The links below provide an outline of the material for this lesson. Be sure to carefully read through the entire lesson before returning to Canvas to submit your assignments.
The emphasis on precipitation and surface flows in Lesson 2 is necessary to gain familiarity with how these sources are generated and how they become essential resources for both aquatic ecosystems and human use. The lesson and readings will present information about precipitation and surface flows, especially for rivers, as two of the three major water sources. The lesson will also help you become familiar with basic hydrologic terms, so you can explain the movement of water through the hydrologic cycle, and as the driver of dynamism in rivers, including floods and droughts.
By the end of this lesson, you should be able to:
This lesson is one week in length. Please refer to the Course Calendar in Canvas for specific time frames and due dates. To finish this lesson, you must complete the activities listed below.
Requirements | Assignment Details | Access/Directions |
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To Watch |
Watch/Read through the following PowerPoint files.
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Registered students can access the Microsoft PowerPoint files under Lesson 2 in Canvas. |
To Read |
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To Do |
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If you have any questions, please post them to our Questions? discussion forum located under Orientation and Resources on Canvas. While you are there, feel free to post your own responses if you, too, are able to help out a classmate!
…nearly everybody has a creek gurgling through their memories, a confiding waterway that rose in the spring of youth.
Peter Steinhart, The meaning of Creeks. Audubon, May 1989, p.22-23
Gaining an understanding the sources of water is fundamental to learning about water resources. Lessons 2 and 3 focus on the three primary sources of freshwater; precipitation and surface flows (Lesson 2), and groundwater (Lesson 3). In reality, of course, these sources comprise a highly integrated system known as a hydrologic cycle, which is a conceptual model representing both storage of water (referred to as reservoirs or pools), and the flows or exchanges between those reservoirs.
In Chapter 2 of the text, the important terms evapotranspiration, interception, and condensation are described. Formal definitions can be found in the text's glossary, beginning on p.351. Further information is available from the USGS weblink listed in Lesson 1 [2].
Water vapor in the atmosphere modulates the adsorption and emission of infrared radiation, representing the heat generated by solar radiation. As we will see in Lesson 5 on climate types, the tropics receive about twice the amount of solar radiation as polar regions, and thus, the natural global energy budget flows from the tropics along the equator toward both poles. Circulation patterns of water vapor in the atmosphere are the major driver of that energy cycle (60%), along with oceanic currents (40%) (Stevens 2011, from Sitch and Drake 2014, Chapter 2).
As we learned in Lesson 1, the phase changes among the liquid, solid, and gaseous states of water govern these processes. When water evaporates from the surfaces of soil, vegetation, or liquid water, energy is absorbed. When the reverse process occurs through condensation, energy is released as water vapor reverts to liquid water (a characteristic known as the latent heat of vaporization, Sitch and Drake 2014). When surfaces cool, water vapor condenses, producing dew (changing from a gas to a liquid). When this occurs in the atmosphere, the condensing water vapor eventually becomes a form of precipitation (i.e., rain, snow, sleet, etc.). As precipitation falls, it may be intercepted by plant or land surfaces before flowing across the ground to a body of water, or infiltrate into shallow or deep groundwater.
Differential patterns of precipitation across the Earth's surface, driven by these complex interactions between energy and water, create weather and ultimately define climate (further considered in Lesson 5). Climate variability, then, ranges between fairly predictable temperature and precipitation conditions and less predictable extremes, such as horrific storms and severe drought. Although not a major topic for this course, water vapor's influence on the natural greenhouse effect, estimated to be 60% (Kiehl and Trenberth 1997, from Sitch and Drake 2014), is exacerbated by release of human-generated greenhouse gases, primarily from burning of fossil fuels. Refer to the most recent reports of the Intergovernmental Panel on Climate Change [3] (IPCC, 2022, http://www.ipcc.ch/ [3]) for additional information.
Evapotranspiration is comprised of two interacting processes, the physically-based evaporation of water from surfaces, and the biologically-mediated transpiration of water loss from the stomata (specialized cells) of vascular plants that allow the “escape” of liquid water to water vapor as a means of cooling leaf surfaces (see Lecture 2.1 PPT). Again, once water vapor from any source accumulates sufficiently in the atmosphere, precipitation occurs, returning liquid water to the oceans and land surfaces.
