Watershed

Every drop of rain that falls on your landscape ends up somewhere. Where that water falls and where it ends up is called a watershed.

Rainwater may be absorbed by plants, sipped by local wildlife, or it may evaporate back into the atmosphere. Rainwater can also wash away valuable topsoil, carrying fertilizers and pollutants into our groundwater supplies, rivers, lakes, and oceans. In fact, the California Native Plant Society tells us that urban drool is the #1 source of ocean pollution. To prevent water waste, pollution, and runoff, a new approach to landscape design was created to protect our precious watersheds.

Watershed approach to landscape design

The watershed approach to landscaping uses garden design, installation, and maintenance methods that take advantage of natural processes to create spaces that are water efficient, while providing abundant plant growth, good habitat, and an enjoyable place to be. The watershed approach captures, cleans, and collects rainwater to slow, spread, and redirect its flow in ways that reduce the need for other irrigation. The benefits of using a watershed approach include:

• improved habitat
• increased biodiversity
• pollution reduction
• flood control
• reduced nutrient leaching
• carbon trapping
• water conservation

How much rainfall do you get?

Every yard is different, but your average 2700 sq. ft. roof in San Jose, California can collect more than 25,000 gallons of water each year! Your mileage may vary, but you can use the USGS rainfall calculator to determine how much rain water falls on your roof in any given storm, simply by entering your home’s footprint dimensions (length x width) and the amount of rainfall measured by you* or reported by your local news station. During a storm that drops one inch of rain onto a half-acre lot turns out to be nearly 14,000 gallons of water! Rain barrels, ponds, swales, and filtration tanks are all different ways you can collect rainwater.

DIY rain gauge

* You can measure your garden’s rainfall with a store-bought gauge, or you can easily make your own with an old plastic soda bottle. Simply cut the top portion off and invert it into the bottle to work as a funnel. After it rains, use a ruler to measure how many inches of water was collected. Using multiple bottles will give you a more accurate reading. I urge you to record the rainfall collected for a more realistic idea of what your garden is working with each year.

​(I put mine in a flower pot to prevent tipping.)

Where does that water go?

All too often, rain water falls on buildings, roads, and concrete, where it collects pollutants and debris, and carries them to our groundwater. Our garden plants never have the opportunity to soak it up. On the other end of the spectrum, rain water either floods an area, carrying away valuable topsoil, or it gets stuck in one place, where soil, plants, and organisms begin to rot. Using the watershed approach removes those problems by studying where water comes from, where it goes, and taking actions that redirect water flow to be more efficient and environmentally sound.

First flush and absorption areas

First flush refers to the first 3/4 to 1 inch of rain that falls after a dry period. This rain water contains higher levels of pollutants and debris than the rain that follows. Can you filter those pollutants out or redirect this water to less vulnerable areas? After that water is dealt with, how much permeable soil is needed to absorb your expected rainfall? First, you will need to know how deeply your soil absorbs water. You can determine this by going outside after a few days of rain and digging in with your shovel. How far down did the rain actually go? This number can help you determine how big of an absorption area you will need for the expected rain.

Example: You live in a 1,000 square foot house in San Jose, CA, where you receive an average of 15 inches of rain each year. Using the USGS rainfall calculator, you would discover that your house can collect 9,351 gallons of water in a year. To absorb all that water, you would need to divide the volume of water by 7.48 for a per foot absorption area. (There are 7.48 gallons of water in a one cubic foot of space.) This gives you 1250 square feet needed to absorb all that water, assuming that your soil absorbed water down to a depth of one foot. If it only went down 6 inches, the 1250 sq. ft. figure would have to be doubled. If you don’t have that much space, how can you prevent runoff? What if that’s not enough water?

What are other sources of irrigation water?

Be sure to check with your local municipality for laws regarding water collection (there have been countless wars started over water rights). You can collect water from your bath or shower, as it heats up, in a bucket. You may be able to redirect the outflow from your washing machine to irrigate ornamentals. Even the water left over from cooking pasta and vegetables makes useful water for the garden.

How much water do you really need?

There is no excuse for wastefulness when it comes to water. You might be surprised at how little water you and your garden actually need. Our household has reduced water consumption to only one-fourth of what it was three years ago and we get more production from the garden! This is possible by:

• reusing water
• inspecting and repairing leaky sprinklers (we stopped using them altogether)
• installing native and other plants that are more suitable to your microclimate
• improving soil structure with compost
• reducing evaporation with mulch
• collecting water from the roof with rain barrels
• getting rid of unused lawns (out front)
• providing plants with only as much water as they will actually use, when and where they need it
• sheet mulching to reduce competition from weeds
• watering early in the morning at ground level with drip lines and soaker hoses
• identifying areas with poor drainage and redirecting water toward areas that need the water

Other factors to consider when using the watershed approach:

• Where does water go when your soil is saturated?
• How much area is covered with impervious materials, such as concrete, and where does that water go?
• Are their slopes on your property that can cause faster runoff? Can they be terraced?
• Does your landscape have chronic problem spots?
• Are there any migratory birds, insects, or animals that you would like to provide for? (Monarch butterflies are one example.)

Start using the watershed approach in your yard by asking yourself these questions:

1. Where does rainwater currently go when it rains on your yard or garden?
2. Can you collect rainwater from downspouts in rain barrels or other containers?
3. Can you redirect rainwater to areas that can make better use of it?
4. Does flowing rainwater move topsoil when it rains and how can this be altered?
5. Have you installed plants that thrive in your microclimate?

What’s really nice about the watershed approach is that it takes advantage of natural processes that have evolved over thousands of years to work without any help on our part. Native plants and those suited to your microclimate require less care, which translates into less work, less expense, and a healthier environment.

Activity: Inventory your landscape’s water needs

1. Start by capturing a Google maps satellite image of your property. (You may want to print out a few copies - and color coding the information makes it easier to use)
2. Label as many plants as you can and learn more about their water needs. You may want to sort plants by water use: red for high, orange for medium, or yellow for low)
3. Label rain gutter downspouts and the route taken by rain water from those downspouts.
4. Label sprinklers, drip lines, soaker hoses, and other forms of irrigation.
5. Identify areas with poor drainage.
6. Use this information to find areas where water can be collected, slowed down, or redirected to reduce water waste and runoff.

Every drop of water that you are able to use more efficiently protects the environment and your bank account.