1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Soil Moisture?

  1. Oct 28, 2009 #1
    Hi all,

    I'm not sure if this is the right thread to post this topic.

    I want to simulate soil moisture and how much it decreases given a certain temperature.
    For example at temperature 25 degrees Celsius , what is the rate of decrease of the soil moisture? The kind of soil is clay suitable for agriculture.
    Is there a general equation for this?
    Please guide me.

    Thanks in advance
  2. jcsd
  3. Oct 28, 2009 #2
    I remember calculating this many years ago. I believe you need to know the air temperature (as you stated), vapor pressure of the moisture (does this depend on soil?), the humidity, the wind speed (maybe), and the diffusion equation. If you can cover the soil with straw or corn stover, the diffusion will be less.
    Bob S
  4. Oct 28, 2009 #3


    User Avatar
    Gold Member

    There is nothing really cut and dried because soils vary so much. What you call "clay" wouldn't be what I would call clay, if it is indeed suitable for agriculture. I'm used to speccing and testing blue marine clays for the construction of clay-core dikes for waste treatment ponds, and for the isolation of leachates from solid wastes. You wouldn't want to try to grow anything in that.

    Clay as a generic term can mean a high concentration of fines, some silt, some sand and some organic materials. That could possibly describe the soil in my garden-spot when I initially moved here (neglecting the ever-present rocks). What I'm getting at is that soil-classifications are not monolithic and need to be backed up with quite a bit of laboratory testing before you can predict their properties with any confidence.
  5. Oct 28, 2009 #4
    Thanks so much for replying.

    The problem is I'm a computer-science graduate and I don't have any knowledge regarding this topic. I'm not even sure it is called clay (as you stated it shouldn't)

    All I know is it is a soil suitable for agriculture, the temperature varies from 20 to 30 degrees Celsius throughout the year. Humidity is moderate, it can be a coastal city though.

    I don't know anything about the diffusion equation.

    What assumptions should I make about the rest of the parameters you stated? and what equation should I use in calculating the moisture?
  6. Oct 28, 2009 #5


    User Avatar
    Gold Member

    One thing that you are missing (and it is critical) is "how fast can water migrate through the soil and evaporate if the soil is wetter than the air?" This is something that is determined through testing in the field. When building clay-core dikes, we had to test the clays in the lab, in situ, and as-deposited because for the layers of clay to bond well, their moistures had to match well. Some soils that can be characterized as "clays" dry fairly well, while others develop a rather hard external crust that helps resist migration of water and retains the moisture in the underlying soil. I'm not a theorist, but spent a few seasons in the trenches as a soil scientist in construction, so most of my work was practical and driven by experimentation.
  7. Oct 28, 2009 #6
  8. Oct 30, 2009 #7
    I tried reading the PDF and I didn't understand a single word. I think it needs a vast background on geophysical issues.

    Is there a simple equation for this with simple variables that I can find and assume? I don't need the model to be accurate. Just accurate enough to model the soil in general, it is a simulation using wireless sensor networks. I can make any number of assumptions within the model.

    I'm sorry for my laziness but I really couldn't get anything.

    Thanks in advance.
  9. Oct 30, 2009 #8

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    I don't think so. People go to school for years to answer these kinds of questions. I knew a civil engineer who specialized completely in soils.
  10. Oct 30, 2009 #9
    I totally agree with you .. but usually complexity comes for accuracy. If accuracy is not required simple equations are formed. right?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook