SSACgnp GB 661 MCR 1 2 Comparing Stream

SSACgnp. GB 661. MCR 1. 2 Comparing Stream Discharge in Two Watersheds in Glacier National Park Does stream flow come equally from all parts of a watershed? Use USGS stream data to find out. Core Quantitative Literacy Topics Rates: temporal and spatial Supporting Quantitative Literacy Topics Ratios Intensive vs. extensive quantities Unit conversions Core Geoscience Issue Hydrology: watersheds and stream discharge Mark C. Rains Department of Geology, University of South Florida © 2010 University of South Florida Libraries. All rights reserved. This material is based upon work supported by the National Science Foundation under Grant Number NSF DUE-0836566. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. 1

Getting started After completing this module you should be able to: • Understand the difference between intensive and extensive quantities. • Know how to convert quantities involving units of time. • Know how to calculate ratios. • Define “watershed, ” “orographic lifting, ” and “stream discharge. ” • Know that orographic lifting cools air masses, causes condensation and precipitation, and results in higher stream discharge per unit watershed area in high-elevation areas. Montana And you should also know where Glacier National Park is. 2

The setting – Glacier National Park is part of the Waterton-Glacier International Peace Park, which was recognized as one of the world’s great treasures by being designated a World Heritage Site in 1995. Here, Precambrian sedimentary rocks have been thrust upward and eastward and now sit atop younger Cretaceous rocks. For the past 75 million years, erosion processes have carved these rocks into steep-sided mountains and valleys filled with lakes, rivers, alpine meadows, and old-growth forests. These mountains form part of the Continental Divide, with water on the west slopes ultimately discharging from the Columbia River to the Pacific Ocean and water on the east slopes ultimately discharging from the Mississippi River to the Gulf of Mexico or the Nelson River to Hudson Bay. 3

Watersheds There are many watersheds within the Park. A watershed is a geographic area that contributes surface-water flow to a specific location. Watersheds are typically named after the river(s) by which they are drained. For example, the Nelson River watershed refers to the entire area that contributes surfacewater flow to the Nelson River where it discharges to Hudson Bay. However, smaller watersheds can be defined for any specific location within a given larger watershed. We will do this for parts of the Grinnell Creek and Swiftcurrent Creek watersheds, where the Grinnell Creek watershed is nested within the Swiftcurrent Creek watershed. The Grinnell Creek and Swiftcurrent Creek watersheds occur within the Nelson River watershed. Karl Musser, Creative Commons, Attribution-Share. Alike 3. 0 4

Orographic lifting and precipitation Elevation plays a major role in controlling precipitation, with higher amounts of precipitation typically occurring at higher elevations and on windward slopes due to orographic lifting. On windward slopes, air is deflected upwards and cools as it rises. (Of course air cools as it rises— this is why snow falls in the mountains!) If the air cools enough, then the air temperature will reach the dew point, which is the temperature at which it is saturated and can hold no more water. At this point, clouds form and rain or snow may fall. On the leeward side of mountains, air warms as it descends. If it warms enough, then the air temperature will go above the dew point, meaning that it is no longer saturated; not only can the air continue to hold water but it can also take up more water through evaporation. If this occurs, then the dry leeward slopes are said to be in a rain shadow (Endnote 1). Orographic lifting affects the Park, too—West Glacier (3200 ft and west of the Continental Divide; 29 in. rainfall), Summit (5200 ft and on the Continental Divide; 40 in. ), and East Glacier (4800 ft and east of the Continental Divide; 30 in. ). (Western Regional Climate Center data for 1971 -2000). 5

Stream discharge is the amount of water flowing in a river. It is expressed in units of volume per unit time, such as cubic feet per second (i. e. , ft 3/s or cfs). We plot discharge on hydrographs, such as the on the right, which shows mean daily discharge on Swiftcurrent Creek at Many Glacier, one of the locations we will study in this module. Hydrographs typically show time on the x-axis and discharge on the y-axis. Discharge is generated when snow melts and/or rain falls. When warm rain falls on recently fallen snow, then large floods, often called “rain-on-snow” events, can occur. Such was the case on Swiftcurrent Creek in early November 2006 (Endnote 2). US Geological Survey 6

