Impact of Deep Soil Layer on Terrestrial Microwave Emission for a Bare Agricultural Field

Transcription

1 RADIOENGINEERING, VO. 6, NO. 3, SEPEMBER 07 7 Impact of Deep Soil ayer on erretrial Microwave Emiion for a Bare Agricultural Field Cihan DOGUSGEN (ERBAS) Dept. of Electrical and Electronic Engineering, Itanbul Yeni Yuzyil Univerity, Yilanli Ayazma Street 6, 3400 Itanbul, urkey cihan.dogugen@yeniyuzyil.edu.tr Submitted July 0, 06 / Accepted March 7, 07 Abtract. In thi tudy, we invetigated the deep (emiinfinite) oil contribution to the brightne temperature at.4 GHz calculated through a modified incoherent radiative tranfer model. We reproduced the meaured brightne temperature collected by a dual -band radiometer in a bare agricultural field. We found that excluion of a emi-infinite oil layer in the incoherent model ignificantly decreaed the brightne temperature when the meaurement depth in the model wa cloer to the emitting depth, which i the firt few centimeter from the top of oil. he maximum brightne temperature difference between the cae with and without the emi-infinite layer in the incoherent model were computed to be K,.889 K, K and K for the meaurement depth of 4 cm, 5 cm, 8 cm and 6 cm, repectively. Baed on a comparion with another coherent radiative tranfer model, we oberved that incluion of the deep oil layer ignificantly improved the preciion of the incoherent model regardle of the meaurement depth. Our reult could be one example of improving the accuracy of radiative tranfer model, which might be applied to other radiative tranfer model and increae the preciion of oil moiture retrieval calculation. Keyword Radiative tranfer model, oil moiture, bare oil, incoherent model, brightne temperature, microwave radiometry. Introduction Soil moiture i a reervoir of the water cycle that impact precipitation pattern [], drought and flood [], phyical, chemical and biological procee that occur within the oil [3], agricultural productivity [4], weather forecating and eaonal climate prediction [5] by controlling the energy and moiture exchange between the atmophere and land urface. Microwave radiometry i an effective way for oil moiture retrieval. he appropriate wavelength for oil moiture meaurement fall in -band ( to GHz) at a wavelength of cm (.4 GHz) operating at near-nadir look angle and horizontal polarization [6], [7]. he cm wavelength allow ignificant penetration through vegetation and decreae the normalized urface roughne [8]. Global meaurement of oil moiture firt tarted in 009 with ESA Soil Moiture and Ocean Salinity SMOS miion [9]. he Soil Moiture Active Paive SMAP miion of NASA followed in 05 [0]. Radiative tranfer theory originate from the law of energy conervation []. hi method ha long been ued to explain cattering, emiion and aborption of the microwave radiation within a defined media []. It allow u to quantitatively interpret the impact uch a oil propertie, vegetation, and oil urface roughne on the emitted radiation of the oil. Variou method have been developed in order to model the brightne temperature [3 6], which i a meaure of radiated power. In thi article, we provided a modification to a previouly developed incoherent radiative tranfer model to include the deep oil layer contribution to the modeled brightne temperature. We hypotheized that deep oil layer affected the emiion characteritic (and therefore emiivity) of the oil, and the amount of that impact depended on where the meaurement depth, which i the deepet point through the oil where the dielectric contant can be defined, wa choen. o verify our hypothei, we tarted with a fictitiou air dry oil wet oil ytem and computed the reulting incoherent emiivity profile for the cae with and without the deep oil layer. hen we applied the two verion of the incoherent model to a dataet collected in Michigan, USA to ee the ignificance of the difference in brightne temperature in Kelvin. We organized our article a follow: In Sec., we decribed material and method including the field meaurement along with their analyi. Furthermore, we explained the radiative tranfer model (original and the modified incoherent model a well a the coherent model) and auxiliary model (AEX model, dielectric mixing model and oil urface roughne model). In Sec. 3, we preented our reult and aement related to the reproduction of meaured brightne temperature, and analyi of deep oil layer impact. In Sec. 4 we provided our concluion. DOI: 0.364/re EECROMAGNEICS

