Lab1_Hydrologic_Cycle
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1 Water Planet Lab 1: The Hydrologic Cycle This exercise was inspired by Module 1 of the GLOBE Watershed Dynamics Project. The modified and updated exercise was developed by Matthew Rossi (a key contributor on the original GLOBE project) and Kelin Whipple at Arizona State University (ASU). The associated Web App was designed by Kelin Whipple at ASU using the ArcGIS Web AppBuilder. This laboratory exercise aims to develop your quantitative understanding of the hydrologic cycle and water availability. For all labs, answers and responses must be entered in Canvas in the corresponding Laboratory Assignment. We have posted video guides to each lab that introduce the problem and provide helpful tips for working through the lab and using the tools (Web Apps and Excel spreadsheets) provided. If you have questions, first check to see if they have already been answered in the Hydrologic Cycle Q&A Forum in Canvas. If not yet addressed, post your question and we will respond as soon as possible. You are also encouraged to answer or add to questions posted by others. Lab Section Purpose Part 1 Evaluate the water budget for the U.S. by looking at precipitation, evapotranspiration, and surface runoff data. Part 2 (1) Identify regional patterns in the annual water balance and further investigate local controls on evapotranspiration. (2) Investigate the role of seasonality on interpreting these patterns in the water budget. The Hydrologic Cycle The hydrologic cycle is critical to all of life, and describes the movement of water from place to place, and how water is transferred among oceans, atmosphere, ice, snow, surface water, groundwater, and plants. Fundamental to understanding the water cycle are the principles of conservation of mass
(water can change phases –
liquid, gas, solid –
and can move from place to place, but the total amount of water remains unchanged) and conservation of energy
(energy and energy transfers are involved in all movement of water and all phase transitions –
solar energy input to the oceans and atmosphere drive the water cycle and govern the magnitude of fluxes such as precipitation, evapotranspiration, and runoff). Thus the hydrologic cycle is driven by the conservation of energy (all energy input into the system is used by the system to move and exchange water) and is subject to the conservation of mass. You will learn about the partitioning of water into various components of the hydrologic cycle, the relative magnitude of various fluxes of water (rates of exchange between components, such as precipitation and evapotranspiration) and the controls on these fluxes that affect our daily lives. In this lab, you will examine real data on the hydro-climatology of the US (
North America Regional Reanalysis or NARR
) made available through a series of powerful but easy-to-use web interfaces for viewing and querying the data.
2 Part 1: Regional Patterns in the Water Balance, USA
Cues –
your directions for completing the lab (correspond with directions in the online lab session available as video you can watch) Italicized –
Commands or entries executed or completed by the student within the GIS tool Bold –
Window, layer, or window names displayed by the GIS tool Underlined –
A variable selected from pull-down menu Q1, Q2, etc. –
Questions to be answered by the student in the associated Canvas Lab Assignment Opening the Webtool
Cue 1.
Launch the Annual Water Balance Explorer and familiarize yourself with the various control buttons
(see Figure 1). Take a minute to click each button
to see what it does. Additional, similar web apps with the same functionality but different data layers will be used later in this exercise. Data layers are controlled in the Layers
dialog. All the layers shown on the map have a check next to their name in the layer list. You can show and hide these layers by clicking on the check box. In this App three layers are available: Annual Precipitation, Annual Evapotranspiration, and Annual Runoff. Leave Annual Precipitation as the only visible layer. You can also see State lines, State Capitals (white) and other Cities of Interest (black), two Ocean points of interest (red), and a terrain basemap. Click on Legend to open a display of the map legend
. Annual Precipitation is given in centimeters (cm). Note that this “precipitation” variable includes the “water equivalent” of all rain, snow, sleet, hail, fog deposition, etc, that falls on average each year. The Legend dialog will show the legend for all visible layers. Using your mouse you can click on all Cities and “
Ocean points of interest
”
to open a dialog that displays the value of the variable displayed in the map as “RASTERVALU”. In this case the layer displays Annual Precipitation in centimeters (cm), so the dialog in Figure 2 shows that Nashville, TN receives 121.17 cm of total precipitation per year. Figure 1. Annual Precipitation WebApp Figure 2. Dialog displaying point values (RASTER VALU) in cm
3 Cue 2.
Click on any city or point of interest
to find
the total annual precipitation predicted by NARR for some of the cities shown on the map (e.g., Olympia, Phoenix, Oklahoma City, Atlanta): click on those locations and record the values reported in the dialog as “RASTERVALU”
(see Figure 2). Then go to
the US Climate Data webpage to find
the annual precipitation for these cities based on local rain gauges. If a city is listed twice, choose the first option. Q1.
Which city showed the largest discrepancy between NARR estimates and the mean annual precipitation values reported on the US Climate Data website (use first entry for each city)? a) Phoenix b) Olympia c) Atlanta Q2.
What are some reasons that the NARR values might differ somewhat from actual climate records? a) Different periods of record b) Local topographic controls c) Limited spatial resolution of the NARR data d) NARR is not data but a model-based interpolation of data from many stations e) all of the above Cue 3.
Now Use the Slider tool to uncover the basemap and find the Mississippi River (you may search on google if you are unfamiliar with the Mississippi River)
. Hint: the Mississippi river runs through St. Louis, Missouri, which is visible on the basemap. Q3.
Which Capital Cities are on the Mississippi River? (choose all that are correct) a) Baton Rouge, LA b) Little Rock, AR c) Topeka, KS d) Columbus, OH e) St. Paul, MN North American Regional Reanalysis (NARR) Our first step is to evaluate the accuracy of the NARR dataset. Notice that the NARR provides a value for everywhere in the US (with a spatial resolution of 32 km for each grid cell). As you may have guessed, there are not evenly spaced climate stations across the US making measurements of all components of the water cycle –
so you might wonder “W
hat is the NARR anyway? What does Reanalysis mean?”
It seems to imply that the data was analyzed incorrectly the first time, such that someone had to do it over. Actually, NARR is a set of outputs from a sophisticated climate model based on mass and energy balance. This model uses all available measurements (in this case from 1979-2020) to guide the model. This data “assimilation” of empirical data ensures that the model reasonably matches observations where and when we have them, and makes predictions for everywhere else that are consistent with physical laws. Thus, NARR is a hybrid of observations and model predictions. All major cities in the US have long term climate records. A good test of the model is to make sure NARR can reproduce these records.
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4 Q4.
How would you describe patterns in total annual precipitation of the entire country east and west of the Mississippi River?. a) There is lots of local variability, but no systematic differences are apparent b) Generally dry/very dry West of the Mississippi and wet/very wet to the East c) Very dry East of the Mississippi and much wetter to the West Explore the Annual Evapotranspiration Data Cue 4. Click on the Layers button to open the Layers dialog. Deselect the Annual Precipitation layer and Select the Annual Evapotranspiration layer (see Figure 3)
. Notice that the Annual Evapotranspiration map appears and that the color of the map and its associated legend changes to reflect annual evapotranspiration data. Also notice that all variables are reported as average totals per year in centimeters (cm). Remember
, clicking on a city or point of interest
opens a dialog showing attributes for that point, the value of the displayed laye
r (Annual Evapotranspiration in this case) is reported as the “RASTERVALU”, again in centimeters. Figure 4. Annual Evapotranspiration Map Figure 3. Layers List, Annual Water Balance Explorer
5 The Water Budget A water budget is a way to think about all of the inputs and outputs of the water cycle. It relies on the principle of conservation of mass
—
water moves from place to place sometimes by changing phase (i.e. liquid, gas, solid), but the total mass of water does not change. How water is partitioned into different reservoirs is critical to understanding the behavior of Earth’s climate system. In fact, some of the biggest uncertainties in predictions of modern global climate change are due to an incomplete understanding of exactly how the water budget will be affected. At the global scale, mean annual evapotranspiration must exactly balance mean annual precipitation if the water content in the atmosphere is maintained. As you might expect, there is significant spatial and temporal variability that prevents the water budget to locally balance. Figuring out how to adapt to this variability is critical to water resource management. Luckily for us, there is generally a surplus of evapotranspiration over the oceans that brings a surplus of life-sustaining rainfall over landmasses. This surplus rainfall eventually makes it back to the oceans through other pathways and is what gives us moist soils, rivers, lakes, and groundwater. We can calculate the terrestrial water budget by considering all of the inputs and outputs such as precipitation (P), evapo-transpiration (ET), surface runoff (SR), subsurface flow (SSF), and changes in storage (
S). We can represent this mathematically by: P = ET +
SR + SSF +
ΔS
Comparing Annual Evapotranspiration to Annual Precipitation Using the Slider Tool In this section you will use the Slider Tool to compare annual precipitation with annual evapotranspiration. This is the first step in analyzing the regional water balance and water availability patterns in the US. Cue 5. Click on the Layers button to open the Layers dialog. Select the Annual Precipitation layer so that both it and the Annual Evapotransipiration layer will be visible (see Figure 5)
. This will allow you to directly compare the two layers using the Slider Tool. Cue 6. Click the Swipe Tool and make sure the Annual Precipitation layer (the upper layer) is the Swipe layer (Figure 6). This will allow you to view the spatial relationships between Annual Precipitation and Evapotranspiration by slowly moving the slider bar back and forth. Figure 5. Layers List, Annual Water Balance Explorer
6 Using the Slider Tool with these two layers visible, explore the relationship between them and answer the following question: Q5. How do you interpret the patterns of annual evapotranspiration evident on the map? a) Other than in cool areas (PNW and NE), evapotranspiration mostly tracks with annual precipitation b) Evapotranspiration amounts largely mimic regional temperature patterns (cooler in the north and in the mountains) c) The Southeastern US has generally low evapotranspiration. Cue 7. Click on any city or point of interest
to find
the both the total annual precipitation and annual evapotranspiration estimated by NARR
. Note: with multiple layers visible, clicking on a point will open an attributes dialog for each visible layer. You must cycle through the dialogs (
“
1 of 2
”
or “
2 of 2
”
in this case) using the arrow at the top right. Be sure to note which Attribute you are viewing (i.e., Capitals_evap vs. Capitals_precip in this example). Q6. In which region are annual evapotranspiration amounts highest? a) Southeast b) Pacific Northwest c) Northeast d) Rocky Mountains (parts of Colorado and Wyoming) e) Southwest Figure 7. Navigating Multiple Attribute Dialogs Figure 6. Swipe Tool, Annual Precipitation over Annual Evapotranspiration
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7 Q7. In which region are annual evapotranspiration amounts lowest? a) Southeast b) Pacific Northwest c) Northeast d) Rocky Mountains (parts of Colorado and Wyoming) e) Southwest Q8. Recall that evapotranspiration is known to depend on temperature, humidity, and wind speed
, but can be limited by water availability over land
. Given this, why is evapotranspiration lower in the PNW than in the SE despite the fact that the PNW is much wetter? a) Precipitation is less in the PNW, so there is less water available. b) Hotter temperatures in the SE enhance evapotranspiration relative to the PNW. c) The PNW is covered in old growth forest that transpires at a much lower rate than the agricultural fields of the SE.
d) The PNW is far more humid than the SE, which greatly reduces evapotranspiration. Q9. Why is evapotranspiration (ET) so low in the SW despite the high temperatures and low humidity, both of which are expected to enhance evapotranspiration? a) There are almost no plants, so there is negligible transpiration. b) ET is limited by how much precipitation falls throughout the year.
c)
Most rain falls at night in this climate, so evapotranspiration is limited. Investigating patterns of annual precipitation, evapotranspiration and surface runoff Before you begin, use your new familiarity with the annual precipitation and evapotranspiration patterns across the US in to ponder what the patterns in surface runoff will look like. For example, what do you expect annual surface runoff to be like in places with high annual precipitation? Are there circumstances that could disrupt a simple relationship between runoff and precipitation?
Q10. What factors could reduce surface runoff in areas with high annual precipitation? a)
Increased vegetative cover enhances transpiration, thus reducing runoff. b)
Increased vegetative cover enhances infiltration by increasing soil porosity (root holes, high organic content), thus reducing runoff. c)
Where high precipitation is paired with warm temperatures, evapotranspiration is relatively high, which reduces runoff. d)
All of the above
8 Surface Runoff In the next part of the lab, you will explore surface runoff data. This will get you one step closer to calculating a full water budget. Surface runoff (together with a significant fraction of the subsurface flow) is the water that flows down creeks, streams, and rivers. Runoff is reported as flow volume per unit area (32x32 km pixels for NARR data) and is reported as cm (volume/area = cm
3
/cm
2 = cm) There are many factors that affect the timing and quantity of surface runoff that is generated during precipitation events including:
The amount that actually reaches the surface (e.g. doesn’t get intercepted by vegetation)
Precipitation rate (faster means more surface runoff)
Form of precipitation (i.e. snow, rain, sleet, hail, etc.)