The problem We have two watersheds, one nested inside the other: the Grinnell Creek watershed, which is 1. 10 square miles in area and above 6322 ft in elevation, and the Swiftcurrent Creek watershed, which is 30. 9 square miles in area and above 4877 ft in elevation. Air flow in the Park is generally from west to east, coming over the ridge over the Grinnell Creek watershed and descending into the valley over the Swiftcurrent Creek watershed. Therefore, the Grinnell Creek watershed is at the top of the orographic lift, while the Swiftcurrent Creek is in the rain shadow. In a typical July, snow and ice continue to melt throughout the Grinnell Creek watershed (see left, in July 2009) but are completely melted in most of the lower Swiftcurrent Creek watershed (see right, again in July 2009). These facts suggest the following hypothesis—in July, stream discharge is generated at a greater rate in the Grinnell Creek watershed than in the Swiftcurrent Creek watershed. Question: Is this true? How could we find out? What data can we use? 7

Getting some data, 1 US Geological Survey stream gages “USGS 05013900 Grinnell C at G Glacier nr Many Glacier MT” and “USGS 05014500 Swiftcurrent Creek at Many Glacier MT” record stream discharge at Grinnell Creek and Swiftcurrent Creek, respectively. The stream gage on Grinnell Creek has been discontinued; the stream gage of Swiftcurrent Creek is still operable and data are uploaded in real time to the Internet by satellite link. Click on the link to the gage on Swiftcurrent Creek if you are connected to the Internet. What is the current discharge? Return to Slide 15. In our exercise, we will work with data summarized for July 2004 -2008. On the left, is a photograph of the stream gage on Grinnell Creek. The corrugated standpipe is a stilling well, which is connected to the stream by a subsurface pipe. Therefore, water levels in the standpipe are controlled by water levels in the stream. The housing protects the stilling well head and the stage recording instrumentation. On the right, is a schematic of a typical stream gage. US Geological Survey Retrieve the data on the next slide. 8

Getting some data, 2 Here are some data. Click on the icon and save the file immediately to your computer. It lists the mean stream discharge in cubic feet per second (i. e. , ft 3/s) at the Grinnell Creek and Swiftcurrent Creek gages in July during the years 2004 through 2008. These data are monthly means, or averages, calculated from data collected every 15 minutes at each gage. The hydrograph on Slide 6 is a plot of daily means calculated from data collected every 15 minutes at the Swiftcurrent Creek gage for Water Years 2004 -2008. Water years run from October 1 through September 30 and are named for the ending year. Thus, Water Year 2004 begins October 1, 2003, and ends September 30, 2004. Note two characteristics of this small data set. First, the mean monthly stream discharge varies —on Grinnell Creek it varied between 26. 2 ft 3/s and 36. 4 ft 3/s, and on Swiftcurrent Creek it varied from 139. 3 ft 3/s to 308. 4 ft 3/s. Second, it is difficult to make comparisons from a table of numbers, even when there are only two columns of data and a few rows such as in this table. Certainly, the values in Column B are smaller than the values in Column C, and the lowest and highest values in Column B correspond to the lowest and highest values in Column C. But, can you answer the question yet? Go back and read the hypothesis if you have forgotten it. (Slide 8) 9

