# Soil Water Chapter 5. The 2 kinds of quantities commonly used as a basis for water potential are volume and weight (not mass). Energy per unit volume.

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Soil Water Chapter 5

The 2 kinds of quantities commonly used as a basis for water potential are volume and weight (not mass). Energy per unit volume (E / V = [F x L]/ [A x L] = F / A = P) has the dimensions of pressure, whereas energy per unit weight, a force (E / W = F x L / F = L) has the dimension, length. Water potential in pressure is commonly used with biological systems, and water potential as length is used in engineering.

Higher potential, higher in the gravitational field, no, so water moves downhill.

Obviously, down there.

The pressure differential, air > water, is given by the above expression, where γ is surface tension and R is radius of the sphere. Notice the bubble is concave into the water.

So, concave into water film means the pressure in the water is < pressure in air.

Water enters the osmometer through a semipermeable membrane and rises in the tube. The top of the water in the beaker is at atmospheric pressure (call it zero net pressure) but at the same level inside the osmometer, there is a standing column of water. The negative osmotic potential just balances the positive pressure potential.

There can be only positive pressure potential (below a free water surface, or water table, in saturated soil) or negative matric potential (unsaturated soil), not both. Matric potential is negative pressure, or tension. In the absence of a semipermeable membrane, only gravitational potential and either pressure or matric potential affect water flow.

.. These properties of water are responsible for its cohesion unto itself and its adhesion to hydrophilic surfaces. So, when some water molecules stick to a surface, they bring their buddies along.

Yes, like with capillarity. This is a derivation of the capillary rise equation. Note height of rise, h, is inversely related to radius of tube, R. ρ is density of water and g is acceleration due to gravity. Pressure outside the capillary tube at the water surface is zero and also inside the tube because the negative pressure in the water at the top of the tube is just balanced by the positive pressure of the water column.

Soil physicists have used capillary tubes of different radii as models for soil pores to explain water movement in soil. Note that soil pores of a particular radius are filled with water only to the height that corresponding capillary tubes are filled. Of course soil pores are very short, not long capillaries.

Here are two capillary pores, one with a small radius and one with a larger radius. When tension is applied to the bottom of each, the larger one empties first because it can’t withstand as great of tension as the smaller capillary. The same thing holds for water in soil pores. Consider the next slide.

The decrease in water content of a soil as tension on it increases is due to pores draining, first the largest and lastly the smallest. There is a continuum of pore sizes so this decrease is smooth.

These dots are supposed to be pore size distribu- tions. So, which would be the sand and which would be the loam and clay? And what would be the effect on the soil moisture characteristic curve?

Besides the rapid decrease in water content with increasing tension (more and more negative matric potential), you might suspect the pink soil to be the sand also based on the lower water content at saturation (recall, generally higher bulk density, thus, lower total porosity, in sand than clay).

Same idea holds here. It’s a matter of pore size distribution – compact a soil and you decrease total porosity and reduce the number of large pores.

You’ve done gravimetric. It’s very accurate but destructive. Attenuation of fast neutrons by interaction with H nuclei (calibrate the instrument at different know soil water contents) can be related to water content. Time domain reflectometry is newer technology that relates changes in dielectric constant to water content. Non-destructive methods.

Simple device consisting of rigid tube with a porous ceramic cup on the end. Fill with water, cap and stick in the soil. The greater the tension in the soil water, the greater the tension in the tube. The latter is read by vacuum gage or pressure transducer. Works fine at lower tensions, i.e., not dry soil.

These considerations allow you to sort of derive this. Obviously, volumetric flow (e.g., cm 3 h -1 ) is directly related to cross sectional area. Everything else the same (soil, length of it and area), there is greater flow when there is more standing water but there will always be some minimum flow provided you add water. Thus, flow is proportional to water depth. However, flow is inversely related to length of flow through soil –resistance.

At this point, we’ve Q is proportional to A x (D + C) / L. At zero depth of water on the surface of saturated soil of different lengths of flow, there is the same flow, and the only way this can be the case is if the unknown constant, C, is actually L. Thus, Q is proportional to A x (D + L) / L but it is going to vary with the soil, i.e., with the pore size distribution, bigger pores, faster flow and conversely.

This is the typical case, unsaturated. Here, despite no difference in gravitational Potential, water moves from left to right. This, of course, is due to the difference in matric potential.

Application of Darcy’s Law is not straightforward for unsaturated soil. A big issue is that conduc- tivity decreases as water content decreases. This is because the area for flow decreases as the soil dries and the path that water moves becomes longer. More importantly, flow is restricted to smaller and smaller pores, through which water moves slower.

Like is always the case everywhere and every time, it goes down a potential gradient, from higher to lower.

Well, isn’t the vapor pressure of water higher above relatively hot water than cold water? Don’t solutes reduce water vapor pressure? In fact, the soil may become so dry that adhesion of film water to soil solids actually reduces water vapor pressure.

If you let a saturated soil drain, it drains fast at first but slows. This is the behavior of a clay and a sand. The early thinkers on the matter concluded that water was draining under the influence of gravity and since drainage was so fast, that portion of the maximum water content was not really available to plants. So, gravitational water was plant-unavailable. Regardless of soil type, the tension of soil water when this gravitational water has drained is about - 0.2 or - 0.3 or - 0.33 bar (depending on authority).

This is a soil moisture characteristic curve, no?

Plants are goners when they can’t uptake water against the tension at which it is held by soil solids. They permanently wilt and the associated tension is about -15 bar. So, plant- available water is in between, no?

Hygroscopic water

Plant available water depends on texture. Field capacity and wilting points are determined from moisture characteristic curves. Max at about silt loam.

Organic matter is good in this way, too, no?

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