Infiltration rate (affected by landuse, groundcover, geology, ground saturation, as discussed in lecture) Similar factors also influence the proportion of subsurface flow that contributes to river flow. Exploring Total Annual Surface Runoff Cue 8. Click on the Layers button to open the Layers dialog. Deselect the Annual Precipitation and Annual Evapotranspiration layers and Select the Annual Runoff layer instead (see Figure 8)
.
9 Cue 9. Click on the Legend button to view the scale on the Runoff color ramp. Note that while both Annual Precipitation and Annual Evapotranspiration maps were scaled between 0-200 cm, Runoff is scaled to the much smaller range of 0-20 cm (again this is mean annual runoff)
. Discussion Point 1. Think about the factors that affect surface runoff discussed in lecture and summarized in the Grey Box above. Can you explain the low surface runoff in the Southwestern US or the high surface runoff in the Rocky Mountains in Colorado (West of Denver) by considering these factors?
(
Optional Post to Yellowdig
) Cue 10. Click on the Layers button to open the Layers dialog. Select the Annual Precipitation layer so that both it and the Annual Runoff layer will be visible
. Use the Slider Tool to explore
the relationships between Precipitation and Runoff. Are there any surprises? Areas with surprisingly high or low runoff given the Precipitation? Return to the Layers dialog and make both Annual Evapotranspiration and Annual Runoff visible
. Now use the Slider Tool to explore
the relationships between Evapotranspiration and Runoff. Does this help explain any of the surprising patterns in Runoff? Calculating a Water Budget for Individual Cities You are now ready to gather data for annual water budgets for selected locations within the US. Remember that these budgets may not perfectly balance because we haven’t included two important outputs
—
subsurface flow and changes in storage (see Grey Box, page 5 of this lab handout)
. We’ve chosen a set of cities that represent much of the major regional differences in water availability around the US, emphasizing an East-West transect (
cities of Figure 8. Using Layers Button to Set Layer Visibility
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10 interest are shown with black dots and labeled
). Reminder
: with multiple layers visible, clicking on a point will open an attributes dialog for each visible layer. This can be very efficient, but you must cycle through the dialogs using the arrow at the top right to see values for each visible layer and you need to take care to note which Attribute you are viewing (i.e., Cities_evap, Cities_precip, or Cities_runoff). Cue 11. Download the Excel spreadsheet WP_lab1_water_cycle_2021.xlsx from the course website (Canvas). Move the file into a water_planet folder on your computer where you will be able to access it later. As you proceed, make sure to save your spreadsheet
frequently. Cue 12. Open the spreadsheet and make sure you are viewing the first worksheet, Water Balance 1 (left hand tab at the bottom). Fill in all missing values of precipitation, P, evapotranspiration, ET, and surface runoff, SR for each location. A few cities have been completed for you as examples. Check to confirm
you can reproduce the numbers we filled in for you. SAVE
your spreadsheet when done. Cue 13. Take a close look at the bar graphs and scatter plot
that automatically fill in as you enter data. The bar charts are handy visual representations showing the relative magnitudes of P, ET, and SR and will help you see how the water budgets vary regionally. The scatter plot provides the best way to assess the relationship between precipitation and surface runoff. Note
this plot excludes the outlier point of Aspen, CO that will be discussed later. Use the data, this chart, and the maps in the Explorer (the Slider Tool may be helpful)
to answer the following questions: Q11. In general, what fraction of annual precipitation actually moves directly into streams and rivers as surface runoff (without infiltrating into the ground)? a)
Very little b)
About half c)
Generally greater than half d)
Almost all Q12.
What is the relationship between precipitation and runoff? a) Precipitation and Runoff are positively correlated. b) Precipitation and Runoff are negatively correlated. c) There is no correlation between Precipitation and Runoff
11
12 Part 2a: Seasonal Patterns in the US and Water Balance 2 In this part of the lab exercise you will use a number of different Web Apps (Explorers) to visualize, compare, and analyze seasonal patterns of precipitation, evapotranspiration, and surface runoff and to further investigate controls on the regional patterns in the water balance. It will be convenient to keep the various Web Apps open in different tabs within your Browser while you work. Exploring seasonal precipitation In this Exercise you will use a number of different Web Apps (Explorers) to visualize, compare, and analyze seasonal patterns of precipitation, evapotranspiration, and surface runoff and to further investigate controls on the regional patterns in the water balance. It will be convenient to keep the various Web Apps open in different tabs within your Browser while you work. Cue 14.