2 7 C. DOGUSGEN (ERBAS), IMPAC OF DEEP SOI AYER ON ERRESRIA MICROWAVE EMISSION FOR A BARE. Material and Method. Meaurement he data ued in thi tudy come from Hornbuckle et al. [7] and Dogugen & Hornbuckle [8]. he experimental field wa located in outheatern Michigan, USA, and wa planted in maize. Dimenion of the ite wa 800 m (E W) and 400 m (N S) Soil texture conited of 6.% and, 55% ilt, 8.9% clay (ilty clay loam). Row were planted E-W with an average pacing of 0.77 m. Figure how a photograph of the experimental ite. We collected meaurement of volumetric oil moiture content at.5 cm and 4.5 cm through the oil repreenting the 0 3 cm and 3 6 cm average water content, repectively. We meaured the oil temperature in a imilar approach. A micrometeorological tation collected everal micrometeorological parameter uch a precipitation, wind peed, relative humidity, air temperature and downwelling olar/atmopheric radiation. A truck-mounted, direct ampling digital radiometer [9] meaured the h-pol brightne temperature of a part of the field that wa not planted. he radiometer wa deployed at the height of 0 m with an incidence angle of 35 and a footprint of approximately 40 m. It operated at.4 GHz, and collected meaurement every minute with a preciion of K. Half power beamwidth level (E and H plane) of the radiometer antenna were. Side lobe level were 0 db.. Data Analyi We were intereted in the hort-term (5 hour) variation of brightne temperature. In order to undertand the effect that influence the brightne temperature change within the time period of interet, how the oil moiture and oil temperature varied hould be conidered. Figure illutrate the variation of 0 3 cm average oil moiture, oil temperature at.5 cm from the top of oil, and meaured brightne temperature. A expected, the brightne temperature decreae in an almot linear fahion with the increaing oil moiture content. he oil temperature ha a decreaing trend a well. We modeled the oil moiture and oil temperature profile through a land-urface model: Fig.. Picture of the experimental area. he photograph wa taken after a rain event on day of year 45. Fig.. Zero to 3 cm average oil moiture (top), oil temperature at.5 cm from the top of oil (middle), and meaured brightne temperature (bottom) on day of year 43 and 44. No precipitation wa recorded during the time of interet. Atmopheric and and-surface Exchange Model (AEX) [0]. More information on AEX i given in Sec he reaon we ued a model uch a AEX i due to the difficulty of ampling at all depth in field experiment, wherea AEX can compute the oil moiture and oil temperature value of all oil layer. Hence, by uing AEX, the preciion of the radiative tranfer model of interet wa ignificantly improved. Furthermore, the agreement between the meaured and modeled oil moiture/oil temperature of interet wa verified in a previou tudy []..3 Modeling We reproduced the meaured brightne temperature through an incoherent radiative tranfer model that we modified to add the deep oil contribution to oil emiion. Original verion of the model wa developed by Burke et al. []. Initially, we alo ued a coherent radiative tranfer model to verify that the incoherent model i capable of reproducing the meaured brightne temperature. After the verification tep, we quantified the amount of emiion generated by the deep oil layer through comparion computation (with and without the deep oil layer) and plot. Furthermore, we explored the effect of depth at which the deep oil layer tart. Other than the radiative tranfer model, we utilized the AEX model, a oil urface roughne model, and a dielectric mixing model throughout the tudy. We ued AEX to generate the two input of the radiative tranfer model: Volumetric oil moiture and the oil temperature profile. Soil urface roughne model generated an amount of extra emiion, added to the emiion of the pecular urface, becaue of the rough nature of the oil. he dielectric mixing model produced the oil dielectric contant profile, another input for the radiative tranfer model. he oil urface roughne model and the dielectric mixing model were embedded in the radiative tranfer model programming code while AEX wa run a a eparate model.

