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Which factor affects evaporation more: temperature, humidity, or air movement? (slrpnk.net)
submitted 6 months ago by to c/water@slrpnk.net
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I live somewhere in the Rio Grande river valley. The farm land here uses flood irrigation to water. For those unfamiliar, water from the river is diverted into various irrigated ditches, those ditches can then be used to flood entire fields.

As I was walking home with my dog from our morning river hike, I started to wonder about conditions for peak evaporation. We water our plants at home in the evening as the sun is setting and the temperature drops. But how much does that change evaporation? Is the 20ish% relative humidity more of a factor? Or is air movement?

Many of the farms seem to irrigate in the mornings, when it is still relatively cool. But while it was 64°/17.7° when I left the house at 6:30 this morning it will be 85°/29.4° by 11am and the daily high of 91°/32.7° by 2pm. Giving the ground very little time to absorb the water. Would it be better to water at 8pm at sunset giving over 12 hours of cooling temps?

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[–] 3 points 6 months ago*

The short answer, is that it's complicated.

The humidity you see on the weather is what's called Relative Humidity, which means that it's already correlated with temperature. As the temperature rises, so does the maximum amount of water the air can hold before it starts precipitating back out. So 20% humidity at 60F will have a smaller volume of water in the air than 20% at 80F.

If you've reached a certain volume of water in the air, and the temperature falls, you can reach the dew point, which is when the air can't hold the same amount of water and deposits it as dew. This is typically when people water, as the air simply cannot absorb moisture until the temperature rises again and the dew starts to evaporate.

Air movement can exchange saturated air with dry air, or vice versa, so it varies as to how much it affects evaporation.

Soil temperature also plays a role, as warm soil will evaporate more than cool soil. This is part of what can make agrivoltaics so dang handy, the solar panel shade will keep soil temps down and increase water retention.

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