Water is necessary for life as we know it. So much so that astronomy research efforts have focused on finding planets with water above all else.
Closer to home, water attracts wildlife and is great for plants. While not everyone can live near a river or lake, everyone has the option of building a water feature. In this article we will cover how to make a small wildlife pond or water feature that can be maintained year round with only rainwater.
How much water does it take?
According to this paper from TX state, the average Net Evaporation rate is between 16 – 32 inches (per year) in the Texas Hill Country. This means that, throughout the year, each square foot of water surface loses 15 – 32 inches more than what’s directly collected by rain landing on it.
This report is based on pan evaporation, a technique in which water is placed in a relatively small container and the amount of water is measured over time. This is not a perfect method, as there are a lot of differences between a pan and a lake or river, but the consensus seems to be that the derived evaporation rates are proportional to those in natural bodies of water.
In the Hill Country in particular, the evaporation rate increases in the summer and decreases in the winter, relative to spring and fall.
According to this EPA report, the average pan evaporation in inches for Austin, TX is as shown in the graph.
If we want to maintain a water feature for local wildlife in Hays county, we need to contend with the evaporation as seen above. One excellent way to do that is by taking advantage of the average 35 inches of rainfall in the area with rainwater collection. Below is a plot of the average precipitation and high and low temperature in San Marcos, TX (source).
Comparing the evaporation plot and the rainfall plot, we can see that a lot of rainfall happens in May and June, the two months leading up to the part of the year with the highest average evaporation.
How many inches of rainfall collection do we need to account for each square foot of water?
Summing across the bars in the first plot for annual evaporation, we get that Austin has a total of 73.2 inches of evaporation on average across the year. Hays county gets an average of 36 inches of rainfall a year (source). Assuming that Austin (in Travis county) and Hays county are close enough for our math, that means that we need about twice as much surface area of roof capturing rain as we have surface area of water in order to make up for evaporation. Of course we’re ignoring that the water feature also directly captures rainfall. It’s better to be safe than sorry for our calculations though, so we’ll ignore that bit.
How much rain water storage do we need?
Looking at the rainfall and evaporation graphs above, we can see that the biggest difference between rainfall and evaporation in a single month is in July or August, where the rainfall is about 2 inches and the evaporation is about 10 inches. We’ll need enough rainwater stored in a tank to make up for the 8 inches of difference.
How much water would we need stored for the most dry month of the year?
Let’s suppose we have a water feature with 16 sq ft. of surface area that is at least one foot deep. If we’re losing the top 8 inches of every exposed square foot of water, then we’re losing (16 cu. ft. * 8/12) = 10 ⅔ cu. ft. of water. According to the data, we typically get 2 inches of water in August. Supposing our rain water collection is off of a guest house or other outbuilding with a 24 sq ft.roof. That would collect 2” * 24^2 / 12” = 96 cu. ft. of water in August. There would be plenty of water even in the worst month of the year, so we could simply use the smallest tank required: one that equals the water loss of 10 ⅔ cu. Ft. of water. For reference, a chest freezer holds about 10 cu. ft.
Keep in mind that these numbers are all based on averages. In a drought, we should expect to have less rain and more evaporation. It may be best to at least double the rainwater collection surface (and storage) to water surface area ratio to make it through tough times. Keep in mind that animals will also be drinking the water, so plan for as much leeway as you’re comfortable with.
Important considerations for your water feature
Your water feature is an ecosystem. That means that predators will come and try to eat your fish and any other critters that are attracted to your water feature. This is simply a part of nature, but if you favor birds and other animals that are typically prey, you can make sure that a small area around your water feature is clear of bushes, shrubs, and tall grasses that can hide predators.
Another thing to consider is the topography of the area surrounding the water feature. If it is not at the top of a slope, it will gather a lot of water and detritus when it rains. You may be able to work around this with a berm or some decorative rock landscaping.
If you want to have fish (which is probably a good idea as they will eat mosquito larvae and frog eggs) you should make sure that the water feature is at least 2 feet deep in some parts so that your fish can hide from predators. Deeper water is also better for many aquatic plants, which will help keep the water clean.
Other wildlife should be able to easily climb out of the water, so there should be a sloped area on at least one edge. Don’t forget this part or you may accidentally trap animals in your water feature.
Your water quality depends on a lot of factors. One of the easiest to manage is how much sun it gets. Shade helps prevent overheating and algae growth; however, if you plan to introduce aquatic plants you will want the appropriate amount of sun for them.
You may want to filter your water for a better look or for a better ecosystem for fish and other life. If you have plants in your water feature then you may be able to support a small population of fish without further intervention. Even better, you can use an organic filtration method known as a bog filter if you want to keep your water clear and get a bonus garden out of your water feature.
A bog filter is simply an area of 12 inch deep pea-sized gravel through which water is recirculated from the pond. The rule of thumb is that you should have a bog filter sized around 1/5th the surface area of the water. Any amount will help and provide an excellent place for plants to grow. Bog filters can be active or passive. An active one will require a pump and will be more effective at a smaller size. If your water feature is not near an outlet, you may wish to use a solar pump, which can be found fairly cheap online. Here is a youtube video explaining active bog filtration.
A simple water feature will do just fine with some plants, tiny fish, and some shade. If you want to make it even more attractive to animals you can also add a bit of flowing water. The rule of thumb is to have a pump that can move twice as many gallons per hour as your water feature holds.
The actual construction
These two articles (1, 2) cover pond construction in depth. Here in the Hill Country, we don’t have to worry about freezing much, so concrete construction will provide you with a low maintenance water feature.
Another approach is to buy a liner from a water garden supply store and simply lay it in the hole you’d like to turn into a water feature. You can secure it with rocks around the edges.
A third option is to spread clay along the bottom of your water feature. Unfortunately, here in the Hill Country you are unlikely to have enough clay in the ground without supplementing it. Clay that you bring in may also need to be compressed. Here is an article on how to line a pond with clay. This is a great approach because it doesn’t require putting plastic in the ground, but you will need to be aware of how much runoff the area gets as it can wash clay away.
The short version is that you need to make a hole in the ground, make it so that your water won’t soak into the ground, hook it up to your rainwater tank, then enjoy the new ecosystem you have created. There is a lot of details to think about but your efforts will be rewarded with all the wildlife you’ll get to view.