UZIG-News - November 1995 - Issue 3

++ National UZIG meeting in San Diego ++

Plans are afoot to hold our next national meeting in San Diego in February 1996. Terry Rees (tfrees@usgs.gov) of the California District is making the arrangements for the first get-together since we met in Georgia in 1992. Watch email for further announcements.

++ Email addresses ++

Contributers and others mentioned in UZIG-News are identified by the brief version of their email address, as used within the USGS system. To send email from outside the system, all that is needed is to append @usgs.gov to the internal address given. Remember that contributors want to have whatever questions or comments arise--the purpose of UZIG-News is to enhance communication! 

++ Use of UZIG email list ++

We now have a system mailing list, installed on the mprcamnl server, that includes the email addresses of all UZIG members. Using this list, the single address
uzig@mailrcamnl.wr.usgs.gov
will send email to everyone. Keep in mind, though, that more than 200 busy people will receive the message. To see who's on the list, when logged in to the USGS-WRD unix system use the command
show_list uzig@mailrcamnl.wr.usgs.gov
(One potential problem: if the list is not up-to-date, for example if an address on the list has recently been changed or deleted from the system, show_list gives an error message instead of the list.) To send a message to the maintainer of the list (presently jrnimmo), send email to the address
uzig-request@mailrcamnl.wr.usgs.gov
Requested additions and deletions are always welcome. ++ Evolution of UZIG-News ++

The last item in this issue is a different type of article, somewhat longer and more formal than would normally be in a newsletter. It is a verbatim draft of a portion of a USGS TWRI (Techniques of Water-Resources Investigations) being prepared by the Office of Ground Water on the use of pressure transducers in water-resources studies. Written by Rick Healy, this excerpt describes applications of pressure transducers in unsaturated-zone studies. The reason for including it in this form, as opposed to a shortened newsletter-format article, is that before publication, Rick (rwhealy) would like to get critical comments and suggestions from interested UZIG members.

By Carole L. Thomas (clthomas)

How much water infiltrates in an arroyo channel? In New Mexico arroyos are a vital source of aquifer recharge, and the answer is crucial. A team consisting of USGS district and NRP personnel studied selected reaches of Grant Line Arroyo and Tijeras Arroyo in Albuquerque to collect the necessary information for infiltration during 1988-92. Essential factors are the instantaneous streamflow-loss rates, wetted-channel area, and instantaneous evaporation rates.

For each of 21 selected reaches, streamflow at a downstream gage was subtracted from streamflow at an upstream gage to obtain a streamflow-loss rate in cubic feet per day. Streamflow at the two points was measured with 3- and 6-inch flumes. The streamflow-loss rate was then divided by the wetted-channel area, measured with a rolling tape, to give a streamflow-loss rate per unit area. The evaporation rate for saturated sediments within a braid of the stream, measured with a portable field chamber, was subtracted from the streamflow-loss rate per unit area to obtain the infiltration rate in feet per day.

Evaporative losses were small--at most 2 percent of the infiltration rates--so calculated infiltration rates were not sensitive to the evaporative water loss from the stream surface. Though not significant as corrections to infiltration, these evaporation and evapotranspiration rates are interesting in themselves. Factors of significant influence include vegetation, availability of water at the land surface, availability of energy to enable a change of state, existence of a vapor concentration gradient, and atmospheric turbulence. Daily evaporation and evapotranspiration rates ranged from about 0 over unsaturated, unvegetated soil during very cold months to about 7.2 millimeters per day over saturated stream sediments during hot months. Measured instantaneous rates were as high as 28 millimeters per day. The rates were similar for Grant Line and Tijeras Arroyos.

Instantaneous infiltration rates varied with the daily temperature fluctuations and also with the length and location of each selected reach. For example, on May 19, 1992 the infiltration rate for Tijeras Arroyo above Four Hills Bridge increased 22 percent when the temperature rose from 12 to 21 degrees Celsius. Spatial variations in infiltration are assumed to result mainly from variations in the sediment intrinsic permeability.

