If hikers are recommended to drink 24 ounces of water per hour, we can calculate how much water they should drink every 10 minutes.
Since there are 60 minutes in an hour, drinking 24 ounces in an hour means:
\[ \frac{24 \text{ ounces}}{60 \text{ minutes}} = 0.4 \text{ ounces per minute} \]
Now, to find out how much water is consumed in 10 minutes:
\[ 0.4 \text{ ounces per minute} \times 10 \text{ minutes} = 4 \text{ ounces} \]
Therefore, the amount of water in the canteen should decrease by 4 ounces every 10 minutes, which means the change in the amount of water is negative.
So, the answer is: the amount of water in the canteen should decrease by 4 ounces every 10 minutes, and this change is negative.