Launch the Seasonal Precipitation Explorer. Click on the Layers button
to see which season is currently displayed. Use this tool to make different layers visible. Remember you can use the Slider Tool to directly compare two layers, but make sure only two layers are visible at a time. Seasonal Water Budget of the U.S.
Seasons are divisions of the year that mark predictable changes in weather. In the mid-latitudes (like the U.S.), the year is often divided into four seasons. However, there can be different ways to divide up the year. For e
xample, you may hear of astronomical seasons (defined by Earth’s orbital position) versus meteorological seasons (defined by weather conditions). The two definitions are related. Since we are interested in how the hydrological cycle is affected by seasonality, we will use the convention of defining meteorological seasons using the Roman calendar:
Winter: December, January, February (DJF)
Spring: March, April, May (MAM)
Summer: June, July, August (JJA)
Fall: September, October, November (SON)
You have already explored patterns in the total annual amount of precipitation that falls across the United States. Just as different parts of the US receive different amounts of annual precipitation, they also receive different amounts of seasonal precipitation. Understanding when a region will be wet, or dry, is very important to human habitability. For example, farmers make decisions on which crops to plant and when to plant them based on seasonal precipitation patterns. As you might expect, the timing of precipitation is fundamental to water availability –
i.e., how much is lost to evapotranspiration and how much makes it to rivers and streams.
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13 To investigate seasonal variation in the water cycle, we will focus on four geographic regions: Region
Acronym
Representative point to gather data Pacific Northwest
PNW
Olympia, Washington
Southwest
SW
Phoenix, Arizona
Rocky Mountain West
Rockies
Aspen, Colorado Southeast
SE
Atlanta, Georgia
Cue 15.
Explore the patterns of Seasonal Precipitation by turning on different pairs of layers and using the Slider Tool. Q13.
Which season is most unlike the others, with a distinct pattern of high precipitation across the continental US (use the Slider to compare seasons)? a) Winter (DJF); b) Spring (MAM); c) Summer (JJA); d) Fall (SON) Cue 16.
Open the spreadsheet
that you worked on in Part 1 of this Lab (
WP_lab1_water_cycle_2021.xlsx
) and move to the second worksheet –
the second tab named Seasonal Variation
Figure 9. Winter (DJF) Precipitation Map
14 Cue 17.
Fill in
the precipitation amounts that go into your excel file for all 4 regions for each season by clicking on the representative Cities in the Seasonal Precipitation
Explorer. Remember the Seasonal Precipitation amount is given as “RASTERVALU” in cm and that yo
u can see data for all four seasons if you make all 4 visible and then cycle through the Attribute Dialogs. Do take care to check which season’s Attribute dialog you are looking at each time –
they do not always appear in the same order. Cue 18.
Save the updated spreadsheet. Use the data in the spreadsheet and the maps of seasonal precipitation to answer the following questions: Q14.
Which of the four regions receives the highest amount of winter precipitation? a) Pacific Northwest; b) Southwest; c) Rockies; d) Southeast Q15.
Which receives the highest amount of summer precipitation? a) Pacific Northwest; b) Southwest; c) Rockies; d) Southeast Exploring Seasonal Patterns of Surface Runoff Cue 19.
Launch the Seasonal Runoff and Seasonal Evapotranspiration Explorers
. Click on the Layers button
to see which season is currently displayed. Use this tool to make different layers visible. Remember you can use the Slider Tool to directly compare two layers, but make sure only two layers are visible at a time. Working in Excel Spreadsheets are designed to make repeated calculations for large tables of data and to visualize that data in charts. Often times you will want to organize data and plots in Excel into different worksheets. This is what has been done for you in WP_lab1_water_cycle_2021.xlsx
. Use the tabs at the bottom of your Excel window to switch which worksheet you are viewing. The Seasonal Variation
worksheet is organized by season. You will be asked to determine seasonal water budgets just like you did in Part 2 for the annual data. To finish this lab, you will need to fill out all of the empty white cells
by using the NARR map tables (precipitation and surface runoff). The blue cells have been filled out for you (evapotranspiration). We have entered formulas in the tan cells, so that the worksheet will automatically calculate these values for you (effective precipitation and water excess). This data is linked to another worksheet called Seasonal Variation Charts
. Plots will automatically be generated in Seasonal Variation Charts
once you complete entering data into the Seasonal Variation
worksheet.
