Laboratory Method for Estimating Water Retention Properties of Unsaturated Soil

Transcription

1 Artcle Laboratory Method for Estmatng Water Retenton Propertes of Unsaturated Sol Sabr KANZARI 1,2,*, Mohamed HACHICHA 1 and Rachda BOUHLILA 2 1 Laboratory of Valorzaton of non Conventonal Water, Natonal, Insttute of Research of Rural Engneerng, Waters and Forestry of Tuns, Tunsa 2 Laboratory of Modelng of Hydraulc and Envronment, Natonal Engneerng School of Tuns, Tunsa ( * Correspondng author; e-mal: sabr.kanzar@gmal.com) Receved: 17 January 2012, Revsed: 18 Aprl 2012, Accepted: 2 November 2012 Abstract Sol hydraulc propertes are necessary for modelng water flow and solute transport n the vadose zone. However, drect measurement of these characterstcs n feld condtons s tedous, tme-consumng and expensve. In ths study, a laboratory method was used to characterze sol water retenton curves of three sol samples n the regon of Bouhajla (Central Tunsa). For expermental purposes, volumetrc water content and pressure head values were measured usng the gravmetrc method and Watermark sensor, of a small dsturbed sol core, respectvely, durng a dryng cycle under the effect of evaporaton. The van Genuchten model was ftted to the measured retenton curves wth the RETC software to determne resdual water content (θ r ), saturated water content (θ s ) and the two shape parameters; α and n. Strong correlatons were found between the ftted and measured retenton curves. The van Genuchten model was also ftted to the retenton curves measured by pressure chamber (as the reference method). The results were evaluated by calculatng the root mean square error (RMSE) and the geometrc mean error rato (GMER). Statstcal analyss proved the success of the proposed method for estmatng van Genuchten sol retenton parameters of the studed sols. A Mann-Whtney test performed at the sgnfcance level of 0.05 showed no sgnfcant dfference between the two methods. Keywords: Evaporaton method, parameter optmzaton, RETC, sol retenton curve, Bouhajla Introducton Salnzaton rsk assessment of sols and aqufers n ard and sem-ard regons requres knowledge of the evoluton of water movement and solute transport n subsurface flow. Durng the last decades, a large number of numercal models have been developed for the smulaton of water flow and solute transport n the unsaturated zone. Nevertheless, ther use n feld condtons s often lmted by the lack of characterzaton of retenton propertes. In stu feld measurements of sol retenton propertes are tedous, costly, tme consumng and are not accurate because of expermental shortcomngs and hgh spatal and temporal varablty. Therefore, the retenton propertes of unsaturated sols are often estmated ndrectly from other sol propertes usng pedotransfer functons (PTFs) [1,2] or determned n the laboratory [3,4], whch allow hgher spatal and temporal resoluton. The evaporaton method s one of the most wdely and easly used methods to determne the retenton curve and hydraulc conductvty of unsaturated sols. Ths method s based on measurng both sol mosture and pressure head durng a sol dryng cycle under the effect of evaporaton. The method developed by Wnd [5] ntroduced an teratve graphcal procedure to estmate, frstly, the water retenton curve from average sol mosture and pressure head readngs, and to determne hydraulc conductvtes from measured pressure head profle and varatons n water content dstrbuton. Although, n general, fve tensometers were used Walalak J Sc & Tech 2012; 9(4):

2 n measurements rangng from 50 cm to at least 850 cm, n evaporaton methods, several authors have proposed to reduce the number of tensometers to 2 [6-8]. However, Wessolek et al. [9] and Smunek et al. [10] have used only one tensometer n small sol cores and showed that ths method s able to accurately estmate sol hydraulc characterstcs. Furthermore, as an alternatve to the Wnd Algorthm, the analyss of water flow durng an evaporaton experment can be performed by usng optmzaton algorthms. The RETC software [11] whch s based on the Levenberg-Marquardt optmzaton algorthm s often used for estmatng sol hydraulc parameters by fttng water retenton and hydraulc conductvty models to measured data. The overall objectve of our study s the numercal smulaton of water movement and salts transfer n Bouhajla (Central Tunsa), characterzed by salne sols [12], to try to assess groundwater contamnaton rsk. The specfc objectve of ths paper s to estmate sol water retenton propertes of three sols from Bouhajla by an evaporaton laboratory method. Ths method s to montor the water content by the gravmetrc method and the pressure head by a Watermark sensor whch allows a wder measurement range than the conventonal tensometer (0 cm to 1,990 cm [13,14], durng a dryng cycle of a small sol contaner under the effect of evaporaton and usng the RETC program to determne the van Genuchten model parameters from measured retenton curves. Materal and methods Laboratory experment Three sols presented n Table 1 were sampled from a land parcel ( N; E) n the vllage of Bouhajla (Central Tunsa). The sol samples were crushed and then placed n small clear plastc contaners (6 cm dameter / 12 cm long). A Watermark sensor (Irrometer Inc., USA) was mplanted n the mddle of each sol layer (Fgure 1). Table 1 Sol partcle sze analyss of the three sols. Sol Clay (g kg -1 ) Slt (g kg -1 ) Sand (g kg -1 ) Texture (USDA * ) Sand Loam Clay * Scheme: Unted States Department of Agrculture Fgure 1 Schematc and photo of sol laboratory experment. 362 Walalak J Sc & Tech 2012; 9(4)

