I used a number of buckets (Diamter 20 cm and length 20 cm) driven into the ground in different spots of the area and I measured their weight on daily bases for 10 days. The problem is that I am conducting this experiment in Malaysia and the weather here is tropical so we have rain practically everyday in different times which lead to very fluctuating weight measurements.
Hi,
I'm a computer scientist, so its not my area of expertise. But I would keep two buckets at each location and measure one of the buckets on even days and the other bucket on odd days. So each bucket measures 2 days but with 1 day overlap. This should smooth out anomalies that are caused by varying measuring times.
A different, more technical approach would be to use some small microcontroller (like an arduino) with a servo to open and shut a lit on the bucket at specific times.
Cheers,
Jochen
You need a lysimeter and a pluviometer. You measure the weight of (soil + water) - precipitation. That should give you an estimate of evaporation. See my poster in EGS in Vienna 2011 or 2012. Good luck.
I did that, Professor Pavia and since it is raining practically everyday or every two days, I couldn't have a good serie of values. Is there any other method in that case?
See this work. It might help
Modeling Soil Surface Energy Fluxes from Solar Radiation, Latent Heat, and Soil plus Air Temperature Measurements
Dear Ghaieth,
your experiment with the buckets had to fail because you have two high weight values with their inaccuracies to subtract to get a small evaporation value. Also you complicate matters when the buckets are not open at the bottom (no drainage above saturation of the soil) and not cylindrical. in shape. But even cylindrical open micro-lysimeters will not be helpful because of drained water being unknown.
Only an accurate continuous weighing lysimeter, with drainage water measured below the soil, would be able to perform the job by measurement alone. Although here also you have high weights to subtract to find the soil evaporation, the weighing is accurate and continuous, while drained water can easily be determined. In periods that (that is as soon as) no water drains from saturated soil and the surface evaporation is the only weight loss, you will get the data you wanted.
This is expensive equipment but you are looking for values of evaporation under very difficult conditions.
Cheers, Kees
Thank you Professor Stigter. I understand that my method is not accurate but I don't have that material right now. Is there any simple method that can give reasonable results in these weather conditions?
Basically we must then simplify the reasoning. An old, still valid approach may be used with ”the three phases/stages of drying of bare soil”.
In the first stage the surface is wet. It ends when the complete surface (of a lysimeter or a soil column in a micro-lysimeter) is dry. Then further evaporation will slowly create a dry layer of soil. This is called “self mulching” (because a mulch changes the original surface layer) and evaporation diminishes more and more (because of the resistance of the dry soil layer to water vapour transport). As long as no rains fall, the third stage of drying occurs when drying slows down to negligible values and the soil has a water content that does compare with the wilting point of cops grown on these soils.
2. For your studies:
- as long as the surface is wet, you may want to treat it as potential evaporation of water; there are several formula for that.
- as soon as the surface of a soil column in a micro-lysimeter is dry and no water drains anymore from the soil column, the only weight loss is that of evaporation from the surface.
- in the attached paper you may see how this was applied in a soil with sufficient drying to reach this stage of evaporation without drainage.
For your case, each time when it rains you get to a period of calculated open water evaporation. You should investigate with open micro-lysimeters (hanging?) whether for your soil the drainage stops before or after the surface got completely dry. In the former case you can weigh the micro-lysimeters on an accurate balance on a say 24 hrs basis. In the latter case you must collect and weigh the drained water as well until it ends.
So in the extreme, if it rains every day, the soil surface may never dry up and you only have to calculate open water evaporation.
Cheers, Kees
Great, keep me informed about difficulties and results.
My e-mail is [email protected]
Best wishes, Kees