A night of heavy rain being slapped against the window, intense lightning beaming into the room as if the lights were turned on, and a loud clap of thunder that rattles the windows will awaken most anybody, but for most Central Texans it will be met with a smile on their face. The following morning everybody seems friendlier than ever, as people begin to compare rainfall totals and talk about the nights weather over a cup of coffee. The recent droughts have taken a toll on area water supplies and this has made everyone excited and cautiously optimistic when we DO actually receive some much needed rainfall that the drought may be breaking. The most often asked question that I hear is, “what did that rainfall do to our lake and aquifer levels?”
There are MANY factors that come into play that figure into how much runoff and recharge the creeks and rivers will get from any given rainfall event. When asked how much rainfall will it take to fill the lake back up, or to refill the aquifer, there really are no simple cut and dried answers. The hydrologic cycle can be viewed as a complex algebra equation that contains many changing variables. Some of the variables that affect the amount of rainfall that will runoff into creeks and streams to recharge aquifers and refill lakes are:
- Amount of rainfall
- Intensity of rainfall
- Rainfall coverage
- Terrain slope
- Vegetation type
- Soil type
- Land use
- Soil moisture
- Time of year
Perhaps the two most important factors to consider when recharge and runoff are desired to refill water sources, would be the soil moisture and the time of year. The time of year actually can limit many of the other factors listed such as evaporation, transpiration, and to some extent land use and vegetation type. Below I have a hydrograph of the Ellenburger-San Saba aquifer in the city of Burnet. At the bottom of the graph I also have included daily rainfall events.
As you can see from the hydrograph, there are only two time periods in the past 4 years that significant recharge has occurred for the Ellenburger-San Saba aquifer in this area. Both instances occurred between October through late May. This is the best time for water to runoff and recharge the aquifers because many plants are dormant through winter, evaporation is low, and the water consumption is low.
If you look closely at the rain events on the bottom of the graph, you will notice that the frequency of rain events is high. This is the most important factor, because several rain events in a row separated by no more than a few days, will saturate the soil and allow the next rain storm to produce greater runoff. There are instances where we have received many inches of rain with little recharge. For example in early September 2010 there were nearly 6.5 inches of rain recorded in Burnet from Tropical Storm Hermine, unfortunately the rain fell on very dry soil, as there was no rainfall for more than a month in advance of this system.
When we look at a broader picture of the entire hydrologic cycle and area water supplies, they all react the same way, no matter if it’s surface water or groundwater. The 6 aquifers in this region all have a local outcrop area that recharges the entire aquifer. This makes the aquifers in our area susceptible to seasonal climate changes. Now this is only true for the outcrop portions of aquifers, if you are in an area that is far removed from the outcrop, there will be little change regardless of the climate.
Below I have a graph of the monthly averages of both Lake Buchanan and Travis. The Trends are exactly the same as the aquifers trends.
As mentioned earlier there are numerous factors that influence the impacts of a given rainfall event. Since January 2008, there have been only two short periods of time that there was sufficient runoff to partially recharge the aquifers and lakes.The rainfall events were frequent enough to allow the soil to become saturated before heavy rainfall events occurred during the late fall and winter months. This is the pattern to hope for and to be aware of that will allow the water resources of the area to increase.
El Nino Update
March-April-May 2013 precipitation outlook from NWS Climate Predication Center 2012
In the next couple of the months the District will begin to install satellite telemetry on a few selected monitor wells, that will allow for “near real time” viewing of the aquifer levels. The District will work in cooperation with the Texas Water Development Board, which has 150+ wells across the state equipped to view “near real time”. You can view TWDB monitor well site at http://www.twdb.state.tx.us/groundwater/data/waterlevel.asp. Once at the site, you can also download the files to be viewed in Google Earth.
Stay tuned for future updates and thanks for reading,