Why is this critically important to patterns of freshwater use (coming up in Lessons 7 and 8)? Because over 50% of the world's potable water comes from rivers – the water people consume. So, let's turn our attention to Chapter 3, to examine the hydrologic behavior of flowing waters in rivers and streams.
Rivers and streams, and their associated floodplains, are common delivery systems for moving freshwater across a landscape to receiving bodies of water, basins, or the oceans. Thus, we will focus more on riverine systems than lacustrine (lakes) or palustrine (vegetated wetlands) systems. The readings and Lectures 2.1 and 2.2 PPTs were chosen to familiarize you with how river systems function across the globe. They provide important habitats for a tremendous variety of species of plants and animals, fish being perhaps the most obvious water-dependent group. They also serve human needs by providing water supplies for drinking, agriculture, wastewater, energy and industrial needs, transportation, and recreation. Extreme events, such as floods and droughts, unfortunately, cause destruction to natural ecosystems, property damage, and loss of human life. Floods are created by high volume rainstorms or snowmelts, or by ocean-generated storms such as hurricanes and typhoons. Droughts, essentially a severe shortage of rainfall or snowmelt over extended periods of time, can damage the same systems, but in different ways. The Lecture 2.2 PPT provides an overview.
The importance of the headwater portions of watersheds to the overall health of aquatic ecosystems in many regions cannot be overemphasized. For example, in the Mid-Atlantic Region of U.S., in the area surrounding Washington, DC, headwaters typically comprise about 67–75% of the contributing area of any given watershed. That is, the combined areas of terrestrial habitats, wetlands, floodplains, and headwater streams occupy two-thirds to three-quarters of the total area of the drainage basin for larger rivers (Brooks et al. 2013). Given this influence on downstream portions of large river watersheds, understanding the impacts of human activities on the ecological structure and function of headwater or tributary portions of watersheds is foundational for optimizing their conservation and management. It is also critically important to understand the linkages and contributions of headwaters to the downstream portions of watersheds where large rivers and broader floodplains dominate.
Increasingly during the past decade, ecologists and hydrologists have moved toward integrating the traditional studies of the upstream–downstream gradient of rivers to produce a more comprehensive view of all the aquatic components of watersheds, and their interactions with terrestrial areas (Fig. 2.1, Brooks et al. 2013). The dimensions of these systems include: longitudinal or first dimension, river channel, streambanks) with lateral or second dimension (e.g., floodplains, riparian corridors, wetlands) and vertical or third dimension (e.g., groundwater flows, hyporheic zone) portions. As you work through the materials, try to visualize how water flows through this ecosystem from a 3-dimensional spatial perspective, and with regard to time. When you are in the field, near a stream, lake, or wetland, try to conceptually imagine how water is supporting and flowing through that system. As you begin to understand the principles that explain surface and groundwater flows between the river channel and other components of the riverine ecosystem, your understanding of how hydrological and ecological processes operate in rivers, and how they affect human use of these vital resources, will come into focus.
The separation of aquatic components of watersheds is common in regulatory and management contexts where streams and rivers are treated as separate entities from lakes, wetlands, and estuaries. In the past, such a separation was more for the convenience of defining and managing these units, than it was based on any ecological principles. There are important hydrological and ecological linkages between streams, rivers, and their adjacent floodplains and wetlands, together, comprising a riverine ecosystem. When the interactions with terrestrial components are considered, a holistic aquatic landscape or watershed can be visualized. Increasingly, syntheses in the literature have begun to move beyond the stream or river channel alone, to incorporating linkages between rivers and the landscapes in which they flow, thus recognizing longitudinal, lateral, and vertical aspects of the riverine network (Brooks et al. 2013). We begin focusing on this theme of integrating waters here in Lesson 2, and we will further develop it in Lesson 3, especially for Assignment 3.1.
Perusing the readings for Lesson 2, in the order listed, will have you progress from how precipitation and surface flow sources are generated, and how they are manifested in riverine systems (Chapters 2 and 3 of Holden, 2014). Chapter 4 covers surface water quality in some depth. It can be skimmed to become familiar with the types of stressors that degrade water quality. The last reading covers the integration of waters, primarily for the Mid-Atlantic Region of the U.S., but it provides a thorough background, and extensive literature citations, for comprehending current thinking about riverine systems, especially with regard to hydrology, energy flow, and the three components of water quality – physical, chemical, and biological.
