The effect of rock cover fraction on the retrieval of surface soil moisture at L-band

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1 18 th Wold IMACS / MODSIM Congess, Cains, Austalia July 2009 htt://mssanz.og.au/modsim09 The effect of ock cove faction on the etieval of suface soil moistue at L-band N. Ye 1, J.P. Walke 1, R. Panciea 1, D. Ryu 1, C. Rüdige 1, and R.J. Guney 2 1 Deatment of Civil and Envionmental Engineeing, Univesity of Melboune, Austalia 2 NERC Envionmental Systems Science Cente, Univesity of Reading, United Kingdom n.ye@civenv.unimelb.edu.au Abstact: The Soil Moistue and Ocean Salinity (SMOS) mission, develoed by the Euoean Sace Agency (ESA), will be launched in the second half of It will be the fist L-band (~1.4 GHz) assive micowave satellite secifically designed fo global soil moistue obsevations, with an exected accuacy fo the etieved soil moistue of ~0.04 m 3 /m 3. While assive micowave obsevations have been widely acknowledged to give the most accuate infomation on soil moistue, these sensos ae chaacteized by low satial esolution footints, being on the ode of 50km. One of the key difficulties with obsevations at this scale is the heteogeneity that exists in land suface featues. Howeve, ast and cuent soil moistue etieval algoithms have tyically assumed a homogeneous ixel aoach. Thus, in ode to maximize the soil moistue etieval accuacy, the vaious land suface featues that exist in a satellite footint should be taken into account. While the SMOS etieval algoithm distinguishes between thee diffeent suface tyes (bae soil, hebaceous and woody vegetation), at this stage it does not take into consideation othe sub-ixel effects such as the bightness temeatue contibution fom oen wate bodies, ock cove o uban aeas, which ae exected to affect the oveall soil moistue etieval accuacy fo many ats of the wold. This study exloes the imact of suface ock on the etieval of suface soil moistue fo shot vegetation coveed fields by comaing etieved soil moistue estimates with and without accounting fo the esence of ock in a synthetic famewok. Fist, the micowave obsevation used to etieve soil moistue is simulated by accounting fo the contibution of ocks to the oveall emission, assuming that ock behaves like vey dy soil with a fixed dielectic constant and a smooth suface. The soil moistue is then etieved using the homogeneous ixel aoach. The simulation of micowave emission is based on a eesentative shot gass field with vaious suface ock cove factions, soil moistue contents and vegetation conditions. The esults illustate that ock induced soil moistue etieval eo is deendent on soil moistue and vegetation wate content since the bightness temeatue diffeence between soil and ock imacts the soil moistue etieval eo. The omission of ock cove fom the etieval algoithm leads to an oveestimation of the bulk soil moistue content fo low soil moistue conditions and an undeestimation fo high soil moistue conditions. Taking 30% ock cove faction as an examle, the maximum eo in the bulk soil moistue estimation is as much as 0.04 m 3 /m 3 in bae soil and u to 0.10 m 3 /m 3 in wet soil coveed by shot gass vegetation. It should be noted howeve that these esults may be highly deendent on two key assumtions of this ae; i) that ock can be modelled as a smooth suface, and ii) that thee is no vegetation cove ove the ock. Keywods: assive micowave, soil moistue, emote sensing, ock faction 3761

2 1. INTRODUCTION The Soil Moistue and Ocean Salinity (SMOS) mission, which will be the fist dedicated soil moistue satellite, is cuently scheduled fo launch in the second half of SMOS consists of an L-band (~1.4 GHz) assive micowave senso that will ovide multi-angula obsevations in dual olaization using a new synthetic aetue technique (Ke et al., 2003). Passive micowave obsevations at L-band have been shown to be the most omising of the diffeent emote sensing techniques fo outine maing of suface soil moistue at global scales, due to its ability to enetate cloud, its diect elationshi with soil moistue though the soil dielectic constant, and a educed sensitivity to land suface oughness and vegetation cove (Jackson and Schmugge, 1989; Njoku et al., 2002). While thee have been a numbe of satellites oeating at fequencies above 6 GHz (e.g. SMMR, AMSR-E, and WindSat), this is the fist sace-bone senso to make long-tem measuements at L-band. Ove the ast thee decades the quality of soil moistue etieval fom assive micowave emote sensing has been significantly