Measurements in the Agre forest for
EUROSTARRS-2001

Jean-Christophe Calvet

► To cite this version:

Jean-Christophe Calvet. Measurements in the Agre forest for EUROSTARRS-2001: Report of Contract N°10167/01/NL/SF to ESA-ESTEC. CNRM, Université de Toulouse, Météo-France, CNRS. 2002. meteo-04249333

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Distributed under a Creative Commons Attribution 4.0 International License
CENTRE NATIONAL
DE RECHERCHES METEOROLOGIQUES

GROUPE DE METEOROLOGIE A MOYENNE ECHELLE

MEASUREMENTS IN THE AGRE FOREST
FOR EUROSTARRS-2001

Report of Contract N°10167/01/NL/SF to ESA-ESTEC

by

J.-C. CALVET

OCTOBER 2002
**Résumé – Mesures en forêt d'Agre pour EuroSTARRS-2001.** Météo-France et l'Agence Spatiale Européenne ont réalisé dans la région de Toulouse, une campagne de mesure aéroportée visant à tester un nouveau concept de satellite d'observation de la Terre, pour des applications en météorologie. Il s'est agi d'acquérir un catalogue de "signatures" de divers types de surfaces, dont des surfaces urbaines, avec un radiomètre en bande L (fréquence de 1,4 GHz). Ce type d'instrument permet d'estimer le contenu en eau des sols et l'estimation de cette variable depuis l'espace devrait contribuer à l'amélioration des modèles hydrologiques et météorologiques. C'est l'avion de recherche de l'Agence Spatiale Allemande (DLR), et le radiomètre STARRS du service de recherche de la Marine Américaine (NRL) qui ont été utilisés dans cette campagne de mesure internationale. Les vols ont eu lieu en Allemagne, en France et en Espagne. La région de Toulouse a été survolée le 20 novembre 2001 : forêt d'Agre près de Montauban, ville de Toulouse, et les Pyrénées au niveau du Lac d'Oô. Le CNRM a organisé ces vols et réalisé des mesures de terrain en forêt d'Agre, destinées à analyser les données radiométriques. Ce rapport présente les données obtenues in situ lors du passage de l'avion.

**Abstract – Measurements in the Agre forest for EuroSTARRS-2001.** Météo-France and the European Space Agency have performed an airborne campaign in the region of Toulouse, aiming at assessing a new concept of earth observation satellite, for an application in meteorology. The objective was to investigate the L-band emission signature of various surface types, including urban surfaces. L-band radiometry permits to estimate the soil water content. Estimating this variable from space should improve the hydrological and meteorological models. The research plane of the German Spase Agency (DLR) and the STARRS radiometer of the US Marine National Research Laboratory (NRL) were used in this international observational campaign. The flights were performed in Germany, France, and Spain. The region of Toulouse was flown over on 20 November 2001: the Agre forest near Montauban, the city of Toulouse, and the Pyrenees at the level of the Oô lake. CNRM has organised the planning of these flights and made field measurements in the Agre forest, in order to analyse the radiometric data. This report presents the obtained *in situ* data during the over-flight of the forest.
# Contents

<table><tr><td>1. Introduction</td><td>p. 4</td></tr><tr><td>2. Material and methods</td><td>p. 6</td></tr><tr><td>3. Results</td><td>p. 11</td></tr><tr><td>4. Conclusion</td><td>p. 17</td></tr><tr><td>Acknowledgements</td><td>p. 17</td></tr><tr><td>Appendix A</td><td>p. 18</td></tr></table>
# 1. Introduction

Numerous studies have shown that L-band radiometry is a relevant technique to monitor soil moisture. An estimation of the temporal evolution of soil moisture can provide significant improvements of meteorological and hydrologic predictions (Calvet and Noilhan 2000). For this reason, a number of projects have been initiated to implement a L-band radiometer on a satellite platform. This study was performed in the framework of the Soil Moisture and Ocean Salinity (SMOS) Mission, the second Earth Explorer Opportunity Mission of the European Space Agency (ESA). SMOS is currently being studied in Phase B and is scheduled for launch in 2006.

