SEPTEMBER 1996 INFILTRATION PROJECT PROPOSAL
15 SEP 96
At sundown Ann Carr and I arrived in Jeffrey City, WY with a small water-tank trailer in tow and with the equipment needed to make simultaneous triplicate infiltration runs using open-top (OT) and closed-top (CT) infiltrometers. That's six runs at once.
16 SEP 96
In the morning Ann and I enjoyed a radiation safety training session conducted by Trinidad Herrera. Mr. Herrera is to be commended for his lively (animated) presentation of what all too often becomes little more than a boring video tape poorly prepared.
In the afternoon several staff members of Shepherd Miller, Inc. (SMI) joined Ann and me in looking for three appropriate field sites for making infiltration runs. As we thought that the granite outcroppings might play a major hydrologic role, we first considered on outcropping in the SW Valley as a potential site. We selected a location (designated as A site) at the west edge of the outcropping in a miniwash for a set of runs - three with OT and three with CT infiltrometers. While there we discussed what might be the role of the outcroppings in watershed hydrology.
Later that afternoon we located the other two sites (B & C sites) in close proximity to an outcropping in the NW Valley. B site was similar to A site, geomorphologically, whereas C site was about 20 meters out from the edge of the outcrop in a sparse stand of native vegetation.
The surface soil or mineral material at the A & B sites was a wind-blown and water-washed sand, whereas the soil at C site was a wind-blown sand with a negligible amount of organic matter.
We decided against making a set of runs on the capped area (as originally planned) because of a thick layer of rock (rip-rap for erosion protection) on top of the cap which was incompatible with the small dimensions of the infiltrometers. Also, at best, infiltration measurements would be dominated by lateral flow in the rocky layer rather than vertical downward flow through the cap. (Please note that for the sake of brevity, the foregoing description of site selection reflects a bit more timely decisiveness than actually occurred.)
17 SEP 96
With the help of several SMI staff members in the trucking of equipment to the remote A site, we made a set of six runs - 3 OT and 3 CT. Infiltration rates were so high at the first OT run that we had to lift the supply cylinder to increase the inflow rate. Fortunately we didn't encounter that problem again. Infiltration was much higher under the hydraulic conditions provided by the OT infiltrometer than under the CT infiltrometer. The OT infiltrometer operates with plus one (+1) inch surface hydraulic head, whereas the CT infiltrometer operates with about an average of minus one (-1) inch head. This is a difference of only 2 inches (5 cm or millibars); however it makes all the difference in the world of water infiltration into soils. The small negative head blocks downward water flow in macropores as these pores (which would otherwise dominate infiltration) are venting soil air upward during infiltration. Thus a large difference between infiltration as measured by the two methods indicates a large contribution of macropore bypass flow.
By the end of this set of six runs we had learned that our initial plans to run all six simultaneously was a bit too ambitious. The best we could do was to have 2 or 3 in various stages of progress at the same time.
18 SEP 96
Early in the morning during a chilling drizzle, Ann and I carried equipment from our pickup and trailer to B and C sites which were readily accessible from the main road. By the time we had deposited the equipment (including a good supply of water) onto a slab of granite outcrop, the drizzle was changing to wet snow. We decided breakfast was an excellent option to give the storm a chance to pass--2 inches of snow had accumulated in well-insulated spots by the time we left the restaurant. To kill time we made a trip down to Muddy Gap for newspapers and gas. Then we drove back to Jeffrey City and on toward Riverton until confronted with blinding fog. As we had no good reason for going there in the first place, we returned to Jeffrey City to find the sky clearing overhead.
19 SEP 96
We arrived at the frosty and icy B and C sites before sunrise which was delayed somewhat by the high granite outcrop on the nearby eastern horizon. The precipitation on the 18th (estimated at ¼ inch) had not been enough to measurably affect the infiltration measurements. I staked out the locations for the 12 remaining infiltration runs with many little red flags to protect them from foot traffic while Ann was sieving soil (sand to be used to seal the infiltration cylinders against lateral flow of air and water.
We completed both sets by 4:00 p.m. In the meantime we enjoyed visits first by Trinidad Herrera and later by John Gearhart. Now it was time to load equipment, return borrowed items and head back to Tucson via Rock Springs and U.S. 191. We were happy with the little project. Things had gone well in spite of the storm. The data I thought would prove to be most interesting.
DATA OVERVIEW
Infiltration rates were generally high with two notable exceptions (both with CT infiltrometers) with OT infiltration being much higher then CT infiltration. The table below gives 15-minute cumulative infiltration since a comparison of longer duration infiltration was not possible without excessive extrapolation because many of the OT runs were shortened by very rapid infiltration and early exhaustion of the water supply.
Fifteen-Minute Cumulative Infiltration in Centimeters
Replication:
*Value derived by extrapolating data to 15 minutes because infiltration rate was so rapid that water supply ran out in just 9 minutes.
**Mean ratio of infiltration under the two types of cylinder infiltrometers, open top (OT) and closed top (CT).
The above table shows that OT was 2, 6 and 7 times greater than CT infiltration for A, B, an C sites, respectively which indicates macropores are dominating the infiltration process especially at the B and C sites. OT infiltration at the B site was twice that of the C site which is attributed to the closer proximity of underlying rock fissures. The extremely low CT infiltration of the B & C sites, Rep. 2 was due to an underlying layer of coarse sand which amounts to a capillary discontinuity in the pathways of infiltrating water. Such a discontinuity does not throttle OT infiltration as water moves down rapidly via macropores under the force of gravity, thereby rapidly saturating the coarse sand layer. During the course of these runs we thought the infiltrometer must be sitting on top of a granite boulder with zero hydraulic conductivity. But in digging down after the run we found the coarse sand which would produce a near zero hydraulic gradient and thus the very low infiltration.
