If 1” of rain falls, how much does the lake rise?

You might have a decorative rain gage outside your window to quickly let you know how much rain fell on your yard last night. These handy gadgets are a great way to determine if your lawn and garden need additional watering.

So, it must work the same way for calculating how much a lake will rise after a rainfall event, right?

Well, not exactly. Things are rarely as simple as they first appear.

So, if 1 inch of rain falls on a Brazos River Authority water supply reservoir, does the lake rise 1 inch?

First, it’s important to remember that if rain has fallen on the lake, it’s more than likely made a splash on adjacent properties. A portion of that rain could then “runoff” the land and drain into the reservoir, contributing to an increase in that reservoir’s level, said Peyton Lisenby, Ph.D., BRA water resources planner.

The amount of runoff and subsequent lake level rise depends on several hydrologic and geographic factors, such as the geographic distribution of rainfall, the intensity with which the rain falls, and the infiltration and storage capacity of the surface the rain falls on.

What?

Basically, it’s

• where the rain falls,
• how heavy the storm is,
• if the rain sinks into the ground or runs off and
• the amount of water already in the lake.

OK, but if I live next to a lake and 1” of rain falls, the portion that hits my driveway and “runs off” to the lake isn’t likely all going to make it there.

Right.

Hydrologists know that the portion of total rainfall that actually “runs off” to the reservoir is always less than the total amount of rainfall. Lisenby said that the difference between the total rainfall amount and the runoff amount is called the runoff ratio.

A runoff ratio can vary from 10% to 95% depending on the type of surface the rain falls upon and the intensity with which the rain falls.

Paved surfaces, for instance, yield higher runoff percentages, while permeable surfaces, such as soils, yield lower runoff percentages since the water sinks, or infiltrates, into the ground. Also, a more intense rainfall generally yields higher runoff volumes because the ground can’t soak up the water as fast as it’s raining.

“Moreover, the distribution and intensity of rainfall can change significantly across rainfall events, along with the soil moisture content and infiltration capacity of the land surface, so that there is no constant runoff ratio for any landscape,” Lisenby said.

He said that predicting how much a reservoir may rise in response to rainfall is inexact.

But, we can make estimates using a few known quantities.

Let’s use the BRA water supply reservoir, Lake Granbury, as an example.

When the lake is relatively full, it takes about 650-acre feet of inflow to raise Lake Granbury about 1 inch. An acre-foot is the amount of water needed to cover one acre (43,560 square feet) with one foot of water. That’s equal to 325,851 gallons of water, enough to cover an area about the size of a football field with one foot of water.

So, if 100 square miles of land surrounding Lake Granbury were entirely paved with concrete and asphalt and received an evenly distributed but intense ¼ inch of rain, the event would generate about 1,300-acre feet of inflow, and the lake might come up about 2 inches, Lisenby said.

If that same 100 square miles of land were made up of deep, sandy soil and received the same evenly distributed but intense ¼ inch of rain, the event would generate about 130-acre feet of inflow, and the lake would come up less than ¼ inch.

“The reality of lake level rise after an actual rainfall event is much more complex than either of these scenarios because of the many different kinds of land surfaces and the uneven distributions of rainfall amount and intensity,” Lisenby said.

There is a handy tool available to view a BRA water supply reservoir elevation without having to do any tricky math.

Go to www.BrazosBasinNOW.org. From there, click on “Reservoir Elevation.” Now zoom in to one of the lakes and click on the associated dot. A box will pop up showing the latest elevation readings and how that number has changed over the past several days. www.BrazosBasinNOW.org will also give you a look at the streamflow coming into a reservoir as well as how much rain has fallen in the area.

For any questions about the data or understanding the information on www.BrazosBasinNOW.org, please contact us at information@brazos.org or call 888-922-6272.