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SSC100 Lecture 10/29/97

Approximate contents of my lecture

Announcements

Class overview

Review of water basics

Water flow under sat conditions

Water flow in stratified soils

Water flow to plants

Review of 6 water basics

1. Water always flows from higher potential to lower potential... this could mean from -10 kPa to -11 kPa
2. Water movement can only be predicted if we know the driving forces acting on water (total potential) and the constraints to flow (integrated into the hydraulic conductivity term)
3. The components of total water potential include

• gravitational (can be positive or negative depending on reference point)
• pressure (soil is typically saturated; positive)
• matric (soil is typically unsaturated; negative)

Saturated water movement is generally slowed by the colloidal content of the soil; unsaturated water movement is aided by colloidal content. as the total surface area over which water can be held increases (eg as montmorillonite or organic matter content increases), the Y m decreases, meaning water has less energy available to move relative to a soil with a smaller colloidal content (recall we define colloidal as particles having a diameter less than 1 um).

• osmotic (requires a semi-permeable membrane such a root to influence water movement; negative)

4. Saturated flow is dominated by macropores: as macropore volume increases, flow under saturated conditions will increase; because flow through micropores is so much more restricted, they have little influence on total flow rates... remember Q=kr^4
1. Unsaturated flow is dominated by micropores: under unsaturated conditions, increased macropores will not increase flow since these will drain quickly under the influence of gravity
2. Pore size distribution, pore connectedness, pore tortuosity all influence k and therefore are critical to predicting flow under unsaturated conditions. It is not enough to know what the total pore volume is; what is important is the relative distribution of pores. As pore connectedness increases, flow increases; as pore tortuosity increases, flow decreases.

Figure _____ in your book shows the relative pore size distributions for three different soil types.

Figure 5-10 in your book reemphasizes the importance of particular pore sizes to hydraulic conductivity. As soon as water begins to drain from larger pores, the hydraulic conductivity of a sandy loam, for example, falls dramatically. Notice that at certain low matric potentials, a clay soil actually conducts water more quickly than does the sandy loam.

1. Water flow under saturated conditions

Remember from Willi’s example that flow through a pore 1 mm radius (Flow=1^4=1 cm3/min) will be equal to flow through 10,000 pores each with a radius of 0.1 mm (Flow=.1^4=.0001 cm3/min).... why does this make sense? What’s holding up the additional flow?

As a result, most saturated water movement is through largest (water-filled) pores.

You can appreciate how a given textural class and structure might influence saturated water movement

2. a. Unsaturated flow

Remember the 1^st rule of water movement... it still applies: water moves from higher potential to lower potential

Unsaturated flow is primarily driven by differences in matric potential

2. b Hydraulic conductivity under unsaturated conditions

** water movement in unsaturated conditions is dominated by flow through micropores **

6. a. Horizontal flow

See your figure on the handout.... always note what the total potential is at each end of a soil column because only by comparing total potential can we determine whether flow will occur and if so, in what direction. In this case, we have no influence of gravity acting on the column.

2. Vertical flow downward: Straightforward case of gravity combining with pressure to create a very high head.

3. Vertical flow upward: For example, water may flow a watertable up to the surface soil which is drying and therefore has a very low matric potential... only the water held in thin films on the surfaces of clays may remain after being baked in a hot Davis summer.

1. Flow in stratified soils (soils with different textures) is reduced and is not as fast as in unstratified soils.

1. Sand over clay

Because flow is blocked by the smaller pores that characterize a clay, water will pond above the clay until sufficient pressure builds to begin to force the water closer to the smaller pores through them.

One way to think about this is to consider what happens when 4 highway lanes merge into 2: before too long, traffic gets backed up.

1. Clay over sand

This blockage of flow in this instance may seem counterintuitive, especially when one thinks about how well sand drains. Why does water perch when it reaches the sand? Some combination of forces must be holding those water molecules back, right at the clay/sand edge. In fact, water will not enter the sand until the interface becomes saturated so that the matric potential of the clay is overcome. Recall that as a soil dries, its matric potential becomes more and more negative. As a soil becomes wetter and wetter, its matric potential becomes less negative. Since water flows from higher potential to lower potential, water will have a tendency not to leave the overlying clay so long as there are some matric potential forces acting on it. As soon as the water has ponded to a certain point, and saturation has increased, then the matric potential is less negative, so flow can occur from higher to lower potential, in this case the still dry sand.

2. Water flow in "non-homogenous media"

1 Plant pots: Clay over sand problem... putting gravel at bottom of a pot will not improve drainage. Why?

2 French drains: coarse gravel around foundation will only drain when saturated.

3 Animals remain dry in animal burrows because the surface tension of water prevents it from moving into air very easilty. However, once the soil is saturated, a positive pressure develops and the burrow could fill with water.

4 If the root ball of a tree is packed in a different soil texture then water flow will be reduced.

6.7 Water flow to plants... consider a Soil-Plant-Air Continuum (SPAC)

In order for a root to take up water, the potential in the root must be lower than that of the bulk soil. A root accomplishes this by taking advantage of its semipermeable membranes to accumulate solutes. Accumulated solutes reduce the osmotic potential of a root, so that water will flow from higher potential to lower potential. A root will have a more difficult time of extracting water from a salty soil.

This movement of water from the soil to a root is only part of a continuum. As a plant transpires (moves water through its phloem and releases it from its leaves), water is moved through a plant and back into the atmosphere, from where it rains down again into the soil.