Exploring the data, 1 The data are given in ft 3/s, but we are working on the time scale of a month. Therefore, let’s start by changing the units of the data to cubic feet per month (i. e. , ft 3/mo). To do this, we simply need to multiply our values in ft 3/s by the number of seconds in a month. Few of us know the number of seconds in a month off the top of our heads, so let’s do this in a series of steps using the number of seconds in a minute, the number of minutes in an hour, the number of hours in a day, and the number of days in the month of July, all of which are data that we do know off the top of our heads. Click forward and watch the red slashes fly in to cancel the units, with seconds in the numerator (i. e. , the top of the fraction) canceling seconds in the denominator (i. e. , the bottom of the fraction), and so on, until cubic feet (i. e. , ft 3) is left in the numerator on the far left and month (i. e. , mo) is left in the denominator on the far right. If we simultaneously multiply all of the numbers, then we arrive at a discharge 87, 048, 000 ft 3/mo for the month of July 2004. We could do this for each value using a calculator or, worse yet, a piece of paper and a pencil. However, we could do this much more efficiently using a spreadsheet. 10

Exploring the data, 2 Put the following function in Cell E 2: =B 2*60*60*24*31. Hit the Return key. Select Cells E 2: E 6, then “fill down” by holding down the Control key and hitting the D key. Now select Cells F 2: F 6, then “fill right” by holding down the Control key and hitting the R key. We still haven’t resolved the problem introduced at the bottom of Slide 9, i. e. , that it is difficult to make comparisons from a table of numbers, even from a simple table such as this. Therefore, let’s simplify our work by calculating the mean monthly discharges for each creek for the entire period of record. Start by putting the following function in Cell E 7: =AVERAGE(E 2: E 6). Now “fill right”, using the instructions above. This produces a pair of numbers (in Cells E 7 and F 7) that each represents all the available July discharge data for the respective watersheds. Last, reformat Cells E 2: F 7 so there are “ 1000 separators” (Excel language for commas) and no digits past the decimal point. Your spreadsheet should now look like this: Certainly the July discharge of Swiftcurrent Creek is much larger than that of Grinnell Creek. In fact, the discharge of Swiftcurrent Creek is more than 6 times the discharge of Grinnell Creek (536 million/ 85 million). Does that mean the hypothesis is incorrect? 11

Thinking about the problem, and getting a plan It’s no great surprise that the discharge of Swiftcurrent Creek is larger than the discharge of Grinnell Creek, which flows into it. Swiftcurrent Creek drains a larger area (watershed) than Grinnell Creek! To compare the two, we need to remove the disparity in the areas. We want to compare the discharge per unit area for the two creeks, not their discharges. Discharge per area is a rate. Recall, the hypothesis (Slide 8) referred to the rate that discharge is generated in the watershed. Rate is a particular type of ratio. A ratio, in general, is the quotient of two numbers or quantities. Often ratios are used to compare the relative difference in the sizes of the numbers or quantities. For example, the July discharge of Swiftcurrent Creek (536 million ft 3/mo) is 530% larger than the July discharge of Grinnell Creek, which is found from the ratio of the absolute difference in their sizes (536 – 85) to the smaller size (85) and multiplying by 100 to make it a percent (note the million ft 3/mo cancel out in the ratio). In ratios of this type, the result has no units. Other types of ratios have mixed units. Discharge (ft 3/mo) and discharge per unit area (ft 3/mo/mi 2) are examples. Rates have mixed units and typically include the word per. Temporal rates are one common type; temporal rates are per time. An example is velocity (distance per time); another is discharge (volume per time). Spatial rates are another common type; spatial rates are per distance, per area, or per volume. An example of the first is gradient (elevation change per horizontal distance); an example of the second is population density (number of people per area); and example of the third is density (mass per volume). The ratio of discharge to drainage area (watershed) gives the areal rate of discharge production in the watershed. We simply need to divide the two discharges we have by the drainage areas of the respective creeks. 12

Carrying out the plan So, now we test the hypothesis and calculate the areal rates. Put the following function in Cell H 2: =E 7/1. 1; similarly, put the following function in Cell I 2: =F 7/30. 9. Last, reformat Cells H 2 and I 2 so there are “ 1000 separators” and no digits past the decimal point. When we do all this, we obtain ratios with the units cubic feet per month per square mile (i. e. , ft 3/mo/mi 2), and we can affirm or reject the hypothesis. Your spreadsheet should now look like this. Have we affirmed our hypothesis? Return to Slide 15 Note that we haven’t proven our hypothesis to be true. We have merely affirmed it. We technically don’t prove hypotheses to be true, because hypotheses are negated the first time they are shown to be not true. Think about it—if the sun were to rise in the west tomorrow, then it would no longer be true to say that the sun rises in the east despite the fact that neither you nor anyone else had ever before seen it rise in the west. 13

