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Harvesting the Rain Part 1: Passive rainwater harvesting

Arizona Daily Star


This dry winter is the perfect time to think about rain harvesting in your yard or home. Rainwater harvesting is either passive or active. In passive rain harvesting, your soil is the storage medium for the rain, while active rainwater harvesting involves the use of tanks, gutters and similar equipment.

Passive rainwater harvesting is the cheapest — you don’t need to spend money on tanks and you can do the work yourself. This article is the first of two and will discuss passive rainwater harvesting, while an upcoming article in the Jan. 16 Tucson Garden Guide newsletter will focus on active rainwater harvesting.

Harvesting the rain has many advantages. For one, it will reduce your water bills by utilizing free water for your landscape plants. Rain is the best type of water for plants, as it helps flush salts and minerals through the soil. In this respect, it is far superior to irrigation with treated water, which actually concentrates salts and minerals in the soil, contributing to caliche development.

When done correctly, rainwater harvesting can also eliminate water erosion on your property and help with flooding. In addition, in urban areas, containing the water that falls on your property can greatly help reduce the runoff of water in the streets — a huge source of water pollution in our ephemeral rivers and streams. 

Despite being a desert, our Sonoran landscape gets a decent amount of rain most years. We have two rainy seasons: the summer monsoon and the winter rains. In total, we get an average of 11 inches of rainfall per year. That may not sound like much, but each square foot of your yard collects 0.6 gallons of water for every inch of rainfall. So for example, over a 6,000 square foot yard (a little over a tenth of an acre) that adds up to about 3,700 gallons per inch of rain, or 40,000 gallons per year of free water.

The basic idea behind passive rainwater harvesting is that you shape and contour your soil to maximize water collection and infiltration. You create structures from the soil and rocks (called earthworks) to channel the flow of water where you want it. Thanks to gravity, water will always flow downhill, so you can shape the soil so that the water flows along a certain path and into basins where it will infiltrate.

One very simple example is building a depression around a newly-planted tree. When it rains, water will flow into the depression and collect there, allowing it to infiltrate into the soil rather than running off. Mulching the depression will help prevent evaporation of the water and is a key step in rainwater harvesting. Other basic water harvesting earthworks are berms (basically little hills, usually 6 inches or less in height) and basins (also called swales). To create these, scoop out the soil from an area you want to collect the water (the basin) and pile the soil on the downhill side like a little fort (the berm) to keep the water from running off. You can also construct terraces, rock dams, gravel drains, and various other earthworks to channel your rainfall. Which earthworks you use depends on the slope of your land, how fast the water flows across it, and how much of the water you want to harvest.

Another key aspect of passive rainwater harvesting includes the use of landscaping materials that are permeable to water for your hardscape areas. Brick pavers, for example, allow water to infiltrate between their edges, while a solid concrete deck or asphalt driveway does not. Permeable hard surfaces help to prevent water runoff, which ends up flowing down our streets causing flooding and washing oil, gasoline, and other pollutants into our rivers.

Since passive rain harvesting only involves moving soil and rocks, it can be a DIY project if you need to save money. It can be a lot of work, though, particularly if you are doing it all by hand. The City of Tucson has been offering a rebate for homeowners installing rainwater harvesting systems since 2012, so you can check out their website for more information. If you would like help in designing or installing a rainwater harvesting system, local nonprofit Watershed Management Group provides those services.

It can also be difficult to create an extensive passive water harvesting system if you have a lot of existing vegetation that you want to keep. You don’t want to damage tree and shrub roots, for instance, while excavating your earthworks. In situations like this it is usually best to pile berms around the drip line of the plants rather than to dig in the vicinity of the roots and risk harming them. Using a lot of mulch can also help you conserve water in your landscape.

The other downside to passive rain harvesting is that you only have the water when the rain falls. You can spread out the benefits of it over time by using mulches to prevent water evaporation from the soil. Sooner, or later, though, our hot sun will dry out the soil. In order to keep using the free rain, you will need tanks to store the water over a long time period. This is active rainwater harvesting, and it will be the focus of the next article.

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