# Evolution of microclimate following small patch de-sealing and revegetation in urban context

Antoine Morel, Laure Vidal Beaudet, Lydia Brialix, Dominique Lemesle, Adeline Bulot, Sophie Herpin

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Antoine Morel, Laure Vidal Beaudet, Lydia Brialix, Dominique Lemesle, Adeline Bulot, et al. Evolution of microclimate following small patch de-sealing and revegetation in urban context. Urban Climate, 2025, 61, pp.102371. 10.1016/j.uclim.2025.102371. hal-05023368

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# Evolution of microclimate following small patch de-sealing and revegetation in urban context

Antoine Morel<sup>a,b,c,*</sup>, Laure Vidal-Beaudet<sup>a,b</sup>, Lydia Brialix<sup>a,b</sup>, Dominique Lemesle<sup>a,b</sup>, Adeline Bulot<sup>c</sup>, Sophie Herpin<sup>a,b</sup>

## ARTICLE INFO

Keywords:

De-sealing

Soil construction

Urban greening

Urban microclimate

Microclimate measurements

Thermal comfort modeling

## ABSTRACT

This research explores the impact of de-sealing a 64 m² patch (Oasis) after introducing indigenous vegetation, four trees and herbaceous species, on local microclimate conditions in an urban context. Over two years, microclimate variables were monitored and thermal comfort was evaluated with the Universal Thermal Climate Index (UTCI). The Oasis experienced a noticeable reduction in Ts compared to surrounding asphalted areas, with a daily mean difference in maximum surface temperatures of 18.4 °C the first summer and 23.0 °C the second, attributed to the development of the herbaceous layer, which covered 60 % of the Oasis three months after sowing and above 90 % from the second year. This Ts is partly responsible for improving thermal comfort during the day. Tree shading induced further local cooling, with a decrease of up to 8 °C of UTCI in shaded areas. No evidence of lower nocturnal Ta compared to sealed reference was shown, possibly due to the small size of the patch and air mixing with the surroundings. However, as the low vegetation grew, the nocturnal de-sealed patch Ta got closer to mature meadow values. Modeling using UMEP and SOLWEIG projected further improvements in thermal comfort in the coming years during the daytime considering future tree dimensions. During nighttime, the trees would induce a significant radiative trapping. This study demonstrates that small-scale de-sealing and revegetation projects can provide meaningful microclimate improvements in urban environment as early as the first year. In the following years, the development of vegetation further helps cities to adapt to climate change.

## 1. Introduction

The Intergovernmental Panel on Climate Change (IPCC) states with a high level of confidence that extreme temperatures and heat waves have intensified in cities, and the vulnerability of populations will continue to increase with further warming (Pörtner et al., 2022). On a global scale, the proportion of the world's population living in cities is expected to reach 68 % in 2050 (United Nations, 2018). The common actual form of urbanization induces the artificialization of natural land, and in European countries, this urban sprawl represented an average annual increase of 429 km² between 2006 and 2015 (EEA, 2021). Combined with more and more frequent extreme temperatures and the urban heat island effect (Taha, 1997), growing urbanization will lead to a local increase of heat

<footnote><sup>a</sup> Institut Agro, EPHOR, 49100 Angers, France</footnote>

<footnote><sup>b</sup> IRSTV, FR CNRS 2488, 44321 Nantes Cedex, France</footnote>

<footnote><sup>c</sup> Institut Agro, Ecole Supérieure des Agricultures, INRAE, BAGAP, 49100 Angers, France</footnote>

<footnote>* Corresponding author at: Institut Agro, EPHOR, 49100 Angers, France.</footnote>

<footnote>E-mail address: antoine.morel@agrocampus-ouest.fr (A. Morel).</footnote>
Notations

<table><tr><td>ET</td><td>Evapotranspiration</td></tr><tr><td>MRT</td><td>Mean radiant temperature [°C]</td></tr><tr><td>RH</td><td>Relative humidity [%]</td></tr><tr><td>SVF</td><td>Sky view factor</td></tr><tr><td>Ta</td><td>Air temperature [°C]</td></tr><tr><td>Ts</td><td>Surface temperature [°C]</td></tr><tr><td>UHI</td><td>Urban heat island</td></tr><tr><td>UTCI</td><td>Universal Thermal Climate Index [°C]</td></tr><tr><td>VWC</td><td>Volumetric water content [m³/m³]</td></tr></table>