At Grant Line Arroyo, 11 measurements of instantaneous infiltration rates ranged from 0 to 0.6 foot per day with a median of 0.1. At Tijeras Arroyo, 160 measurements ranged from 2.28 to 30 feet per day with a median of 5.86. Infiltration at Grant Line Arroyo generally percolates less than 5 feet and is assumed to produce no recharge at the water table estimated to be about 600 feet below land surface. In contrast, infiltration at Tijeras Arroyo is known to percolate below 10 feet, and is assumed to produce recharge at the shallow water table, about 30 feet deep. Thus the hydrologic influence of the two arroyos differs markedly.

by Rick Healy (rwhealy)

(Note: This article has been through review, but before it is published, Rick would like to get feedback from any interested UZIG members. Not enough detail? Too much detail? Pertinent references? There are 2 figures which could not be transmitted electronically. Figure UZ1 is a simple diagram of a transducer connected to a tensiometer. Figure UZ2 shows a schematic of a field site where tensiometers and soil-gas piezometers are installed from land surface. Rick will fax a copy of the figures, or the complete text with figures, to anyone that is interested.)

Pore space within the unsaturated zone is occupied by either water or air. Water is held under tension (i.e., pressure is less than atmospheric) by the capillary attraction of small pores. Devices called tensiometers are used to measure water pressure under unsaturated conditions. Air pressures in the unsaturated zone may be greater than or less than atmospheric pressure and are measured with soil-gas piezometers. Pressure transducers can be used in conjunction with either of these devices.

Tensiometers

Tensiometers are instruments that are designed to measure the total head and pressure head, hp, of water in the unsaturated zone. Because hp is generally less than atmospheric, transducers used with tensiometers must be capable of measuring negative pressure or vacuum. Tensiometers are functional when hp is greater than about -8 m of water in gage pressure. Principles of tensiometer operation and design are described in Gardner and others (1922), Cassel and Klute (1986), and Stannard (1986). A typical design is shown in Figure UZ1. The body of the tensiometer is a PVC tube filled with water. The bottom of the tube contains a porous ceramic cup that is inserted into the soil. This membrane forms a continuum with the soil pores, allowing soil-water pressures to be transmitted to the tensiometer. The top end of the tensiometer is fitted with some sort of pressure sensing device. Original tensiometer designs used mercury or water manometers or vacuum gages. Gage or absolute pressure transducers are now more commonly used (for example, Yeh and Wierenga,1986) because they allow automatic recording. Tensiometers are usually installed vertically from land surface, although construction of a trench or tunnel permits horizontal installation.

Measurements of water pressure in the unsaturated zone are desirable at a number of different depths in order to quantify the hydraulic gradient and infer the direction and magnitude of water movement. A typical field site might be equipped with 10 or more tensiometers. Switching valve systems have been used in the past to scan several tensiometers with a single transducer. Yeh and Wierenga (1986) used a hypodermic needle attached to a pressure transducer to manually record several hundred tensiometer readings in a short period of time. The needle was inserted through a rubber stopper at the top of each tensiometer. With the advent of inexpensive pressure transducers (less than about $25.00 each) it has become feasible in most studies to fit each tensiometer with a separate transducer. This avoids complicated switching valves and retains the ability for automatic recording.

