Soil Moisture Measurement in Heterogeneous Terrain

Transcription

1 Soil Moistue Measuement in Heteogeneous Teain Melin, O. 1, J.P. Walke 1, R. Panciea 1, R. Young 1, J.D. Kalma 2 and E.J. Kim 3 1 Depatment of Civil and Envionmental Engineeing, The Univesity of Melboune, Austalia 2 School of Engineeing, The Univesity of Newcastle, Austalia 3 NASA Goddad Space Flight Cente, Geenbelt, USA omelin@unimelb.edu.au Keywods: soil moistue senso, calibation, uncetainty assessment, soil moistue pattens. EXTENDED ABSTRACT Reliable soil moistue measuement ove lage aeas is much needed fo both hydologic modelling and emote sensing applications. Fo collecting such data, potable electonic sensos offe a pactical altenative to gavimetic measuements. The convesion of the measued electical output to soil moistue is nonetheless a non-tivial task as it depends on soil type and tempeatue. In this study, diffeent calibation appoaches of the Stevens Hydapobe soil dielectic senso opeating at 50MHz ae tested with National Aibone Field Expeiment (NAFE) data. The objective is to evaluate the impact of soil type and tempeatue on the senso esponse and test the applicability of a geneal calibation equation. Duing NAFE, a spatially enabled platfom (Hydapobe Data Acquisition System, HDAS) was used to collect extensive measuements of nea-suface soil moistue. HDAS is a handheld system integating the soil dielectic senso and a pocket PC with GPS eceive allowing fo diect stoage of location and measuement with GIS softwae. HDAS measuements ae composed of the dielectic constant (DC) of the soil-wate mixtue, soil tempeatue, soil moistue content, salinity and conductivity. A diect compaison between the factoy calibation and gavimetic soil moistue measuements indicates that the senso esponse has a loss of sensitivity at soil moistue ove 25 % v/v in clay. On the othe hand, the eal component of the measued elative DC is found to be moe stongly coelated to gavimetic obsevations than the pedicted soil moistue. Following these obsevations, two calibation appoaches based on the measued DC wee tested. The fist is deived by aveaging the slope obtained with vaious soil types (geneal equation). The second uses the atio of the imaginay to eal component of DC (loss tangent) to descibe the diffeence in soil popeties (loss-coected equation). Results indicate that the calculated loss tangent is able to explain most of the vaiability among soil types. The oot mean squae eo (RMSE) of the pedicted soil moistue is deceased fom 4.0% v/v with the geneal equation to 3.3% v/v with the loss-coected equation. A thid-ode polynomial egession between the factoy equation and obsevations gives the best oveall accuacy with a RMSE of 2.7% v/v. Howeve, the loss-coected equation is moe obust as it does not loose sensitivity above 25% v/v and is moe eliable than the soil type dependent polynomial egession. Pevious analyses have shown that the Hydapobe senso is also sensitive to soil tempeatue. In this study, the tempeatue effect on the eal component of the measued DC is evaluated with sand and clay in diffeent moistue conditions. With sand, tempeatue is found to have a negligible effect with the lagest effect on eal DC fo a 15 C tempeatue incease (elative to 25 C) of about -0.6, coesponding to a soil moistue change of about -1% v/v. With clay, the obseved tempeatue effect on DC fo a 15 C incease is about 2 at 30% v/v and 4 nea satuation, coesponding to a soil moistue incease of about 3% v/v and 5% v/v espectively. It is also found that the factoy tempeatue coection algoithm inceases the tempeatue effect on the measued eal DC. Consequently, a new coection is deived based on the loss tangent, to account fo diffeent tempeatue effects accoding to soil type. The loss-coected equation, including the poposed coection fo tempeatue, is finally applied to the data fom the National Aibone Field Expeiment. Maps of soil moistue at 250m esolution ove an aea of 27km 2 ae pesented fo thee sampling days following a ainfall event. Such spatial data will be available fo calibation/validation of hydologic models, emote sensing of soil moistue and undestanding contols on spatial pattens in soil moistue. 2604

2 1. INTRODUCTION Rapid measuement techniques using electonic sensos such as time domain eflectometes, capacitance, impedance and dielectic sensos offe an altenative to destuctive and time consuming gavimetic sampling. Howeve they equie caeful calibation to convet the senso esponse to soil moistue in diffeent soils and tempeatue conditions (Cosh et al. 2005). The National Aibone Field Expeiment (NAFE) is a seies of two soil moistue-dedicated expeiments undetaken in South-Easten Austalia (Walke et al. 2005, 2006). NAFE 05 was undetaken duing a 4 week intensive peiod in the Goulbun Rive catchment and NAFE 06 duing a 3 week intensive peiod in the Muumbidgee catchment, New South Wales. Duing NAFE, top 5cm soil moistue was measued extensively fom paddock to egional scales using a spatially enabled platfom (Panciea et al. 2006) based on the Hydapobe (Vitel 1, 1994). The Hydapobe, heeafte efeed to as the soil moistue senso, is a soil dielectic senso opeating at 50MHz with an embedded themisto in the pobe head. At each measuement point, a volumetic soil moistue value is infeed fom the eal component of the measued elative dielectic constant (DC) and the conductivity (linked to salinity) fom the imaginay component. Because the eal component of DC (ε ) may vay with tempeatue, a tempeatue coection is poposed by the manufactue that uses the measued soil tempeatue (assumed to be the tempeatue of the pobe head). The wate content is then calculated fom the tempeatue-coected eal DC via one of thee possible manufactue s calibation equations (fo sand, silt and clay). Independent evaluations of the pefomance of this senso wee made by Seyfied and Mudock (2002, 2004) and Seyfied et al. (2005). Seyfied and Mudock (2002) epoted that the thee calibation cuves povided by the manufactue do not effectively descibe obsevations, and that soil tempeatue effects may be significant. Seyfied et al. (2005) developed two multi-soil calibation equations: a geneal calibation equation and a calibation equation that incopoates the effects of soil popeties. The objective of the pesent study is to evaluate the impact of soil type and tempeatue on the senso esponse and test the applicability of a 1 Mention of manufactues implies no endosement on the pat of the authos geneal calibation equation to the NAFE data set. In paticula, the two calibation equations of Seyfied et al. (2005) ae tested and compaed to a 3 d ode polynomial egession in tems of accuacy and obustness. The analysis is based on fou distinct data sets, one collected in the field (NAFE 06) and thee in the laboatoy using NAFE 05 and NAFE 06 samples that include a wide ange of soil types anging fom sand to clay. 2. DATA Among the fou datasets used in this study, thee wee obtained in the laboatoy (Temp 05, Lab 05 and Lab 06) and one in the field (NAFE 06). These datasets wee all collected in the NAFE famewok with the aim of facilitating calibation of the soil moistue senso. Duing NAFE 06, volumetic samples wee collected at five pe-defined locations within each of six focus fams (denoted by Y1, Y2, Y7, Y9, Y10 and Y12) see Walke et al., this issue. These locations wee chosen to cove a ange of soil type and moistue conditions. The five gavimetic points emained unchanged thoughout the field expeiment so that each themogavimetic measuement was associated with a given soil but with time vaying moistue conditions. A Hydapobe eading was taken at each themogavimetic point, and a soil sample was collected at exactly the same location. In the case when the pobe was modifying the soil suface (e.g. soil stuck on the pins of the pobe), the soil sample was collected at the middle of a 10-20cm wide tiangle of thee successive Hydapobe measuements. Gavimetic sampling was undetaken as much as possible at the same time on evey sampling day, so as to meet simila tempeatue conditions. Soil samples wee pocessed using the standad themogavimetic appoach. Lab 06 complements the field data of NAFE 06 with a set of thiteen soil samples. Soil samples wee collected on the same fams as fo NAFE 06, but locations wee in geneal diffeent fom the gavimetic points of the field expeiment. Lab 05 is an identical laboatoy expeiment except that soil samples wee fom the NAFE 05 Goulbun Rive catchment egion. Eight soil samples wee used, one on each of the eight focus fams. [Note that this dataset does not stoe the output voltages]. The infiltation-addition method was applied to all soils of Lab 05 and Lab 06 by pouing wate on the top of the containes, and allowing samples to satuate fo a minimum of 24 hous. A senso was then inseted into the containe and samples wee 2605

