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Name: Matt
Status: Educator
Grade: A-Level
Location: N/A
Country: England
Date: December 2007


Question:
What are the average latent heat levels for the following soil types: clay, sand, and silt. My study is focusing on sites with a northern European climate and figures in MJ/m3 would be appreciated.



Replies:
Matt,

Unfortunately I cannot answer your question directly because latent heat flux is dependent not only on soil type but on soil moisture content, intensity of solar radiation impinging on the soil, the temperature of the soil, the composition of the soil (soils are not generally one pure type, but a mixture of types), soil texture, and how much surface litter and vegetation is above the soil surface.

Please excuse me if I repeat things below that you already know, but we have a wide audience that may not know the following.

One thing that can be said is that soil type does affect evaporation if all other things are equal. A sandy soil does not hold water nearly as well as a clay, the water tending to drain out of the sandy soil whereas clay soils tend to be able to hold more water and hold it more tightly because the clay particles are smaller and have smaller spaces between them. Therefore, it is generally more difficult to evaporate water from a clay soil than from a sandy soil.

As the size of soil particles becomes smaller, the porosity of the soil becomes larger (indicating the total amount of "air" space between soil particles), in the order of sand, loam, silt, clay. The larger the porosity of the soil, the more tightly water is held between the soil particles, and the more difficult it is for water to drain out of the soil and for it to be evaporated. Organic matter in the soil further helps to absorb water and keep it in the soil, so the extent of organic content is also important in water retention and evaporation.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory



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