A study was conducted that updated methods for estimating the magnitude and frequency of floods in ungaged urban basins in Georgia that are not substantially affected by regulation or tidal fluctuations. Annual peak-flow data for urban streams from September 2008 were analyzed for 50 streamgaging stations (streamgages) in Georgia and 6 streamgages on adjacent urban streams in Florida and South Carolina having 10 or more years of data. Flood-frequency estimates were computed for the 56 urban streamgages by fitting logarithms of annual peak flows for each streamgage to a Pearson Type III distribution. Additionally, basin characteristics for the streamgages were computed by using a geographical information system and computer algorithms. Regional regression analysis, using generalized leastsquares regression, was used to develop a set of equations for estimating flows with 50-, 20-, 10-, 4-, 2-, 1-, 0.5-, and 0.2-percent annual exceedance probabilities for ungaged urban basins in Georgia. In addition to the 56 urban streamgages, 171 rural streamgages were included in the regression analysis to maintain continuity between flood estimates for urban and rural basins as the basin characteristics pertaining to urbanization approach zero. Because 21 of the rural streamgages have drainage areas less than 1 square mile, the set of equations developed for this study can also be used for estimating small ungaged rural streams in Georgia. Flood-frequency estimates and basin characteristics for 227 streamgages were combined to form the final database used in the regional regression analysis. Four hydrologic regions were developed for Georgia. The final equations are functions of drainage area and percentage of impervious area for three of the regions and drainage area, percentage of developed land, and mean basin slope for the fourth region. Average standard errors of prediction for these regression equations range from 20.0 to 74.5 percent.

The U.S. Geological Survey (USGS) South Atlantic Water Science Center (SAWSC) Georgia office, in cooperation with local, State, and other Federal agencies, maintains a long-term hydrologic monitoring network of more than 340 real-time continuous-record streamflow-gaging stations (streamgages), including 10 real-time lake-level monitoring stations, 67 real-time surface-water-quality monitors, and several water-quality sampling programs. Additionally, the SAWSC Georgia office operates more than 180 groundwater monitoring wells, 39 of which are real-time. The wide-ranging coverage of streamflow, reservoir, and groundwater monitoring sites allows for a comprehensive view of hydrologic conditions across the State. One of the many benefits of this monitoring network is that the analyses of the data provide a spatially distributed overview of the hydrologic conditions of creeks, rivers, reservoirs, and aquifers in Georgia. Streamflow and groundwater data are verified throughout the year by USGS hydro graphers. Hydrologic conditions are determined by comparing the results of statistical analyses of the data collected during the current water year 1 (WY) to historical data collected over the period of record. Changing hydrologic conditions emphasize the need for accurate, timely data to help Federal, State, and local officials make informed decisions regarding the management and conservation of Georgia's water resources for agricultural, recreational, ecological, and water-supply needs and for use in protecting life and property.

In 2011, new record-low monthly discharge occurred at 52 of 113 streamgages that have 20 or more years of data. These 52 streamgages are located throughout Georgia. Most of the State received lower-than-normal precipitation; from central Georgia to Florida, the State received 50-75 percent of normal precipitation. Normal is defined as a 30-year average for 1971-2000. New record-low 7-day average discharge occurred at 24 of 113 streamgages that have 20 or more years of data in 2011. The majority of these streamgages were located in southern Georgia. These maps represent hydrologic conditions during the 2011 WY compared to available historical data. The colors represent runoff (flow per unit area) as a percentile of long-term averages. Runoff was calculated for each basin and assumed to be uniform over the entire basin area. Only streamflow stations with a complete daily-flow dataset for the 2011 WY were used (U.S. Geological Survey, 2012c). For the first quarter of the 2011 WY (October-December 2010), much of the State was observing "below normal" and "much below normal" runoff conditions as a result of extreme temperatures and lack of precipitation during the preceding summer months of the 2010 WY (A). Little to no precipitation kept the majority of the State in drought during the second and third quarter of the 2011 WY (B, C). After receiving 50-75 percent of normal precipitation from central Georgia to Florida during the 2011 WY, the majority of the State was in an extreme drought during the fourth quarter as runoff was "much below normal," and large areas of the State observed some of the lowest runoff conditions on record (D).

The United States Geological Survey (USGS) Georgia Water Science Center (GaWSC) maintains a longterm hydrologic monitoring network of more than 260 realtime streamflow stations and more than 100 noncontinuous streamflow and water-quality sampling stations throughout Georgia. One of the many benefits of the data collected from this monitoring network is that it allows for the analysis of the overall hydrologic condition of the rivers and streams of Georgia. The following figures were developed using daily, monthly, and yearly statistics from the 2007 Georgia Water Science Center Annual Data Report (ADR).

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