3 oven died at 45 C, with soil moistue measuements made peiodically until dy. Temp 05 was a laboatoy expeiment specifically designed to quantify the tempeatue effect on the soil moistue senso. The infiltation-addition method was applied to the soil samples of Lab 05 by pouing diffeent amounts of wate to get diffeent moistue conditions fom dy to satuated soil. Sealed samples wee allowed to equilibate in the oven at tempeatues of 20, 30, 40, 50 and 60 C. 3. TEMPERATURE EFFECT Seyfied and Mudock (2002) estimated that the tempeatue effect of a 40 C tempeatue change was about 4-6% v/v depending on soil type. In this section, the Temp 05 dataset is analysed to confim this assetion and a coection fo tempeatue effect on the measued eal DC is deived. Results of the Temp 05 expeiment ae pesented in Fig 1. As the manufactue s tempeatue coection amounts to calculating the coect DC at 25 C, the tempeatue effect on the eal and imaginay component of DC is evaluated elative to 25 C. As no measuement was made at that tempeatue, the DC at 25 C is linealy intepolated fom the DC measued at 20 C and 30 C. Fig. 1a and 1b show the change in DC fo a 15 C incease elative to 25 C as a function of soil moistue. Both the measued DC and the DC coected fo tempeatue effect by the manufactue s algoithm ae pesented fo compaison. Tempeatue has a diffeent effect on the eal and imaginay components of the measued DC. Fo the imaginay component, the tempeatue effect is always positive and geneally inceases with soil moistue (Seyfied and Mudock, 2004). The manufactue s algoithm was able to efficiently coect fo this effect (see Fig. 1a). The tempeatue effect on the eal component of the measued DC diffes with soil type. With sand, the effect is slightly negative nea satuation. This can be explained by the fact that soil wate in sand has dielectic popeties simila to those of pue wate (Seyfied and Mudock, 2004). In that case, the tempeatue coection poposed by the manufactue is in good ageement with obsevations. With clay, the tempeatue effect is positive and inceases with soil moistue. The obseved change in eal DC ove the 15 C tempeatue incease is about 2 at 30% v/v and 4 at 40% v/v, coesponding to an estimated soil moistue change of about 3% v/v and 5% v/v espectively. In this case, it is found that the coection poposed by the manufactue is not satisfactoy as the eo on the measued eal component of DC is inceased fo all soil samples (see Fig. 1b). As the factoy tempeatue coection fo eal DC was found to be unsatisfactoy, a new fomula is poposed. The coection equation is based on the obsevations that (i) the tempeatue effect diffes lagely with soil types; (ii) the tempeatue effect is significant fo clay and inceases with soil moistue. As the manufactue s tempeatue coection amounts to calculating the coect dielectic constants at 25 C, ou coection equation is also elative to 25 C, and can be witten as co [ 1 K( 25) ] ε = ε T, (1) co with ε the tempeatue-coected eal DC, ε the measued eal DC, T ( C) the senso tempeatue and K ( C -1 ) a constant. As the tempeatue effect diffes with soil types (negative with sand and positive with clay), paamete K was coelated with the loss tangent to integate the effects of soil dielectic popeties. Paamete K is computed as the diffeence between DC at 40 C and 25 C divided by the tempeatue change (15 C) and the loss tangent tan δ is computed as ε i tan δ =, (2) ε Figue 1. Effect on the uncoected and manufactue coected DC fo a 15 C incease in soil tempeatue: a) the imaginay component of DC; and b) the eal component of DC. In (c), the tempeatue effect pe degee (K) on the eal DC is shown as a function of loss tangent. 2606

