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Only in specific locations do dams prevent drought, this can be viewed by comparing locations on ether side of Lake Volta in Ghana. If you are to the west (down wind) then during a general drought period rainfall increases by 12 to 14%. If to the east rainfall is reduced by 10 to 12%. This increased rainfall is the result of there being less cloud cover during the drought phase, thus higher solar energy to the lake, greater evaporation, thus higher rainfall down wind.

To the greater extent dams hold back fresh water that would normally 'float' upon the salty water when it reaches the sea. This also provide a temperature difference between the sea water and the river water. This damming effect reduces the appearance of these fresher water lakes that appear on the sea surface during rainy season or snow melt runoffs. Consequently evaporation is reduced with reduced rainfall down wind. This can be demonstrated by comparing mid summer runoff at Aswan on the Nile with reduced rainfall in Sub Saharan Africa (SSA):10N to 13N, Sudan westwards (Nicholsons dataset). The northerly (Hadley cell) wind brings the moisture south into SSA, but there is a notable year on year reduction in this moisture budget, drought. This same scenario probably applies to the effects of the dams on the US west coast affecting the Mid Western moisture budget (Ferrel Cell). Since there are 40,000 'large' dams, it remains to be concluded whether or not they are relevent in the global climate areana. £50 says they are!

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Q: How dam prevent drought?
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