Gamma-ray beam attenuation as an auxiliary technique for the evaluation of the soil water retention curve

Atenuação de feixe de raios gama como técnica auxiliar na determinação de curvas de retenção de água no solo

GAMMA-RAY BEAM ATTENUATION AS AN AUXILIARY TECHNIQUE FOR THE EVALUATION OF THE SOIL WATER RETENTION CURVE

O.O.S. BACCHI^1,5; K. REICHARDT ^1,2,5; J.C.M. OLIVEIRA⁴; D.R. NIELSEN³

¹Laboratório de Física do Solo-CENA/USP, C.P. 96, CEP: 13400-970 - Piracicaba, SP. ²Depto. de Física e Meteorologia-ESALQ/USP, C.P. 9, CEP: 13418-900 - Piracicaba, SP.

³Dept. of Land, Air and Water Resources, University of California, Davis, USA.

⁴Bolsista da FAPESP.

⁵Bolsista do CNPq.

ABSTRACT: The soil water retention curve is fundamental for the hydraulic characterization of a soil and has many applications in agricultural research as well as in practical agriculture. A new procedure for soil moisture and soil bulk density evaluation inside closed pressure chambers through gamma-ray beam attenuation is presented. The proposed procedure presents several advantages in relation to the traditional process: avoids the need of continuous sample manipulation; minimizes the problem of hysteresis; allows a more precise evaluation of soil moisture by taking into account changes of soil bulk density due to swelling or shrinking on addition or removal of water; allows frequent evaluation of soil moisture without the need of opening the pressure chamber; allows a more precise judgement of equilibrium; reduces drastically the time of the determination of the retention curve and allows easy automation of data acquisition by a computer.

Key Words: gamma attenuation, gamma radiation, soil water retention

ATENUAÇÃO DE FEIXE DE RAIOS GAMA COMO TÉCNICA AUXILIAR NA DETERMINAÇÃO DE CURVAS DE RETENÇÃO DE ÁGUA NO SOLO

RESUMO: A curva de retenção da água no solo é fundamental para a caracterização hídrica de um solo e tem muitas aplicações na pesquisa agrícola assim como na agricultura aplicada. Um novo procedimento para determinação da umidade e densidade do solo no interior de câmaras de pressão utilizadas para deter minação de curvas de retenção é apresentado. O procedimento proposto apresenta várias vantagens em relação ao processo tradicional, tais como: evita a necessidade de constante manipulação das amostras; minimiza o problema da histerese; permite uma determinação mais precisa da umidade por levar em conta possíveis alterações na densidade do solo devido à expansão e/ou contração durante a adição ou retirada de água; permite constante avaliação da umidade do solo sem a necessidade de abertura da câmara; permite um julgamento mais preciso do ponto de equilíbrio após cada período de extração; reduz drasticamente o tempo necessário para a determinação das curvas de retenção e permite fácil automação do sistema de aquisição de dados com ajuda de um computador.

Descritores: atenuação de raios gama, radiação gama, retenção de água, curva característica

INTRODUCTION

The traditional method of determining the water retention function involves the establishment of a series of equilibria between water in the soil sample and water at a known potential. The hydraulic contact of the soil sample and the water at the reference potential is made through a porous plate, membrane or other porous media, depending on the pressure to be used. The required apparatus is well known by soil physicists and the main differences between systems are related to the range of matric pressure head in which the retention measurements are to be made and can vary also with the form of pressure head application: by suction (suction cells) or by pressure (pressure cells) (Klute, 1986). This traditional procedure has not received significant improvement during the past 50 years and many important practical problems still remain. The main difficulties of operation of such apparatus are the correct judgement of the equilibrium condition and the long time required for data acquisition. In the case of pressure cells, the chamber must be opened after each equilibrium, the soil sample must be weighted, re-saturated and placed again in the chamber in order to be submitted to a new pressure. If a different sample is used for each pressure step, the problem of variability and representability is introduced. Other important practical problems like hysteresis, soil losses during sample manipulation and changes of soil bulk density during experiments are also very frequent in such determinations and are not very easy to be avoided and accounted for. For very low extraction pressures there is a significant effect of the gravitational potential gradient on the water extraction of samples of different highs, which is not accounted for when soil moisture is evaluated gravimetrically, and a correct interpretation of the phenomena is not possible.

The gamma beam attenuation technique has been used in many soil physics studies and some new potential applications have been recently proposed, as is the case of the soil particle-size analysis (Vaz et al.,1992; Oliveira et al., 1997) and soil structure evaluation (Oliveira et al., 1998).

