Transect Soil Water Potential 1986 - 1987

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Date range: 

1986-02-27 to 1987-12-18

Original investigator: 

Walter G Whitford

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Soil water potential recorded on data sheets for psychrometers along treatment and control transects every 5 stations at 5cm, 15cm and 30cm depths.

Soil temperatures at these depths and calibration slope and intercept for each psychrometer are included in order to convert microvolt readings to -bar readings.

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Data file information for the following Jornada data set: Transect Soil Water Potential 1986 - 1987


Field data sheets

Readings from Wescor screen-cage soil psychrometers taken every 2 weeks using WESCOR HR-33T microvoltmeter. Half of control and matching half of treatment transects read on one day. Other half of control and treatment readings done the following day. PROCEDURE FOR CLEANING AND CALIBRATING SCREEN-CAGE SOIL PSYCHROMETERS PSYCHROMETER CLEANING PROCEDURE 1. Use wooden dowel to remove screen-cage by slipping it over the end of the screen-cage on psychrometer. Grasp near base of dowel with plyers and hold firmly to psychrometer boot while pulling the screen-cage straight out from the boot. Use great care while removing the screen-cage to avoid damage to the very fragile junction wires mounted on the end of the boot within the screen- cage. Inspect junction wires to insure that they are not broken. If broken, save psychrometer for possible repair. 2. Using 95% ethanol in a water bottle, wash the junction wires and the boot thoroughly. 3. Using deionized water in a water bottle, rinse the junction wires and the boot thoroughly. 4. Using compressed air that has been filtered with cotton to remove possible oil droplets added from air supply, thoroughly dry junction wires and boot. Pay special attention to the groove which the screen-cage slips down into since water left there will be forced up under the screen-cage when it is replaced. (The screen-cage consists of 2 layers of wire mesh. The inner mesh is impermeable to liquid water and thus will trap water which will later affect calibration values.) When dry, carefully set aside so that there is no chance that the junction wires will be damaged while the screen-cage is being cleaned. 5. To wash the screen-cage, hold with forceps and swish vigorously and thoroughly in boiling deionized water. While still holding with forceps, use filtered compressed air to thoroughly dry inside and out. Set screen-cage back in wooden dowel without touching it (to avoid contamination of screen-cage with salts from hands). Carefully replace screen-cage on psychrometer. If there are any loose or dangling wires on the screen-cage or caught in the boot-groove, carefully cut these off (if attached) or pull free (if in boot-groove). 6. Inspect psychrometer cable for punctures. If present, use electrical tape and tightly wrap punctures to prevent water damage when immersed in water bath. 7. Screen-cage psychrometer is now clean and care should be taken to insure that it remains so. 8. Record psychrometers in data book labeled "Soil Psychrometers" in numerical order by I.D. # along with "pi-v" value and calibration values as these are acquired. CALIBRATION PROCEDURE A. Calibrating Psychrometers 1. Prepare the following calibration solutions: 0.05m NaCl = 2.923g NaCl in exactly 1L deionized water 0.5m NaCl = 29.227g NaCl in exactly 1L deionzied water *1.0m NaCl = 58.454g NaCl in exactly 1L deionzied water 1.5m NaCl = 87.681g NaCl in exactly 1L deionzied water *Need only be used if unable to calibrate using 1.5m NaCl. 2. Construct calibration chambers as follows: a. Use clean 13x100 test tubes (1 per psychrometer). Store upside-down in test tube rack. b. Insert 1 rolled filter paper ( 9cm diameter) as far as possible into test tube . (Rolling the filter paper around a pencil and then sliding it off works well and is the right size for the test tube.) c. Add 2ml of 0.05m NaCl to each test tube making sure to wet filter paper as it is added. d. Place a pre-drilled and cut #000 stopper around the cable of each psychrometer at a distance from the screen-cage end such that the end of the psychrometer will be 1.5-2 inches from the bottom of the test tube when inserted. It should not be immersed in calibration solution at bottom of test tube at any time. Carefully work the stopper as far into the test tube as possible so there is a complete seal around the psychrometer cable. It may be necessary to add a small quantity of high vaccuum grease around the cable to insure a tight seal. e. Place test tube rack with test tube calibration chambers into a water bath that has been preheated to 25 degrees C. (Preheat with water bath covered by aluminum foil to prevent evaporation and to help maintain temperature.) Leave the minimum length extended out of water bath that is required to reach microvoltmeter (HR33T). Keep the rest of the cable in water bath and seal waterbath with aluminum foil. It is very important that psychrometers be maintained at constant temperature as an improper cover over the water bath will encourage temperature fluctuation which will result in faulty calibration. f. Allow psychrometers to stabilize for approximately 3 hours before proceeding with calibration procedure given below. 3. The following applies to use of HR33T Dew Point Microvoltmeter in calibrating psychrometers: a. Set range switch to anticipated range. b. Set function switch to SHORT. c. Make sure toggle switch is in uV mode. d. Turn instrument on. e. Plug psychrometer into instrument (match the notch in plug to notch in socket). f. Turn function switch to READ. g. Get temperature reading by switching toggle switch to C. Check range and use correct scale on meter (top 0 to 10 scale for microvolt readings of 0-10 and readings of 30-100, bottom 0 to 3 scale for microvolt readings of 0-30 and 100-300). Wait for temperature to stabilize before taking a reading. h. Switch back to uV mode. i. Calibrate instrument according to the individual psychrometer's cooling coefficent, "pi-v". (# found on tag on psychrometer lead or on data sheet) by: (1) Adjustment must be made to the cooling coefficent (pi-v) according to the difference bewteen the soil temperature and 25 degrees C. The correction factor, (T - 25degrees C) X 0.7, must be added to the factory specified "pi-v" found on the psychrometer's lead in order to obtain the correct cooling coefficient to be used in dewpoint measurements. (where T = soil temperature) (2) Press in and hold down "pi-v" button while turning "pi-v" set knob to the adjusted cooling coefficient that was just calculated. ADJUST USING TOP SCALE. (3) If "pi-v" is not known, see NOTE to determine its value. j. Set the zero by turning the zero offset knob until the meter needle is exactly on zero. Use coarse and fine adjustment knobs as needed. The instrument is now ready to take dew point readings. * k. Turn fuction switch to COOL for 20 seconds for 0.05m NaCl calibration solution. l. Turn function switch to D.P. (dew point). Needle will drop. Take reading when needle stops. If needle stops briefly then begins to drift slowly, take reading at first stop. Be sure to read on correct scale and range. This value is the dew point in uV and should be recorded in SOIL PSYCHROMETERS data book. m. Turn function switch to SHORT before unplugging psychrometer. n. To retake a reading, switch function knob to HEAT for 5 to 10 seconds, turn function switch to SHORT, then repeat steps f - m after waiting 5 minutes. o. When reading a number of psychrometers at the same time, it is not necessary to turn the machine off between readings, but the function knob must be switched to SHORT before plugging or unplugging psychrometer. NOTE: Determine "pi-v" as follows: (1) Allow psychrometer to equilibrate in a dry calibration chamber in water bath at 25C. (2) With the FUNCTION switch in the READ position, push "pi-v" button and adjust top scale to about 70 using "pi-v" set knob. (2) With the RANGE switch set to 30 uvolts, adjust the ZERO OFFSET control to bring the meter reading to zero. It is necessary to accurately zero the meter before determining "pi-v". (3) Switch the FUNCTION switch to COOL for a few seconds, then switch to DEW POINT. Depending upon the position of the "pi-v" SET knob, the meter will tend to (a) fall back toward zero, (b)hold steady, or (3)climb upscale. Rotate "pi-v" SET until the meter holds relatively steady and the reading is between 15 and 30 on the bottom scale. (If the needle is rising or pegged upscale, rotate the control clockwise.) (4) When the "pi-v" button is depressed, the meter will give a reading proportional to the position of the "pi-v" SET control. The numerical value of the reading on the 100 uvolt scale is the cooling coefficient, "pi-v", of the psychrometer being used. * Cooling time is dependent on soil moisture. Cooling times required will vary from 5 seconds or less for very wet soils to 30 seconds or more for dry soils. Once the necessary cooling time has been determined for a given water potential range, this time should be used for all measurements within that range, including calibration measurements. To determine cooling time: The time it takes during COOL cycle for needle to stop (or almost stop) when calibrating the first couple of psychrometers is the time to use for the rest of the psychrometers for that particular calibration solution. If during COOL, needle rises higher than the adjusted cooling coefficient, "pi-v", the calibration solution is "too dry" to read. This is most likely to happen with the 1.5m NaCl solution. If it does, you must recalibrate that psychrometer using 1.0m NaCl solution. 4. Make sure HR33T microvoltmeter is switched OFF when finished with your readings. 5. You have now finished calibrating your psychrometers with the first of 3 calibration solutions. You must now --- re-clean the psychrometers as per the Cleaning Procedure and repeat steps 2 through 4 of the Calibration Procedure --- for calibration solutions 0.5m NaCl and 1.5m NaCl. Only for those psychrometers that have a "pi-v" too low to read using 1.5m NaCl will you have to use the calibration solution of 1.0m NaCl. 6. PROCEDURE AT END OF EACH CALIBRATION You need to carefully remove stoppers from test tubes without pulling on psychrometer cable. Clean all psychrometers as per procedure. Rinse stoppers well. They must be dry before reusing. Store in plastic bag until ready to use. Pull used filters out of test tubes with forceps and return test tubes to stockroom for cleaning. Get clean 13x100 test tubes from stockroom and rinse these thoroughly with deionized water. (They may be "clean" but they've been sitting in the open collecting dust.) You can either (1)dry these upside down in a test tube rack or, (2)if you will need them soon, shake excess water out of them, place them upright in test tube rack, cover them completely with paper towels to keep out air-borne contaminants, and set them in a drying oven for a couple of hours. B. Performing Linear Regression 1. You must verify that the values recorded for the calibration solutions confirm a "good" psychrometer. Also, slope and y- intercept must be calculated for later conversion of field data. 2. Using the HP11C calculator, calculate m(slope), b(y-intercept), and r(correlation coefficient) for each psychrometer: a. Clear memory by pushing "f" "E" b. Enter data pairs: uV value "ENTER" *calibration soln in bars " " Repeat for each calibration solution used, for each psychrometer. d. Push "f" "y,r" "x y" and record the resulting r in data book. * 0.05m NaCl = 2.34 bars 0.5m NaCl = 22.81 bars 1.0m NaCl = 46.40 bars 1.5m NaCl = 71.34 bars Value should be no less than 0.9995. If it is less, evaluate uV readings for that psychrometer and redo any or all of calibration readings as necessary. If r remains low, this psychrometer must be considered "bad". Save it for possible repair. e. Push "f" "L.R." and record resulting y-intercept in SOIL PSYCHROMETERS data book. f. Push "x y" and record resulting slope in SOIL PSYCHROMETERS data book. CONVERSION OF MICROVOLTS TO -BARS: BARS = (DEWPOINT - INTERCEPT) DIVIDED BY SLOPE.

