In calculating the Equivalent Uniform Depth, at least there are 3 analysis models. Below calculation of Equivalent Uniform Depth using 3 analysis.

**1.Arithmetic average method (algebra)**

Arithmetic average method is the simplest to calculate the average rainfall on a catchments area. Calculate the mean rainfall with rain falling and the measurement data at the same time summed and then divided by the number of stations. The result of this method is good if rainfall stations spread evenly in Catchments area, and relatively equitable distribution of the entire watershed.

**2.Thiesen Polygon Method**

These methods also account ratio area of each station representing the surrounding area. In an area within the Basin is considered that the rain is similar to that which occurs at the nearest station, so that rain was recorded at a station representing the extent of the rain. This method is used if the spread of rainfall stations in the area of interest is uneven.

Thiesen polygon method used to calculate average rainfall areas. Thiesen polygon is fixed for a certain rain network. If there are changes in rainfall station networks such as removal or addition of rainfall stations, the polygon thiesen network must be changed.

An example Of Polygon thiesen Map

**3. Isohyets Methods**

Isohyets is an imaginary line that connecting each points with the same depth of rain. In isohyets method, considered that the rain in an area between two lines isohyets is evenly distributed and equal to the average value of both these isohyets line.

Value of the isohyets line generated from interpolated rainfall depth at the station the rain. The more detailed the process of interpolation, a line that produced more detail.

Figure below shows the results isohyets making and analysis.

Basically, each method has advantages and disadvantages their own. Thiesen Polygon useful to analyses the rainfall distribution with a long data, because the rain station network can be used for different years.

Isohyets have the ability to display a gradual rain, but that must be considered is the level of contour lines detailed. Due to the interpolation more detail, the result will be different.

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*For The Sake Of Education and Information*

*Reference : Triatmodjo, bambang,2008, Hidrologi Terapan, Beta Offset *

*By : Gepenx 2011*

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