15 Cue 20.
Explore the maps using the Slider Tool to get a feel for seasonal patterns of precipitation and runoff. Do the differences make sense given the seasonal patterns of evapotranspiration?
Cue 21.
Return to the spreadsheet
(
WP_lab1_water_cycle_2021.xlsx
) and make sure you are still on the Seasonal Variation
worksheet. Cue 22.
Fill in
the surface runoff and evapotranspiration amounts that go into your excel file for all 4 regions for each season by clicking on the representative cities
in each Web App.
Remember the Seasonal Runoff or Evapotranspiration amounts are given as “RASTERVALU” in cm and that you can see data for all four seasons if you make all 4 layers visible and then cycle through the Attribute Dialogs. Do take care to check which season’s Attribute dialog you are looking at each time –
they do not always appear in the same order. Cue 23.
Save the updated spreadsheet. Using the Maps, the Slider tool, and the Seasonal Variation Charts in the spreadsheet, answer the following questions
: Q16.
During which season is ET the highest across the US? a)
Winter b)
Spring c)
Summer d)
Fall Comparing Seasonal Precipitation and Seasonal Surface Runoff
Cue 24.
Launch the Winter Precipitation-Runoff and Spring Precipitation-Runoff Explorers. Use the Slider Tool to directly compare Precipitation and Runoff in Winter and Spring. Figure 10. Winter (DJF) Runoff Map
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16 Q17.
As you might have expected, there is generally a strong correlation between surface runoff and precipitation in both Winter and Spring. One important exception is the very high Spring runoff in the Rocky Mountains (near Aspen) despite only moderate precipitation. What most likely accounts for this high amount of Spring runoff in the Rockies? a)
Spring runoff is enhanced by snowmelt in this mountainous area. b)
Potential evapotranspiration is very low due to cool Spring weather. c)
Actual evapotranspiration is relatively low due to the moderate precipitation. d)
High Spring winds in the Rocky Mountains enhance evapotranspiration. Cue 25.
Launch the Summer Precipitation-Runoff and Fall Precipitation-Runoff Explorers. Study these maps
using the Slider Tool and ponder
the seasonal differences in the relation between surface runoff and precipitation. Can you make sense of the patterns you see? Now let’s return to
the spreadsheet (
WP_lab1_water_cycle_2021.xlsx
) to look at these patterns in a little more detail. The data you entered into the spreadsheet have been automatically graphed in order to help you interpret regional differences in the seasonal water budget. Cue 26.
Switch to
the Seasonal Variation Charts
worksheet by clicking on its tab at the bottom of the Excel window. The 6 charts in this worksheet help illustrate the major regional differences in the seasonal water budget. Use these plots to answer the following questions. When trying to interpret Figure 11. Winter Precipitation and Runoff Map
17 these plots, make sure you understand what is being plotted on the x-axis and the y-axis. For instance, there are two surface runoff versus season plots, but they are not the same. The y-axis covers a much smaller range of values on the second version of the plot. This will help you see smaller differences in the data. Q18.
In the SE, why does surface runoff peak in the Spring while precipitation peaks in the Summer (the 6
th
plot may be helpful)? a)
Because rainfall intensity is greater in the Spring b)
Because the ground is still frozen in the Spring, which inhibits infiltration c)
Because in the heat of the summer, evapotranspiration is very effective d)
Because of all of the spring snow melt Q19.
Three of the four regions have very similar seasonal patterns and magnitudes of evapotranspiration. However, while the Phoenix ET also peaks in the summer, it has a much lower magnitude than all the other sites. Why? a)
Phoenix has little vegetation so most of the rainfall runs off, greatly reducing evapotransporation b)
Monsoon storms bring lots of cloud cover, which greatly reduces evapotranspiration c)
In Phoenix’s arid climate, evapotranspiration is strongly water
-limited d)
“Cool” desert nights greatly reduce the total annual evapotranspiration.
18 Part 2b: A Closer Look at the Controls on Evapotranspiration By this point, you should have noticed that evapotranspiration plays a dominant role in the water cycle and in the availability of surface water both throughout the year and seasonally. To examine this in more detail, we will use the annual precipitation, evapotranspiration, and surface runoff data you collected in Part 1 of the lab along with additional data, to try to develop a more complete picture of the water balance across the US. We will continue to use the Excel spreadsheet to do this. Cue 27.
