February has become the traditional month for reviewing water conditions. All of the data for the major water condition indicators have been collected and analyzed, so an evaluation of 2007 is in order.There are three measurements used to evaluate a water year: rainfall, streamflow and ground water levels. Rainfall is measured by observing rain gauges. In our local watersheds, there are many rain gauge readers who are recording daily rain - fall levels throughout the year. This information is summarized monthly and sent to the Chester County Water Resources Authority which prepares rain - fall charts for the entire county.
Streamflow measurements are made on a regular basis by stream gauges installed by the U.S. Geological Survey. On the Red Clay there is a stream gauge just below the confluence of the east and west branches near Marshall's Bridge and further south in Delaware at Wooddale and Stanton. On the Brandywine there are many stream gauges, the oldest at Wilmington, Chadds Ford, Downingtown and Modena. These stream gauges automatically record levels on a 15 minute interval. This information is collected by the U.S. Geological Survey and reported on their Web site and through annual publications.
Some stations on the Brandywine have records going back nearly 100 years. Conse - quently, streamflow averages have been established for each day of the year as well as monthly averages. In addition, historic high and low levels have been established. These historical data help us to make comparisons with current flows.
The third indicator is ground water level. This is measured at locations throughout the different watersheds. On a monthly basis measurements are made of unused wells in different geologic formations. One well, located in the Doe Run Water - shed which is a tributary to the west branch of the Brandywine, has a continuously reading gauge providing ground water level information on a daily basis. Because of the long history of ground water measurements, Chester County has established levels for normal, drought watch, drought warning, and drought emergency conditions.
With these indicators in mind, let's take a look at the results of 2007. By most standards, 2007 was a fairly boring water year. There were no major floods or drought. In fact, most months were typical for rainfall with the exception of September. September was one of the driest months in many years, recording less than half an inch of rain. Nevertheless, by the end of the year, rainfall over the local watersheds had averaged close to 48 inches, which is a very normal year. Some areas saw rainfall closer to 44 inches and others received over 50 inches of rain.
Since water conditions are dependent on rainfall, a normal rainfall year generally produces near normal ground water and streamflow levels. These two indicators mirror rainfall levels but respond a little bit later. For example, ground water levels will tend to trail rainfall levels by two to three weeks. Stream - flow levels are very similar. Consequently, when rainfall was near or above normal in the early part of the year, streamflows and ground water levels were also above normal. Conversely, when rainfall dipped dramatically in September, both stream - flow and ground water levels followed. When rainfall came back strong in October and December, both ground water and streamflow levels rose to normal or slightly above normal levels.
2007 continues a five - year record of normal or above normal rainfall conditions. The last major drought was 2001/2002. It might seem that five years is a long time without a dry year. It is difficult to predict what type of water year we will have since the science is not that precisely developed. Forecasters will look at past water years and see if any patterns can be identified. There is also some speculation that climate change may be involved in predicting water patterns as well.
Since I am not bashful about predicting what sort of water year 2008 will be, I will go on record that we are overdue for a dry year and that rainfall for 2008 will be about 41 inches. This would not be considered a drought, although the pattern of rainfall might result in some sort of a drought status during the year. We are entering 2008 in good condition with ground water and streamflows at normal levels. How well that continues remains to be seen, but my money is on a drier year for 2008.
? Robert G. Struble Jr. is executive director of the Brandywine and Red Clay valley associations, the oldest and second oldest watershed organizations in the country. He lives in East Bradford Township.