<figure><img src="image_2.png" /><figcaption>**Fig. 1-** Quicklook of the multiangular observations of L-band brightness temperatures performed on 20 November 2001 by the STARRS radiometer mounted on the research plane of DLR. Note the sharp contrast between the North and the South of the Pyrenees: the low brightness temperatures observed in Spain denote soil conditions close to saturation. The Agre forest site is indicated.</figcaption></figure>
Past field experiments were devoted to study the potential of L-band measurements for monitoring surface soil moisture over, mainly, agricultural surface types. In particular, the effect of vegetation cover has been studied, as well as soil temperature, and soil surface roughness. Other parameters such as soil texture, bulk soil density and atmospheric effects have a smaller (second-order) influence but should also be accounted for. In the framework of the SMOS mission, the EUROSTARSS airborne campaign was carried out in November 2001 to support the soil moisture algorithm development and to investigate the L-band signature of different forest types. Multiangular V-polarization observations were acquired by the L-band STARSS microwave radiometer from the Naval Research Laboratory (USA) over specific sites and over long transects in France and Spain. Measurements were also carried out over the Ocean. The main studied land sites were:

* The Nezer forest (Les Landes region near Bordeaux, France): coniferous forest.

* The Agre forest (near Toulouse, France): a mixed deciduous and coniferous forest.

* The urban area of Toulouse, France.

* The mountainous region of the Pyrenees, France.

* Valencia (Spain): a variety of natural and agricultural vegetation covers.

The STARSS instrument has six antennas pointing at +/-7°, +/-22° and +/-38° across track (the 3dB antenna beamwidth is about 15°). The instrument was installed onboard a Dornier 228 aircraft operated by DLR Germany and was tilted by 12° to enable measurements over a larger range of incidence angle θ (up to 55°). Low altitude flights (at altitude Hc of about 300 m) were made over the forest and urban sites, while combined low (Hc=500m) and high altitude flights (Hc=2.5km) were made in the Valencia region.

The sites of Agre, Toulouse, and the Pyrenees, were observed on 20 November 2001. Fig. 1 presents a quicklook of the L-band observations. This report presents the in situ measurements performed in the Agre forest and in one adjacent agricultural field, during the EUROSTARRS flight of 20 November 2001.
## 2. Material and methods

### 2.1. The Agre forest

The Agre forest (or Montech forest) is characterised by a large diversity of forest stands: 12 tree species (Table 1), including broadleaf and coniferous trees (mainly sessile oak and Corsican pine, respectively), 26 canopy types (mature timber, coppice, plantations, etc.) organised in 145 well-defined stands (Fig. 2). The terrain is flat (altitude ranging from 100 to 130 m). Fig. 3 shows an aerial photograph of the forest.

<table><caption>**Table 1-** Species composition of the Agre forest in 1992 (data from ONF).</caption><thead><tr><th>Species</th><th></th><th>Surface (ha)</th><th>Surface (%)</th></tr></thead><tbody><tr><td>Sessile oak</td><td>Quercus petraea</td><td>837</td><td>57.3</td></tr><tr><td>Corsican pine</td><td>Pinus nigra laricio</td><td>190</td><td>13.0</td></tr><tr><td>English oak</td><td>Quercus robur</td><td>178</td><td>12.2</td></tr><tr><td>Hornbeam</td><td>Carpinus betulus</td><td>115</td><td>7.9</td></tr><tr><td>Red oak</td><td>Quercus rubra</td><td>45</td><td>3.1</td></tr><tr><td>Maritime pine</td><td>Pinus pinaster</td><td>33</td><td>2.3</td></tr><tr><td>Caucasian fir</td><td>Abies nordmanniana</td><td>18</td><td>1.3</td></tr><tr><td>Atlas cedar</td><td>Cedrus atlantica</td><td>10</td><td>0.7</td></tr><tr><td>Weymouth pine</td><td>Pinus strobus</td><td>9</td><td>0.6</td></tr><tr><td>Robinia</td><td>Robinia pseudoacacia</td><td>7</td><td>0.5</td></tr><tr><td>Scots pine</td><td>Pinus sylvestris</td><td>5</td><td>0.3</td></tr><tr><td>Douglas fir</td><td>Pseudotsuga menziesii</td><td>2</td><td>0.1</td></tr><tr><td>Empty stands</td><td></td><td>12</td><td>0.8</td></tr></tbody><tfoot><tr><td>TOTAL</td><td></td><td>1461</td><td>100</td></tr></tfoot></table>

