**:**
## Most often interested in quantifying rainfall over an entire watershed. Has to be inferred from some sort of weighted average of available point measurements P(xi) ## Several methods to determine weights. All require
## Arithmetic Average ## Arithmetic average: ## Method OK if gages distributed uniformly over watershed and rainfall does not vary much in space.
## Theissen Method ## Weight ( )is a measure of rain-gage contributing area. Assumes rain at any point in watershed equal to rainfall at nearest station. ## To determine ( ):
## Isohyetal Method ## most accurate method if have a sufficiently dense gage network to construct an accurate isohyetal map. Can account for systematic trends, i.e., orography, distance from coast.
## Return Period ## Return period (T) of an event is the average time (recurrence interval) between events greater than or equal to a particular magnitude. ## For example, 25 year return period storm occurs on average once every 25 years and has a probability of 1/25 of occurring in any one year.
## To estimate return period from flood/drought/rainfall records ## Select annual maximum/minimum of a particular duration from historical record to form annual maximum/minimum series. ## Rank annual maximum/minimum from largest to smallest (or smallest to largest if interested in drought) -
## What is probability T-year return period will occur once in N years?
## Examples ## For example, 10 year return period storm has prob. of occurrence 0.1 in any 1 year. How probable once in 10 years? - T = average recurrence interval for event =10 years
- Probability of occurrence in any one year = 1/T=0.1
- Probability = 1 - (1-1/10)10 = ______ at least once in ten years
## What is the probability that a 20 year return period storm occurs at least once in 10 years?
**Dostları ilə paylaş:** |