Isohyetal Method and Thiessen's Mean Method

The average depth of annual precipitation in a catchment basin can be calculated in different ways. Two popular methods are the Isohyetal Method and the Thiessen's Mean Method. Both methods consider the importance of individual raingauge stations, but they differ in their approach.

Isohyetal Method:

The Isohyetal Method is a graphical method that involves drawing contour lines of equal precipitation intensity, called isohyets, on a map. The isohyets are then used to calculate the average depth of precipitation for the entire catchment basin. The method assumes that precipitation is evenly distributed within each contour line.

Advantages of Isohyetal Method:

- It takes into account the spatial variability of precipitation within the catchment basin.
- It can provide a detailed map of precipitation intensity, which can be useful for planning and design purposes.

Disadvantages of Isohyetal Method:

- It requires a large number of raingauge stations to accurately estimate the isohyets.
- It assumes that precipitation is evenly distributed within each contour line, which may not be true in reality.

Thiessen's Mean Method:

The Thiessen's Mean Method is a numerical method that involves dividing the catchment basin into polygons based on the proximity of raingauge stations. The precipitation data from each raingauge station is then averaged within its respective polygon, and the average precipitation values are used to calculate the overall average depth of precipitation for the entire catchment basin.

Advantages of Thiessen's Mean Method:

- It takes into account the importance of individual raingauge stations in the catchment basin.
- It is a simple and straightforward method that requires only the location and precipitation data of raingauge stations.

Disadvantages of Thiessen's Mean Method:

- It assumes that precipitation is evenly distributed within each polygon, which may not be true in reality.
- It does not take into account the spatial variability of precipitation within the catchment basin.

Arithmetical Method:

The Arithmetical Method is a simple method that involves adding up the precipitation data from all raingauge stations in the catchment basin and dividing the total by the number of stations. This method assumes that all raingauge stations are equally important and that precipitation is evenly distributed throughout the catchment basin.

Advantages of Arithmetical Method:

- It is a simple and straightforward method that requires only the precipitation data of raingauge stations.

Disadvantages of Arithmetical Method:

- It does not take into account the importance of individual raingauge stations in the catchment basin.
- It assumes that precipitation is evenly distributed throughout the catchment basin, which may not be true in reality.

Conclusion:

Both the Isohyetal Method and the Thiessen's Mean Method take into account the importance of individual raingauge stations in the catchment basin, while the Arithmetical Method assumes that all stations are equally important. The Isohyetal Method is a graphical method that takes into account the spatial variability of precipitation within the catchment basin, while the Thiessen's Mean Method is a numerical method that divides the catchment basin into polygons based on the proximity of raingauge stations. Both methods have their advantages and disadvantages, and the choice of method depends on the availability of data and the specific requirements of the study.