### 2.2. The STARRS flight over the Agre forest

Sixteen flight lines of about 7 km each were performed over the Agre forest on 20 November 2001, 1300 LST. The flight lines were made as low as possible (altitude above the ground of 300 m), and one flight line was made every 200 m. A quicklook of the obtained brightness temperatures is presented in Fig. 3.
<figure><img src="image_3.png" /><figcaption>**Fig. 2-** The stands of the Agre forest. The forest stands where in situ measurements were performed are highlighted (©IGN, ©ONF). The red spot indicates the agricultural field of Lacoyne.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>**Fig. 3-** The Agre forest: (left) IGN aerial photograph (Aerial photograph: Mission 2000 FD 46-82/250, plates n°1208 and 1236 ©IGN - Paris - 2002, Authorisation n°212034), (right) quicklook of L-band brightness temperatures. The in situ measurement sites are shown. The red line superimposed to the aerial photograph indicates the axis of the flight lines.</figcaption></figure>
## 2.2. The measurement sites

Field measurements of surface temperature, water content of the soil surface and of the litter (or mulch) were be performed over 17 forest stands (Fig. 2 and Table 2) and one agricultural field.

<table><caption>**Table 2-** Description of the 17 selected measurement sites of the Agre forest (data from ONF), and of the agricultural field (AgF). The forest stand numbers correspond to those of Fig. 2.</caption><thead><tr><th>Stand</th><th>Latitude (°E)</th><th>Longitude (°N)</th><th>Description</th></tr></thead><tbody><tr><td>AgF 13</td><td>43.967610<br>43.971363</td><td>1.2732225<br>1.2649337</td><td>Lacoyne agricultural field. Harvested maize field with crop residues.<br>Very dense bushes of sessile oak (&lt;3 m height, 16-22 years, &lt;5 cm diameter, 3000 per ha). Representative of stands No 3, 4, 13.</td></tr><tr><td>17</td><td>43.967320</td><td>1.2579452</td><td>Plantation of Corsican pine (10-20 m, 15-27 years, 12-20 cm diameter, 1100-1600 per ha). One strip every 3 m. Very dense herbaceous undergrowth. Representative of stands 17, 47-2, 85, 95, 105, 116, 119, 122-1, 123, 133-3, 134, 142-2.</td></tr><tr><td>18</td><td>43.968398</td><td>1.2621968</td><td>Thinned out sessile oak forest. Few seed trees (30 to 100 per ha) were left in order to obtain a natural regeneration of the forest. Very dense bushes of either sessile oak or hornbeam have grown between the seed trees (&gt;10 years, &gt;2 m height). Representative of stands No 1, 2, 6, 10, 14, 16, 18, 31-1.</td></tr><tr><td>26</td><td>43.961021</td><td>1.2659672</td><td>Timber of Corsican pine (&lt;25 m, 34-40 years, &gt;20 cm diameter, 150-1400 per ha). Strips (one every 3 m) perpendicular to the forest axis. Representative of stands 20, 26, 38, 51.</td></tr><tr><td>30</td><td>43.958065</td><td>1.2663617</td><td>Mature timber of sessile oak (&gt;20 m height, &gt;90 years, &gt;40 cm diameter, 70-150 per ha), low density (open canopy), little undergrowth vegetation. Representative of stands No 31-2, 29, 30, 36, 37, 42, 43, 49, 52.</td></tr><tr><td>34</td><td>43.960481</td><td>1.2748727</td><td>Coppice of sessile oak (&gt;16 m height, 40-60 years, &gt;15 cm diameter, 150 per ha). Representative of stands No 31-2, 34, 35-1, 62-2, 83-2, 83-3, 86-1, 110-3, 118-2, 131-2, 138.</td></tr><tr><td>52</td><td>43.952289</td><td>1.2745096</td><td>Mature timber of sessile oak (&gt;20 m height, &gt;90 years, &gt;40 cm diameter, 70-150 per ha), low density (open canopy), little undergrowth vegetation. Thick litter layer. Representative of stands No 31-2, 29, 30, 36, 37, 42, 43, 49, 52.</td></tr><tr><td>87</td><td>43.943006</td><td>1.2791123</td><td>Timber of sessile oak (&gt;20 m height, &gt;40 years, &gt;25 cm diameter, 150-400 per ha). Representative of stands No 5, 44, 45, 70, 75, 76, 77, 81, 82, 87, 88.</td></tr><tr><td>106</td><td>43.938703</td><td>1.2914132</td><td>Bushes of sessile oak (&lt;6 m height, 15-30 years, &lt;5 cm diameter, 2000 per ha). Representative of stands No 8, 84-2, 89, 90, 96, 97-1, 106, 113, 114, 115.</td></tr><tr><td>109</td><td>43.936814</td><td>1.2825343</td><td>Coppice of sessile oak with timber (&gt;20 m height, &gt;60 years, &gt;25 cm diameter, 250-400 per ha). Representative of stands No 71, 72, 78, 79, 80, 83-1, 109, 117, 118, 136, 137, 138, 139.</td></tr><tr><td>111</td><td>43.935734</td><td>1.2760869</td><td>Undisturbed mature timber of sessile oak (&gt;20 m height, &gt;90 years, &gt;40 cm diameter, 70-150 per ha), with younger trees forming a relatively dense high coppice. Representative of stands No 9, 12, 15, 19-3, 27, 28, 44, 46, 48, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62-1, 64, 65-1, 67-1, 68, 73, 84, 86-2, 99, 100, 101, 102, 107, 108, 110, 111, 120, 121.</td></tr><tr><td>124</td><td>43.934355</td><td>1.2867786</td><td>Coppice of sessile and english oak with timber (&gt;22 m height, 90-140 years, &gt;40 cm diameter, 70-150 timber per ha). Representative of stands No 19-1, 50-1, 98, 112, 124.</td></tr><tr><td>125</td><td>43.934975</td><td>1.2897290</td><td>Plantation of english oak (&gt;5 m height, 18-22 years, &gt;4 cm diameter, 2000 per ha). Representative of stands No 7, 11, 107-2, 125, 126, 127, 129, 144.</td></tr><tr><td>132</td><td>43.930349</td><td>1.2845516</td><td>Timber of red oak (&lt;25 m height, 15-80 years, 2-50 cm diameter, 1000-2000 per ha). Representative of stands No 21-2, 44-2, 45-2, 69-1, 74, 128, 132.</td></tr><tr><td>133</td><td>43.929141</td><td>1.2822351</td><td>Young plantation of Atlas cedar (&lt;20 m, 15 years, &lt;20 cm diameter, 1600 per ha). Representative of stands 122-2, 133-2, 142-1.</td></tr><tr><td>134</td><td>43.930697</td><td>1.2781405</td><td>Plantation of Corsican pine (10-20 m, 15-27 years, 12-20 cm diameter, 1100-1600 per ha). One strip every 3 m. Very dense herbaceous undergrowth. Representative of stands 17, 47-2, 85, 95, 105, 116, 119, 122-1, 123, 133-3, 134, 142-2.</td></tr><tr><td>135</td><td>43.929856</td><td>1.2750696</td><td>Thinned plantation of Corsican pine (10-20 m, 15-27 years, 12-20 cm diameter, 700-800 per ha). Well-defined strips (one every 3 m with one over five lacking). Very dense herbaceous undergrowth. Representative of stands 135, 140, 141, 143 (strips perpendicular to the forest axis). Representative of stands 91, 92, 93, 94 (strips parallel to the forest axis).</td></tr></tbody></table>
## 2.3. The measurement method