The pressure, P, recorded in units of head by the pressure transducers in Figure UZ2 is equivalent to the total head at the measurement point of the tensiometer (the porous cup). P accounts for both pressure head, hp, and gravity head, Z, and can be written as P = hp - Z. Z is the difference in height between the sensing element of the transducer (Z = 0) and the porous cup and is positive in the downward direction. The pressure head at depth Z is calculated as hp = P + Z. If, in Figure UZ2, the water table is at Z1 then by definition hp1 would be 0 and P1 would equal -Z1. Selection of the most appropriate pressure range for transducers depends on the specific application. A range of 0 to -8 m of water would be sufficient for field studies. A smaller range would allow for more accuracy and may be desirable for laboratory experiments or detailed field studies. In selecting the pressure range, the gravity component Z must be considered. If the anticipated range of pressure heads for a study is between 0 and -50 cm, but Z is 200 cm, then the transducer must be capable of sensing pressures in the range of -200 to -250 cm. The need to account for Z places a limitation on the depths to which tensiometers can be installed from land surface. Some researchers avoid this limitation by installing a transducer near the ceramic cup and burying it along with the rest of the transducer. In other studies trenches, tunnels, or caissons have been constructed to allow horizontal installation of tensiometers. This permits deeper installation and minimizes Z so that smaller pressure range transducers can be employed. When transducers are used with tensiometers, a barometer should also be used to account for barometric fluctuations.

Water within a tensiometer is under a vacuum, so air tends to slowly come out of solution and accumulate within the tensiometer. Design of tensiometer/transducer systems should minimize adverse influences of air accumulation. The air will expand and contract as its temperature rises and falls. This will show up in the recorded data as oscillations in pressure head that are correlated with air temperature. Most tensiometers are equipped with a flushing valve to permit periodic removal of air. Tensiometers that are completely buried do not have that capability, thus their usefulness for many studies is limited. Ideally both the tensiometer and transducer should be well insulated and shielded from direct solar radiation. This may be accomplished in field studies by enclosing the top of the tensiometer and transducer is a container that can be buried a few inches below ground.

Soil-Gas Piezometers

Measurements of soil-gas pressures may be made in conjunction with measurements of soil-water tension or for determining barometric efficiency of geologic units (Weeks, 1978) or for monitoring of air pumping tests (Baehr and Hult, 1990). The latter two operations are designed for calculating the air permeability of partially saturated porous media. Data is obtained from piezometers that are constructed in the unsaturated zone by drilling a borehole and installing a small diameter tube (usually not greater than 0.25" in diameter) that runs from the measurement point (Z1 in Figure UZ2) to land surface. The borehole is then sealed and a pressure transducer is attached to the top of the tube. Determination of barometric efficiency relies on measuring changes in barometric pressure and the time lag it takes for those changes to be propagated down to the piezometer. The analysis requires a barometer in addition to the gage or absolute transducer attached to the tube. Gage transducers with a pressure range of a few inches of water are usually preferred for this application because they encompass the typical range in diurnal barometric fluctuations. Either absolute or gage transducers with a wider range of pressure may be used for monitoring pneumatic tests.

References

Baehr, A. L., and Hult, M. F., 1991, Evaluation of unsaturated zone air permeability through pneumatic tests: Water Resources Research, v. 27, no. 10, p. 2605-2617.

Gardner, W., Israelson, O. W., Edlesfsen, N. E., and Clyde, D., 1922, The capillary potential function and its relation to irrigation practice, Phys. Rev., v. 20, p. 196.

Cassel, D. K., and Klute, A.,1986, Water potential: Tensiometry, in, A. Kulte, ed., Methods of Soil Analyses, Part I - Physical and Mineralogical Methods, Second Edition, American Society of Agronomy, Inc., and Soil Science Society of America, Inc., Madison, WI, p. 563-597.

Stannard, D. I.,1986, Theory, construction, and operation of simple tensiometers: Ground Water Monitoring Review, v. 6, no. 3, p. 70-78.

Weeks, E. P., 1978, Field determination of vertical permeability to air in the unsaturated zone: U. S. Geological Survey Professional Paper 1051, 41 p.

Yeh, T-C. J., and Wierenga, P. J., 1986, Observations of spatial variability of soil-water pressure in a field soil: Soil Science, v. 142, no. 7, p.

List of Figures

1) Schematic design of soil-water tensiometer.

2) Schematic design for field tensiometers and a soil-gas piezometer. Pi is pressure recorded by transducer i and is equivalent to total head of the tensiometer; Zi is depth of tensiometer cup i beneath the reference depth Z = 0 (elevation of transducer sensing element); and hpi is pressure at Zi.