graph shows Evapotranspiration (ET). The green curve is the historical average, and yellow line is the current year. The lower graph shows precipitation events throughout the year.

The graph of evapotranspiration over the year can be presented in two ways.  It can be plotted as a cumulative graph, where the total ET over the season can be seen. This graph will rise steadily from zero at the start of the graph and will be steepest when daily ET is highest. This type of plot is useful for tracking the total amount of water needed for irrigation over a whole season.  You will note a small tick box where you can also choose “Not cumulative in graphical display”. Click on this option and re-examine the graph. Now, the daily amount of ET is shown. This graph will rise and fall over the year as climate conditions change and is useful for seeing how daily climate impacts ET.

Once you have examined the graphs, answer the following questions:

  1. Annual cumulative evapotranspiration is a curve, with the highest point in the summer and lowest in the winter. Based on your understanding of evaporation and transpiration, briefly explain why.
  2. Examine the local precipitation events and the yellow line indicating evapotranspiration. Is there a correlation? Why or why not?
  3. What other data set would be good to see here that could impact evapotranspiration? Why?
  4. Precipitation events occur in the winter, but the evapotranspiration curve does not change. Why is this?

EX3: Completing a Water Budget

A water budget in an area can be useful for city planning, or for irrigations needs. Indeed, after examining water budgets, city planners and engineers in the Okanagan built reservoirs to capture spring runoff for agricultural and city needs.

Here, we will examine a water budget local to your area. Referring to your instructor’s directions and the supplemental materials section, below, complete the water budget for your local area (in Worksheets).

Water Budget Location: _______________________________________________

Once complete, answer the following questions. Be sure you are doing/presenting your own work.

    1. What is the driest month (Precipitation)?
    2. What is the month of greatest moisture stress (Deficit)?
    3. Are 1. and 2. the same? If not, why not?
    1. What is the wettest month (Precipitation)?
    2. What is the month of greatest moisture surplus?
    3. Are 1. and 2. the same? If not, why not?
  1. What is causing most of the seasonality, annual temperature differences or precipitation? Be sure to consider potential evapotranspiration (PE) in your discussion.

EX4: Plotting a Water Budget

Once you have completed EX3 (above), plot your data using Excel (or similar) plotting software to create a visual water budget (See Figure 2 as an example). You will plot your PE, P and AE values for the year. You may draw by hand (and scan back in) or use PowerPoint or Word to create polygons for Recharge, Deficit, Utilization and Surplus zones. Note that recharge ends once the WHC has been reached. Be sure to include title, axis labels, appropriate legend and other chart elements for full marks.

Reflection Questions

In general, it is accepted that British Columbia’s climate will become hotter and dryer in most places due to climate change. Considering precipitation, temperatures, and evapotranspiration (both AE and PE) examined in this lab, take 15-20 minutes to answer the following questions:

  1. How do you think ‘climate change’ might impact your study area’s water budget in 50 years? Include trends in inputs (P) and outputs (PE, AE) in your discussion and why. Be sure to mention deficits and surpluses as well.
  2. What sorts of things can be done to remediate future water budget challenges? If you mention reservoir or similar, refer to your water budget and where the water would come from. Can human usages be changed (consider that populations will only increase)? Include at least three examples in your discussion.
  3. Describe how your farmwest.com evapotranspiration curve will change with climate changes.  Describe how this would influence farmers’ actions for their crops.
  4. The simple monthly bucket model used in this lab considered the soil water storage as a ‘bucket’. Is this strictly true of the ground?  Describe where else water in the soil goes (refer to the water cycle)? When during the year is this mostly likely to happen?

Worksheets

Water Budget Worksheet

Supplemental Materials

Instructions for doing a water balance analysis

  1. Your instructor will supply mean monthly temperature, PE and P, plus a value for the water holding capacity (WHC) for the soil at your site. The WHC is equal to the maximum value of soil water storage (ST).
  2. You can start your calculations at any time in the water balance, preferably at a time you know something about the soil moisture conditions (i.e. either full or empty) e.g. assume soil moisture is full — at WHC — in January: enter your WHC value in the ST column for January.
  3. Calculate the P – PE, or ‘Diff’ values for your location.
  4. Determine your change in storage (\DeltaST) value. Based on the Diff (P-PE) value, you can determine the following

If the Diff value is positive, there is extra water, so

    • Fill up the soil moisture reservoir (if applicable) if it is not full (equal to the WHC). \DeltaST will be positive.

If ST (prior month) + Diff is greater than WHC,

\DeltaST = WHC – ST (prior month)

Remainder of Diff becomes S (see 8, below).

If the Diff value is negative, there is a water need

    • Determine the amount of moisture to be extracted from the soil (\DeltaST will be negative).

If ST (prior month) + Diff is greater than zero, then there is sufficient soil water remaining

\DeltaST = -Diff

If ST (prior month) + Diff is less than 0, the soil moisture storage is empty,

\DeltaST = -ST (prior month), or 0

Remainder of Diff becomes D (see 7, below)

  1. Calculate ST

If Diff is negative

ST (current month) = ST (prior month) + ΔST (current month)

If Diff is positive

If ST previous month = WHC

ST (current month) = WHC

If ST previous month is less than WHC

ST (current month) = ST (prior month) + Diff

OR

ST = WHC if (ST previous month + Diff) is greater than WHC

  1. Calculate AE

If Diff is positive (more than enough water from precipitation for the month)

AE = PE

If Diff is negative (we are taking water out of the bucket, or ST)

AE = P – (\DeltaST)

  1. Calculate Deficit (D)

D = PE – AE

  1. Calculate Surplus (S)

If Diff is negative

S= 0

If Diff positive then

If ST (prior month) = WHC

S = Diff

If ST (prior month) is less than WHC

S = Diff – (WHC – ST prior month)

  1. Balancing the budget

Continue your calculations for all months of the year until you fill in the table and return to the month you started with. If at this point your ST is not the same as the value you began with, you need to repeat the entire procedure until you “balance’ the water budget, and the values for ST in the starting month match. Very rarely you may have to repeat the entire procedure twice to balance the water budget.

Checking your results: Annual AE + Annual D = Annual PE and Annual AE + Annual S = Annual P

Attributions

References

Christopherson, R.W. et al., (2019). Geosystems: An Introduction to Physical Geography (4th ed). North York, Ontario: Pearson Canada Inc.

Environment and Climate Change Canada. (2013-09-09) Water basics: the hydrologic cycle. Accessed 2020-06-26.  https://www.canada.ca/en/environment-climate-change/services/water-overview/basics/hydrologic-cycle.html

Environment and Climate Change Canada, (2019-06-11). Canadian Climate Normals 1981-2010 Station Data. http://climate.weather.gc.ca/climate_normals/index_e.html, Accessed 2020-07-15.

Farmwest.com (2020). Evapotranspiration. https://farmwest.com/climate/et. Accessed 2020-07-15.

WebWIMP version 1.02, 2009, implemented by K. Matsuura, C. Willmott and D. Legates at the University of Delaware in 2003. http://climate.geog.udel.edu/~wimp/ Accessed 2020-07-13.

Media Attributions

  • Figure 8.1 A simplified bucket model. © Johannes Feddema is licensed under a CC BY (Attribution) license
  • Figure 8.2 Simplified modeled water balance of the South Okanagan based on Table 8.1 © G. Krezoski is licensed under a CC BY (Attribution) license