imoved. Moeove, Owe et al. (2008) have ecently ublished a soil moistue oduct coveing the yeas using the available high fequency data fom a seies of satellites. While seveal of these ecent soil moistue oducts show a good elationshi with in-situ obsevations fo aeas of low vegetation cove (Rüdige et al., 2009), thei etieval eo is tyically above 0.06 m 3 /m 3. Futhemoe, all ast soil moistue etieval algoithms have been develoed assuming a homogeneous land suface cove, with the excetion of the SMOS algoithm (Ke et al., 2007), which allows fo thee diffeent suface tyes within the satellite footint. Howeve, the influences of suface ock, wate bodies, o uban centes within the senso s field of view ae not cuently accounted fo, and this will intoduce a so fa lagely unquantified uncetainty in the soil moistue etieval accuacy (Delwat et al., 2008). This ae exloes the fist of these effects in a synthetic study in ode to quantify this effect. To date only a few exeiments (e.g. Jackson et al., 1992; Moneis et al., 2008) have been conducted to examine the effect of ocks on soil adiometic emission at L-band. Those studies identified thee main chaacteistics that affect the soil esonse: 1) ocks do not absob any aeciable amount of wate but they can occuy a lage at of the soil sace; 2) the low dielectic constant of ock educes the aveage dielectic constant of the ock-soil mixtue; and 3) the esence of ock may incease the suface oughness. The combined imact of these thee effects is to alte the land suface emission in a comlex way. Moeove, as ocks do not contibute to the soil moistue content, data collected with otable soil moistue obes duing field camaigns will tyically give an oveestimation of the bulk soil moistue content, as the obes can only be inseted into soils having a elatively low ock faction. While the effect of the suface ock faction on the micowave emission fom soil has been studied in elation to its emission chaacteistics (Jackson et al., 1992; Moneis et al., 2008), its effect on soil moistue etieval accuacy has not yet been exloed. This study simulates the imact of ock on bightness temeatue obsevation and the subsequent soil moistue etieval, using the L-band Micowave Emission of the Bioshee (L-MEB; Wigneon et al., 2001, 2007) model, which is the basis fo the SMOS Level 2 etieval algoithm. Consequently, the elationshi of etieved soil moistue to bulk soil moistue unde the esence of suface ock is exloed. 2. METHODOLOGY The effect of ock faction on soil moistue etieval is studied hee by comaing synthetic tuth soil moistue with etieved soil moistue fom adiometic obsevations at L-band. The obsevations wee synthetically geneated by using the tuth soil moistue and ou cuent undestanding of ock effects on micowave emission. Moeove, they wee calculated fo a ange of ock cove factions, vegetation conditions and soil moistue content. The ock cove effect on soil moistue was studied by subsequently etieving the soil moistue without taking the suface ock faction into consideation. The etieved soil moistue was then comaed with the oiginal tuth bulk moistue. Essentially, the test consists of the following thee stes: 1) assigning aametes and ancillay data fo the adio bightness model; 2) geneating synthetic bightness temeatue obsevations; and 3) etieving soil moistue and vegetation wate content simultaneously using the same aametes and ancillay data as used to deive the obsevations but without accounting fo the esence of ock, as descibed in the following sections Land Suface Reesentation A land suface eesentation was ceated to simulate the field conditions of vaying ock faction in vaious vegetation conditions anging fom bae soil to shot gass. This land suface eesentation consists of a ock comonent without any ovelying vegetation coveage and a soil comonent coveed by shot gass. Each comonent was assumed to be homogeneous and indeendent fom each othe, which means 3762