APPENDIX
Infiltration Data
Eighteen page attachment gives data for eighteen infiltrometer runs (6 runs at each of three sites) both numerically and graphically.
Symbol examples:
OT-1 = open-top infiltrometer, replication no. 1
CT-1 = closed-top infiltrometer, replication no. 1
Infiltrometer Photographs
Photos of the infiltration sites with brief descriptions are attached
Infiltrometer Sketches
Granite Outcrop Hydrology
Although its risky to extrapolate point infiltration measurements to watershed infiltration, much of the groundwater recharge is probably occurring within and at the edges of the granite outcroppings for these reasons:
• Such granite is typically deeply fissured providing rapid bypass routes for infiltrating snowmelt and rainwater, whereby surface water can become groundwater in a matter of minutes.
• Some of the edges of the outcroppings are diked with wind-blown sand deposits (dunes), which pond or confine outcrop runoff to miniwashes until it infiltrates.
• There is very little vegetation and soil on the outcrop to dissipate precipitation as evapotranspiration (ET).
• Conditions on and at edges of outcrop are conducive to high bypass infiltration.
• Surface micro and macroroughness provides for water ponding and thus the hydraulic head required to drive water into and down through the granite fissure. This roughness also readily vents displaced air, thereby preventing the build-up of pneumatic pressure in fissures to retard bypass water flow.
• An abundance of macropores terminating at the surface in both depressed and elevated areas to facilitate the exchange of air and water across the air-earth interface, thereby favoring rapid bypass infiltration.
In conclusion a large (significant) portion of the water produced by the large and infrequent rain and snowstorms probably infiltrates on or at the edges of the granite outcroppings and then quickly contributes to groundwater recharge by the process called rapid bypass flow. This could be greater than 50% of the precipitation for single events producing more than one inch of water. Snowdrifts at the edges of the outcrops may also contribute to groundwater recharge upon melting.
In contrast the outlying wind-blown sandy areas with sparsely scattered native vegetation probably contribute very little to groundwater recharge as most of the precipitation is dissipated as ET.
Infiltration measurements with open and closed-top infiltrometers, certainly support the foregoing conclusions. The extremely high infiltration as measured with the open top infiltrometer operating with 1 inch of ponded water suggests that water can move downward fast enough to infiltrate the rapid onrush of runoff from the granite outcrop above. The large difference between open top and closed top infiltration shows that macropores are indeed contributing greatly to infiltration.
Soil or AEI Bubbling Pressure
The Bubbling Pressure (BP) of the soil or Air-Earth Interface (AEI) is the amount of soil air pressure required to force air bubbles upward through a saturated soil surface. BP is a function of soil texture being inversely related to primary particle size. Thus, a sandy soil will have a lower BP than a clayey soil. BP is also a function of soil structure with a well-aggregated soil having a lower BP than a structureless soil. Macropores which open through the soil surface such as invertebrate burrows greatly reduce BP because of their relatively large diameter and lack of tortuosity.
Soil pores may be idealized as glass capillary tubes, where BP and capillary rise (CR) are equal balancing pressures or forces. The pressure (force) is inversely proportional to the diameter of the capillary Dc as shown in the table and sketch below.
Dc (mm)
BP(cm)
CR (cm)
10 0.3
10 -1 3.0
10 -2 30.0
CR (cm) VS Dc (mm)
BP and CR of sandy soils is on the order of 3 cm ( @ 1") of water pressure for an equivalent pore diameter of 1 mm. BP tends to rise with the duration of infiltration because the air escape route becomes longer, more tortuous and more constricted.
Closed-Top Infiltrometer
The foregoing mini-essay on BP is helpful if not essential to understanding the function of closed top infiltrometers. These infiltrometers simulate the natural process of air pressure rise beneath the wetting front by creating a vacuum above the ponded surface water to induce counterflow of air and water across the soil surface simultaneously. Unlike open-top infiltrometers with positive surface heads (SH), simple closed top infiltrometers operate with a negative surface head equal to or slightly greater than the BP of the largest soil pore connected to the soil surface. The simple example sketched below will help in understanding the fundamental difference between open and closed top infiltrometers.
Selected References
• For effects of soil air pressure and macropores on infiltration:
Dixon, R.M. 1966. Water infiltration responses to soil management practices. Ph.D. Theses. University of Wisconsin, Madison. 175p.
Dixon, R.M. and A.E. Peterson, 1971. Water movement in large soil pores: A Channel System Concept of Infiltration. Research Report No. 74. College of Agriculture, University of Wisconsin, Madison.
• For design and function of closed-top infiltrometers:
Dixon, R.M. 1975. Design and use of closed-top infiltrometers. Soil Science Society of America Proceedings 39:755-763.
• For the effects of a coarse buried sand layer on infiltration:
Gardner, W.H. 1962. How water moves in soils. Crops and Soils 15:7-11.
• For least-square fitting of infiltration data to linear equations:
Dixon, R.M., J.R. Simanton and L.J. Lane 1978. Simple time-power
functions for rainwater infiltration and runoff. Arizona-Nevada Acad.
and Am. Water Resources Association 8: 79-89.
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