The method proposed here is an association of the conventional extraction pressure cells with the gamma beam attenuation technique for a continuous soil moisture evaluation during water retention determinations.

Being inspectional, the method minimizes the hysteresis problem, since samples are saturated only once, and avoids sample disturbance by manipulation. It allows a more precise judgement of the equilibrium, since soil moisture can be continuously monitored inside the chamber and, for the same reason, reduces drastically the time required for the whole retention curve determination. The more precise judgement of equilibrium is related to the possibility of frequent evaluations of soil water content, in each sample inside the chamber, which is not the case in the conventional procedure, where there is no precise quantitative indication of the volume of water out-flow from each sample inside the chamber. The time of equilibrium is established empirically. The new approach also permits the scanning of the soil water distribution within the sample, which is of great importance for heterogeneous samples, specially in the very wet range, when soil water content can significantly change with height. Scanning the sample, a criterium to choose the beam location to have a "representative soil volume" can be estabilished.

Regarding the judgement of the equilibrium condition, since the gamma ray system can be computer aided, the proposed auxiliary technique allows the use of a software specifically elaborated for data acquisition which could be settled in order to compute the gamma-ray intensity I during the water extraction process. The computer would calculate minimum permissible changes in I (according to equations 8 and 9), and indicate automatically the right moment to change the chamber pressure to a new desired value. It is also obviously possible that the same software makes all the calculations to convert I values into final q values and presenting finally the complete retention curve with its fidutial limits. A more complex system could also be idealized in order to provide automatic changes of extraction pressures.

Another very important advantage of the proposed auxiliary technique, when using a dual-gamma-source, is a more accurate determination of q when changes of soil bulk density occur during experiments due to swelling or shrinking (Phogat, V.K. et al., 1991). It also permits evaluation of soil moisture at different positions in the soil sample allowing the interpretation of the effects of potential gradients on water extraction. During transient conditions it is also possible to evaluate soil hydraulic conductivity.

PROPOSED METHOD AND MATERIALS

As an illustration of the idealized system, Figure 1 shows an available commercial low pressure extraction vessel, CAT. No.1600 - Soil Moisture Equipment Co., adapted with two acrylic windows in order to minimize the gamma beam attenuation by the vessel steel walls. In this case it would be possible to monitor at a time only one soil sample inside the vessel that would be positioned in the gamma beam direction that crosses diametrically the vessel through the two acrylic windows. Especially constructed extraction vessels could be totally made in acrylic, or any other low gamma attenuation material, and other shapes could also be designed in order to allow multiple sample monitoring at the same equilibrium periods. Sample scanning automation could also be incorporated to the system that would allow automatic changes of samples and its positioning at the beam direction. A more compact system configuration could also be idealized in order to have a specifically dedicated equipment for the proposed application.

Figure 1
- Scheme of the gamma ray beam attenuation system associated to an adapted extraction pressure vessel.

A basic gamma ray beam system as shown in Figure 1 can be easily assembled, as described by Phogat et al.(1991). It can be composed by two commercially available ¹³⁷Cs and ²⁴¹Am gamma sources, with adequate selected activities, mounted in a Pb shield castle opposite to a NaI(Tl) scintillation detector attached to a photomultiplier tube. The signal from the photomultiplier tube passes through a base-preamplifier before reaching an amplifier. Other components of the detection system include a high-voltage supplier, a channel analyzer and a counter. The counter, with an RS-232 interface, can make communication with an IBM PC, which allows an easy automation of data acquisition. The beam can be collimated at different cross section sizes according to desired gamma source intensities and/or the desired sample volume to be analyzed. The Pb shield castle can have a mechanism by which the two sources can be alternately brought into line with the colimator and detector or the two sources can be placed one in front the other in order to produce a single beam of two gamma energies.

It is possible to use a double energy gamma ray beam or the two single energy beams alternately for a simultaneous determination of soil water content (q) and soil bulk density (d_b) or one of the two single energy gamma beams for the determination of q, when d_b is known. Equations 1 and 2 can be applied individually, for a single energy gamma ray attenuation process, or as a two equation system in a double energy gamma ray attenuation process, according to Figure 1.

(1)

(2)

where I_o is the intensity of the gamma beam (cps) after attenuation by the two acrylic vessel windows and the two soil container walls, representing therefore the attenuation by the system without the soil sample; I is the intensity of the gamma beam (cps) after attenuation by the sample; m_s and m_w (m².Kg^-1) are the mass attenuation coefficients for the soil and soil water, respectively; X (m) is the soil sample thickness, d_b (Kg.m^-3) is the soil bulk density and q (m³.m^-3) is the soil water content. Subscripts Am and Cs on I, I_o, m_s and m_w indicates specificity of each variable in respect to the used gamma ray source. Air attenuation is neglected.

