(b) The following table gives 10 years of annual data plus the totals for that 1
ID: 116853 • Letter: #
Question
(b) The following table gives 10 years of annual data plus the totals for that 10 year period fora 100km2 catchment. i. What is the average annual runoff depth? ii. Estimate the average annual evapotranspiration? iii. What assumption(s) did you make in answering ii? (1 marks) (2 marks) (2 marks) Year Precipitation (mm) Potential Runoff (ML) Evapotranspiration 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Total 855 735 840 900 725 780 870 810 795 760 8070 1300 1400 1320 1280 1410 1380 1350 1330 1300 1380 13450 10000 7500 9800 11200 7100 7200 0500 8000 6800 7200 85300 (5 marks)Explanation / Answer
i. Total annual runoff in ML= 1000+7500+9800+11200+7100+7200+10500+8000+6800+7200=(85300)ML
=85300x106L= 8.53x1010L
Now, 1000litres = 1metre3
Therefore, 8.53x1010L= 8.53x107m3
Catchment area = 100km2=100x106m2=108m2
108m2x runoff depth= 8.53x107m3
runoff depth= (8.53x107m3)/ 108m2
average runoff depth= 0.853meter
average annual runoff depth= 0.853/10meter = 0.0853meter= 85.3mm
ii. Total runoff= 85300ML = 853mm
Total potential evapotranspiration=13450mm
Total precipitation= 8070mm
Total precipitation=Total runoff + Actual evapotranspiration
8070= 853+Actual evapotranspiration
Actual evapotranspiration= 7217mm
Average annual evapotranspiration= 7217/10mm
=721.7mm
iii. The assumption made in ii. part is that in a system where mass of water is contant that is water added by precipitation can only convert into runoff water or its evapotranspiration can take place.
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