The readings for Lesson 2 are substantial; please budget your time carefully and allow sufficient time.
Chapter 2 of the text by Sitch and Drake provides a global perspective on our water cycle. Note that Figure 2.1 - a global hydrologic cycle - is similar to the more local hydrologic cycle you will attempt to construct for Assignment 3.1 in Lesson 3. All of the readings and PowwerPoint files for Lessons 2 & 3 are aimed at helping you do just that!
Holden (2020) – Sitch and Drake, Chapter 2 – The changing water cycle (p.23-52)
This reading is from the required course text.
Chapter 3 of the text provides more water hydrology and flow characteristics with which you should be familiar.
Holden (2020) – Holden, Chapter 3 – Surface water hydrology (p.61-94)
This reading is from the required course text.
The Lecture PPTs 2.1. and 2.2 are designed to cover the primary sources of water and lead you to an solid understanding of a hydrologic cycle - something that is relatively easy to conceive of, but much, much harder to measure. The Lecture 2.1 PPT introduces you to the surface water portion of the hydrologic cycle. Water budgets are briefly illustrated, but you will not be expected to compute a budget. A series of concepts and terms are introduced following along with the Background and Chapter 2 of the text. Watersheds (and HUCs) are described and illustrated with an emphasis on the dynamic nature of rivers and streams. We finish up with the climate predictions affecting water from the IPCC 6th Assessment reports (Reports — IPCC [4]). Remember to refer to the text's glossary and/or the USGS website for clarifying or supplementary information on these topics.
Lesson 2 – Lecture 2.1 – Surface Water and Precipitation - Brooks.
Registered students can access the GEOG 431 Lesson 2 Lecture 2.1 file located under Lesson 2 in Canvas.
Lecture 2.2 PPT covers the extremes of the water sources - floods and droughts - using lots of images and illustrations.
Lesson 2 – Lecture 2.2 – Floods and Droughts - Brooks)
Registered students can access the GEOG 431 Lesson 2 Lecture 2.2 file located under Lesson 2 in Canvas.
The Brooks et al. 2013 chapter provides you with an integrated view of aquatic habitats, not only the physical and structural attributes, but through processes and functions as well. There is quite a bit of ecological information packed into this reading, the details of which I would not expect you to master from this one chapter. The emphasis is on headwater systems in the upper reaches of watersheds, but much of the information is pertinent to freshwater resources in general, even though the subject matter is the Mid-Atlantic Region. Other chapters of this book, and similar ones, are available to you through Google Scholar and Penn State's arrangements with major publishers like Springer.
Brooks, RP, C Snyder, MM Brinson. 2013. Aquatic Landscapes: the importance of integrating waters. Chapter 1 (p. 1-37) in RP Brooks and DH Wardrop (eds.) Mid-Atlantic Freshwater Wetlands: Advances in science, management, policy, and practice. Springer Science+Business Media, 491+xiv pp.
The Brooks, Snyder, Brinson reading is available through Google Scholar [1]. Search for "Aquatic Landscapes: the importance of integrating waters." Registered students can access this reading in the Lesson 2 module in Canvas.
Chapter 4 of the text focuses on the chemistry aspects of water quality, particularly for major stressors on and threats to water resources. I suggest skimming this chapter and only delving into detail on topics that interest you.
Holden (2020) – skim Chapman et al., Chapter 4 – Surface water quality (p.99-142)
This reading is from the required course text.
There are no deliverables for Lesson 2. Please focus your time on the readings for the week and on your discussion post.
The emphasis in Lesson 2 on precipitation and surface flows was to gain familiarity with how these sources are generated, and how they become essential resources for both aquatic ecosystems and human use. Having completed the readings and reviewed the PPT lectures, circle back to the introductory page of Lesson 2 and make sure you can explain the basic aspects listed under Learning Outcomes, as well as the bolded terms through the lesson.
It's now time to move on to Lesson 3 and continue to assimilate additional knowledge about freshwater sources - this time, focusing on groundwater. For Assignment 3.1, you'll be applying your knowledge about precipitation, surface water, and groundwater and using it to create a hydrologic cycle for a geographic area near to your home base.
You have reached the end of Lesson 2! Double check the to-do list on the Lesson 2 Overview page [5] to make sure you have completed all of the activities listed there before you begin Lesson 3.