A full tutorial is available on the Hydrology For Beginners YouTube Channel
The isohyetal method is a widely recognised technique to estimate average or mean rainfall over a catchment area by using contours (isohyets) that connect points of equal precipitation.
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Steps of the Isohyetal Method
Draw Isohyets
Plot rainfall data from various gauges on a map of the catchment area and draw smooth contour lines (isohyets) connecting points of equal rainfall.
Calculate Areas Between Isohyets
Measure the areas between each pair of neighbouring isohyets using planimeters, grid counting, or digital mapping tools.
Determine Mean Rainfall for Each Zone
For each zone between adjacent isohyets, calculate the average rainfall as the mean of the two isohyet values bounding the zone.
Compute Weighted Average
The mean rainfall is then computed by weighting each zone's mean rainfall by its area. The formula is:
Example Calculation
Suppose there are three isohyet zones:
Zone 1: 1–2 cm rainfall, covers 40 km²
Zone 2: 2–3 cm rainfall, covers 20 km²
Zone 3: 3–4 cm rainfall, covers 10 km²
Calculation:
Advantages of the Method
Accounts for topographic and meteorological influences on rainfall.
More accurate than the arithmetic average or the Thiessen polygon method, especially in rugged terrain or where rainfall varies significantly.
Allows expert judgment to refine rainfall estimation.
Limitations
Requires expertise and is more time-consuming than other methods.
Subjectivity in drawing isohyets may affect accuracy.
In summary, the isohyetal method provides a reliable estimate of mean rainfall for a catchment by combining spatial rainfall data, mapping, and calculation of weighted averages based on area.
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