4 whee ε i is the measued imaginay DC. This quantity is popotional to the enegy dissipation expeienced by the input voltage. Fig. 1c illustates the elationship between the estimated K and the loss tangent computed with the Temp 05 data set. A linea egession gives K = 0.011tanδ with a coelation coefficient of CALIBRATION APPROACHES Fig. 2 shows the vaiations of the tempeatuecoected senso esponse with laboatoy datasets Lab 05 and Lab 06. The soil moistue calculated by the manufactue s algoithm (option silt) and the eal DC measued by the senso ae both plotted against gavimetic measuements. Note that the manufactue s ecommendation is to set the pogamming option fo silt when the soil type is unknown. The eal DC could not be computed with data set Lab 05 as the input data of the algoithm (voltages) ae not stoed. Fig. 2a indicates that thee is a loss of sensitivity of the pedicted soil moistue θ silt at soil moistue ove appoximately 25% v/v. Howeve, Fig. 2b shows that thee is good elationship with DC until satuation. The coelation coefficient is inceased fom 0.90 in Fig. 2a to 0.94 in Fig. 2b. These esults ae consistent with Seyfied et al. (2005), who developed a calibation equation of the pobe diectly fom the dielectic constant [note: they used the manufactue s tempeatue coection]. They deived a linea elationship between θ and ε given by θ = A ε + B, (3) with A and B soil-dependent paametes. In that study, a geneal equation was deived by aveaging the paametes obtained with measuements made on 20 diffeent soil types. Thei geneal calibation equation (A=11.0; B=-18.0 % v/v) was found to be supeio to any of the thee equations povided by the manufactue. Seyfied et al. (2005) then coelated the diffeence between the measued and pedicted soil moistue with the loss tangent at satuation ( tan δ s ). The loss tangent was used fo coecting the obseved diffeences between individual soil calibations. Since most of the vaiation in soil calibations was due to vaiations in the slope A, the loss-coected A paamete value A was based on a egession between A lc and tan δ s. The new loss-coected calibation equation was witten Figue 2. Senso esponse as function of soil moistue: a) soil moistue pedicted by the factoy calibation equation (option silt); b) eal DC measued by the senso. ( ε ε ( = 0) ) θ = A θ, (4) lc with A 1.53 tanδ (% v/v) and lc = s ε = 2.7 at θ = 0 (Seyfied et al., 2005). Note that B was eplaced in (4) by A lc A thid appoach consists of fitting the senso soil moistue to obsevations using a polynomial egession 3 θ = aθ silt + bθ silt + cθ + 2 silt d, (5) with θ silt the soil moistue pedicted by the manufactue s calibation equation (option silt) and a, b, c and d fou fitting paametes. As an illustation of the thee calibation appoaches, the polynomial equation (5) and the geneal equation (3) ae plotted in Fig 2a and 2b espectively. It is appaent that the geneal equation is moe linea than the polynomial equation and fits elatively bette the senso esponse with the ange of soil types of NAFE. Note that the loss-coected equation (4) cannot be 2607

5 plotted in Fig 2 as the pedicted soil moistue is also a function of the imaginay component of DC. 5. MULTI-SOIL CALIBRATION In this section the geneal equation (3), the losscoected equation (4) and the polynomial egession (5) ae successively applied to the NAFE 06 datasets. The diffeent appoaches ae then assessed in tems of accuacy and obustness. To apply the loss-coected equation to the oving measuements made duing NAFE 06, which uses the loss tangent measued at satuation, one needs to assume that the loss tangent is constant (i.e. does not depend on soil moistue). Fig. 3 shows the vaiation of tan δ as a function of soil moistue at six pemanent sites in the NAFE 06 aea. The loss tangent at satuation vaies fom 0.4 (Y1) to 1.5 (Y12) with a vaiability attibuted to soil moistue anging fom 0.2 to 0.8. At most sites, the value at satuation appeas to be eached at about 15% v/v, which means that the losscoected equation can be eliably applied fo soil moistue values above 15% v/v. Note that below 15% v/v, the diffeence between soil types is still geate than the diffeence by soil moistue. A second assumption is that the tempeatue measued by the senso located in the head of the pobe epesents the top 5cm soil tempeatue. Fig. 4 plots the senso tempeatue measued in the field by the oving measuements as function of the 2.5cm tempeatue measued continuously at the pemanent sites in the sampling aea. The standad deviation between oving and stationbased measuements is about 2 C, which is much smalle than the ange coveed by tempeatue values (15 to 35 C). In the wost case whee the diffeence in tempeatue is maximum (10 C), and with a high loss tangent (1.5), the pedicted maximum eo on the measued eal DC is about 10% of its value, coesponding to an eo in soil moistue of about 4% v/v at 30% v/v