waves severity (Masson-Delmotte et al., 2021). The restoration of sealed soil and the process of revegetation are among the solutions put forward to reduce the impact of urbanization on the environment, while improving quality of life and enabling adaptation to global changes. In this sense, de-sealing is gaining increasing attention from urban planners, as shown by Vieillard et al. (2024). This author's definition of de-sealing is based on the European Commission (2012), Maienza et al. (2021) and Tobias et al. (2018) studies: "removing the impermeable covering layer, decompacting and replacing the underlying materials or restructuring the soil profile in order to re-establish an effective connection with the subsoil and develop a new soil use or to replace it by a more permeable cover". De-sealing is often associated with revegetation, which consists of introducing any type of vegetation (trees, shrubs, or herbaceous plants) where it was no longer present. Indeed, soil de-sealing could improve local microclimate conditions through water infiltration and increased evapotranspiration (ET), while tree planting can provide shading for thermal comfort improvement. This is why soil de-sealing is a solution increasingly encouraged by public policies to achieve 'no net land take' by 2050 in accordance with the European Green Deal and the Soil Thematic Strategy in Europe (European Commission, 2020).

There is an increasing interest in the subject of urban vegetation and its impact on thermal comfort (He et al., 2023; Jin et al., 2024), studying street tree alignments (Wang and Akbari, 2016), grass parking areas (Bouzouidja et al., 2021b), green roof (Coutts et al., 2013) or else urban parks (Park et al., 2017; Das et al., 2022; Bodenan et al., 2023). If herbaceous vegetation can reduce *Ta* (Bowler et al., 2010) through transpiration, the induced increase in water vapor content can limit the improvement of thermal comfort. Considering trees, their shading increases thermal comfort during the day (Bowler et al., 2010) depending on various parameters. However, they may also reduce radiative cooling from the ground to the atmosphere during the night (Taleghani et al., 2014). As for herbaceous vegetation, it induces a heat mitigation during the day and the night (Takebayashi and Moriyama, 2009). The combination of grass and trees further reduces heat stress, creating comfortable thermal conditions throughout the day (Shashua-Bar et al., 2011). Yet, these studies about urban greening do not explicitly imply de-sealing, which means they do not take into account soil construction (and soil moisture capacity) and vegetation development over time.

There are a few examples of de-sealing and revegetation studies prompted by urban planning with a temperature mitigation objective. Several studies focus on methods for prioritizing such areas (Gupta and De, 2024; Herpin et al., 2022). One of the most known projects that produced climate data is the Garibaldi street in Lyon, France, where a 3- to 5-lane boulevard completely sealed except for existing rows of trees was transformed into a greener space with herbaceous plants, shrubs and more trees. Renard et al. (2019) used remote sensing to evaluate the evolution of the surface temperatures (*Ts*) before (2001 to 2005) and after (2013 to 2016) the de-sealing. No evidence of *Ts* decrease was shown by this study. Meteorological sensors were installed after the de-sealing in 2018 to compare conditions between the vegetated and sealed sections of the street. Around solar noon between June and August 2020, 2021 and 2022, the vegetated sections showed a *UTCI* reduction by 3 to 5 °C, compared to sealed sections (Ségur and Bensaoud, 2023). This difference is at least partly due to the pre-existing trees. In the Paris suburbs, in Aubervilliers, another de-sealing project called the "Tertiary forest" converted a parking surface of 1200 m² to a treed space (72 trees of 4 m height: *Acer campestre* L. (SAPINDACEAE), *Quercus ilex* L. (FAGACEAE), *Ulmus 'Resista' (ULMACEAE*) – with a mineral but permeable ground. Local measurements were processed before (2018) and after (in 2020 and 2021) the operation (Parison et al., 2023). *UTCI* decreased by 3.1 °C on a 24 h-average radiative day, especially thanks to the tree shading during daytime. Another Parisian project, called the "cours Oasis" (Oasis courtyards), aims at replacing sealed pavement in primary schools by altering materials and new vegetation planting. The original goal was to achieve 20 to 30 % vegetation in every courtyard, along with the use of woodchips, light-colored asphalt, porous concrete, or permeable asphalt to replace the existing asphalt. However, in some cases, these objectives could not be fully achieved. Ten courtyards were instrumented between 2019 and 2022 to evaluate the microclimate impacts of the de-sealing. The results are hardly comparable as each of the courtyard is unique but overall, the 24 h average *Ta* were a little bit increased (0.11 to 0.56 °C) for 6 of the 10 courtyards (Karam, 2023) probably due to the use of materials with higher albedos. Modeling has also already been used for evaluating revegetation projects in cities (Cortinovis et al., 2022; Morille and Musy, 2017). Some authors used models to evaluate the effect of urban park configuration (size, composition, shape, etc.) on their cooling ability. For example, Yu et al. (2023) used ENVI-met to assess the impact of 15 different plaza designs on thermal comfort, each with varying configurations of water bodies, buildings, and green spaces. Local weather data can also be used in predictive models (Yang et al., 2024, Yang and Wang, 2023).