### 2.3.1. Soil temperature

At each forest site, one automatic probe was placed horizontally at the soil-litter interface. The temperature was recorded hourly and a continuous time series was obtained. The measurements started one day before the flight. At the agricultural site, a small gutter was digged at the soil surface. The probe was placed at the soil surface, into the gutter, and covered with a soil layer of a few mm. Before the experiment, all the probes were intercalibrated in order to unb bias the obtained temperature values.

### 2.3.2. Surface soil moisture

At each site, the surface soil moisture (0 to 6 cm depth) was measured by using two methods: gravimetric and FDR (frequency domain reflectometry). At stand N°30, four additional gravimetric measurements were performed in order to detail the soil moisture profile of the top soil layer (from the surface down to 10 cm).

The gravimetric method consisted in collecting one cylindrical soil sample at each site by using a steel cylinder of a volume of 4.352 10⁻⁴ m³. After collecting the soil sample, it was placed into an air-tight plastic bag. The soil sample was oven-dried at 105 °C during 48h in a ventilated oven. The sample's weight was measured before and after the drying in order to estimate the gravimetric water content and the dry bulk density. The soil volumetric water content was obtained by multiplying the two quantities.

The additional detailed measurements of stand N°30 were made by collecting different soil layers of a thickness of 1 cm (and an additional layer of 0.5 cm at the soil surface). The same oven-drying procedure as before was applied but the volumetric moisture values were obtained by using the cylinder-derived bulk density values.