and 5% v/v at 40% v/v. In geneal, the tempeatue measued by the soil moistue senso is within 2 C of the 0-5cm soil tempeatue, yielding a soil moistue eo less than 1% v/v. The tempeatue coection of equation (1) is applied to the measued eal DC of the NAFE 06 dataset. To do so, we assumed a linea tempeatue effect in the tempeatue ange expeienced duing the expeiment (15 to 45 C). Results obtained with the geneal equation and the loss-coected equation of Seyfied et al. (2005) ae then compaed in Fig. 5a and 5b espectively. The use of the loss tangent educes the RMSE of the pedicted soil moistue fom 4.0% to 3.3% v/v. Figue 3. Loss tangent (tan δ) vesus soil moistue at six stations of the Muumbidgee netwok. Figue 4. Roving vesus station-based soil tempeatue measuements fo thee diffeent days duing NAFE 06: a typically cold (16 Nov), dy (9Nov) and wet (13 Nov) day. This impovement confims the existing elationship between the loss tangent and the change in measued eal DC among soil types, including the assumption that the measued loss tangent is tan δ. The loss-coelated paamete s A lc is then fitted to the NAFE 06 data set. A linea egession between the measued A and tan δ gives A lc = 4.3tanδ (% v/v). With the new slope, the RMSE of the loss-coected equation is slightly deceased to 3.1% v/v. A thid ode polynomial egession between the soil moistue computed by the manufactue s calibation equation and obsevations is deived 3 θ = ( θ silt θ silt θ silt + 210) /100 and esults ae plotted in Fig. 5c. The RMSE of the pedicted soil moistue is 2.7% v/v, which epesents the best fit among the fou calibation equations poposed. Howeve, when using the whole NAFE 06 data set to compae the

6 polynomial and the loss-coected equations (see Fig 5d), one obseves that thee is a loss of sensitivity of the soil moistue pedicted by the polynomial egession at about 25% v/v. This finding is consistent with the esults obtained in the laboatoy (Fig. 2) showing the satuation of the factoy-pedicted soil moistue at gavimetic measuement ove 25% v/v.. In fact, the appaently bette esults obtained with the polynomial equation is an atefact of the NAFE 06 soil and moistue conditions, with the soil in the study aea being elatively homogeneous (mainly clay), and the ange of soil moistue values measued elatively low. It is expected that the polynomial equation (o any equation fit to the senso measued soil moistue) would induce systematic eos with soils that ae non-epesentative of whole aea (in paticula sand fo NAFE) and fo soil moistue values above 25% v/v. In this egad, the multi-soil calibation equation of Seyfied et al. (2005) with the tempeatue coection developed hee is a moe obust appoach fo an opeational application. Figue 5. Diffeent calibation equations: the geneal (a); the loss-coected (b) and the polynomial fit (c) equation. The loss of sensitivity of the polynomial fit at soil moistue ove 25% v/v is illustated in d) with the whole NAFE 06 data set. 6. APPLICATION The calibation equation of Seyfied et al. (2005) including the tempeatue coection deived in this pape is applied to the NAFE data with the assumption of a constant loss tangent. As the calibation of the slope with the NAFE 06 data did not significantly impove the accuacy of the pedicted soil moistue (eo of 3.1% instead of 3.3%), the slope of Seyfied et al. (2005) is used instead of the calibated one. An illustation of the calibated data is povided in Fig. 6. The soil moistue maps obtained on 13, 14 and 16 Novembe 2006 at thee fams Y2, Y9 and Y12 ae pesented. A ainfall of about 15mm occued in the sampling aea on Novembe. The geneal dying of the study aea is clealy visible fom an aveage of about 25% v/v on 13 Novembe down to 15% v/v on 16 Novembe. Ove the dying peiod, the spatial vaiability within fams Y9 and Y12 is mainly due to iigated cops; Y2 is dy land pastue while Y9 and Y12 ae copping fams with some iigated cops (maize and wheat). Satuated soils ae appaent in the iigated aeas at the south-west cone of Y9 and the middle of Y12. To assess the impact of the loss tangent on calibated data, the loss tangent is computed on the wettest day of the field campaign (13 Novembe). Only the measuement points with a soil moistue value highe than 30% v/v ae used, giving an aveage of soil moistue of about 35% v/v fo Y2, Y9 and Y12. The computed loss tangent vaies Figue 6. Examples of nea-suface soil moistue maps using the calibation equations pesented in this pape. fom 0.7 to 2.2 in Y12 (mean 1.2), fom 0.3 to 1.8 in Y9 mean (1.0) and fom 0.4 to 1.1 in Y2 (mean 0.9). The pedicted maximum diffeence in soil moistue between the minimum (0.3) and maximum (2.2) loss tangent is about 8% and 10% v/v at 30% at 40% v/v soil moistue espectively. 7. CONCLUSION The objective of this study is to evaluate the impact of soil type and tempeatue on the senso 2609