Looking back Which is heavier, a kilogram of basalt (a volcanic rock) or a kilogram of water? There are two likely answers this question. First, one might answer “basalt” – of course a rock is heavier than water; it sinks in water. Or, one might say – hey, this is a trick; they weigh the same; each one weighs a little more than 2 lbs (1 kg). In the first case, the interpretation of “heavy” is density, mass per unit volume. The density of basalt is 3. 3 g/cm 3. The density of water is 1. 0 g/cm 3. Density is an intensive property: if you cut the rock in half, the density of each half is still 3. 3 g/cm 3. That’s because density is an intensive property – the intensity of mass. The second interpretation, on the other hand, is that “heavy” refers the extensive property – the amount of mass. Cut the rock in half, and each half is 0. 5 kg. Density and mass are two different kinds of properties. This module illustrates the distinction between extensive and intensive properties. Using an extensive property, the discharge of Swiftcurrent Creek is 6. 3 times the size of Grinnell Creek. But the watershed of Swiftcurrent Creek (30. 9 sq miles) is 28 times the size of Grinnell Creek’s watershed (1. 1 sq miles). So, of course the intensive property, discharge per area, would be larger for Grinnell Creek than Swiftcurrent Creek. In fact the areal rate of discharge for the Grinnell Creek watershed is 4. 5 times (28/6. 3) that of the Swiftcurrent Creek Watershed. Hydrologically, the take-home message is that the larger areal rate of discharge generation (from precipitation, snowmelt) occurs at the higher elevation. This accords with the phenomenon of orographic lift, as well as the observation on Slide 8 that led to our hypothesis. 14

End-of-module assignment 1. 2. 3. 4. Print and turn in your spreadsheet. Answer the questions on Slide 8. Answer the question on Slide 13. What would the discharge per month in Grinnell Creek be if it maintained its climatic and geologic properties but were as large as the Swiftcurrent Creek watershed? 5. Compute discharge per month per square foot for both watersheds. How deep would the water be if all of this discharge were spread evenly over the surface of the entire watersheds at a single point in time? Hint: Be sure and cancel as many of the units as possible. 6. Compute the ratio of discharge per month per square mile in Grinnell Creek to discharge per month per square mile in Swiftcurrent Creek. Calculate the difference in the discharge per month per square mile between the two creeks. By what percent does the larger discharge per month per square mile exceed the smaller? 7. You are a watershed manager living on the east slope of the Northern Rocky Mountains just south of Glacier National Park. You are primarily interested in maintaining natural discharges in your larger rivers located in the lower elevations of your watershed. You can only protect part of your watershed. Would you preferentially protect land in the higher elevations or the lower elevations? Why? 15

Endnotes 1. The effects of orographic lifting can be seen throughout the American West where air flow is generally west to east, i. e. , from the Pacific Ocean across the continent. In the figure above, note that precipitation is generally highest in the mountains (e. g. , the Sierra Nevada, the Cascade Mountains, and the Rocky Mountains) and generally lowest in the rain shadows immediately east of the mountains (e. g. , the Great Basin and the Plains). Return to Slide 5. 16

Endnotes (cont. ) 2. This high flow occurred at a different time of year than any of the other high flows. The other high flows occurred in late spring and summer, when snow that has been stored throughout winter was slowly released by warm weather. This high flow occurred in early fall, when an early-season cold snowfall was immediately followed by a late-season warm rainfall, thereby quickly releasing both the melted snow and the new rain simultaneously. Return to Slide 6. 17