9.1.2. Steps of Construction

1. Plot the catchment area on a map and mark the location of all rain gauge stations.

The first step is to clearly outline the catchment area on a map where you want to estimate rainfall. This involves identifying the geographical boundaries of the region. Then, locate all the rain gauge stations within this catchment area. These stations are where rainfall data has been collected, and they will serve as the basis for the estimation of average rainfall across the entire area.

Think of this step as laying out a treasure map. You first draw the outline of the land where you’d like to explore (catchment area) and then place markers where you found clues (rain gauge stations).

2. Connect adjacent stations with straight lines to form triangles (Triangulation).

After plotting the catchment area and rain gauge locations, the next step is to connect each adjacent station with straight lines, creating triangles. This technique, known as triangulation, helps in defining the relationship between nearby rain gauges and establishes the framework for detailed rainfall estimation across the catchment.

Imagine you are connecting dots in a dot-to-dot drawing. Each rain gauge is a dot, and by connecting them, you form shapes (triangles) that help you visualize the area influenced by each station.

3. Draw perpendicular bisectors of each line connecting two adjacent stations.

In this step, you need to create perpendicular bisectors for each line connecting adjacent stations. A perpendicular bisector is a line drawn at a right angle to a line segment and cuts it into two equal parts. This helps to determine the boundaries of influence for each rain gauge station by indicating points that are equidistant from the two stations.

Think of this step like marking the center point of a bridge. By placing lines directly in the middle and drawing straight up (perpendicular), you can see how far each station has an effect on the surrounding area, almost like marking safe zones for each side of the bridge.

4. The intersection of perpendicular bisectors creates polygons around each station.

The crucial part of this method occurs when you draw the perpendicular bisectors. The points where these lines intersect define polygons around each rain gauge station. These polygons represent the area that each station influences for rainfall measurement, helping to organize the whole catchment area into manageable sections.

Imagine drawing fences around several houses in a neighborhood. Each fence encloses a home's yard and marks where that home's influence (like a local store's delivery radius) extends.

5. Each polygon defines the area of influence of that rain gauge.

At this stage, the polygons created in the previous step explicitly define the area of influence for each rain gauge station. This means that the rainfall collected at that specific station is assumed to represent the average rainfall within that polygon. Understanding this influence is key when calculating the average rainfall across the entire catchment area.

Think of each polygon as a school district. Just as students in a district fall under a particular school’s influence, each rain gauge affects only the area within its polygon, where its rainfall data is applicable.

6. Measure the area of each polygon using a planimeter or CAD tools.

After establishing the polygons, the next task is to measure the area of each one accurately. This can be done using tools like a planimeter (a device for measuring areas) or through computer-aided design (CAD) software. Knowing the area of each polygon is essential for calculating rainfall weighted by its area.

Think about measuring the area of a garden for planting. By using a ruler or software, you can find out how much space you have to grow plants, just like finding out how much rainfall data each polygon represents.

7. Calculate weighted average rainfall as:

P_avg = (Σ (A_i × P_i)) / A_total

where: – A_i = area of the ith polygon – A_total = total catchment area – P_i = precipitation at ith station.

Finally, you can compute the weighted average rainfall across the catchment area. This involves multiplying the area of each polygon by the precipitation measured at its respective rain gauge station, summing all these values, and then dividing by the total area of the catchment. This calculation ensures that areas with more significant influence on rainfall are given appropriate weight in the overall average.

Consider a cake made up of different sized slices. If each slice represents a different rainfall measurement influenced by different areas, to determine how sweet the whole cake tastes (the average rainfall), you have to weigh each slice (area) according to its size (influence).

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