7 esponse and test the applicability of a geneal calibation equation to the NAFE data set. The tempeatue effect on the soil wate senso esponse is evaluated with sand and clay ove a ange of moistue conditions. With sand, the tempeatue appeas to have a negligible effect with the lagest tempeatue diffeence (15 C) estimated to have only about 1% v/v impact on the soil moistue value. With clay, the obseved tempeatue effect is moe significant with a soil moistue change up to 5% v/v. With ou data set the manufactue s algoithm inceases the tempeatue effect on eal DC. A simple coection is deived based on the obseved elationship between the elative effect on eal DC and loss tangent. The geneal and loss-coected calibation equations of Seyfied et al. (2005) ae applied to the tempeatue-coected dielectic constant, and compaed to obsevations. Results indicate that the computed loss tangent is able to explain most of the vaiability among soil types. The RMSE of the pedicted soil moistue is educed fom 4.0% to 3.3% v/v. A thid-ode polynomial egession between the manufactue-simulated and the obseved soil wate content gives the best oveall accuacy with a RMSE of 2.7%. The losscoected equation is howeve moe obust than the polynomial egession fo diffeent soil types, and at soil moistue ove 25% v/v. The tempeatue coection and the loss-coected equation ae applied to the NAFE data set. As an illustation of the calibated data, a time seies of soil moistue maps at 250m esolution is pesented. The tempoal and spatial vaiability is high with nea-suface soil moistue values coveing the full ange fom nea 0 to 40% v/v. Such spatial data will povide the gound tuth that can be used fo calibation/validation of hydologic models and emote sensing techniques. 8. ACKNOWLEDGEMENT We wish to thank Daniele Biasioni and Hannah Meade fo thei laboatoy wok, and all the NAFE paticipants fo field calibation and data collection. The National Aibone Field Expeiments have been made possible though ecent infastuctue (LE and LE ) and eseach (DP ) funding fom the Austalian Reseach Council, and the collaboation of a lage numbe of scientists fom thoughout Austalia, the United States and Euope. Initial setup and maintenance of the study catchments was funded by eseach gants (DP , DP and DP ) fom the Austalian Reseach Council, the CRC fo Catchment Hydology, and NASA. 9. REFERENCES Cosh, M.H., Jackson, T.J., Bindlish, R., Famiglietti, J.S., and Ryu, D., Calibation of an impedance pobe fo estimation of suface soil wate content ove lage egions. Jounal of Hydology, 311: Panciea, R., Melin, O., Young, R., and Walke, J.P, The Hydapobe Data Acquisition System (HDAS): Use guide. Repot, Univesity of Melboune. Seyfied, M., and Mudock M., Measuement of soil wate content with a 50-MHz soil dielectic senso. Soil Sci. Soc. Am. J., 68: Seyfied, M.S., and Mudock, M.D., Effects of soil type and tempeatue on soil wate content measuements using a soil dielectic senso. P1-13. In I.C. Paltineanu (ed.) Fist intenational symposium on soil wate measuement using capacitance and impedance, Beltsville, MD. 6-8 Nov Seyfied, M.S., Gant, L.E., Du, E. and Kumes, K., Dielectic Loss and Calibation of the Hyda Pobe Soil Wate Senso. Vadose Zone J., 4: Vitel, Inc Hyda soil moistue pobe use s manual. Vesion1.2. Vitel Inc., Chantilly, VA. Walke, J.P., Hacke, J.M., Kalma, J.D., Kim, E.J. and Panciea, R., National Aibone Field Expeiments fo Pediction in Ungauged Basins. In A. Zege and R. M. Agent (Eds), MODSIM 2005 Intenational Congess on Modelling and Simulation. Modelling and Simulation Society of Austalia and New Zealand, Decembe 2005, Walke, J.P., Melin, O., Panciea, R. and Kalma, J.D., National Aibone Field Expeiments fo Soil Moistue Remote Sensing, 30th Hydology and Wate Resouces Symposium [CD-ROM]. The Institute of Enginees Austalia, Launceston, Austalia, 4-8 Decembe,