Since the exponents of equations 1 and 2 represent ln (I/I_o), when X is common for all samples, these equations can also be expressed only in terms of attenuation coefficients, as suggested by Phogat et al. 1971, as follows:

(3)

(4)

where and are the linear attenuation coefficients (m^-1) for a wet soil for both Am (60KeV) and Cs (660KeV) gamma ray beam, respectively.

For a single energy gamma ray attenuation process one can use equation 3 or 4 according to the selected gamma source. In this case the evaluation of d_b would be necessary at the end of the experiment by any conventional methodology in order to calculate q values. The use of a single energy system is recommended only for non swelling soils when no changes of d_b is expected during the experiment. In this case, from 3 or 4 we have:

(5)

It should be noted that in equation 5, the respective values of I and I_owhich are actually measured, are inside each attenuation coefficient.

For a double energy gamma ray attenuation process it is possible to evaluate q and d_b simultaneously using the system of equations composed by equations 3 and 4. In this case, q values will take into account the changes in d_b due to possible soil shrinking after each water extraction. The system of equations 3 and 4 can be solved for d_b andq to give:

(6)

(7)

The system has to be initially tested for the statistical deviations of I and I_o in order to evaluate the effect of this variation on q and d_bestimates and to know the sensibility of the system . This evaluation is also important for a correct judgement of the equilibrium condition according to some statistical criteria. Each equilibrium point is reached when variations of I are in the limits of the normal statistical variation of detection system. The precision of d_band q determinations can be evaluated, similarly to Gardner et al.1972 and Phogat et al. 1991, according to the following equations:

(8)

(9)

where

Phogat et al.(1991), working with a similar attenuation system in a Computer Assisted Tomograph (CAT), using gamma sources of ¹⁶⁹Yb (63.1KeV) and ¹³⁷Cs (660KeV), show that s(d_b) and s(q) can be reduced to acceptable levels by using longer counting times, larger than 35 seconds. The authors concluded that for tomography purposes, this increase of the counting time can be unacceptable, resulting in a total scanning time that severely limits the speed and flexibility of the system and, hence, its usefulness in studying these properties in situations where they change rapidly with time. This is not the case of the proposed application where the gamma beam would be monitoring q and d_bat only one representative soil volume, equivalent to the gamma beam volume crossing the sample in only one direction. Taking into account that the main objective is the evaluation of q and d_b at equilibrium conditions, 60 s interval between measurements, or even more, would not be a limiting factor.

REFERENCES

GARDNER,W.H.; CAMPBELL, G.S.; CALISSENDORFF, C. Systematic and random errors in dual gamma energy soil bulk density and water content measurements. Soil Science Society of America Proceedings, v.36, p.393-398, 1972.

KLUTE, A. Water Retention: Laboratory Methods. p.635-662. In: Klute, A. (Ed.) Methods of soil analysis: Part I. 2. ed. Madison: ASA/SSSA, 1986.

OLIVEIRA, J.C.M.; VAZ, C.M.P.; REICHARDT, K.; SWARTZENDRUBER, D. Improved soil particle-size analysis by gamma-ray attenuation, Soil Science Society of America Journal, v.61, p.23-26, 1997.

OLIVEIRA, J.C.M.; APPOLONI, C.R.; COIMBRA, M.M.; REICHARDT, K.; BACCHI, O.O.S.; FERRAZ, E.; SILVA, S.C.; GALVÃO FILHO, W. Soil structure evaluated by gamma-ray attenuation. Soil & Tillage Research, v.48, p.127-133, 1998.

PHOGAT, V.K.; AYLMORE, L.A.G.; SCHULLER, R.D. Simultaneous measurement of the spatial distribution of soil water content and bulk density. Soil Science Society of America Journal, v.55, p.908-915, 1991.

VAZ, C.M.P.; OLIVEIRA, J.C.M.; REICHARDT, K.; CRESTANA, S.; CRUVINEL, P.E.; BACCHI, O.O.S. Soil mechanical analysis through gamma ray attenuation. Soil Technology, v.5, p.319-325, 1992.

Recebido para publicação em 15.04.98

Aceito para publicação em 15.08.98

Publication Dates

• Publication in this collection
  14 May 1999
• Date of issue
  1998

History

• Accepted
  15 Aug 1998
• Received
  15 Apr 1998