1. TITLE 1.1. Data set identification Soil water potential measured by tensiometer 1.2. Revision date of this document (yy/mm/dd) 05/99 2. INVESTIGATOR(S) Identify the Principal Investigator for this data set, including general affiliation, if applicable. Name Andre Chanzy Address INRA, Laboratoire de Science du Sol 84914 Avignon Cédex 9 Tel (33) 4 90 31 61 29 Fax (33) 4 90 31 62 44 E-mail achanzy@avignon.inra.fr 3. EQUIPMENT 3.1. Instrument description. Measurements were done with tensiometers which were either equipped by a mercury or an electronic manometer (Model SKT850C of SDEC manufacturer) 3.1.2. Mission objectives. Monitor the soil water potential profile within the top 0-130 cm soil layer. At the bottom of the profile two tensiometers were installed at 110 and 130 cm to measure the soil water potential gradient at 120 cm in order to implement the Darcy law to estimate the drainage flow. 3.1.3. Key variables. Tensiometer measured directly the soil water potential. The matric potential is reported in the data files. The depression is expressed in meter of water. 3.1.4. principles of operation. See the abundant literature on tensiometers 3.1.5. Instrument measurement geometry. The porous ceramic cup in contact with the soil has a length of 6 cm and a diameter of 2 cm.
3.1.6. manufacturer of instrument. SDEC France - Scientific Instrumentation - Z.I de la Gare - 37310 Reignac/Indre France (Europe). Tel : 33 2 47 94 10 00 / Fax : 33 2 47 94 17 13 WEB site : http://www.sdec-france.com 3.2. calibration. 3.2.1. frequency of calibration. SKT850C manometer were calibrated prior to the experiment. Calibration were done within the [-8 0] meter range at different temperatures from 5 to 50 C. The calibration were done against reference pressure values delivered by a DRUCK DPI 602. 3.2.2. Other calibration information. The calibration relationship accounts for the temperature in the following relationship : Pressure = a+a'*t + (b+b'*t)*u where T is the temperature of the manometer and U is the tension delivered by the manometer. Calibration coefficients are given in the following table. Id b b' a a' 231-0.005612 1.111E-06 2.274347-0.001533 232-0.005745 1.998E-06 2.344755-0.001861 233-0.005757 1.341E-06 2.343328-0.000357 234-0.005714 1.614E-06 2.300091-0.001526 235-0.005712 2.387E-06 2.381298-0.003356 236-0.005848 6.124E-07 2.394454-0.001301 237-0.005636 1.034E-06 2.244756 0.000894 238-0.005711 1.106E-06 2.277355-0.000900 239-0.005725 1.437E-06 2.315224-0.000867 240-0.005840 1.447E-06 2.382393-0.001900 241-0.005713 1.656E-06 2.315693-0.001650 242-0.005740 0.000000 2.273000 0.000000 243-0.005684 1.259E-06 2.271993-0.000697 244-0.005657 5.806E-07 2.271426-0.000686 245-0.005741 1.093E-06 2.247297 0.001026 246-0.005731 8.468E-07 2.269583 0.000329 247-0.005733 1.308E-06 2.348162-0.002732 248-0.005506 1.006E-06 2.225065-0.000936 249-0.005693 1.897E-06 2.280843-0.001338 250-0.005753 3.122E-07 2.424425-0.002977 251-0.005795 1.049E-06 2.363461-0.002556 Id is the manometer identification
4. PROCEDURE 4.1. Data acquisition methods. Tensiometers were installed in validation and calibration fields. Two sites of measurement were installed in each field. At each site, we installed a set of 5 (20 50 80 110 130 cm for the validation field) or 7 (10 20 30 50 80 110 130 cm for the calibration fields) tensiometers. The tensiometer site were located in the vicinity of a neutron probe access tube. The tensiometers were aligned with the neutron access tube along the tillage direction. The distance between two tensiometer is about 15 cm. The correspondance between the tensiometer site and the number of the neutron probe access tube is given below : Field Site Access tube number 101 1 1 101 2 2 102 1 4 102 2 5 120 2 9 121 1 12 121 2 13 203 1 6 203 2 7 214 1 10 214 2 11 One can note that there is no tensiometric measurements in field 101 during the second period of measurements and in field 501. The soil was too dry during the whole period of measurement to implement the tensiometry. 4.2. Spatial characteristics. 