LWI Region 3 Modeling Overview

4.1.2 Aerial Reduction Factor (ARF) Development Using MetVue AREAL REDUCTION FACTOR (ARF) DEVELOPMENT AND APPLICATION IN ROM MODELS

Background { Applying point precipitation data (e.g., from NOAA Atlas 14) to large HUC8 watersheds requires ARFs to account for the reduction in rainfall depth as storm area increases. { Historically, ARFs—also referred to as Depth-Area Reduction (DAR) factors—have been used to reflect this spatial reduction. Technical Paper 40 (TP40) was previously used for basins < 400 mi.²

Need for ARF in Region 3 { Region 3 contains HUC8 watersheds ranging from 500 to over 2,000 mi,² necessitating the use of ARFs or DARs to accurately model storm impacts. { The U.S. Nuclear Regulatory Commission (NRC) developed national ARF datasets using gridded PRISM precipitation data (Figure 4-6). { Applying a single ARF across an entire watershed may lead to significant underestimation of rainfall in small, upstream headwater basins. { Smaller drainage areas typically require ARFs closer

Source: U.S. NRC

to 1.0, reflecting minimal reduction, while downstream areas require lower ARFs due to accumulated flow area.

Figure 4-4: Accumulated rainfall for a tropical storm, September 10-17, 2009, is presented in the background (color) for Lower Mississippi River Forecasting Center. Precipitation pattern for Sep 16, 23hr is shown in gray color. The eye of the tropical storm is to the left of Region 3 basins.

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LWI REGION 3 TRAINING

LOUISIANA WATERSHED INITIATIVE