3 Table 1. Land suface aametes used in the soil moistue etieval algoithm (afte Panciea et al., 2009). Sand Content S (%) 67 Vegetation stuctue tt H 1 Roughness H R 0.5 Clay Content C (%) 15 Vegetation stuctue tt V 1 Roughness exonent N RH 0 Bulk Density ρ (g/cm 3 ) 1.1 Scatteing albedo ω H 0 Roughness exonent N RV 0 Vegetation aamete b 0.15 Scatteing albedo ω V 0.05 suounding vegetation has no attenuation imact on assive micowave emission fom ock comonent. Moeove, the soil comonent was assumed to have soil, vegetation and oughness chaacteistics simila to those found at Roscommon fam in the Goulbun Rive Catchment (Panciea et al., 2009). Fo all cases, the land suface was assumed to have no elief changes in ode to emove any toogahic effects on the soil moistue etieval, and the ock comonent suface was assumed to be smooth. Consequently, the bulk moistue of the ixel is calculated by: SM bulk = ( 1 f ) SM soil, (1) whee f and SM soil ae the ock cove faction and soil comonent moistue in the ixel, esectively Synthetic Bightness Temeatue Geneation Eleven aametes (Table 1) wee used to geneate synthetic bightness temeatue and subsequently invet soil moistue fo each scenaio, using L-MEB. Fo simlification uoses, the temeatues of ock, soil and vegetation wee assumed to be identical and wee set to 300 K. At the same time, the soil comonent moistue SM soil was vaied fom to m 3 /m 3 in m 3 /m 3 stes, and the Vegetation Wate Content (VWC) values of soil comonent vaied fom 0.0 to 1.0 kg/m 2 in 0.02 kg/m 2 stes coveing the whole ange of VWC of shot vegetation. This allowed the ock imact to be exloed fo a tyical land suface unde a ange of soil moistue and vegetation conditions. The total bightness temeatue of the land suface was assumed to be the sum of the bightness temeatue deived fom the two suface comonents weighted by its ock cove faction: TB = ( 1 f ) TBs. + f TB., (2) whee TB is bightness temeatue; subscits, s, and ae the signal olaization (, hoizontal o vetical) fo the soil (s) and ock () comonents. In the τ-ω model (Mo et al., 1982), the bightness temeatue emission fom the soil-vegetation laye is defined as the sum of thee tems: 1) the uwad vegetation emission scatteed by the atmoshee, 2) the downwad vegetation emission eflected by the soil and attenuated by the vegetation laye and then scatteed by the atmoshee, and 3) the soil emission attenuated by the vegetation laye: TBs. = ( 1 ω ) (1 γ ) (1 + γ Γs. ) Tv + (1 Γs. ) γ Ts, (3) whee T v and T s ae the effective vegetation and soil temeatues; and ω and γ ae the single scatteing albedo and tansmissivity of the vegetation laye. The eflectivity of a ough soil suface Γ s. is a function of the smooth soil suface eflectivity Γ * s., the incidence angleϑ, and the oughness aametes H R and N R (Wang and Choudhuy, 1981; Wigneon et al., 2001) given by: * N Γ.. ex[ cos R s = Γs H R ( ϑ)]. (4) The smooth suface eflectivity Γ * s. is calculated though the Fesnel equations as a function of olaization, elative dielectic constant of the soilε ' + ε" i, and the incidence angleϑ. Soil dielectic constant can be estimated fom the soil comonent moistue SM soil though the mixing model of Dobson et al. (1985). The tansmissivity of the vegetation laye γ is detemined by the vegetation otical deth at nadi τ NAD and the aamete tt that coects the otical deth fo non-nadi views at each olaization by: γ = ex[ τ NAD ( tt sin ( ϑ) + cos ( ϑ)) cos ( ϑ)]. (5) Accoding to Ke et al. (2007) ocks can be assumed to behave like vey dy soil, and thei dielectic constant anges fom 2.4 to 9.6 at fequency of 400 MHz and 35 GHz (Ulaby et al. 1986). Ulaby et al. (1990) measued dielectic oeties of 80 ock samles at fequency fom 0.5 to 18 GHz in stes of 0.1 GHz fo the eal at. The esults illustate that eal at of dielectic constant of ock ( ε ' ) is indeendent of fequency, but has a coelation with the ock bulk density ( ρ ) as: ε ) ρ b ' = (1.96 ± (6) Fo the uose of this study, it is assumed that ock behaves like a vey dy bae soil with a smooth suface, and a temoally stable dielectic constant i, which is the aveage value of dielectic constant measuement of ock samles at L-band in Jackson et al. (1992). Consequently the imotant assumtions hee ae that 1) the ock emission behaves like a smooth suface and 2) thee is no vegetation laye ove the ock. The adiometic emission fom a ock suface can thus be calculated though: b 3763

4 TB. = ) T, (7) ( 1 Γ. whee T is the ock suface temeatue, Γ. is the ock suface eflectivity calculated though Fesnel equations, assuming that the ock suface is smooth and hoizontal that the incidence angle is equal to that fo the soil suface. The sensitivity of soil moistue etieval to the eal at of ock dielectic constant, as well as to ock ough and ock temeatue, is examined late Soil Moistue Retieval The suface soil moistue content was etieved fom the synthetically geneated bightness temeatue obsevations that accounted fo the ock faction, by using the L-MEB model without consideing a ock cove faction. The nonlinea otimisation scheme used fo the etieval is an iteative algoithm that otimises soil moistue and vegetation wate content simultaneously. It aims to minimize the diffeence between the adiometic obsevation and the modelled bightness temeatue. When the minimized diffeence exceeds the assigned toleance, a non-convegence flag is etuned. The ock induced eo is theefoe the diffeence between the tuth soil moistue and the suface soil moistue etieved fom the synthetic fowad obsevation. A numbe of scenaios wee simulated to identify the effect of the ock cove unde vaying conditions, by changing the ock cove faction, suface soil moistue, incidence angle, and vegetation wate content. 