Posted: 5:59 am Friday, July 14th, 2017
By Kirk Mellish
Most counties and cities in Metro Atlanta still have water use restrictions in place. The reason is not the lack of rain or a drought classification, but rather concern for water supply going forward the rest of the summer and fall.
Keep in mind, it may rain somewhere almost every day, it does NOT rain everywhere every day. And the region that needs the most rain is too small to adequately feed Lake Lanier.
A combination of humidity, wind speed and temperature impact water evaporation and can be 25 million gallons a day or more for large lakes, drops of a foot a month or more are not uncommon.
Only the Northwest part of Georgia has a short-term rainfall deficit while longer term drought is gone for most of the state at least for now:
Cobb County Water Resource Manager Kathy Nguyen explains:
“With all the recent rain I just wanted to clarify a few things. Cobb County, as well as 11 other counties remain under a Level 2 Drought Response. Lake Lanier provides 70% of the region’s drinking water, through direct withdrawals or releases into the Chattahoochee River. The drought conditions have abated with recent rainfall, but Lake Lanier has failed to refill. It likely will not until the fall and winter recharge season, provided we remain in a similar climate pattern.
The state regulatory agency that declares drought response levels has not indicated that they plan to lift the level 2 declaration until Lake Lanier has recovered. Currently the Army Corps is managing within their rules, but with a focus on protecting Lanier since downstream reservoirs have recovered and have a larger drainage basin.
To briefly explain the situation with Lake Lanier, the Lake has a very small drainage basin, it must rain steadily and consistently in that basin to affect the Lake. Pop up storms are helpful in mitigating the need to irrigate and improving soil moisture. Unfortunately, they are not enough, in most cases, to even replace the volume of water lost daily to evaporation in Lake Lanier during the heat of the day.
The rainfall we are getting is helpful. Most yards and plants only need 1-inch of water weekly. Using a rain gauge you can measure the rainfall received at your property and only need to supplement that natural rainfall on your allowed watering days. Also if you use a rain barrel to collect the rainfall you can direct it to plants or use it for hand watering. This prevents excessive runoff and provides a source of water for supplemental irrigation. Please remember that unfiltered rainwater should only be used on non-potable plants. It should not be used on food gardens, because it can contain bacteria that could be harmful.
Inflows into Lanier from July 2016-April 2017 were the lowest on record.
This also happens to coincide with the recharge season. Even with a typical pattern for Georgia of high heat and high humidity days, giving way to isolated cloud burst thunderstorms this time of the year and that type of precipitation is not effective in recharging stressed water resources, particularly Lanier. Lanier holds roughly 60% of the storage for the Chattahoochee system but has only 9% of the available drainage area.
The metro area is in the headwaters of most of the regional river basins, meaning we always have the shallowest and narrowest rivers to depend on.
Cobb Water understands the frustration related to the ongoing drought response declaration, but caution is needed to protect the drinking water source. We appreciate everyone’s continued patience and compliance with the rules.”
There are numerous methods to estimate water loss from a lake: evapotranspiration calculations using the Priestly-Taylor method, the Penman method, the DeBruin-Keijman method and the Papadakis method among others. It’s science and math.
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