Quantifying the Effect of Topography on Extreme Precipitation within the General Edgar Jadwin Watershed in Northeastern Pennsylvania

The Probable Maximum Storm (PMS) estimates for General Edgar Jadwin Dam, with a drainage area of approximately 64.5 square miles, were calculated for the watershed using NOAA’s HydroMeteorlogical Report (HMR) 51 (Probable Maximum Precipitation Estimates, United States East of the 105th Meridian), and HMR 52 (Application of Probable Maximum Precipitation Estimates - United States East of the 105th Meridian). A comparison between 50 mi² and 100 mi² was calculated using modern Graphical Information System (GIS) methods to determine the ideal size which should be used and how the size would result in differing Probable Maximum Precipitation (PMP) amounts for different duration. Results for commonly used duration of 72 hours were 30.67 inches for 50 mi² and 30.51 inches for 100 mi² for the watershed. At a duration of 6 hours the difference was 20.33 inches at 50 mi² and 19.88 inches for 100 mi² for the watershed.

Since General Edgar Jadwin Dam resides in the "stippled region" illustrated in HMR 51, this study also incorporated an analysis of the effect of local topography on precipitation patterns. The topographic effects, however, necessitated the use of several additional reports as guidance to quantify the impact of orographic enhancement of extreme rainfall events for the study. By implementing the techniques outlined in NOAA Technical Memorandum NWS HYDRO 39 (Probable Maximum Precipitation for the Upper Deerfield River Drainage Massachusetts/Vermont), NOAA Technical Memorandum NWS HYDRO 41 (Probable Maximum Precipitation Estimates for the Drainage Above Dewey Dam, Johns Creek, Kentucky), HMR 55A (Probable Maximum Precipitation Estimates – United States Between the Continental Divide and the 103^rd Meridian), and HMR 59 (Probable Maximum Precipitation for California), the orographic component of PMP for the region was calculated and a basin-specific estimate of PMP was determined. These steps revolved around the development of a K-factor (orographic enhancement factor) specific to the General Edgar Jadwin watershed. The K-factor calculation relied upon a combination of historical storm intensity observations (M-factor) and a ratio of precipitation frequency depths in the orographic and non-orographic region surrounding the basin (T/C ratio).

The storm intensification factor (M) is a dimensionless number that represents the percentage of Free Atmospheric Forced Precipitation (FAFP) the occurred during the most intense or core precipitation event. For many storms, this value varies with duration as more convective storms have greater intensities during shorter periods of time. This number, however, can be quite subjective as the “core precipitation event” is not always clear. In this example a value for M was calculated to range from 0.77 to 0.81 through the different selected durations ranging from 1 hour to 72 hours.

The ratio of T/C is defined as the orographic factor portion of K. The numerator (T), which can literally be gleaned from NOAA Atlas 14 publications, is easily obtained while the denominator (C) can now be more objectively calculated through the use of modern GIS methods. Since the convergence component (C) of the orographic factor is based upon non-orographic precipitation, it is integral to identify locations that are deemed to be “non-orographic”. A first step in this process was the digitization of the “stippled region” from HMR 51, which was developed by the authors of that publication to identify orographic regions in that study. Then as outlined in HMR 59, Step A in the development of C requires taking the 100-year, 24-hour dew point climatology values at the observed elevations and adjusting them to 1000-mb. Next is to create a smoothed contouring map across the area which represents the 1000-mb convergence component. Since the precipitation frequency estimates are heavily impacted by localized extreme storm events that have occurred in the relatively short period of extreme storm observations, this step requires a significant amount of smoothing through the identification and removal of outlier data points to create a relatively smooth pattern reflective of obvious moisture sources. As one would expect for this specific example, the effects of the Atlantic Ocean heavily influences an overall pattern that decreases in a northwesterly direction. The final step as outlined in HMR 59, step B requires taking those values and adjust them to the surface elevation. This completes the process to acquire C, which through a simple mathematical calculation, a final T/C value. For the General Edgar Jadwin Basin the average T/C ratio within the basin is 1.065, or a 6.5% increase in precipitation due to orographic enhancement. Of note, the highest values (around 1.15) was located in the far northwestern portion of the basin while the lowest percentages (near 1.00) are near the mouth of Dyberry Creek. These results correlate well with the observed topography as the former is mostly 1500-2000 feet above sea level and the latter 1000-1500 feet above sea level.

The final results at a 72-hour duration showed a total orographic enhancement factor (K-factor) of 2.4%. A range of the orographic enhancement factor (K-factor) was found to vary from as low as 2.2% at 24 hours to as high as 2.6% at 6 hours. One note of concern, if incorporation of storm specific data (the intensity of observed storms) into an analysis of the orographic effects of terrain in other locations could breed to the potential for a “double-counting” in the PMP process. Specifically, the “intensity” of the “core precipitation event” is already quantified within the Depth-Area-Duration (DAD) tables that are subsequently transposed to the follow-on orographic region. For this reason, the most appropriate portion of the HMR formula to quantify orographic enhancement of extreme storms may be the T/C ratio.