3. RESULTS AND DISCUSSION 3.1. The Effect of Rocks on Bightness Temeatue Due to Eqn. 2, the emission esonse of ock mixed ixels is combined by the micowave chaacteistics of the ock comonent and soil comonent. The cuves of synthetic bightness temeatue vay fom the bightness temeatue cuves of soil comonent to those of ock comonent as ock cove faction inceases. Fig. 1 illustates the changes in bightness temeatue simulated as a function of bulk soil moistue, incidence angle, and vegetation wate content fo vaious ock cove factions unde secific conditions. Simila esults can be gained on othe shot vegetation conditions. In Fig. 1(a), the bulk soil moistue and soil comonent VWC ae fixed at 0.15 m 3 /m 3 and 0.5 kg/m 2 esectively, eesenting a shot gass coveed dy soil condition. Clealy, as incidence angle inceased, the imact of ock cove faction to V-olaized bightness temeatue deceased. The sensitivity of V-olaized bightness temeatue to ock cove faction eaches a minimum value when the incidence angle is aound 60º. Given these esults, it may be ossible to estimate the ock cove faction fom the multi-incidence angle esonse, as will be available fom SMOS. In Fig. 1(b), the incidence angle is fixed at 42.5º, which is the fixed incidence angle of SMOS L1C bowse oducts (McMullan et al., 2008), and soil comonent vegetation wate content is again fixed at 0.5 kg/m 2. As exected, the sensitivity of bightness temeatue simulations to soil moistue deceases with inceasing ock cove faction. Due to Eqn. 1, the ange of bulk soil moistue is esticted by the ange of soil comonent moistue SM soil and ock faction f, thus limiting bulk moistue to less than 0.6 (1 - f) assuming the wate content caacity of soil comonent is at satuation. These esults ae in ageement with those obtained by Jackson et al. (1992) and Moneis et al. (2008), who showed that the bightness temeatue emissions fom soils with highe ock cove factions become less sensitive to changes in the Figue 1. Synthetic bightness temeatue simulations fo vaying ock cove factions as a function of (a) incidence angle; (b) bulk soil moistue; and (c) soil comonent vegetation wate content. 3764

5 Figue 2. Relationshi of etieved soil moistue (to ow) and vegetation wate content (bottom ow) with bulk soil moistue fo diffeent ock cove factions f and vegetation conditions: (a) and (d) VWC=0.0 kg/m 2 ; (b) and (e) VWC=0.5 kg/m 2 ; (c) and (f) VWC=1.0 kg/m 2. suface soil moistue content. In Fig. 1(c), the bulk soil moistue and incidence angle ae again fixed at 0.15 m 3 /m 3 and 42.5º, esectively. In addition, Figs. 1(a)-(c) imly that the synthetic bightness temeatue in H olaization is moe sensitive to ock cove faction than that in V olaization The Effect of Rocks on Soil Moistue Retieval Figs. 2(a)-(c) illustate the elationshi between tuth soil moistue and etieved soil moistue acoss thee diffeent vegetation conditions. Soil moistue etieval eo is not only deendent on ock cove faction but also on bulk soil moistue and vegetation wate content. In some bulk soil moistue and vegetation wate content conditions, ock cove faction has little imact on soil moistue etieval accuacy. Fo examle, in Fig. 2(b), when the bulk soil moistue aoaches 0.15 m 3 /m 3, etieved soil moistue is equal to bulk soil moistue in all anges of ock cove faction. It can be seen that the soil moistue etieval model will oveestimate the bulk soil moistue in dy conditions and undeestimate it in wet conditions if the esence of ock is not accounted fo. In the whole ange of bulk soil moistue, esecially unde wet soil conditions, ock induces a soil moistue etieval eo exceeding the 0.04 m 3 /m 3 eo budget of SMOS fo most cove factions. Howeve, a low ock cove faction in dy soil conditions leads to a etieval eo smalle than 0.04 m 3 /m 3. Additionally, the highe the VWC of the soil comonent, the lage the soil moistue etieval eo incued. Figs. 2(d)-(f) illustate the