The FDR measurements were made by using a portable ThetaProbe (Delta-T devices). Five to twelve measurements were made for each site.

### 2.3.3. Surface soil texture

All the dried cylinder samples were sent to INRA-Laboratoire d'Analyse des Sols, Arras, France, for a granulometric analysis.

### 2.3.4. Litter water content

The litter samples were collected by using a 0.5×0.5 m² frame. The frame was placed onto the surface, and all the coherent organic elements located inside the frame were collected in an air-
tight plastic bag. The litter sample was oven-dried at 60 °C during 48h in a ventilated oven. The sample's weight was measured before and after the drying in order to estimate the water content and the dry matter of the litter.

### 3. Results

Table 3 summarises the obtained values of soil temperature and soil and litter water content, together with soil texture. The agricultural site presents the highest values of bulk density and surface soil water content ($m_v$). The high density value observed at this site is representative of the typical soil compaction of agricultural soils. The forest sites present much lower values: for 7 of them, the density is lower than 1000 kg m$^{-3}$, indicating a large macroporosity.

<table><caption>**Table 3-** Obtained values of surface soil temperature and soil moisture ($T_s$ and $m_v$, respectively), and litter water content, dry matter, and relative moisture content on 20 November 2001 at 1300 LST, over the 17 selected measurement sites of the Agro forest, and over the agricultural field (AgF). The soil texture (i.e. the percentage in mass of clay, fine silt, coarse silt, fine sand, and coarse sand particles) is detailed. The forest stand numbers correspond to those of Fig. 2.</caption><thead><tr><th>Stand</th><th>Ts (°C)</th><th>Average<br>$m_v$<br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>Litter<br>water<br>content<br>(kg m<sup>-2</sup>)</th><th>Litter<br>dry<br>matter<br>(kg m<sup>-2</sup>)</th><th>Litter<br>relative<br>moisture<br>content</th><th>Soil<br>dry<br>bulk<br>density<br>(kg m<sup>-3</sup>)</th><th>Clay<br>(%)</th><th>Fine<br>silt<br>(%)</th><th>Coarse<br>silt<br>(%)</th><th>Fine<br>sand<br>(%)</th><th>Coarse<br>sand<br>(%)</th></tr></thead><tbody><tr><td>AgF</td><td>6.81</td><td>0.35</td><td>0.99</td><td>0.70</td><td>0.59</td><td>1487</td><td>18.5</td><td>21.2</td><td>30.8</td><td>16.9</td><td>12.6</td></tr><tr><td>13</td><td>6.94</td><td>0.18</td><td>0.93</td><td>0.67</td><td>0.58</td><td>909</td><td>19.3</td><td>27.9</td><td>30.3</td><td>13.3</td><td>9.2</td></tr><tr><td>17</td><td>5.81</td><td>0.15</td><td>1.55</td><td>1.77</td><td>0.47</td><td>776</td><td>19.5</td><td>24.8</td><td>23.3</td><td>12.5</td><td>19.9</td></tr><tr><td>18</td><td>7.81</td><td>0.16</td><td>1.21</td><td>1.30</td><td>0.48</td><td>548</td><td>23.7</td><td>27.2</td><td>20.9</td><td>12.3</td><td>15.9</td></tr><tr><td>26</td><td>7.56</td><td>0.12</td><td>2.34</td><td>1.48</td><td>0.61</td><td>1067</td><td>17.9</td><td>30.7</td><td>33.6</td><td>13.4</td><td>4.4</td></tr><tr><td>30</td><td>7.69</