Click on
the Water Balance 2 worksheet in Excel. There’s a lot of information in this this data table, so take a moment to familiarize yourself with each column of data. The first three columns of data (P, ET, SR) are the values you entered in the Water Balance 1
worksheet (numbers in italics
). The next two columns, Effective Precipitation (EP) and Water Excess (WE), are computed from the data using the formulas given in the header (tan columns, numbers in bold
). Effective Precipitation
is the net precipitation remaining after accounting for losses due to evapotranspiration. This is the total amount of water available to flow across the surface and subsurface. Water Excess
is a term we’ve invented to describe the amount of effective p
recipitation that does not contribute to surface runoff. In other words, this is the water that infiltrates into or that is extracted out of the subsurface. Some of this water quickly flows into rivers, lakes, and wetlands; some makes it into deep groundwater reservoirs and flows very slowly. Potential Evapotranspiration
is a very important concept that helps explain many of the patterns you have seen in the regional and seasonal water budgets. Potential evapotranspiration is just what it sounds like –
it is an estimate of how much evapotranspiration would occur if there was enough water available. Actual evapotranspiration (ET) must always be less than or equal to the potential evapotranspiration (PET). Generally we can expect ET to reach the value of PET only over oceans and large lakes. Aridity Index
is the ratio of potential evapotranspiration (PET) to precipitation (P). The higher this ratio is, the more water-starved, or arid, the climate is. Although an Aridity Index greater than one appears to imply that all water will be lost to evapotranspiration (i.e., no runoff), it is important to realize that the Aridity Index (AI) is based on annual totals of precipitation and evapotranspiration. As you learned above, the water budget changes importantly with the seasons. Additionally, during individual storm events evapotranspiration often cannot keep up with precipitation. This means that runoff can occur even in the most hyper-arid settings during storms. For these reasons, we define arid regions to have an AI > 5 and semi-arid regions to have and 5 > AI > 2.
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19 The next two columns, Mean Annual Temperature (MAT) and Annual Potential Evapotranspiration (PET), are taken from the same NARR dataset you have been using. However, since they are not included in the Web Apps you have been using, we provided the data for you. You will calculate the final column, Aridity Index (AI), in the steps below. Cue 28.
Complete the data table by entering the formula
for computing the Aridity Index in each cell of column I. You are using the spreadsheet like a calculator. To do this: (a) Click
on an empty cell in the column (b) Type
=
(c) Click
on the cell for the value of PET for the same location (the cell number will appear after the equal sign) (d) Type
/
(e) Click
on the cell for the value of P for the same location (the cell number will appear after the division sign) (f) Hit
Enter
. (g) Repeat
for all cells in the Aridity Index column. Q20.
Which cities have an Aridity Index greater than 5? a)
Los Angeles b)
Las Vegas c)
Phoenix d)
Denver e)
Lubbock f)
Oklahoma City g)
Little Rock h)
Santa Fe Q21.
Compare this measure of “arid
ity
” to the more traditional
(simpler but limited) definition that defines aridity as places that receive less than 25 cm of annual precipitation. You can use the Water Balance 2
table in your spreadsheet or the Annual Water Balance Explorer to find locations with an Aridity index greater than 5 (Arid by this measure) and yet receive more than 25 cm of precipitation. Which cities have an Aridity Index greater than 5 but also a mean annual precipitation greater than 25 cm (
that is arid based on Aridity Index but not based on just precipitation
)
? a)
Los Angeles b)
Las Vegas c)
Phoenix d)
Denver e)
Lubbock f)
Oklahoma City g)
Little Rock h)
Santa Fe Tip
:
You can Copy
the formula from the one cell and Paste
it into the other cells (all at once or one at a time –
either way) and the formula will update automatically to use values of P and PET from the appropriate rows in the table.
20 Whew. It can be difficult to get a quantitative sense of the relationships among different components of the water budget (P, ET, SR, Effective Precipitation, and Water Excess) and the controls on evapotranspiration (ET) that are so critical to water availability just by looking at a map or a data table. Perhaps we can gain some insight into these relationships by graphing the data. Cue 29.
