Lake Level and Rainfall – Lake Conroe, July 2012

Over the last few weeks, there have been numerous reports on the evening news of torrential rainfall and flooding in Harris and Montgomery counties and other surrounding communities. These
reports have caused many people to question why Lake Conroe is still two feet below normal pool level. The simple answer is that there just hasn’t been enough rainfall in the right locations.

In order for rainfall to impact the lake level in Lake Conroe, it has to fall in Lake Conroe’s watershed. A watershed is the area of land that drains into a particular lake or reservoir. This area is
sometimes called a drainage basin. Lake Conroe has a relatively small watershed (444 square miles) compared to Lake Houston’s watershed (2,700 square miles), so rain must fall within a relatively small target area to impact Lake Conroe’s water level.

Lake Conroe’s watershed is located north of the Lake Conroe dam, and most of the heavy rain and flooding over the past few weeks occurred in Harris County and southern Montgomery County. As of July 23rd, the rainfall gauge here at the Lake Conroe dam has received just over 27 inches of rain since January 1, 2012. With an annual average rainfall of 48 inches for this part of the state, that amount is considered to be about normal for the seven-month period from January through July. During this period of time, Lake Conroe’s water level has risen approximately 6 ½ feet. Although we missed the majority of the heavy rains over the past few weeks, Lake Conroe did receive some rain and rose approximately 8 inches.

The graph below shows both lake levels and releases from Lake Conroe since January of 2010. The green line indicates releases made either from excess runoff in early 2010 or releases made to the City of Houston in response to the exceptional “drought of record” in 2011. As the graph shows, there have been no other releases from Lake Conroe during this period.