(b) For the last 10 years, the average annual rainfall for the Wandering River c
ID: 116854 • Letter: #
Question
(b) For the last 10 years, the average annual rainfall for the Wandering River catchment was 700 mm, the average annual potential evapotranspiration was 1100 mm and the average annual runoff from the catchment was 14,000 ML. The Wandering River catchment area is 100 km2 (i) What was the annual average runoff depth? (2 marks) (ii) Estimate the runoff coefficient (also known as the runoff ratio). (1 mark) iii Estimate the annual average actual evapotranspiration in mm (1 mark (iv) What assumption did you make in answering(iii)? (1 mark)Explanation / Answer
1) Annual average runoff depth = average annual runoff from the catchment/total catchment area
= 14000/100
= 140mm/a
2) runoff coefficient = average annual rainfall/average annual potential evapotranspiration
= 700/1100
= 0.63
3) annual average actual evapotranspiration will be same as average annual potential evapotranspiration = 1100mm
4) Actual evapotranspiration depends not only on atmospheric factors but on the
amount of moisture and the nature of the vegetation at the surface. A barren and
totally dry ground surface, as in some deserts, will generate no actual
evapotranspiration, though its potential rate would be high , so we assume that that amount of moisture and nature of the vegetation will be same for both.
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