Chapter 2
Assessing Your Site's Water Resources

But especially as I drink the last of my water, I believe that we are subjects of the planet's hydrologic process, too proud to write ourselves into textbooks along with clouds, rivers, and morning dew. When I walk cross-country, I am nothing but the beast carrying water to its next stop.

—Craig Childs, The Secret Knowledge of Water

It's important to begin harvesting water knowing how your site fits into larger water flows and knowing how much water there is to harvest.

This chapter builds on the principle of long and thoughtful observation by beginning with a description of the hydrologic cycle. This will help you understand your site's water flow in the context of global hydrologic patterns and interconnections. Next the chapter moves to local watersheds and subwatersheds, where small-scale portions of the hydrologic cycle occur. You will learn how to determine the boundaries of your site's subwatershed, where to concentrate your water-harvesting efforts, how to calculate rainfall and runoff volumes that affect your site, and how greywater can add to your site's harvestable water resources. The chapter ends with the story of an Arizona couple striving to live within their site's rainwater budget. Appendix 3 "Calculations" and appendix 5 "Worksheets" are intended to be adjuncts to this chapter.

The Hydrologic Cycle—Our Earth's Circulatory System
The following is primarily drawn with permission from water-harvester Ben Haggard's great little book, Drylands Watershed Restoration:

Of the world's total water, a small percentage is fresh water. The majority of that is tied up as ice in polar ice caps and glaciers. The remainder is continually recycled in order to support the world's living systems. This recycling is known as the hydrologic cycle.

Water is evaporated from the oceans and precipitated as rain or snow over the continents. This water is absorbed by plants and evapotranspired back into the air. This pumping water back into the air by plants accounts for much of our atmospheric water. This water forms clouds and rains again ...Forests play an important role in maintaining [and retaining] rain in the landscape

Raindrops form around ice crystals in clouds. These ice crystals require a nucleus for their formation. Dust, tiny bits of leaf, and bacteria are among the particles that initiate rain. A number of natural systems [such as forests] encourage rain by giving off columns of tiny particles that seed the clouds causing drops to form.1

Raindrops are soaked up by the living sponges of forests, prairies, and desert thornscrub. These, along with their associated leaf drop, topsoil, and the cavities created by burrowing animals, help hold onto that water and slowly release it.

If forests, grasslands, and other rain seeders and sponges are removed from a landscape, the landscape begins to dry. Rain can become less common and vegetation has trouble reestablishing. Rivers and streams become dry.

Ben Haggard continues:

Rivers and streams generally flow throughout the year; in spite of the fact that rain is a localized and fairly infrequent event in arid settings. Even in rainy climates, rain occurs a relatively small percentage of the time. Rivers have a sustained flow because most of the water is actually stored in the soil where it slowly releases into the drainage. In disturbed watersheds, this slow and sustained release is disrupted. Water runs rapidly off the ground's surface rather than soaking into the ground. This process creates floods followed by drought. To repair such a watershed, infiltration of the water into the ground must be increased.

Living systems create complex interactions with water. Water falls as rain. Trees intercept this water, directing it into the ground where a layer of organic material deposited by the trees absorbs and holds it. Some of the water flows slowly through the ground where it supports the growth of forests and the sustained flow of rivers. Rivers act as transportation networks, allowing nutrients from the forests to wash downstream and fish and other animals to swim upstream, importing phosphate and other minerals into the forest. Water in the landscape also attracts and supports wildlife, the active planters, fertilizers, and maintainers of the forest. The forests breathe water back into the air, where it condenses around particles also released by the forests. The clouds form and the entire process repeats itself.2(Reprinted with permission from Dryland Watershed Restoration—Introductory Workshop Activities by Ben Haggard, copyright 1994, Center for the Study of Community.)

Each of us depends on and is a part of the hydrologic cycle. As water moves through the global cycle, so it moves through the watersheds of our communities, the subwatersheds of our individual sites, and our own bodies, which are over 70% water. We can slow, cycle, and enhance that flow as we improve our lives and community by harvesting rainwater. First, we need to identify and thoughtfully observe our watersheds.

