**Thiessen's Polygon & Isohyetal Methods**

Thiessen's Polygon

Thiessen's Polygon is the variant of the Vornoi Diagram where a certain area or space is divided into smaller regions to better understand the performance of the region with respect to an event.

Thiessen's Polygon:The Method

Locate all rainfall stations on a base map and record the rainfall amount.

Connect each station by straight lines with the several nearest stations to form a series of triangles.

Erect perpendicular bisectors on each of these lines and extend them to the intersect with other bisectors, thus forming a series of irregular polygons

Area of each of the polygon is taken.Theissens weights are computed for each area which is calculated by dividing area of each polygon by total area of the basin.

Sum of all the weights should be equal to unity.

In hydrology and climatology,Vornoi Diagrams are referred to as Thiessen's Polygon which is used to study the amount of rainfall affecting a particular region.

A diagram showing thiessen's polygon.

Examples

where,

P1,P2 etc. are rainfall of a particular station.

A1,A2 etc are the area of a particular station.

P is the average rainfall.

Calculations

**Isohyets.**

An isohyet is a line which joins elevations or places having the same rainfall.

No two isohyets cross each other.

A map showing isohyetes can represent the pattern of rainfall.

Isohyetal Method.

Locate all rainfall stations on a base map and record the rainfall amount.

Draw isohyets (lines of equal rainfall) by proportioning the distances between adjacent gauges according to differences in catch.

Then calculate the mean precipitation for the area corresponding to each isohyet.

Calculate the fraction of catchment area under each isohyet,multiply by the mean precipitation for that area and sum to get the catchment average.

where,

P1,P2..are values of isohytes.

a1,a2.. are inter-isohyte areas.

A is the total area.

Calculations

Diagram showing a isohyetal map.

Examples.

**Comparison of Isohyetal and Thiessen's Polygon**

Thiessen's Polygon.

Not very accurate .

If a basin is added or removed the polygon has to be redrawn all again.

Topographic features are not considered.

Orographic features are not accounted for.

Isohyetal Method.

Accurate method,but very laborious.

For each storm, a separate isohyet has to be drawn.

If its done on an annual basis the work will be very laborious and time consuming.

Useful for project studies and storm studies.

**A presentation By:**

Anumol Cherian (12BT03CIV094)

Abel George Philip (12BT03CIV095)

Jacob Baby (12BT03CIV096)

Sairam Zambre (BT11CIV068)

Anumol Cherian (12BT03CIV094)

Abel George Philip (12BT03CIV095)

Jacob Baby (12BT03CIV096)

Sairam Zambre (BT11CIV068)