Where Water Flows in the West

By Brett Bovee

This book is a summary overview of water resource management in the Western United States. The book is organized into three parts, each addressing an important aspect of the subject: Part 1 provides information on natural systems and the basics of hydrology, Part 2 provides an overview of water rights, and the legal concepts that have been adopted to allocate water resources to various uses, and Part 3 provides context for future allocations of water resources by describing influential factors and recent data trends.

• Language: English
• Publisher: BookBaby
• Release date: Dec 1, 2019
• ISBN: 9781543988826

Book preview

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Chapter 1: The Science of Water Flows

A journey to understand water flow in the West must start somewhere, and this book starts with water in its natural state. It seems obvious to start here, and to understand the unobstructed conditions before one comprehends what we have done to modify the flow of water to meet our demands. The science of water flow provides a good foundation because although we have certainly made a lot of modifications, natural flows still describe a lot of what we see in the West. This chapter provides some of the basic ideas behind the movement of water; and if I had to summarize the science of water flow, it would simply be that water has a persistent desire to fall downhill.

Hydrologic Cycle

Rivers are one of nature’s most impressive and inspiring features. They are destinations for our travels, focal points of our memories, and subjects of our books. Rivers are also what most often come to mind when we think of flowing water; but the truth is that water is flowing all around us, under our feet and in the air. Rivers are only the most visible piece of a much larger cycle of water.

It is strangely comforting to be reminded that water on earth is not created or destroyed – it simply moves around¹⁸. Any standard hydrology textbook provides an illustration of the various parts of the ever-continuous hydrologic (or water) cycle, and a typical illustration is shown in Figure 1.1. To understand our relationship with water, it is helpful to divide this cycle into three categories: (1) water that we can see without effort, such as rainfall and river flows; (2) water that is visible if we go looking for it, such as groundwater and soil moisture; and (3) invisible water, such as water vapor in the air. We have managed and monitored the first category for a very long time, evidently since at least 3000 B.C¹⁹. We have done the same for the second category since about the middle of the 20th century (in the Western U.S. at least²⁰) and are continuing to get better at it. We have often considered the last category as lost water, because we are fairly incapable of managing it even if great expense is made to monitor it.

The hydrologic cycle describes the movement of water between phases, with each phase representing either a relatively temporary or long-term state. Phases of the hydrologic cycle include rain, rivers, groundwater, water vapor and clouds, large water bodies, and others. Rivers are a temporary state for water, linking rainfall on the land surface with oceans and other large water bodies. When water is said to be lost, it has simply transformed into a different state within this overall cycle and is only lost to the extent that society cannot really utilize it anymore.

Although rivers and streams make up a relatively small volume (less than ten percent) of the total combined flow in the global water cycle, they remain the focal point of water use and management because they are the terrestrial portion of the cycle on which we are reliant. Rainfall over the world’s oceans is almost four times greater than rainfall over land, but rainfall over land creates rivers, which are a unique and vital resource for us living on the land.

Figure 1.1: Hydrologic Cycle

Comment: Figure from W. Brutsaert. Hydrology: An Introduction. Cambridge University Press. 2005. Notice the counter-clockwise circulation of water flow in the figure from water vapor in clouds to precipitation, to runoff & infiltration, to evaporation & transpiration, and back to clouds. This circulation represents the hydrologic cycle.

This chapter explores the natural flow of water from rainfall to river mouths and coastal estuaries. The exploration starts with why rain falls where it does in the West, follows the fallen rain as it takes one of several routes to become river flow or groundwater, and follows the flow downhill until it reaches the ocean.

Rivers are borne from precipitation, which is a term used to define all types of water falling from the sky, such as rain, snow, sleet, hail, and others. What is lacking in the Figure 1.1 illustration, and what creates water management and conflict hotspots in the West, is the spatial variability in this cycle, and in particular precipitation. Precipitation does not fall uniformly across land masses, creating rain forests in some areas and deserts in others. Most of the Western U.S. is considered semi-arid, meaning that it receives between 10 and 20 inches of precipitation per year. But variations in precipitation are as boldly evident in the West as they are elsewhere in the world. The green lush environment of the Pacific Northwest coast is so very different than the desert landscape of the Southwest.

For many reasons, precipitation can be viewed as a master feature of hydrology which defines the local environment. Plants, animals, cultures, and (long-term) economies are often dictated by how much rain falls at a given location. Figure 1.2 shows a map of average annual precipitation for the United States. This map explains so much of what we see on a cross-country road trip. The Eastern forests, great Midwest plains, Southern bayous, snowcapped Rockies, Western rangelands, and Southwest deserts are all explained, at least in part, by the amount of rainfall. Considering its significance to how rivers are born, and ultimately how much water is available, the next section will discuss climate patterns in the Western U.S. with a focus on precipitation.

Comment: Map produced by the Oregon Climate Service using PRISM data. Oregon State University. PRISM Climate Group. http://prism.oregonstate.edu/. PRISM is a precipitation mapping tool that stands for Parameter-elevation Relationships on Independent Slopes Model.. Some of the more interesting observations are found: (1) in the Midwest states going from 40 inches in the eastern parts to 15 inches in the western parts, which corresponds to the gradual rise in land elevation and rain shadow effects of the Rocky Mountains; (2) in mountainous states with over 40 inches at high elevations down to less than 10 inches at lower elevations; (3) in the coastal Pacific Northwest receiving around 100 inches; and (4) in the far Southwest receiving less than 5 inches.

Western Climate

For water science, climate is a critical subject because it helps explain rainfall amounts, and rainfall amounts largely explain the amount of water that nature provides for a given location. Climate science aims to explain why it rains 10 inches one year, and 30 inches the next year; or why one area of the West is a desert while another area is a rain forest.