Watersheds and Subwatersheds—Determining Your Piece Of the Hydrologic Cycle
A watershed is the total area of land from which water, sediments, and dissolved materials flow by gravity to a particular end point. At the largest scale this endpoint might be a river, lake, or ocean. A watershed is a geographic entity clearly defined by high points or ridgelines that split the flow of water, creating the boundaries of each watershed. Watersheds are made up of many smaller subwatersheds, each defined by lower elevation ridgelines that further split the flow of water and direct portions to particular endpoints. These subwatersheds are made up of a collection of still smaller subwatersheds. If you trace the boundaries of all these subwatersheds, the pattern you'll see looks like pieces of jigsaw puzzle forming an interconnected whole. The terms "watershed" and "subwatershed" are relative terms that can refer to a variety of scales of water drainage areas.

On a large scale, your land will almost surely be part of a regional watershed that drains thousands of square miles of land, creating streams and rivers. My Tucson home is a part of the Santa Cruz River watershed, which covers approximately 8,600 square miles in southern Arizona and northern Mexico. Within this regional watershed, water drains toward me from upslope areas, away from me to downslope areas, and ultimately flows to the Santa Cruz River. The Santa Cruz River watershed is in turn a subwatershed of the larger Gila River watershed, which is is turn a subwatershed of the still larger Colorado River watershed.

	Fig. 2.1A. The top of the watershed
	Fig. 2.1B. Ridge of hill defining watersheds

The land area of a city flowing toward one regional watershed consists of smaller subwatersheds throughout the city, broken up into many smaller neighborhood-sized subwatersheds, made up of property-sized subwatersheds, made up of still smaller subwatersheds consisting of residents' roofs, yards, patios, and driveways. Each small urban-scale subwatershed directs flowing water toward a different urban endpoint. Urban landforms, buildings, and parking lots act as the dividing "ridges" between these tiny watershed. Pitched roofs divide the flow of water between front and back yards. Parking lots act as gradually sloped fields. Roads act as linear ridges if raised, or as drainageways if built as a lower element in the landscape.

Fig. 2.1C. Watersheds and subwatersheds, the larger picture

If your site is at the very top of a hill it is also at the top of a watershed, because all runoff water will be draining off the hill away from your site (fig. 2.1A). If your site is at the bottom of the hill, your site's watershed will be that part of the hill's slope that drains or sheds water toward your property. Most likely, the water on the other side of the hill will drain to a different endpoint, so it will be part of a different watershed (fig. 2.1B). But, if runoff from the other side of the hill does eventually drain toward your site—perhaps via an arroyo or wash curving around the hill—then it too is part of your site's watershed. In addition to this local hill, there may be other areas of land that drain toward your site. If so, the watershed affecting your site is even larger (fig. 2.1C).

Identify Your Site's Watershed and Observe Its Water Flow
To assess your site's water resources, first define the boundaries of your property and the watershed directly affecting your site. A topographic map will give you a general idea of your watershed's "ridgelines"—the tops of slopes that determine if water is flowing toward or away from your location (see box 2.2). You can walk your land in the rain to see which way water runs to help understand land slope and the extent of the watershed draining to your site. Erosion patterns can clue you in to flow patterns when it's dry (see appendix 1). If runoff flows across your land, pay particular attention to what direction it comes from, its volume, and surfaces it flows over. Potential contaminants—such as oil from streets and pesticides from yards and fields—might be picked up and carried in this runoff water (See figure 2.2 as to how you might conceptualize your site's runoff and runon).

Fig. 2.2. An urban home watershed with arrows depicting runoff flow

Create a Site Plan and Map Your Observations
Creating a site plan helps you see and make use of site resources and challenges; integrate your water-harvesting system with the rest of your site (see chapter 4); and place and size water-harvesting earthworks, vegetation, and tanks appropriately.

You can use your own paper (perhaps you can create your own water-harvesting journal) or use the worksheets and grid paper provided in appendix 5.

Leave wide margins around the outside of the paper, and draw your property's boundaries "to-scale" inside these margins. If you choose a scale of 1/8 in = 1 foot, a measured distance of 1 foot on your site will be drawn on your plan as 1/3 inch. Use the wide margins to map the locations where resources—such as runoff from your neighbor's yard—flow on, off, or alongside your site. Draw buildings, driveways, patios, existing vegetation, natural waterways, underground and above-ground utility lines (to avoid damaging them and yourself), and other important elements of your site to-scale on the plan. Make multiple copies of your basic site plan on which to draw a number of drafts of your observations and ideas. (Figure 2.3 is a sample site map).

Fig. 2.3 Site map (overhead/plan view) of a 4,400 square foot property. In an average year of 12 inches of precipitation the site receives 32,912 gallons of rainfall "income."