3 RADIOENGINEERING, VO. 6, NO. 3, SEPEMBER Decription of the above mentioned model are given in Sec..3.,.3. and Incoherent Model and the Re-derived Verion (Incluion of Semi-Infinite ayer) he incoherent model aume that the radiation i incoherent (in which the phae effect i avoided), oil moiture and oil temperature are function of depth only, and oil dielectric and thermal propertie are contant acro layer of finite thickne. he ytem geometry for a one-layer ytem i illutrated in Fig. 3. he obervation angle and thickne of the layer are denoted a and z, repectively. he depth varie acro z direction. Radiation emitted from the one-layer ytem repreented by B i R, B where and R i, i =, are the oil temperature of layer, and the reflectivitie for boundary and boundary, repectively, and () exp z / () i the power lo factor, where i the direction coine for layer, and i the attenuation contant of layer, which i defined a Im r 0 where 0 and r are the free-pace wavelength and relative dielectric contant of layer, repectively. B ( +,) denote the brightne temperature above the firt boundary between air and oil at an obervation angle of. Radiation emitted from the oil for an N-layer ytem i [0] B, R N R R R 3 R R R R N N. N N For an N-layer ytem, we re-derived (4) to take into account the deep oil contribution to the total oil emiion. he re-derivation proce i given in (5) (8). It i clear that we have an extra lat term in our reulting expreion (8) that i miing in (4) due to our boundary condition given by (). We called that extra term a the emi-infinite layer effect or deep oil effect, becaue it i the emi-infinite region below the N th layer that caue the extra emiion term in (8). We denoted that emi-infinite (3) (4) Fig. 3. Soil layer, boundarie of the layer, layer thickne, and the incidence angle for a one-layer ytem. region a (N+) t layer in (8), in which layer depth i not a neceary variable to compute the lat term. Step of our re-derivation are a follow: he total energy received at the bottom of a boundary i due to emiion by all trata in layer i can be expreed a i i i where i and i are the phyical temperature and power lo factor of layer i. For a two-layer ytem, the geometry i hown in Fig. 4. Uing that geometry, we obtain R S S (5). (6) In (6), upercript + and correpond the upper and lower part of boundary, repectively. We utilized the ame logic in the following re-derivation proce., (7) R R, (8) 3, (9) phyical 3 R R 3 3 Fig. 4. A two-layer ytem geometry., (0). ()

4 74 C. DOGUSGEN (ERBAS), IMPAC OF DEEP SOI AYER ON ERRESRIA MICROWAVE EMISSION FOR A BARE Uing () in (0) 3 phyical R3 R 3 Uing () in (9) RR. () 3 R R (3) phyical 3 3. Uing (3) in (8) 3 phyical R3 R 3 Uing (4) in (7) RR. (4) R 3R R phyical 3 3 R S. Uing (5) in (6) R (5) phyical 3R3 R 3 R. R (6) Rearranging (6) R R R R 3 R R R 3 phyical 3. For an N-layer ytem B, S R R R R 3 R R R N 3 N R R R R (7) R N N N N N N. 3NN (8) he lat term in (8) correpond to the deep oil layer contribution..3.. Coherent Model Njoku and Kong [3] coherent radiative tranfer model aume that oil i a layered and a horizontallyhomogeneou medium in which oil moiture and oil temperature are function of vertical direction (z-axi). A the model take the amplitude and phae of the reflection between the layer into conideration, it i regarded a a coherent technique. Horizontally-polarized (h-pol) brightne temperature Bh i given by (9): 0 r (9) k Bh z z z z co '' d where r (z) = r (z) + r (z) and k are the relative dielectric contant, and free-pace wave number, repectively. (z) correpond to the vertical oil temperature profile, and k x = k in. he function (z) i governed by (0): d dz z r z k kx z 0. (0) We ued the coherent model a a reference model whoe reliability wa alo verified by another tudy [4] Auxiliary Model We ued three auxiliary model in conjunction with the radiative tranfer model: A land urface model (AEX), a dielectric mixing model, and a oil urface roughne model: AEX explain energy and carbon exchange in a oilplant-atmophere ytem