Figure 3.

Flood predictability is the highest in the mid-sized catchments. Data from Walnut Gulch Experimental Watershed, 1955–1996. (a) Probability density distributions of rainfall events (x) that either resulted in a flood (f(x|F), shown in black) or did not result in a flood (g(x|NF), shown in grey) for three catchment sizes (i) 0.04, (ii) 9 and (iii) 149 km². f and g show good signal-to-noise characteristics in the small and medium catchments, respectively, but the distributions are indistinguishable in the large catchment, rendering floods unpredictable from rainfall cues. (b) Flood probability in the three catchments (i) 0.04, (ii) 9 and (iii) 149 km² (f(x|F) scaled by α, the prior probability of a flood). Floods are rare in the small catchment, which renders them less predictable.