Interpretation of Precipitation Data Bestudio
Interpretation of Precipitation Data Bestudio
Interpretation of Precipitation Data Bestudio
OF
PRECIPITATION
DATA
PRECIPITATION Chapter 3
• 1. Estimating missing precipitation data at a
station
• 2. Checking inconsistency in particular data at
a station
• 3. Averaging precipitation over an area
Estimating missing precipitation data at a station
Some precipitation stations may have short breaks in the records because of
absence of the observer or because of instrumental failures. It is often
necessary to estimate this missing record.
The station whose data is missing is called interpolation station and gauging
stations whose data are used to calculate the missing station data are called
index stations. Missing precipitation data is estimated by two commonly used
methods.
Px = (Pa + Pb + Pc)/3
Where:
Px = precipitation at the missing location
Pi = precipitation at index station
N = number of rain gauges
Normal ratio method (NRM)
Normal ratio method (NRM) is used when the normal annual precipitation at any of the index
station differs from that of the interpolation station by more than 10%. In this method, the
precipitation amounts at the index stations are weighted by the ratios of their normal annual
precipitation data in a relationship of the form:
Where:
Pm = precipitation at the missing location
Pi = precipitation at index station
Nx = the normal annual precipitation value for the 'x' station
Ni = average annual rain at gauge
N = number of rain gauges
Example
Find out the missing storm precipitation of station 'C' given in the following table:
Station A B C D E
Storm precipitation (cm) 9.7 8.3 ---- 11.7 8.0
There are some widely used methods to compute average precipitation over
an area, but the most common of these used are:
• Isohytal method
Arithmetic Mean Method
The simplest of all is the Arithmetic Mean Method, which taken an average of all the rainfall depths as shown.
Average rainfall as the arithmetic mean of all the records of the four rain gauges, as shown below:
For the given example, the “weighted” average rainfall over the
catchment is determined as:
Isohyetal Method
This is considered as one of the most accurate methods, but it is dependent on
the skill and experience of the analyst. The method requires the plotting of
isohyets as shown in the figure and calculating the areas enclosed either between
the isohyets or between an isohyet and the catchment boundary. The areas may
be measured with a planimeter if the catchment map is drawn to a scale.
Example: Assume that the areas enclosed between two consecutive isohyets and are calculated as under:
Given:
Area I = 40 km2
Area II = 80 km2
Area III = 70 km2
Area IV = 50 km
Hence, these areas may be thought of as corresponding to the following rainfall depths:
Area II: Corresponds to (10 + 15)/2 = 12.5 mm rainfall depth
Area III: Corresponds to (5 + 10)/2 = 7.5 mm rainfall depth
TIME VARIATION
RECORD RAINFALLS