As shown, the few studied de-sealing projects focusing on microclimate impacts mix the establishment of vegetated areas with the use of alternative materials to asphalt (woodchips, porous or light surface covering), which makes it difficult to assess the effect of revegetation process itself. Also, the constructed soil is rarely considered, though it can affect the water availability that supports plant
transpiration. The consideration of all soil and plant (trees, herbaceous spontaneous or introduced plants) effect on thermal comfort is even more scarce. Finally, based on current knowledge, no study has monitored vegetation (both herbaceous and tree layers) development and microclimate evolution over several consecutive years after de-sealing at a fine scale.

Based on the existing gaps in the current literature, the main objective of this study is to evaluate the effect of a de-sealing and subsequent vegetation development on the evolution over time of local microclimate and thermal comfort.

To do so, this study focuses on the de-sealing of a small patch in a parking lot in Angers, France, that reuses part of the excavated material for soil construction. Indigenous trees were planted and indigenous herbaceous species were sown, followed with minimum management. This site was instrumented and monitored for two years, in order to assess experimentally the effect of de-sealing and vegetation development on local microclimate. The use of a numerical model, SOLWEIG, is then proposed to estimate the benefits in the longer term.

## 2. Materials and methods

### 2.1. Studied site

#### 2.1.1. Site location

Angers is a city of 150,000 inhabitants in western France (Cfb Köppen climate), with a typical Urban Heat Island (UHI) (maximum overheating of 2.9 °C) for a medium-sized French city (Gardes et al., 2020). The site is located on the outskirts of Angers where the UHI intensity is quite low (0.7 °C) at night, in a logistic site occupied by local public transport company (Fig. 1). The site is located close to the highway and is a transitional space, on the boundary between a business park, a residential area and fields (Fig. 1b). On a 500 m diameter buffer zone around the site, the impervious rate is 56 % while on average in European union the average sealing rate in cities was 42 % in 2018 (European Commission, 2022). Bocher et al. (2018) work on Reference Spatial Unit (RSU) through the MApUCE project locates the site in an urban typology called “industrial building”. According to the Local Climate Zone (LCZ) classification, the site is mainly included in a “large low rise” zone (Demuzere et al., 2019), which correspond to a mostly sealed surface with large low-level buildings and few or no trees (Stewart and Oke, 2012). Such large sealed areas on the outskirts of major cities are characterized by high Ts during the day, as shown by the Earth Resources Observation and Science (EROS) Center (2019). They are also more prone to de-sealing than city centers, which are very constrained in terms of space and use.

This public transport logistics site itself is largely sealed, with a car parking lot, several bus parking areas, bus lanes and an office space. The experimental set-up, called hereafter the “Oasis”, consist in a de-sealed area of 64 m² in the car parking lot. In October 2022, six adjacent parking lot places were de-sealed to create a vegetated area (Fig. 2). The company intends to create a green cool area accessible to employees.

#### 2.1.2. Soil construction

A soil profile was described until 1.1 m depth (Fig. 3). The first 10 cm was an asphalt sealed cover laying on 10 cm of untreated gravel. The third layer (between 20 and 70 cm depth) was compacted rubble (coarse sandy soil associated with large coarse elements of 10 to 20 cm in diameter) separated from the soil in place by a geotextile. To install the experimental set-up, the first 20 cm were removed, and replaced by topsoil (sandy-clay silt) normally used for urban landscaping (T1), to form a homogeneous surface. Underneath, between 15 and 70 cm depth (T2), the constructed layer was a mixture of the decompacted rubble and imported topsoil (proportions 50/50 v/v). Finally, the deepest layer, below 70 cm depth (T3), composed of the original soil, was only decompacted,

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Left: intensity of Urban Heat Island in Angers urban area: simulation data from the MAPUCE project with night-time Ta difference in a summer situation (situation 2: light northerly wind, adapted from Bocher et al., 2018; Gardes et al., 2020); right: context around the studied site.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Left: View of the future Oasis before de-sealing from the north (photograph taken on July 26th, 2022); right: View from the north of the Oasis after the de-sealing with no vegetation (photograph taken on October 5th, 2022).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Left: Pedological description of the soil before de-sealing (photograph taken on July 27th, 2022), consisting of the original soil in depth and imported materials at the surface. Right: Constructed soil after the de-sealing of the Oasis, composed of topsoil at the surface, followed by a mixture of topsoil and rubble, and the decompacted original soil at depth.</figcaption></figure>

because this part was evaluated as suitable for the development of the rooting system.