Additional information: 

In the spring of 1982, as part of the establishment of the Jornada Long-Term Ecological Research site in southern New Mexico, a 135 ha portion of a 1500 ha, internally drained, watershed was exclosed from grazing by domestic livestock. Prior to exclosure the watershed, as well as the rest of the Jornada basin, had been moderately to heavily grazed for the past 100 years. Concurrent with grazing, the vegetation had undergone a dramatic change from desert grassland, with an almost continuous cover of C4 perennial grasses, to isolated patches of the original grassland in a mosaic with desert shrub dominated plant communities (Buffington and Herbel, 1965). The exclosure lies along a northeast facing piedmont slope at the base of a steep isolated mountain peak, and covers a variety of component landforms from the foot of the mountain to the basin floor. The northeast side of the exclosure is immediately upslope of the College Playa located near the NMSU College Ranch. Three parallel transects (2.7 km in length) run from the middle of the College Playa up into the foot of Mt. Summerford. The Control transect is to the west, the Treatment transect on the east side of the Control transect, and the Alternate Control to the east of the Treatment transect. Each transect is 30 meters wide with a 45 meter buffer zone between each transect. The Treatment transect was treated annually until 1987 with NHNO3 in a concentration equal to 10g N/m2. The station markers at 30 meter intervals along each transect.



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