*3.4. E*ff*ect of Regional Sample Size*

In our analysis, we opted to use the same sample size of the radar record (11 years) to highlight the differences in the estimation methods, i.e., pixel-based estimation vs. The spatial bootstrap approach. However, since one of the advantages of the spatial bootstrap technique is the ability to incorporate more information from neighboring pixels, it is important to assess the effect of the size of the regional sample. Figure 10 compares the percentage change in the mean quantiles and the confidence interval (95–5%) of PFEs when using pixel-based approach and regional spatial bootstrap methods as compared to our reference (i.e., NOAA Atlas 14).

**Figure 10.** Percentage change in mean quantiles and confidence interval (95–5%) when using the spatial bootstrap method with sample size of 11 and 30, compared to NOAA Atlas 14 gauge-based PFES (only 10 and 25-year return periods are shown).

The results are presented at the locations of the three selected gauges in southern Louisiana (indicated in Figure 1). The spatial bootstrap method produced lower mean quantiles (i.e., negative changes) for the three gauges with slight differences when increasing the regional sample size. For example, at the location of gauge (2), a regional sample of size N = 11 would underestimate (as compared to NOAA Atlas 14) the mean 10-year PFE by 28.4% (compared to 30.8% in case of N = 30). Increasing the regional sample of the spatial bootstrap can improve the PFEs estimation in case of gauge (3). The relative change in the mean 25-year quantile for the pixel coincides gauge (3) slightly reduced from −38% (N = 11) to −37% (N = 30). In terms of uncertainty, increasing the regional sample size can result in narrower confidence intervals as compared to gauged-based PFEs. For example, when using a regional sample size of N = 30 at Gauge (3) location, 25-year PFE has a narrower confidence interval with a reduction of −58.9% compared to the gauge-based PFEs (as opposed to a −40.8% reduction when using a sample size of N = 11).