etieved vegetation wate contents concuent with etieved soil moistue shown in Fig. 2(a)-(c). The etieved VWC deceases with inceased ock cove faction fo two easons. Fist, the ock comonent in land suface eesentation was assumed to be vegetation-fee in this study. The esence of ock theefoe educes the aveage VWC ove the entie field. Second, the esence of suface ock inceases the diffeence between H and V olaized bightness temeatue, thus the otimization scheme in L-MEB decease the etieved VWC in ode to minimize the cost function since VWC is much moe sensitive to the seaation than soil moistue. When etieved VWC eaches 0, it can be no futhe imovement in Tb, and a non-convegence flag is etuned since the toleance is not achieved Geneal Sensitivity Study The esults esented so fa assumed a smooth ock suface unobscued by vegetation, with a constant ock and soil temeatue, and a edefined ock dielectic constant. Howeve, the effects of ock on soil moistue 3765

6 Figue 3. Relationshi between bulk soil moistue and etieved soil moistue fo diffeent (a) ock suface oughness; (b) ock dielectic constant; and (c) ock temeatue. etieval accuacy ae not only deendent on the ock cove faction, incidence angle and vegetation wate content that wee assessed, but also on these othe fixed aametes and assumtions. In this section, the sensitivities of the etieved soil moistue to ock suface oughness, ock temeatue and dielectic constant ae examined, based on a tyical ock cove faction of 0.35 (Jackson et al., 1992). Fig. 3 shows the sensitivity of the etieved soil moistue to changes in the ock suface oughness, effective ock temeatue and ock dielectic constant as a function of bulk soil moistue. Clealy, the ock oughness aamete has a significant imact on ock induced soil moistue etieval eo. Since the diffeence between H and V olaized bightness temeatue of smooth ock is much lage than that of soil, the highe the ock suface oughness aamete H R, the smalle the diffeence between the olaization ga of soil and ock comonents, and the smalle the soil moistue etieval eo due to the esence of ock. Moeove, changes in the eal at of the ock dielectic constant and ock temeatue ae not able to change the atten in which the ock cove faction affects the soil moistue etieval eo, but both affect the magnitude of ock induced eos. The ock dielectic constant and ock temeatue influence the etieved soil moistue in diffeent ways; aoximately evey 1.5 incease of eal at of ock dielectic constant leads to 0.02 m 3 /m 3 incease in etieved soil moistue, while highe ock temeatue coesonds to a soil moistue decease of 0.01~0.02 m 3 /m 3 e 10K incease of ock suface temeatue. 4. CONCLUSION A simle shot gass land eesentation was studied to assess the effect of ock cove faction on soil moistue etieval assuming ock behaves like vey dy soil with smooth suface and without attenuation by suounding vegetation. Based on this synthetic study, it was shown that uncetainty in the ock cove faction esults in a significant effect on the etieval accuacy of soil moistue fom emotely sensed bightness temeatue obsevations. Fo bae soil, maximum eo in the bulk soil moistue estimation is as much as 0.04 m 3 /m 3 and u to 0.10 m 3 /m 3 in wet soil coveed by shot gass vegetation (VWC = 1 kg/m 2 ) fo 30% ock cove faction. It was also shown that the omission of ock cove fom the etieval algoithm leads to an oveestimation of the bulk soil moistue content fo low soil moistue conditions and an undeestimation fo high soil moistue conditions. Rock suface oughness, ock temeatue and ock dielectic constant wee found to have a significant imact on the etieval of soil moistue; a fact that has lagely been ignoed in the ast and should be studied futhe. While it may seem at fist difficult to aly the findings esented hee to actical alications, due to the lack of sufficient data on global ock cove faction, it is suggested that the elationshis shown in this study may be exloited to detemine the ock cove faction fo individual footints by using the multi-angula data that will be ovided by SMOS. As the ock faction and its dielectic constant ae constant though time, it may be ossible to estimate the ock cove faction fom multi-angula data. ACKNOWLEDGMENTS This oject is funded by an Austalian Reseach Council Discovey Poject (DP ). Ye Nan is sonsoed by a Melboune Intenational Fee Remission Scholashi (MIFRS). The authos wish to thank Jean-Piee Wigneon fo oviding the souce code of L-MEB. 3766

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