with minimal empirical relationhip. Moreover, application to variou type of crop i poible. AEX utilize the Richard equation, and factor uch a oil urface water, aturated conductivity and oil water potential to compute the oil moiture profile againt time. he oil i divided into layer whoe depth are pecified by the uer. ayer depth do not need to be uniform. An input file include everal land and atmophere parameter (excluding vegetation parameter a we dealt with bare oil) uch a oil bulk denity, oil texture, water vapor preure, precipitation, wind peed, and thermal radiation from ky. By running AEX, we generated the time-dependent oil moiture profile along with the oil temperature profile. Next, we input thoe profile into the radiative tranfer model. he dielectric mixing model [5] evaluate the microwave dielectric behavior of oil-water mixture. hi model aume that the oil-water mixture conit of four component: dry oil, air, free and bound water. he reulting relative dielectric contant i / ii V () i

5 RADIOENGINEERING, VO. 6, NO. 3, SEPEMBER where τ i a contant hape factor, V i and i are the volume ratio and the relative dielectric contant of each component, repectively. Choudhury et al. [6] developed a oil urface roughne model that i baed on tandard deviation of urface height. If the cattering urface i tatitically rough uch that there i no correlation between the amplitude of the wave cattered by two point on the urface, and the urface height ditribution i repreented by a Gauian ditribution with a zero mean and a tandard deviation, then a roughne parameter h can be defined 4. 0 h () Fig. 5. etcae geometry (air dry oil wet oil). Auming that the pecular urface reflectivity R pecular i known Rrough Rpecular exp hco (3) where R rough i rough urface reflectivity. hen the rough urface emiivity E rough i E rough rough R. (4) 3. Reult and Dicuion o undertand the impact of deep oil layer on incoherent emiivity, we firt conidered a tetcae geometry given in Fig. 5. he geometry wa a one-layer ytem (N = ) of dry oil characterized by the relative permittivity of ε r = 4 j0.3. he deep oil layer wa repreented by wet oil with a relative permittivity of ε r = 5 j5. Soil temperature profile wa uniform. Incidence angle wa 35 (conitent with our field meaurement). Figure 6 compare the h-pol emiivitie computed with the incoherent model for the configuration in Fig. 5 with and without the emi-infinite layer. We plotted both emiivity variation againt dry oil depth. Maximum depth of the dry oil layer wa 0 cm. From Fig. 6, it i clear that the emiivity value ignificantly became larger when the emi-infinite layer wa taken into account, which would alo yield an increae in the brightne temperature. Note that when the dry oil depth wa zero, the incoherent emiivity for the configuration without the emi-infinite layer effect wa alo zero. hat wa becaue all the emiivity wa uppoed to contribute from the emi-infinite layer, however there wa no dry oil a the dry oil depth wa zero. Since the emi-infinite layer wa excluded in that plot, the reulting emiivity wa therefore zero. he maximum difference (between the configuration with and without the emi-infinite layer) in emiivity wa , occuring when the dry oil depth wa zero. Next, we explored how the deep oil layer affected the brightne temperature for the experimental data. o do that, we utilized the coherent model a a reference tool in order to verify the degree of agreement with the incoherent model for variou cae. Figure 7 illutrate the firt cae Fig. 6. Incoherent emiivity veru dry oil depth for the tetcae geometry: Impact of the emi-infinite layer. Fig. 7. Agreement between the incoherent and coherent model along with the impact of the emi-infinite layer on the incoherent model. Meaurement depth i 6 cm. (Cae ): We oberved a cloe agreement between the coherent and the incoherent model for a meaurement depth of 6 cm. Furthermore, Figure 7 depict that the impact of emi-infinite oil layer on the brightne temperature produced by the incoherent model wa negligible for that meaurement depth. In that figure, the plot for the cae with and without the emi-infinite layer are inditinguihable a the maximum brightne temperature difference between thoe cae wa only K. Note that all