Calculate Your Site's Rainfall Volume
Once you've defined and mapped the boundaries of your site, use the calculations in box 2.3 to determine average volume of rain falling on your site each year. This is the "income" side of your "water budget." (Again see figure 2.3).

Map Your Site's Catchment Surfaces and Calculate Their Runoff Volumes Roofs, paved surfaces such as driveways and patios, and compacted earth surfaces such as paths are useful catchment surfaces from which to harvest rainfall. Indicate on your plan any catchment surfaces that drain water off your site (for example, a driveway sloping toward the street) and subtract this lost runoff volume from your site's calculated average annual rainwater resources. (You can devise strategies to recapture that lost runoff later.)

Indicate on your plan any catchment surfaces draining water onto your site from off-site (for example, runoff from your neighbor's yard that drains into your yard). Add this bonus runoff (or "runon") volume to your site's calculated average annual rainwater resources. See box 2.4 for the example site map with runoff volumes and runoff coefficients.

Note: This book and the rainwater-harvesting principles emphasize the harvest and utilization of localized runoff and runon high in the watershed before it enters a drainageway. Once the water is in a drainageway, it is to remain there, although its flow can be slowed to allow for more infiltration as is the case with a check dam.

Fig. 2.5. Size of a roof catchment: measuring only the outside dimensions—or "footprint"—the roof's edge. Ignore the roof slope; no more rain falls on a peaked roof than falls on a flat roof with the same footprint.

Where to Get Information On Rainfall Rates and Other Climatic Data
Seek out data on your area's annual rain and snowfall. Note record high and low temperatures in each season to determine suitable plants to grow. Find out the longest periods of drought and of rainfall to get an idea of the volume of water storage to plan for. Check evaporation rates: The higher the evaporation rate, the more important water-harvesting strategies are that limit evaporation losses. Assess prevailing wind direction and intensity—you may want to plant windbreaks irrigated with harvested rainwater.

Good information sources include:

• In the United States: National Weather Service's website at www.wrh.noaa.gov. Locate the weather stations closest to your site and find out their elevations. Download data from those stations that are most like your site.
• In Arizona: Arizona Meteorological Network at ag.arizona.du/azmet. Evaporation rates, prevailing winds, soil temperatures, and minimum/maximum temperatures are listed for various sites. For other states contact your local agricultural extension service for similar meteorological networks.
• The U.S. National Forest Service compiles data for remote weather stations, though the data is not as comprehensive nor standardized as the above two resources. This data can nonetheless be valuable for rural sites since a Forest Service weather station may be closer to a given site than one monitored by other agencies.
• Local airports, since they collect and record climatic data.
• Buy a rain gauge from a hardware or garden store to begin keeping precipitation records for your site.

Estimate Your Site's Water Needs
Determine the "expense" side of your water budget by estimating your household and landscape water needs. Your water bill reflects current water use. The user-friendly website www.h2ouse.org provides water use rates for household appliances, and recommended conservation strategies. Estimated water needs of plants can be obtained from the local agricultural extension office (or see, for example, appendix 4 "Example Plant Lists and Water Requirement Calculations for Tucson, Arizona"). Better yet, take a hike to observe native plants that grow naturally in your area on rainfall alone. Water needs of plants will vary widely depending upon the plant, its size, and the soils, climate, and microclimate in which it is planted. Keeping that in mind, in Tucson, Arizona a mature 20-foot (6-m) tall and wide native mesquite tree will use about 3,000 gallons (11,355 liters) of water per year, while a mature 16-foot (4.8-m) tall and wide exotic non-native citrus tree will use about 8,000 gallons (30,282 liters) per year. 4

Compare your site's water needs to the volume of rain falling directly on, or flowing through, your site. How much of your domestic water needs could you meet by harvesting rooftop runoff in one or more tanks? How much vegetation could you support by harvesting rainfall directly in your soil? How do you balance your water budget using harvested rainwater as your primary water source?

1. Haggard, Ben, Drylands Watershed Restoration: Introductory Workshop Activities, Sol y Sombra Foundation. (Santa Fe: Center for the Study of Community, 1994).

2. Ibid.

3. U.S. Environmental Protection Agency (EPA), "What is a Watershed?" www.epa.gov/win/what.html

4. Figures determined from calculations from Pima County, Arizona, Cooperative Extension, Water Resources Center, Low 4 Program. "How to Develop a Drip Irrigation Schedule" handout.

Copyright © 2006 by Brad Lancaster. All rights reserved.