[Celia Tuttle]

Celia Tuttle asked the Naked Scientists:
   
I live in the desert in Arizona.  How much water is really lost to evaporation when watering flowers with a hose during the heat of a summer day when the temperature is 100 degrees F?  I've heard it's close to 50 %, which seems excessive.  

I love your show and appreciate your help!
What do you think?

[Don_1]

I think it would be difficult to put a figure on the amount of water lost through evaporation. But even in the more temperate climates, such as here in the south of England, it is always advisable to water plants after the sun has gone down.

In Arizona, I should think this would be almost an absolute essential. Using a hose with a sprinkler attached would lead to evaporation while the small water droplets are in the air (there would also be a loss due to wind carrying off a certain amount), then further evaporation on contact with the hot soil, and then still more evaporation due to the water not getting a chance to seep deep enough into the soil where the Sun's effect would be reduced.

You could try a small experiment.

Take a deep seed tray (without drainage holes) and fill it with soil from your garden. Make sure the soil is bone dry. Weigh the soil & tray and place it in a hollow in one of your flower beds. Once it is at the same temperature as the surrounding soil, use a watering can to water the tray with a measured amount of water. 1 litre would be fine, since it will weight 1 kilo. Be careful to get all the water into the tray. After about 5 mins, take the tray out of the hollow and weigh it. Add the tray's dry weight to the weight of the water added and then subtract from its watered weight. This will give you the weight of water left in the soil.

eg:

Dry weight of tray + soil ---------------------- 20kgs
Weight of cold water added --------------------- 1 kg
Total ------------------------------------------ 21 kgs
Weight of tray + soil 5 mins after watering ---- 20.5 kgs

Total water loss 0.5 Kgs Therefore total water loss through evaporation = 50%

[dentstudent]

If you include evapo-transpiration (EVT) from the plants, the figure can get even higher. In some southern European countries (which may be analogous to where you are, CT), the total water restored to the atmosphere from precipitation over forests can reach 90%. The volume of water replaced into the atmosphere depends on the leaf-area-index (LAI) of the plants in question -  the higher the LAI, the more water is lost through EVT.

[Don_1]

You might want to read this http://www.thenakedscientists.com/forum/index.php?topic=20511.25

Watering in strong sunlight can lead to large drops of water, which may not evaporate quickly enough, scalding the leaves. A cool shower after the sun has gone down, however, can be beneficial.


BTW, the experiment I gave, will not allow for airborne evaporation, since water from a can tends to have larger droplets and is airborne for probably less than a second, while from a hose, the droplets will be small and airborne for maybe two - three seconds. Do the same experiment with fresh dry soil again after the sun has gone down and compare the difference.

[Raghavendra]

quite interesting []