Is there a formula for calculating the evaporation of water on dry land?

     I have pigs and during the summer they get hot. I've tried misters and timer controlled sprinklers but neither suited my needs, the pigs either got ahold of the mister line and ripped it down or i just forgot to head out and turn it on and off, and the sprinkler on a timer was never right, on hot days it underwatered leaving the pen dry and dusty which is unpleasent for the pigs and on cold days it overwatered leaving the pen a muddy mess which was also unpleasant for the pigs (contrary to popular belief pigs actually don't like mud its just something cool for them to roll in and natural sunscreen. Pigs would much rather have clear water raining down on them). I decided I'm going to make a simple Arduino controlled pig sprinkler. I'm going to equip it with a temperature and humidity sensor and I want it to plug the data it gets into a formula which calculates how long it will take for the water to evaporate, it will then take that data and use it to calculate the time in between sprinkler runs. But the obvious dilemma here is I cant seem to find a formula which will work. Any help is greatly appreciated! Thanks a bunch.

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Why not put two metal rods in the ground at a suitable location and measure the resistance between them?

Resistance too high and the sprinkler goes on, reaching saturation point and the sprinkler goes off.

Since the water needs some time to evaporate and start the cacle again it should work.

All you need is the right location for the rods and a suitable distance between them

Take a resistance reading when soaking wet, one when semi dry and one totally dry.

Use the values to program your Arduino or to drive some transistors to switch the relay to power the pump/valve.

+1. It's the fundamental basis for a soil RH sensor.

Kiteman2 years ago

Outside the box - wet soil is darker than dry soil.

Set a light to reflect off the soil to a sensor - when the soil gets too light (too dry), the sensor triggers the misters.

Water will evaporate from a puddle, or wet ground, any time the relative humidity of the air above it is less than 100%.

However I don't think there is an easy way to predict how fast water is evaporating. I'm guessing that is a kinetics thing. I mean back in chemistry class I learned about delta-H, delta-S, delta-G, and those described changes in heat energy and entropy associated with a reaction, like

H2O(l) ---> H2O(g)

But regarding the question of, how fast does the reaction occur? That was always a matter of "kinetics"


and then the professor makes some hand-waving arguments


about things that tend to make a reaction go faster, including surface area, mixing, and temperature.

However, I am not sure if you actually need to know this for your application.

If you have a humidity sensor, you can use feedback to tell you when to turn the sprinklers on. E.g. a humidity signal lower than RHlow tells the controller it should turn the sprinklers "on". A humidity signal higher than RHhigh tells the controller it should turn the sprinklers "off"

Also if you log all this data, the RH, the air temperature, the ground temperature, the on-or-off state of the sprinklers, then you might be able to go back and use that data to build an empirical model of how fast water is leaving the ground, and try to discover if it depends on temperature, or what. I mean if you had that data, you would at least know how fast you're putting water into the system, since you'd know often the sprinklers are turning on.

BTW, I like this suggestion from Downunder35m about making a sensor to measure the conductance, = 1/resistance, of the ground. That senor would make sense (no pun intended) if ground wetness is really the quantity you want to control.

kelseymh2 years ago

I doubt that there is a single formula. Evaporation depends on temperature and humidity; but the "temperature" is really a proxy for solar irradiance (heat flux), which depends on atmospheric opacity. That is strongly dependent on local conditions.

You might do better to measure the evaporation rate more directly using an evaporation pan, with monitoring hooked up to either an Arduino or a Raspberry Pi running appropriate data analysis software. Then you can turn the measured rate around to a flow rate. There will be an obvious lag, of hours or even as much as a day, which your controller code will need to take into account.

iceng kelseymh2 years ago