Journal of

Hydrology E LS EV 1E R

Journal of Hydrology 188-189 (1997) 310-329

The effects of laterite and associated terrain components on PBMR response in HAPEX-Sahel W i l l i a m L. T e n g a'*, B h a s k a r J. C h o u d h u r y b, J a m e s R. W a n g c aHughes STX Corporation, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA bHydrological Sciences Branch, NASA Goddard Space Fligt~t Center, Greenbelt, MD 20771, USA CMicrowave Sensors and Data Communication Branch, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA

Abstract Terrain characteristics such as roughness and vegetation have been shown to significantly affect the interpretation of microwave brightness temperatures (TBs) for mapping soil moisture. This study, a part of the 1992 HAPEX-Sahel experiment (Hydrologic Atmospheric Pilot Experiment in the Sahel), aimed to determine the effects of laterite and associated terrain components (i.e. vegetation, soil, and exposed water bodies) on the Ts response of the Pushbroom Microwave Radiometer (PBMR, L-band, 21 cm wavelength), using the NS001 Thematic Mapper Simulator data as a surrogate for ground data. Coincident PBMR and NS001 data acquired from the high altitude (about 1500 m) long transect flights were processed to obtain Tss and radiances, respectively. The transects covered a range of moisture conditions. For this preliminary evaluation, no atmospheric corrections were applied, and the data sets were aligned by matching the acquisition times of the data records. NS001 pixels (about 4 m) were averaged to approximate the resolution of the PBMR (about 450 m), before their flight line data were compared. The laterite plateaux were found to have a surprisingly strong effect on the PBMR Ta response. Ts variations along the flight line could largely be explained by a combination of density and dielectric properties of laterite. The effect of surface moisture was distinguishable from the laterite effect, with the distinction apparently related to the occurrence of ephemeral pools of water after rainfall. Model simulated Tss agreed reasonably well with the observed Tas.

1. Introduction Terrain components, such as vegetation, surface roughness, and soil properties, have been shown to significantly affect the interpretation of passive microwave data for * Corresponding author. 0022-1694/97/$17.00 © 1997- Elsevier Science B.V. All fights reserved PII S0022-1694(96)03 ! 64-2

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mapping soil moisture. These components all affect the microwave emissivity of the ground surface (Wang et al., 1980; Jackson and O'Neill, 1987; Theis and Blanchard, 1988; Choudhury, 1991; Jackson and Schmugge, 1991). They must first be isolated and corrected for before one can interpret emissivity in terms of soil moisture. These terrain effects have been noted in the Pushbroom Microwave Radiometer (PBMR) response during previous large-area field experiments, such as HAPEXMOBILHY (Mod61isation du Bilan Hydrique), FIFE (First ISLSCP Field Experiment, (ISLSCP, International Satellite Land Surface Climatology Project)), MACHYDRO'90 (Multisensor Aircraft Campaign, and MONSOON'90 (Wang et al., 1990; Nichols et al., 1993; Jackson et al., 1994; Lin et al., 1994; Schmugge et al., 1994). The PBMR is an Lband (21 cm wavelength, 1.413 GHz frequency) radiometer designed for large area soil moisture mapping. It has four, horizontally polarized beams centered at 8° and 24 ° from the nadir, each having a beam width of about 16°. At an aircraft flight altitude of H, the resulting ground area coverage of each beam is about 0.3H, and the total cross-track coverage is about 1.2H (Harrington and Lawrence, 1985). These previous studies have shown that vegetation decreases the sensitivity of the PBMR to soil moisture variations. A vegetation canopy absorbs the emission from the soil underneath and adds its own emission to the total radiation sensed by the PBMR. Jackson and Schmugge (1991) have shown that, at L-band, canopy attenuation is low for agricultural crops and grasses. This result does not apply to forest canopies, which have high vegetation water content and, thus, high optical depth. However, by first stratifying the vegetation to screen out forests, the vegetation effect in non-forest areas can be corrected by using an estimate of the vegetation water content and a vegetation parameter. This latter parameter can be taken to be a constant at long wavelengths, such as that of the PBMR. The terrain in the 1992 HAPEX-Sahel experiment (Goutorbe et al., 1997) is markedly different from those of previous large-area field experiments and has been described in detail by Kabat and Prince (1993), Goutorbe et al. (1994), and Prince et al. (1995). Specifically, the presence of laterite as a prominent terrain feature was a key difference between this and previous experiments. This study was aimed to determine the effects of laterite and associated terrain components on the PBMR Ta response. Fig. 1 shows the 1° latitude by 1° longitude area of the experiment, with four north-south long transects and three 'Super Sites' marked. Intensive ground data were collected in these Super Sites. In this paper, the data for long transect 1 (westernmost), which crossed the Niger River, are presented. The terrain covered by the other three long transects was similar to that for long transect 1. The present analysis is mainly focused on two flights for long transect 1, one of which was a 'wet' day and the other was a 'dry' day (August 29, 1992 and September 6, 1992, respectively).

