Many of our readers have expressed bewilderment that lake levels don’t seem to increase in proportion to the amount of rain they observe. They look outside and see an intense downpour, but lake levels appear to remain the same. Some of you have asked directly why this happens, so here’s the explanation.
Where does all that rain go?
The amount of rainfall that ultimately makes it into the reservoirs is highly dependent on many hydrologic factors such as location, intensity, duration, season, temperature, evaporation, and even groundwater levels. If one inch of rain falls uniformly across one of the reservoirs, then that reservoir will theoretically rise by one inch. However, rainfall is seldom distributed uniformly.
When it rains, much of that water gets lost as it falls on dry ground. Some is lost to vegetation, puddles and evaporation. We need enough precipitation to saturate the dry ground before runoff makes its way into creeks and streams, and over time into the reservoir system.
The condition of the ground prior to the storm is very important in the development of runoff. Typically across our basins during the summer, only one to 30 percent of the observed rainfall makes its way into the reservoir. Evaporation takes a big toll on runoff, as well as water already in the reservoir. Measured as you would outflows, the three reservoirs average a total loss of 200 to 1200 cfs, depending on the season. This is purely due to the reservoirs large surface area. Location and spatial coverage of rainfall across the basin affects runoff the most and is probably the least understood phenomenon.
How does the Corps calculate rainfall in the basin?
Before the implementation of Doppler Radar, the National Weather Service (NWS) used ground station rain gages to measure precipitation. While the amount recorded at each ground station was accurate for that particular site, it was difficult to provide an accurate figure to represent spatial coverage over the entire Savannah River Basin.
Today, the added technology from Doppler Radar provides us near-continuous coverage across the southeast. Every hour the NWS creates a gridded rainfall product and we use this to help estimate accumulated rainfall. Using a 1.6 kilometer resolution, the rainfall grid overlays a basin map depicting the drainage area for the reservoir system. From this overlay, we can calculate the rainfall for each hour across the entire area.
Once compiled, we enter the basin average hourly precipitation into our database. Use of the hourly entries from the database helps us develop daily and even monthly average rainfall numbers. By keeping the monthly average in mind, we can also track each reservoir’s rainfall estimate through time. This broad-based data shows precipitation trends which we use to determine if the average is above or below the expected levels for a particular season of the year.
It helps to keep this broader perspective in mind when observing heavy rainfall in your area. It is not uncommon that your friends a few miles away are enjoying a sunny picnic in the park, while any adventure outside for you involves rain boots and an umbrella.