3 Each sol was saturated from the top wth dstlled water and was left to evaporate. Durng the dryng cycle, no devce was used to accelerate evaporaton. Montorng volumetrc water content was performed by a gravmetrc method (weghng scale) and the pressure head by a delectrc method (Watermark sensor). Upon converson of gravmetrc water content to volumetrc humdty, the values of bulk densty were measured usng the cylnder method [15]. The measurements were made daly untl the dgtal meter ndcated h = 1,990 cm whch corresponds to the Watermark sensor lmt. Parameter estmaton The van Genuchten model [16] was used to set the water retenton curve θ (h), whch relates the volumetrc water θ [L -3 L -3 ] content to pressure potental h [L]. Ths functon s θs θr θ r + h < 0 n m 1+ αh θ ( h) = ; θ s h 0 1 m = 1, n > 1 (1) n where θ r s the resdual water content [L 3 L -3 ], θ s s the saturated water content [L 3 L -3 ], h s the water pressure head [L], α [L -1 ] and n [-] are shape parameters. Eq. (1) contans four ndependent coeffcents, represented by the vector b = {θ r, θ s, α, n}. The dfferent parameters are essentally emprcal coeffcents wthout much physcal sgnfcance [17]. Ther values were determned by fttng the retenton model to the observed data usng the parameter optmsaton RETC software [11]. Ths program uses Marquardt s maxmum neghbourhood method to mnmze the objectve functon, O(b): N mn O ( b b ) = θ θ(b) = 1 (2) where θ and θ are the observed and ftted water contents, respectvely, and N s the number of retenton data. Intal values for the sol hydraulc parameters θ r, θ s, α and n were estmated wth the ROSETTA [18] pedotransfer functon usng measured values of sand, slt, and clay content (Table 1 and 2). 2 Table 2 Intal values of van Genuchten sol retenton parameters estmated by Rosetta. Sol θ r (cm 3 cm -3 ) θ s (cm 3 cm -3 ) α (cm -1 ) n (-) Pressure chamber Sol samples were placed n a pressure chamber. The same pressures measured by Watermark sensors were appled to sol cores. For each value of pressure (from 0 cm to 1,990 cm), water content was measured gravmetrcally. The mass of sol sample was determned by subtractng the mass of the contaner and the probe from total weght. The pressure chamber (reference method) was used to valdate the values obtaned by the proposed laboratory method. Sols samples were left 48 h n the pressure chamber. Pressure was changed successvely on the same sample each 48 h. Statstcal analyss To evaluate retenton curves measured by the proposed laboratory method, two statstcal parameters were used: the root mean square error Walalak J Sc & Tech 2012; 9(4) 363

4 (RMSE) and the geometrc mean error rato (GMER). These statstcal parameters are calculated as follows: j ( L P )² = 1 RMSE = (3) j j 1 L GMER = exp ln (4) j = 1 P where L s the value measured by the laboratory method, P s the value measured by the pressure chamber, P s the average value of pressure chamber data and j s the number of observatons. The RMSE and the GMER when equal to 0 and 1, respectvely, correspond to an exact match between observed and ftted data. The GMER value less or greater than 1 ndcates that the correspondng model underestmates or overestmates ftted data. The smaller (closer to 0) the RMSE value was, the better the model was. Statstcal processng was acheved by the STATISTICA software, Verson 5 (Statsoft France, 1997). The non-parametrc Mann-Whtney test was also performed at the sgnfcance level of 0.05 (test s sgnfcant at p < 0.05) to fnd out whether there s a sgnfcant dfference between the proposed method and the pressure chamber method or not. Results Durng the dryng cycle, the pressure head (h) reduced from a saturated pont (h = 0 cm) to a partally saturated pont (h = 1,990 cm) for all the sols. However, the values of volumetrc water content (θ v ) at the saturaton and at the dry end are dfferent between the sols. Sandy sol has the lowest values of θ v at saturaton and at the end of dryng than other sol samples. Sol water retenton ncreased wth sol enrchment by fne partcles (clay + slt). Sol water retenton curves of clay sol showed the hgher values of θ v at saturaton and at dryng and dry whle loamy sol showed an ntermedate mosture state between them and the surface layer. Measured water retenton data obtaned from the laboratory method were ftted by RETC software to determne the van Genuchten equaton parameters (Fgure 2). Strong correlatons were found between the measured and ftted curves, the correlaton coeffcent R² ranged between 0.87 and The values of van Genuchten s equaton parameters and the values of the objectve functon O(b) are presented n Table 3. Table 3 Estmated van Genuchten sol retenton propretes and values the objectve functon from the laboratory method. Sol θ r (cm 3 cm -3 ) θ s (cm 3 cm -3 ) α (cm -1 ) n (-) r² O(b) Walalak J Sc & Tech 2012; 9(4)