td><td>0.16</td><td>1.14</td><td>0.89</td><td>0.56</td><td>1002</td><td>17.5</td><td>28.7</td><td>33.0</td><td>13.4</td><td>7.4</td></tr><tr><td>34</td><td>8.13</td><td>0.19</td><td>1.79</td><td>1.91</td><td>0.48</td><td>613</td><td>24.7</td><td>26.7</td><td>26.1</td><td>13.1</td><td>9.4</td></tr><tr><td>52</td><td>6.88</td><td>0.17</td><td>2.71</td><td>4.62</td><td>0.37</td><td>833</td><td>17.7</td><td>26.2</td><td>29.3</td><td>16.4</td><td>10.4</td></tr><tr><td>87</td><td>7.75</td><td>0.19</td><td>1.21</td><td>0.94</td><td>0.56</td><td>1044</td><td>13.9</td><td>24.8</td><td>32.0</td><td>17.6</td><td>11.7</td></tr><tr><td>106</td><td>6.88</td><td>0.19</td><td>1.51</td><td>1.21</td><td>0.56</td><td>947</td><td>17.3</td><td>27.0</td><td>32.3</td><td>16.1</td><td>7.3</td></tr><tr><td>109</td><td>6.88</td><td>0.16</td><td>0.94</td><td>1.34</td><td>0.41</td><td>1302</td><td>19.3</td><td>25.1</td><td>26.6</td><td>12.6</td><td>16.4</td></tr><tr><td>111</td><td>7.63</td><td>0.19</td><td>1.20</td><td>1.16</td><td>0.51</td><td>1186</td><td>19.2</td><td>22.4</td><td>28.2</td><td>17.2</td><td>13.0</td></tr><tr><td>124</td><td>7.56</td><td>0.15</td><td>1.32</td><td>2.04</td><td>0.39</td><td>1058</td><td>11.6</td><td>23.1</td><td>34.5</td><td>19.8</td><td>11.0</td></tr><tr><td>125</td><td>7.13</td><td>0.16</td><td>1.97</td><td>1.75</td><td>0.53</td><td>1112</td><td>11.2</td><td>25.6</td><td>33.1</td><td>18.9</td><td>11.2</td></tr><tr><td>132</td><td>7.25</td><td>0.19</td><td>1.31</td><td>1.53</td><td>0.46</td><td>941</td><td>17.0</td><td>24.9</td><td>28.3</td><td>13.6</td><td>16.2</td></tr><tr><td>133</td><td>7.44</td><td>0.13</td><td>1.88</td><td>1.25</td><td>0.60</td><td>1202</td><td>17.2</td><td>24.6</td><td>27.4</td><td>15.0</td><td>15.8</td></tr><tr><td>134</td><td>8.94</td><td>0.19</td><td>3.42</td><td>4.42</td><td>0.44</td><td>1069</td><td>16.4</td><td>27.4</td><td>29.7</td><td>11.9</td><td>14.6</td></tr><tr><td>135</td><td>7.69</td><td>0.16</td><td>2.37</td><td>2.34</td><td>0.50</td><td>1248</td><td>16.7</td><td>25.4</td><td>29.5</td><td>12.4</td><td>16.0</td></tr></tbody></table>
The much lower $m_v$ values observed for the forest sites are due to the hydrophobic effect of the forest: a rain occured on 19 November 2002 and the majority of the precipitation was either intercepted by the forest canopy and evaporated, or absorbed by the litter layer, thus preventing the soil surface from rewetting. Fig. 4 shows that there is a good agreement between the FDR and gravimetric methods to measure soil moisture (the root mean square difference is 0.03 $m^3 m^{-3}$). Fig. 5 presents the soil moisture profiles obtained for stand N°30. This figure shows that the average 0-6 cm surface soil moisture values of Fig. 4 corresponded to non-uniform profiles. Whereas the average soil moisture dis not exceed 0.19 $m^3 m^{-3}$ for forest stands (0.16 $m^3 m^{-3}$ for stand N°30), much higher values were observed at the top soil layer of stand N° 30 (up to 0.16 $m^3 m^{-3}$). Conversely, $m_v$ was very dry below 6 cm ($m_v$ was about 0.10 $m^3 m^{-3}$ at 10 cm).