2. Study site description Some key aspects of the study site are summarized here from Kabat and Prince (1993), Goutorbe et al. (1994), and Prince et al. (1995). The overall terrain is composed of a semiperiodic toposequence of plateaux and valleys. The plateaux, occupying about 20% of the area, are capped by laterite (described in the following paragraph), parts of which are

W.L Teng et aL/Journal of Hydrology 188-189 (1997) 310-329

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sand-covered. The laterite has a dark-toned, gravelly surface and is effectively impermeable, which causes extensive runoff during heavy precipitation. On top of the plateaux, in varying densities, are vegetation strips of about 3 to 5 m high, 10 m wide, and varying lengths, aligned along the contours in a roughly concentric pattern. These strips, locally termed "tiger bushes," cover about 30% of the plateau surface (Kabat and Prince, 1993). Surface material in the strips is finer-grained and lighter-colored. Otherwise, there is generally little soil or vegetation on the plateaux. The slope transition from plateau to Table ! Physical and chemical properties of iaterite and its primary constituents

Grain density = (gm cm -3) Dielectric constant b Average composition c (%)

Hematite (Fe203)

Corundum (AI203)

Silica (SiO2)

5.3 25.0 47.5

3.9 12.1 20.1

2.6 4.6 16.6

a Carmichael (1989). b Carmichael (1989). Corresponding dielectric loss factors were not available; all were assumed to be 0.1 (Ulaby et al., 1990). ¢ Persons (1970).

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valley can be abrupt or gradual. The valley floors are filled with deep, bleached, aeolian sands, with a very low water holding capacity, and covered with a generally sparse layer of vegetation (fallow savanna, degraded fallow, and millet crop). The laterite, or ironstone, that caps the plateaux is an indurated material, irreversibly hardened (i.e. cannot be dispersed by shaking in water with a dispersing agent) from plinthite. The latter is a humus-poor, iron-rich, soil horizon, commonly occurring as clayey, red or dark-red mottles, which hardens irreversibly when subjected to repeated wetting and drying (e.g. when exposed by erosion of overlying material). The iron in laterite is ferric oxide or hematite and is the main cementing agent. The relatively insoluble and stable ferric oxide in plinthite is concentrated by the removal of more easily weatherable and soluble compounds. With sufficient amounts of plinthite, a continuous phase is formed in the soil and, when indurated, becomes a massive ironstone layer. Laterites are often found on old, stable geomorphic surfaces, such as plateaux, high terraces, and pediments, with level or relatively gentle slopes. Laterite distribution is largely independent of current rainfall patterns, suggesting that many might have formed under previous, wetter climate (Persons, 1970; Buol et al., 1973; Soil Survey Staff, 1975). Whether or not a soil irreversibly hardens depends, in part, on the chemical balance of the three essential constituents, hematite (Fe203), corundum (A1203), and silica (SiO2) (Table 1). One method of identifying laterites is by calculating an index based on the ratio of these three constituents, %SIO2/60 L = (%A1203 / 102) + (%Fe203 / 160)

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where L < 1.33, 1.33 < = L < = 2, and L > 2 correspond to laterite, lateritic soil, and nonlateritic soil, respectively (Persons, 1970).

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