6 76 C. DOGUSGEN (ERBAS), IMPAC OF DEEP SOI AYER ON ERRESRIA MICROWAVE EMISSION FOR A BARE reult were generated for the h-pol with an incidence angle of 35. Standard deviation of urface height wa taken a.5 cm throughout the computation. hen we conidered another cae (Cae ) for the meaurement depth of 8 cm, a given in Fig. 8. Maximum brightne temperature difference between the configuration with and without the emi-infinite layer for the incoherent model wa 0.5 K. In Cae 3 and Cae 4, we carried out the ame analyi for the meaurement depth of 5 cm and 4 cm, repectively. Maximum difference in the brightne temperature with and without the deep oil layer were.89 K and 6.84 K, for Cae 3 and Cae 4, repectively. Figure 9 and Figure 0 how the aociated plot. We utilized the coherent model plot of Cae a a reference for all cae. In all four cae, it i obviou that the incluion of the emi-infinite oil layer in the incoherent model add extra amount of brightne temperature, which i alo conitent with the tetcae. Alo, the more the meaurement depth decreaed and reached/exceeded the emitting depth toward the top oil, the more the incoherent model without the emi-infinite layer could not reproduce the meaured brightne temperature, which howed that modeling error might occur due to the emi-infinite layer effect when it wa not poible to collect ample of oil moiture a well a oil temperature. he meaurement depth became a le important factor when the emi-infinite layer wa taken into account. hat wa viible in Fig. 5 through Fig. 8 a the incoherent model with the emi-infinite layer cloely followed the coherent model, which actually howed that incluion of a emi-infinite oil layer returned more precie reult even when it wa not poible to ample (oil moiture and oil temperature) deep through the oil. Note that we obtained the emi-infinite oil layer effect on the brightne temperature by applying the boundary condition in (), and eliminating the dependency of the lat term in (8) from the variable R and that require the knowledge of two neighbor layer characteritic and depth. Fig. 8. Incoherent model reult with and without the emiinfinite oil layer along with the coherent reult a a reference. Meaurement depth i 8 cm. Fig. 9. Incoherent model reult with and without the emiinfinite oil layer along with the coherent reult a a reference. Meaurement depth i 5 cm. Fig. 0. Incoherent model reult with and without the emiinfinite oil layer along with the coherent reult a a reference. Meaurement depth i 4 cm. 4. Concluion We oberved trong contribution to the incoherent model brightne temperature of bare oil at.4 GHz due to the deep oil layer. Neglection of the emi-infinite oil layer returned ignificantly lower brightne temperature. arget difference occurred at a meaurement depth of 4 cm. Independent of the meaurement depth, incluion of the deep oil layer yielded agreement with the coherent model that wa ued a a reference model. We alo teted how the deep oil layer affected the emiivity of a hypothetical air-dry oil-wet oil configuration. We obtained ignificantly larger emiivity value when the deep oil layer wa taken into account. Our reult let u better undertand/interpret one of the different effect on meaured brightne temperature that might be adapted to improve other poible radiative tranfer model ued in oil moiture inverion. Furthermore, conideration of the deep oil effect in modeling could compenate for the limited availability of oil moiture and oil temperature data in term of model preciion.

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She i currently an aitant profeor of Electrical and Electronic Engineering at Itanbul Yeni Yuzyil Univerity, urkey. Her reearch interet include microwave radiometry, ynthetic aperture radar data proceing, and antenna deign.