5 Fgure 2 Measured and ftted sol retenton curves of Bouhajla unsaturated sols. The values of these parameters (θ r, θ s, α and n) are very heterogeneous between the dfferent sols. Sols 1 and 3 have the hghest values of n and α, the most senstve parameters to water flow [19]. These layers may be partcular areas for water movement and solute transport n the unsaturated zone of Bouhajla. Good agreement between the ftted and measured (by the pressure chamber) retenton curves are shown n Fgure 2. The RMSE and Walalak J Sc & Tech 2012; 9(4) 365

6 GMER calculated for the dfferent sol samples are close to 0 and 1, respectvely, as shown n Table 4. The GMER values were greater than 1 meanng that the proposed laboratory method may slghtly overestmate the sol water retenton curve. Calculated values of the Mann-Whtney test of the three sols: sand, slt and clay were, respectvely, , and above 5 %. So there s no sgnfcant dfference between the proposed laboratory method and the reference method (pressure chamber) for measurng the sol water retenton curve. Table 4 Statstcal analyss of measured retenton curve and pressure chamber values. Sol RMSE GMER Dscusson The evaporaton method s a wdespread expermental method for estmatng sol hydraulc propertes. In ths research, we have demonstrated the success of estmatng sol retenton parameters by RETC from an evaporaton experment on small sol cores and usng a sngle sensor for measurng the pressure head n lne wth the work of Smunek et al. [10] and Wessoleck et al. [9], and we have extended the range of measurement to 1,990 cm. Estmated values of van Genuchten equaton parameters, especally α and n, are of the same order as the parameters estmated by other authors usng the evaporaton method, for nstance Bruckler et al. [20] and Fujmak and Inoue [6] for sandy loam sols and Basle et al. [21] for sandy clay sols. However, the hydraulc conductvty curve has not been determned assumng that t can be estmated from the equaton of van Genuchten [16]. Smultaneous estmaton of the retenton curve and unsaturated hydraulc conductvty usng the Wnd algorthm [5] could be an nterestng perspectve of ths study. Accordng to Abbas et al. [1] salnty ndrectly affects sol hydraulc propertes by actng on the porosty and permeablty, the study of the effect of salts on these propertes s recommended. Fnally, the Levenberg-Marquardt optmzaton algorthm mplemented n RETC presents some dffcultes to optmze certan parameters of water content and hydraulc conductvty from collected data n feld condtons (Wesselng et al.) [4], use of other optmzaton methods s also suggested. All these recommendatons wll be taken nto consderaton n future work. Conclusons A smple evaporaton method was advanced n ths study for estmatng sol water retenton propretes of an unsaturated zone. The obtaned estmaton results are acceptable and have shown that the van Genuchten retenton curve parameters are dfferent from one layer to another. However, these results allowed us to get an dea of the range of these parameters for each sol layer, especally for the shape parameters α and n. These results are essental for modelng of water flow and salts transfer n the Bouhajla vadose zone. Acknowledgements Ths study was done under the support of INRGREF/ACSAD project Technology Transfer of Bracksh/Salne Waters for Farmers and the PISEAU project Salnty Management for Small Farmers and Long-term Salnzaton Rsk n Central Tunsa. We thank Mr. Selm BEN HAMADI for hs assstance n the lab measurements. We also would lke to thank the valuable comments of the anonymous revewers, whch helped us to mprove the manuscrpt. References [1] Y Abbas, B Ghanbaran-Alavjeh, AM Lghat and M Shorafa. Evaluaton of pedotransfer functons for estmatng sol 366 Walalak J Sc & Tech 2012; 9(4)

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