Pennsylvania Water Science Center
1) Why did the USGS create the Pennsylvania Drought Condition Monitoring web site?
The Pennsylvania Department of Environmental Protection (PaDEP) relies upon a number of parameters to monitor water supply drought conditions--precipitation, stream flows, ground water levels, and the Palmer Drought Severity Index. Information on each of these parameters is collected and reported by different government agencies. When this information was needed to make water-management decisions, PaDEP had to access several sources to get the information they needed, adding time to their decision-making process. In an effort to reduce the information gathering time, the USGS was asked to create a website which combined all parameters needed to assess current drought conditions.
2) Why are the drought status colors for some of the counties different from the icons in the county?
The color displayed for each county is that county's current drought status as declared by the Governor. The color of the icon in the county represents the current drought indicator status for that specific parameter. For example, when looking at the surface-water indicator map, the icon color of green would indicate that the streamflow site used as that county's surface-water indicator sites is currently experiencing flow conditions which would be considered normal for that station. The drought status of the county, as a whole, may be different because other parameters and factors are used to determine the drought condition.
3) If the parameter icons are different colors than the county's declared drought status why doesn't the Governor change the county's drought status?
These parameters are used only as indicators of the four stages of drought that Pennsylvania uses for drought management--normal, watch, warning and emergency. A single parameter or a combination of particular parameters does not automatically establish a stage of drought. Water managers must look at all available current information and forecasts to determine if conditions have changed enough and will continue at that level to justify a change in the county's drought status. The current normal conditions may only be temporary.
4) Why don't some counties have their own ground-water or surface-water indicator sites?
Currently, the USGS does not have suitable ground-water observation wells located in all counties of the Commonwealth. Until such time that each county has it's own representative ground-water observation well, we have determined which adjacent county's well best represents the ground-water conditions of the county that lacks an observation well. We also have determined that those surface-water sites within certain counties currently do not best represent that county's overall streamflow conditions. Sometimes the only surface-water sites within a county are in locations that are affected by regulation or diversion and therefore do not represent "natural" conditions.
5) What does 30-day moving average mean and how is it calculated?
A 30-day average for a particular day is the average value of the previous day's reading and the value recorded for each of the 29 days preceding that day. It is called a moving average because a particular day's 30-day average is determined by using the 30 days preceding that day. Each day, the newest value is added to the 30-day group of data points and the oldest value is removed.
6) If the 30-day moving average line is below the red band, does that mean that the water level is the lowest it's ever been?
Not, necessarily, but it does mean that it is the lowest 30-day average ever recorded. The percentile color bands, as well as the current conditions are based on 30-day moving averages - not instantaneous readings. The data presented on this web site do not show record instantaneous extremes. Instantaneous readings can be viewed by clicking on the link to 'Real-Time Graph' at the bottom of each graph.
7) Where can I find information on historical and extreme conditions for the parameter indicators sites?
A) For historic and extreme precipitation and Palmer Index information go to the following National Oceanic and Atmospheric Administration (NOAA) web sites:
Precipitation information from the National Climatic Data Center:
Palmer Index information from the Climate Prediction Center:
http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/cdus/palmer_drought/
B) For streamflow and groundwater level historic and extreme conditions do the following:
When viewing a particular streamflow or groundwater station duration graph, click on "Link to Real - Time Graph". There you'll see a scroll-down menu titled "Available data for this site" Use that menu to select general site information, statistics, historic data and more. If information or data you require is not available at this site, contact our Information Specialist. Our Specialist's contact information is available at bottom of this FAQ list.
8) Why do some graphs have only green or green and yellow bands and no orange or red bands?
There are two main reasons for this situation to occur. First, the range in water levels or streamflow may be relatively small. The 5th percentile value could be so close to the 10th percentile value that the graph appears to not have a red band. Secondly, we may not yet have sufficient historical record for that station to reliably provide that statistical information.
9) What is the Palmer Index?
The Palmer Index (more properly called the Palmer Drought Severity Index) was developed by Wayne Palmer of the U.S. Weather Bureau (now the National Weather Service) in the 1960's and uses temperature and rainfall information in a formula to determine soil dryness. It has become the semi-official drought index.
The Palmer Index is most effective in determining long-term drought---a matter of several months---and is not as good with short-term forecasts (a matter of weeks).
The Palmer Drought Severity Index is a value computed as a function of both
meteorological and hydrologic data to measure soil moisture conditions. It is
compiled weekly by the Climate Prediction Center of the U.S. National Weather
Service for each of their climatological regions. The computed value ranges
from +6.0 to -6.0. Pennsylvania uses values between -2.0 and -2.99 to indicate
a drought watch condition, between -3.0 and -3.99 to indicate a drought warning
condition and -4.0 or below to indicate a drought emergency condition.
10) Why are different periods of precipitation departure used?
See the explanation at http://pa.water.usgs.gov/monitor/precip/explanation.html
11) For drought conditions monitoring, why are the periods of record for the surface-water and ground-water indicator sites different for the same county or between counties?
U.S. Geological Survey data collection activities have been varied throughout our history. We have collected different data at various locations for various purposes. The drought conditions monitoring program is a relatively new program, but many of the sites in the network have been active for decades, while others are fairly new depending on their original data collection purpose. The reasons for the differences in periods of record are many, but the important thing is that water managers have all the data available on which to base their decisions. One aspect of the USGS mission is to provide the best available data so that management decisions do not include a bias based on inadequate knowledge.
12) Why is it important to know the period of record?
Water managers who use our information must be aware of the periods of record for our sites. Data collected from a site are more representative of climatic conditions if they include measurements during extreme periods of drought and flooding. For example, if data collection at a new site was initiated during a time when steady precipitation was received throughout the period and no flooding occurred nor wells went dry, it would have been collected during normal or non-extreme conditions. If during the next year less precipitation occurred and conditions became drier, we would be measuring record lows and water managers may become concerned. However, since data were collected for only a few years, it would difficult for water managers to know whether the current period is normal for that area and the first few years of data were collected during a wet period, or whether the current conditions are really a dry period and the first few years of data were collected during a normal period of precipitation. The more data that are available will contribute to water managers' understanding of current conditions and how they relate to historical periods. When comparing current conditions with the past, the longer the period of record, the better.
13) How can I find out more information about precipitation and the Palmer index?
In addition to the web sites listed in response A to question 7, visit the following web sites of the National Oceanic and Atmospheric Administration:
http://www.nws.noaa.gov/ (U.S. National
Weather Service Home page)
http://www.cpc.ncep.noaa.gov/ (NWS
Climate Prediction Center Home page)
Pennsylvania Drought Regulations and Water Conservation Tips:
Pennsylvania Department of Environmental Protection (Drought Information Center)
Drought Definitions:
Drought Information Resources:
Drought-Related Frequently Asked Questions:
National Drought Mitigation Center:
Drought Monitor
U.S. Department of the Interior |
U.S. Geological Survey
URL: http://pa.water.usgs.gov/monitor/faq.html
Page Contact Information: Pennsylvania WSC Webmaster
Page Last Modified: Monday, 06-Aug-2007 15:07:14 EDT
