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By subtracting the beginning weight from the ending weight we determined how much water was in each of the tubs. We took the net water weight of the rain gauge tub as the total rain fall. We assumed there was no evaporation in the rain gauge tub,and we assumed that the same amount of rain fell in each of the three tubs. By comparing the net water weight of the soil tub and the open tub to that of the rain gauge tub we were able to determine the percentage of water that evaporated from each of those tubs.
In this experiment, 15.7% of the water that fell on the hard surface evaporated compared to 6.5% of the water that fell on the soil. An important difference between this experiment and the roof of the Ecotrust building is that most of the water that falls on the hard-surface part of the roof drains off immediately. In other words, no mater how much faster evaporation happens on a hard surface compared to the water in soil, only the small amount of water that does not drain is available for evaporation.
Therefore, it is quite possible that, even though water evaporates faster from the hard surface, more total water evaporates from the soil.
Another issue to keep in mind is that this experiment took place during a rainy week in March. There was no bright sunlight during any part of the experiment. It would be interesting to have a similar experiment running for a whole year, measuring the water weight of the tubs each week. This longer experiment would give more complete information about the overall difference in evaporation between the ecoroof and the hard surface roof. |
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