- 2082 Baisakh · 4 marks
Average rainfall by isohyetal method, Bagmati basin
Short answer
Mean depths of intervals: 15, 11, 7, 3 cm. Weighted by areas (125, 117, 108, 92 km²), Σ(A×depth) = 4194. Divide by 442 km² gives average 9.49 cm (about 95 mm).
An isohyet drawn in the catchment of Bagmati river basin with catchment area of 442 sq. km has the following data. Calculate the average rainfall for the basin.
| SN | Range of isohyet (cm) | Area enclosed by isohyet (sq. km) |
|---|---|---|
| 1 | 17-13 | 125 |
| 2 | 13-9 | 117 |
| 3 | 9-5 | 108 |
| 4 | 5-1 | 92 |
The average depth between two isohyets is the mean of their values. Weighted by the area, the catchment average is
| Isohyet range (cm) | Mean depth (cm) | Area (km²) | Area × depth |
|---|---|---|---|
| 17-13 | 15 | 125 | 1875 |
| 13-9 | 11 | 117 | 1287 |
| 9-5 | 7 | 108 | 756 |
| 5-1 | 3 | 92 | 276 |
| Total | 442 | 4194 |
Answer: Average rainfall of the Bagmati basin = 9.49 cm (about 95 mm).
✓ Answer checked by Claude Sonnet 5.5
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