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Developing event-identification threshold

Inspired from ITHACA ERDS (Extreme Rainfall Detection System - http://erds.ithacaweb.org), The extreme rainfall detection is based on the concept of event-identification threshold: an alert is provided if the accumulated rainfall exceeds the threshold.

THRESHOLD

THRESHOLD is AMOUNT OF RAINFALL NEEDED TO TRIGGER A EVENT (flood/flash-flood/landslide) INDUCED BY EXTREME RAINFALL

Frequency analysis

a return period, also known as a recurrence interval (sometimes repeat interval) is an estimate of the likelihood of an event, such as an earthquake, flood, landslide, or a river discharge flow to occur.

For example, a 10-year flood has a \({1 \over 10}\) = 0.1 or 10% chance of being exceeded in any one year and a 50-year flood has a 0.02 or 2% chance of being exceeded in any one year. This does not mean that a 100-year flood will happen regularly every 100 years, or only once in 100 years. Despite the connotations of the name "return period". In any given 100-year period, a 100-year event may occur once, twice, more, or not at all, and each outcome has a probability that can be computed as below.

Given that the return period of an event is 100 years,

\[P = {1 \over 100}\]
\[P = 0.01\]

So the probability that such an event occurs exactly once in 10 successive years is:

\[P(x=1) = ({10 \over 1}) * 0.01^1 * 0.99^9\]
\[P = 10 * 0.01 * 0.914\]
\[P = 0.0914\]

The Threshold

Daily extreme rainfall alert is based on 1-5 days of consecutive rainfall exceeding a threshold, which will develop at pixel level. The long-term historical daily rainfall (2000-2019) is used to define the threshold.

It is based on the maximum of 1-5 days consecutive rainfall in 1 year for the period 2000 - 2019 in (x,y) area, and calculate the percentile using k value:

  • k = 0.5 for rainfall exceeding P50 2 years return period (RP), classified as Moderate Rainfall
  • k = 0.8 for rainfall exceeding P80 5 years RP, classified as Heavy Rainfall
  • k = 0.9 for rainfall exceeding P90 10 years RP, classified as Intense Rainfall
  • k = 0.96 for rainfall exceeding P96 25 years RP, classified as Extreme Rainfall

Return Period Source: http://www.meted.ucar.edu/hydro/basic_int/flood_frequency/print.htm

Extreme rainfall alert

Overlaying the near-real time or forecast data with the threshold is used to obtain an estimate of the area who will experience unusual rainfall at (x,y) location.

Following the Yes or No warnings, there are four different alert levels of Yes can be visualized (moderate, heavy, intense and extreme), based on specific critical rainfall (threshold), defined as the amount of precipitation for a given duration over a specific climatological area.

The alert will generate for 5 different class: 1-day, 2-days, 3-days, 4-days and 5-days ahead.

The Alert!