Click
on the Evapotranspiration_Runoff Charts
worksheet in Excel. Tip
:
If you don’t see this tab, you may need to click
the small right arrow in the lower left corner of the Excel window to scroll through the tabs for the available worksheets. There are three pre-made charts to view (linked to the worksheet you just filled out, Water_Balance_2, and plotting various data for the same familiar cities): (a) Actual and Potential Evapotranspiration versus Temperature (b) Actual and Potential Evapotranspiration versus Annual Precipitation (c) Surface Runoff and “Water Excess” versus Effective Precipitation. Looking at the data in this way can help reveal patterns in the data that may have been perplexing to you over the last couple weeks. Cue 30.
Study
the plot of Actual and Potential Evapotranspiration versus Temperature to answer the following questions: Q22.
Does the relationship between Potential ET and Temperature make sense? a)
Yes –
Potential evapotranspiration shows a strong positive correlation with temperature, with scatter that reflects differences in humidity and wind speed. b)
Yes –
Potential evapotranspiration is negatively correlated with temperature, with scatter may reflect errors in the NARR estimates of climate variables c)
No –
Potential evapotranspiration depends strongly on effective precipitation, and I cannot tell from this plot how effective precipitation varies with temperature, so I can’t be sure what’s going on.
d)
No –
Potential evapotranspiration shows no clear trend with temperature Q23.
What about the relationship between Actual ET and Temperature? Which interpretation best describes the data? a)
Actual ET shows a strong positive correlation with Temperature, with scatter that reflects differences in humidity and wind speed b)
Actual ET depends only on temperature, scatter may reflect errors in the NARR estimates of climate variables c)
Actu
al ET depends strongly on precipitation but I can’t tell from this plot how precipitation varies with Temperature. Therefore, I’d have to say that it does
not
appear that Actual ET and Temperature are correlated.
21 Cue 31.
Next, scroll down
to and study
the plot of Actual and Potential Evapotranspiration versus Precipitation to answer the following questions: Q24.
Does the relationship between Potential Evapotranspiration and Precipitation make sense? a) Yes -- Potential ET shows a strong positive correlation with precipitation, scatter may reflect differences in temperature and wind speed b) Yes -- Potential ET decreases as precipitation increases, reflecting the increase in humidity, scatter may reflect differences in temperature and wind speed c) Yes -- Potential ET is water-limited in the desert SW where temperatures are greatest d) No -- Potential ET should increase with precipitation because it is water-limited Q25.
The solid black line is a “1:1” line where Actual Evapotranspiration and Precipitation
are exactly equal. Sites on land can only plot above this line (evapotranspiration greater than precipitation) if water is being drawn out of storage (e.g. lakes, reservoirs, wetlands, snowpack, soils). Which statement best explains the difference in the relation between actual evapotranspiration and precipitation below and above ~75 cm annual precipitation –
i.e. Why is evapotranspiration essentially equal to precipitation in the dry areas but notably less than precipitation in wet areas? a) In the west, annual precipitation is generally less than 75 cm and PET exceeds 150 cm -- these areas are water-limited (ET limited by P). For P > 75 cm, PET is less (more humid) and more runoff occurs b) For annual precipitation <75 cm, most of the rainfall is intercepted by leaves and grass blades, so it evaporates before reaching the ground. At higher rainfall rates, a greater amount of runoff is always generated c) Rain occurs mostly in the Winter and this is especially true where annual precipitation is higher. Cooler Winter temperatures and dormant vegetation together reduce losses to ET, explaining the increasing fraction of runoff Cue 32.
Finally, scroll down
to the plot of Runoff and Water Excess versus Effective Precipitation (defined as P –
ET) to answer the question below (
Note
that from your data table P ~75 cm corresponds to an Effective Precipitation of ~10 cm): Discussion Point 2.
Now that the data table is complete, the computed values in the Effective Precipitation
and Water Excess
(which includes subsurface flow and changes in subsurface storage) columns are meaningful and the plot of Runoff and Water Excess as a function of Effective Precipitation (Evapotranspiration_Runoff Charts tab, scroll down) has been filled in. Ignoring the negative values of Water Excess in the Rockies (and Denver) (
these negative values probably reflect a period of net loss of snowpack/ice in the Rockies
), describe how the relation between water excess and effective precipitation differs from the relation between runoff and effective precipitation. Hint: consider how much of the effective precipitation goes into surface runoff (SR) vs. “water excess” (WE) for EP < ~20cm and EP > ~20cm. (Optional Post to Yellowdig)
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