<figure><img src="image_5.png" /><figcaption>**Fig. 4-** Comparison of the surface soil moisture ($m_v$) measured by the gravimetric method and by frequency domain reflectometry (FDR). The box, the circles and the pluses correspond to the agricultural field, the broadleaf and the coniferous stands, respectively.</figcaption></figure>

The harvested agricultural site presented crop residues. The coverage of these crop residues was rather low, and their dry matter was lower than the forest stand's litters. However, the water content of these residues approached 1 kg m⁻², which was more than the value obtained for stands N°13 and 109.
The water content of the forest stand's litters was high, up to more than 3 kg m⁻² (stand N°134). Fig. 6 shows that the litter of stand N° 134 consisted of dead needles and mosses, which may absorb a lot of water. There is a good relationship between the litter dry mass and water content because the relative moisture content was close to 0.5 (Table 3).

The surface temperature during the flight and during a 2-day period including the flight-time is presented in Table 3 and Fig. 7, respectively. The range of observed Ts at flight-time was large (from about 6 to 9°C). It was larger than the day before, perhaps because air temperature dropped on 20 November 2001 (the maximum value was about 4°C) and the warmer soil surface cooled at different rates from one stand to another.

<table><caption>**Table 4-** Obtained values of the surface soil moisture (m<sub>v</sub>), as measured by two methods: frequency domain reflectometry (FDR) and gravimetric. In the first case, a portable ThetaProbe instrument was used. In the second case, cylindrical soil samples of known volume were collected, weighed and oven-dried. The forest stand numbers correspond to those of Fig. 2.</caption><thead><tr><th>Stand</th><th>FDR<br>m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>FDR<br>m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>FDR<br>m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>FDR<br>m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>FDR<br>m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>FDR<br>m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>FDR<br>m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>FDR<br>m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>FDR<br>m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>FDR<br>m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th><th>Gravimetric m<sub>v</sub><br>(m<sup>3</sup>m<sup>-3</sup>)</th></tr></thead><tbody><tr><td>AgF</td><td>0.35</td><td>0.40</td><td>0.39</td><td>0.42</td><td>0.34</td><td>0.37</td><td>0.33</td><td>0.33</td><td>0.34</td><td>0.34</td><td>-</td><td>0.31</td></tr><tr><td>13</td><td>0.20</td><td>0.20</td><td>0.18</td><td>0.18</td><td>0.17</td><td>0.14</td><td>0.14</td><td>0.19</td><td>0.23</td><td>0.18</td><td>0.17</td><td>0.17</td></tr><tr><td>17</td><td>0.13</td><td>0.16</td><td>0.15</td><td>0.18</td><td>0.17</td><td>0.15</td><td>0.16</td><td>0.15</td><td>0.16</td><td>0.15</td><td>-</td><td>0.13</td></tr><tr><td>18</td><td>0.12</td><td>0.14</td><td>0.15</td><td>0.23</td><td>0.21</td><td>0.18</td><td>0.15</td><td>0.20</td><td>0.21</td><td>0.16</td><td>-</td><td>0.13</td></tr><tr><td>26</td><td>0.10</td><td>0.10</td><td>0.12</td><td>0.12</td><td>0.10</td><td>0.10</td><td>0.10</td><td>0.12</td><td>0.12</td><td>0.17</td><td>-</td><td>0.13</td></tr><tr><td>30</td><td>0.10</td><td>0.10</td><td>0.10</td><td>0.10</td><td>0.19</td><td>0.19</td><td>0.23</td><td>0.23</td><td>0.20</td><td>0.20</td><td>0.16</td><td>0.16</td></tr><tr><td>34</td><td>0.25</td><td>0.25</td><td>0.18</td><td>0.18</td><td>0.18</td><td>0.18</td><td>0.16</td><td>0.16</td><td>0.20</td><td>0.20</td><td>-</td><td>0.16</td></tr><tr><td>52</td><td>0.17</td><td>0.17</td><td>0.18</td><td>0.18</td><td>0.16</td><td>0.16</td><td>0.21</td><td>0.21</td><td>0.20</td><td>0.20</td><td>-</td><td>0.12</td></tr><tr><td>87</td><td>0.14</td><td>0.13</td><td>0.14</td><td>0.18</td><td>0.19</td><td>0.27</td><td>0.25</td><td>0.24</td><td>0.25</td><td>0.25</td><td>-</td><td>0.15</td></tr><tr><td>106</td><td>0.21</td><td>0.21</td><td>0.15</td><td>0.15</td><td>0.15</td><td>0.15</td><td>0.24</td><td>0.24</td><td>0.21</td><td>0.21</td><td>0.17</td><td>0.20</td></tr><tr><td>109</td><td>0.17</td><td>0.17</td><td>0.16</td><td>0.13</td><td>0.13</td><td>0.14</td><td>0.14</td><td>0.19</td><td>0.19</td><td>0.16</td><td>0.16</td><td>0.16</td></tr><tr><td>111</td><td>0.17</td><td>0.17</td><td>0.15</td><td>0.15</td><td>0.17</td><td>0.17</td><td>0.18</td><td>0.25</td><td>0.25</td><td>0.17</td><td>-</td><td>0.19</td></tr><tr><td>124</td><td>0.16</td><td>0.14</td><td>0.12</td><td>0.15</td><td>0.15</td><td>0.14</td><td>0.14</td><td>0.14</td><td>0.16</td><td>0.13</td><td>-</td><td>0.15</td></tr><tr><td>125</td><td>0.17</td><td>0.17</td><td>0.18</td><td>0.18</td><td>0.18</td><td>0.16</td><td>0.16</td><td>0.14</td><td>0.17</td><td>0.17</td><td>-</td><td>0.13</td></tr><tr><td>132</td><td>0.19</td><td>0.19</td><td>0.22</td><td>0.22</td><td>0.21</td><td>0.22</td><td>0.19</td><td>0.18</td><td>0.18</td><td>0.19</td><td>0.19</td><td>0.17</td></tr><tr><td>133</td><td>0.16</td><td>0.16</td><td>0.15</td><td>0.15</td><td>0.14</td><td>0.12</td><td>0.12</td><td>0.13</td><td>0.13</td><td>0.14</td><td>-</td><td>0.11</td></tr><tr><td>134</td><td>0.19</td><td>0.19</td><td>0.19</td><td>0.19</td><td>0.19</td><td>0.19</td><td>0.17</td><td>0.17</td><td>0.21</td><td>0.21</td><td>0.18</td><td>0.19</td></tr><tr><td>135</td><td>0.15</td><td>0.15</td><td>0.14</td><td>0.15</td><td>0.15</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>0.17</td></tr></tbody></table>
<figure><img src="image_6.png" /><figcaption>**Fig. 5-** Surface soil moisture (m_v) profiles obtained on stand N°30 by using the gravimetric method at four locations.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 6-** Comparison of the litter dry matter with the litter water content. The box, the circles and the pluses correspond to the agricultural field, the broadleaf and the coniferous stands, respectively. The photographs correspond to (from top to bottom): stands N°52 and 134, agricultural field, stand N°13.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 7-** Measured temporal evolution of the surface soil temperature over the considered 17 stands of the Agre forest and agricultural field (solid and dashed thick lines, respectively). The vertical dashed line indicates the time of the flight.</figcaption></figure>
## 4. Conclusion