To understand typical climate patterns, one first has to take a few thousand steps back and look at large air masses that exist in the atmosphere. An air mass represents a large pocket of air that sits in a particular location long enough to acquire the temperature and humidity properties of the surface at that location. Around North America, there are seven dominant air masses that affect climate. At the intersection of the polar air masses to the north and the tropical air masses to the south, patterns of air movement known as jet streams develop that drive dominant weather patterns in the Western U.S.

The jet stream is often discussed in weather reports, usually depicted as a superhighway for winter storms. A jet stream is a fast flowing current of air in the upper atmosphere that moves west to east in North America. Actually, most air currents in the continental U.S. move in a west to east direction, which itself explains a lot of the dominant precipitation patterns. The mid-latitude or polar jet stream typically moves across Canada in the summer months and dips down into the continental U.S. in the winter months. A separate sub-tropical jet stream is located south of the continental U.S. The jet stream pathway is sinuous, with ridges and troughs; and this pathway contributes to the formation and path of low pressure systems.

A basic principle of climate is that warm air rises and cool air sinks, due to density differences. On a large scale of hundreds of square miles, this principle leads to the formation of high pressure (cool air sinking) and low pressure (warm air rising) areas. Low pressure systems are the dominant weather driver in the continental U.S. The location and movement of these low and high pressure systems can be quite variable and are often shown on weather maps to describe the short-term forecast.

Although variations exist, a dominant trend in the winter season is for a low pressure system to develop in the Pacific Ocean off of the western coast of North America, and then to move into the mainland U.S. following the jet stream pathway. This low pressure system dissipates, or breaks apart, over the Rocky Mountains, and then reforms on the eastern side of the mountains and continues across the Midwest towards the Great Lakes. In the summer months, low pressure systems often develop at more northern latitudes near the Canadian border, and high pressure systems (associated with nice weather and a lack of rainfall) dominate much of the Western U.S. Figure 1.3 illustrates some dominant pressure systems for the winter and summer seasons.

A notable characteristic of this dominant low pressure movement track is its avoidance of the Southwest U.S., which has the effect of causing more arid conditions in this part of the country. You may have heard of a weather pattern called El Nino, and perhaps you have even heard of its opposite twin sister, La Nina²¹. These terms refer to the relative latitude (north / south position) at which winter low pressure systems form in the Pacific and move inland across the Western U.S. A strong El Nino signal means that the low pressure systems are expected to form at a more southerly latitude, roughly west of southern California, and move east into the U.S. over the Southwest. The more southern presence of these low pressure systems means that El Nino years bring more winter-time precipitation to the Southwest region. Just the opposite happens in La Nina situations. A strong La Nina signal means that the low pressure systems are expected to form at a more northerly latitude, and move into the West around the Seattle and Vancouver area. La Nina years usually produce more rain for the northern areas of the West, and drought conditions in the Southwest.

The reason that the location of low pressure systems is important is that weather fronts form where low and high pressure areas meet, and weather fronts often produce precipitation. Weather fronts form at the intersection of two air masses of different temperatures, often as a result of low pressure systems moving air masses around in a counter-clockwise direction. Frontal systems are characterized by the transition from one air mass to another and named after the incoming air mass properties. Cold fronts and warm fronts are often associated with precipitation events.

Unlike weather fronts, which cover a large area of usually several hundred square miles, thunderstorms can be localized and caused by small-scale convection (or rising) of air. Convective rain clouds are formed as warm, moist air from the land surface rises up and into the cooler atmosphere, causing water condensation and eventually precipitation. Convective precipitation is typically of short duration with rapidly changing intensity since the clouds have a limited spatial extent.

The same convective air movements that cause thunderstorms can also produce monsoon seasons, particularly in the Southwest. A monsoon condition is simply when the land surface is heated by the sun, causing warm air near the surface to rise, and then resulting in moist air from the ocean or other large water body to move inland to replace the lifted air. This moist air from the ocean causes widespread rain or more commonly localized thunderstorm development in July and August, particularly in the Southwest²².

The map in Figure 1.2 also shows that precipitation in the Western U.S. is largely defined by elevation. Higher elevation areas receive more precipitation than lower elevation areas because of orographic effects. Orographic rainfall effects simply mean that air cools and condenses as it rises, and if condensation is sufficient, then precipitation will occur. Higher elevations usually receive more rainfall because the air is cooler. Aspect, which refers to the cardinal direction, plays an important role when considering orographic effects. Air masses typically move west to east in the Western U.S. due to the dominant direction of the trade winds. When an eastern moving air mass encounters a mountain range, the air will lift, which in turn may cause condensation and precipitation on the western (or windward) aspect of the mountain range. As the air mass continues to move east, it has lost much of its water content, and dry conditions are often found on the eastern (or leeward) aspect of the mountain range. Figure 1.4 illustrates this orographic effect.

Figure 1.3: Typical Pressure Systems by Season

Comment: Low Pressure system follows the jet stream, dipping down into the U.S. during the winter and remaining high in Canada during the summer. El Nino conditions occur when the Low Pressure from the Pacific follows the more southerly track into the Western U.S.

Figure 1.4: Orographic Effects Causing Precipitation

Comment: Air cools and condenses as it rises over a mountain ridge, and the condensing water vapor forms rain clouds and precipitation. After the air has moved over the mountain, the air has lost much of its water vapor and is dry as it descends down the mountain. This process results in wet conditions on the windward (wind-facing) aspects of mountains and dry conditions on the leeward (downwind)