Accurate field saturated hydraulic conductivity—Why is it so difficult?
SATURO makes life a little easier for those who need a faster, more accurate way to measure Kfs in the field
SATURO: Why it’s more accurate
Field saturated hydraulic conductivity, Kfs (cm/s) is a fundamental soil hydraulic property
that describes the ease with which a fluid (usually water) can move through pore spaces or
fractures under field saturated conditions. One of the oldest and simplest methods for in situ
determination of Kfs has involved the measurement of ponded infiltration (D) from within a
single ring (with a radius b) pushed a small distance into the soil (d) (Figure 1). The original
analysis used the measured steady flow rate, Qs (cm3/s) and assumed one-dimensional,
vertical flow to obtain Kfs from Bouwer (1986) and Daniel (1989).
This approach overestimated Kfs due to lateral divergence of flow resulting from the
capillarity of the unsaturated soil and from the ponding in the ring (Bouwer 1986). Attempts
to eliminate flow divergence involved the addition of an outer ring to buffer the flow in the
inner ring (Figure 2). However, the double-ring infiltrometer technique was ineffective at
preventing lateral flow from the inner ring (Swartzendruber and Olson 1961a, 1961b).
More recent research provides new methods for correcting for lateral flow. Reynolds
and Elrick (1990) presented a new analysis method of steady ponded infiltration into
a single ring, which accounts for soil capillarity, depth of ponding, ring radius (b), and
depth of ring insertion (d) and provides a means for calculating Kfs, matric flux (φm), and
macroscopic capillary length (∝). This analysis is known as the two-ponding head approach
(Reynolds and Elrick 1990).
The two-ponding head approach is the technique used by SATURO, though with some
modifications and simplifications. The simplest equation for this calculation is from
Nimmo et al. (2009). They compute Kfs as shown in Equation 1.
where i (cm/s) is the steady (final) infiltration rate (volume divided by area) and F is a
function that corrects for sorptivity and geometrical effects.
Nimmo et al. (2009) gives F as shown in Equation 2
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- D is the ponding depth (cm)
- d is the insertion depth of the infiltrometer (cm)
- b is the radius of the infiltrometer (cm)
- ∆ is the constant for a given infiltrometer geometry; C1d + C2b (cm)
- C1 is 0.993
- C2 is 0.578
- λ is the reciprocal of the Gardner ∝, which is a characteristic of the soil and its initial
water content (cm)
In Equation 2, ∆ is simply Equation 36 of Reynolds and Elrick (1990) multiplied by bπ, which
allows Figure 2 and Equation 2 to be reconciled with Equation 37 of Reynolds and
For two ponding depths, use Equation 3:
Rearranging one of the right terms to solve for λ in terms of Kfs, substituting this for λ in the
other right term, and simplifying yields
- D1 is the actual high pressure head
- D2 is the actual low pressure head
- ∆ is 0.993d + 0.578b (cm)
- i1 is infiltration rate at the high pressure head
- i2 is infiltration rate at the low pressure head
For ∆, d is the infiltrometer insertion depth and b is the infiltrometer radius. For the SATURO,
5-cm insertion ring, d = 5 cm and b = 7.5 cm, so ∆ = 9.3 cm. For the 10-cm insertion ring,
d = 10 cm and b = 7.5 cm, so ∆ = 14.3 cm.
The hydraulic conductivity is then multiplied by the difference in quasi-steady state
infiltration rate for the last pressure cycle and divided by the difference in the measured
pressure head from the last pressure cycle.
Equation 4 is equivalent to Equation 41 from Reynolds and Elrick (1990) and removes the
dependence on soil characteristics and initial water content described by λ.
Save hours of tedious manual labor
The SATURO combines automation and simplified data analysis together in one system. It even computes infiltration rates and field saturated hydraulic conductivity on the fly. The SATURO makes life a little easier for those who need a faster, more accurate way to measure Kfs in the field.
- Bouwer H. 1986. Intake rate: Cylinder infiltrometer. In Klute A., editor, Methods of soil analysis: Part 1—Physical and Mineralogical Methods. 2nd ed. Madison (WI): ASA and SSSA. 825−844. (Article link)
- Dane JH and Topp GC, editors. 2002. Methods of soil analysis: Part 4—Physical Methods. Madison (WI): Soil Science Society of America Inc. (link)
- Daniel DE. 1989. In situ hydraulic conductivity tests for compacted clay. J. Geotech. Eng. 115(9). (Article link)
- Nimmo JR, Schmidt KM, Perkins KS, and Stock JD. 2009. Rapid measurement of field saturated hydraulic conductivity for areal characterization. Vadose Zone J. 8(1): 142−149. (Article link)
- Reynolds WD and Elrick DE. 1990. Ponded infiltration from a single ring: I. Analysis of steady flow. Soil Sci. Soc. Am. J. 54(5): 1233−1241. (Article link)
- Swartzendruber D and Olson TC. 1961. Sand-model study of buffer effects in the double-ring infiltrometer. Soil Sci. Soc. Am. Proc. 25(1): 5−8. (Article link)
- Swartzendruber D and Olson TC. 1961. Model study of the double ring infiltrometer as affected by depth of wetting and particle size. Soil Sci. 92(4): 219−225. (Article link)