Over the land surfaces, the main objectives of the EUROSTARRS campaign were achieved. In particular, it will be possible to improve the modelling of the L-band emissivity over a variety of surface types for soil moisture retrievals, and, more specifically:

* To characterise the dependence of vegetation optical depth as a function of incidence angle for a variety of vegetation covers (in particular very few information is available on the attenuation properties of forests and natural vegetation covers).

* These results are of key importance to parameterise the vegetation models, which are used in the soil moisture retrieval process. Also, over forests, it will be possible to evaluate the potential interest of optical depth to retrieve forest characteristics such as branch biomass or total biomass.

During EUROSTARRS, we tried to characterise the variables which presented rapid changes, such as soil surface temperature and water content. Slow-varying characteristics such as the forest biomass are available from previous forest inventory (Inventaire Forestier National, Nogent-sur-Vernisson, France).

## ACKNOWLEDGEMENTS

Many thanks to the staff of Office National des Forêts, who provided useful maps and data about the Agre forest: Patrice Mengin-Lecreulx, Denis Laurens, Michel Morel, Thierry Sardin, and Jean-Pierre Deruelles. Jérôme Pignard, with Inventaire Forestier National, provided the biomass data.
# Appendix A

Photographs of stands N°30, 111 and 17 of the Agre forest.
## Stands No 30

Mature timber of sessile oak (>20 m height, >90 years, >40 cm diameter, 70-150 per ha), low density (open canopy), little undergrowth vegetation.

Representative of stands No 31-2, 29, 30, 36, 37, 42, 43, 49, 52.

<figure><img src="image_9.png" /></figure>

15 November 2001
Stand No 111

Undisturbed mature timber of sessile oak (>20 m height, >90 years, >40 cm diameter, 70-150 per ha), with younger trees forming a relatively dense high coppice.

Representative of stands No 9, 12, 15, 19-3, 27, 28, 44, 46, 48, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62-1, 64, 65-1, 67-1, 68, 73, 84, 86-2, 99, 100, 101, 102, 107, 108, 110, 111, 120, 121.

<figure><img src="image_10.png" /></figure>

15 November 2001
Stand No 17

Plantation of Corsican pine (10-20 m, 15-27 years, 12-20 cm diameter, 1100-1600 per ha). One strip every 3 m. Very dense herbaceous undergrowth.

Representative of stands 17, 47-2, 85, 95, 105, 116, 119, 122-1, 123, 133-3, 142-2.

<figure><img src="image_11.png" /></figure>

2 October 2001