Many people land on this page of my other blog in search of an answer for this problem:
A circular lake 1.0 km in diameter is 10 m deep. Solar energy is incident on the lake at an average rate of 200 W/m². If the lake absorbs all this energy and does not exchange heat with its surroundings, how long will it take to warm from 10°C to 20°C ?
as seen in the following snapshot:
Now, I do offer a pathway to the solution there but I feel that not providing the final answer is tantamount to cheating on my part. So, this being a ‘Physics blog’, I provide the entire solution here (I don’t know or care what good does it do to those ‘searchers’!):