4.2.1. Spatial coverage. The tensiometric measurement are local and representative of the site (~1 m²). The sites are in the vicinity of the micro-meteorological masts. 4.3. Temporal characteristics. 4.3.1. Temporal coverage. The period of measurements are governed by the soil wetness conditions which allow tensiometric measurements (-8 0 m). Sometimes the range was narrower since air leak in the hydraulic circuit altered the measurements. The air leak were detected either when the rate in water potential decrease suddenly become smaller or when an air volume was observed in the tensiometer. The period when tensiometric measurements are available are given in the following table :
Field Id beginning (DoE) end (DoE) 101 345 484 102 507 568 120 403 536 121 509 633 203 378 464 214 439 555 4.3.2. Temporal resolution. Measurements with SKT850c are delivered hourly. Measurements with the mercury manometer were typically made weekly. However, the measurements were often made with a shorter time interval. 6. DATA DESCRIPTION 6.1. Table definition with comments. Measurements with the mercury tensiometers are given in the following files sp101aaa.dat sp102aaa.dat sp120aaa.dat sp121aaa.dat sp203aaa.dat sp214aaa.dat where : # soil matric potential of water (given in meter of water) # col 1 : field identicator # col 2 : site number # col 3 : year # col 4 : Month # col 5 : Day # col 6 : Day of experiment # col 7 : hour of the day # col 8 : Hour of the experiment # col 9 : matric potential at a depth of 10 cm # col 10 : matric potential at a depth of 20 cm # col 11 : matric potential at a depth of 30 cm # col 12 : matric potential at a depth of 50 cm # col 13 : matric potential at a depth of 80 cm # col 14 : matric potential at a depth of 110 cm # col 15 : matric potential at a depth of 130 cm Measurement with the SKT850c manometers are given in the following files : sa101aaa.dat
sa102aaa.dat sa203aaa.dat where # soil water matric potential measured by automatic tensiometer # col 1 : Field identicator # col 2 : Site number # col 3 : Year # col 4 : Month # col 5 : Day # col 6 : DoE (Day of the experiment) # col 7 : HoD hour (htu) # col 8 : HoE # col 9 : temperature taken for the temperature correction # col 10 : Soil matric Potential (m) at 10 cm # col 11 : Soil matric Potential (m) at 20 cm # col 12 : Soil matric Potential (m) at 30 cm # col 13 : Soil matric Potential (m) at 50 cm # col 14 : Soil matric Potential (m) at 80 cm # col 15 : Soil matric Potential (m) at 110 cm # col 16 : Soil matric Potential (m) at 130 cm 6.2. Sample database data record. For the sp files : 101. 2. 96. 12. 10. 345. 9.0 3129.0-0.08-0.01 0.02 0.15-0.04 0.10 0.08 101. 2. 96. 12. 11. 346. 15.0 3159.0-0.08-0.03 0.01 0.18 0.23 0.57 0.40 for the sa files : 101 1 97 1 23 389 10 4186 9.3-0.13-0.10-0.01 0.17 0.34 0.63 0.46 101 1 97 1 23 389 11 4187 9.8-0.16-0.13-0.02 0.12 0.32 0.61 0.44 7. DATA MANIPULATIONS The data manipulation consists in - applying the calibration relationship for the skt850c. We took the soil temperature measured near the soil surface (0.5 cm in general) to make the temperature correction. - compute the matric potential from the pressure measurement (this calculation accounts for the depth of ceramic cup and the height of the manometer) - filtering the data to remove the measurements altered by air leak in the hydraulic circuit. 8. ERRORS 8.1. Sources of error. - manometer reading - calibration error (5 cm) for the SKT850C - exact depth of the ceramic cup (2 cm)
- exact height of the manometer (1 cm) - error in air leak detection 8.2. Quality assessment. Representation of the temporal evolution of the potential and consistency with climatic condition; Intercomparison between the sites; 8.2.2. Confidence level/accuracy judgement. The water potential have shown high diurnal cycles which are likely not related to the soil water potential. These variations were observed with both manometer types. Such variations is a common pattern observed with tensiometers which origin is not yet clarify. Night measurement seemed to be better than the measurements during the day.