This original method aims to up-cycle the rubble material, reducing dependency on non-renewable topsoil. The rubble was not disposed as non-hazardous or hazardous waste in landfill as usual. The construction of Technosols integrating in place material to reduce the use of topsoil is becoming a more frequently studied scientific question (Knoll et al., 2024; Maienza et al., 2021; Vieillard et al., 2023). The fine fraction of the T2 horizon can actually be considered as the topsoil itself since the reused compacted rubble mainly consists of coarse elements.

### 2.1.3. Vegetation

#### i. *Trees*

A single species was chosen for the planting of 4 trees to create a homogeneous area. The species choice is crucial given the observed mortality of urban trees (Hilbert et al., 2019). A first list of criteria for the choice of this specie was drawn up and compared thanks to the Sésame tool developed by the Cerema (Chrétien, 2019): (1) Drought resistance; (2) Shading and ability to adapt stomatal
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Left: View of the Oasis from the south, with a weather station visible at the center (photograph taken on July 17th, 2024); right: Oasis layout with associated trees and weather stations.</figcaption></figure>

opening; (3) Indigenousness; (4) Absence of superficial roots; (5) Medium size; (6) Ability to support dense soil (urban environment favors compaction). Additional criteria were identified based on Rahman et al. (2020) work on the relationship between species traits, site conditions (climate and land use) and transpiration and shading potential. Criteria considered to have a cooling impact include leaf colour and shape, crown shape and tree size. For these criteria, *Sorbus torminalis* (L.) Crantz (ROSACEAE) stood out as a satisfactory species for the study: simple dark green leaves, pyramidal habit and medium height. Its stemmed form is relatively rare, but seven-year-old specimens (average height 4 m, average crown diameter 1 m) were found in tree nurseries near the study site (30 km).

The trees were planted in staggered rows in December 2022 (Fig. 4) inside 1 m³ planting pits composed of topsoil with the aim to create favorable conditions for tree development, and hence for shading to improve local thermal comfort.

## ii. Herbaceous vegetation

The sowing was carried out in April 2023. The Oasis was divided into 12 plots, enabling to experiment the sowing of two mixes of seeds (sowing densities: 5 and 7 g/m²) as well as an option without sowing in order to let spontaneous vegetation develop. The plots were arranged in an alternating pattern on the Oasis.

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Flowchart describing the overall methodology.</figcaption></figure>
## 2.2. Instrumentation

Fig. 5 outlines the main steps of the methodology implemented in this study, which are further explained later in this section. Three main aspects are addressed: vegetation (including both herbaceous and tree layers), microclimate evolution, and modeling.

### 2.2.1. Soil and atmospheric compartments

Five weather stations were set up for this experiment from April 2023 (Fig. 6). Two stations are located on the Oasis (experimental zone): one on the southern side, always exposed to direct sunlight (De-sealed_south), and the other in the center, benefiting from the shade of the trees at some time of the day (De-sealed_tree). Two other stations are located in a control zone 200 m away from the Oasis. Both stations are located in typical land use situations and are used as positive and negative control to evaluate the effect of de-sealing and revegetation on the Oasis.

- A station on a meadow exposed to direct sunlight with no irrigation (Meadow);

- A station on a sealed asphalt surface exposed to direct sunlight (Asphalt). Asphalt is located 40 m south from Meadow. Its microclimate may thus show some influence of the grass ET occurring in the Meadow, depending on the speed and direction of the prevailing winds. The analysis was thus conducted for radiative days, with low wind speeds (see Section 2.2.3 and Appendix A).

Considering the length of the Oasis (9.4 m), the control zone is located at a distance equivalent to 21 times the Oasis's dimensions. This distance is sufficient to assume no mutual influence between the control and experimental zones, especially since we only consider radiative days with low wind speeds to study air measurements (see Section 2.2.3).

Finally, a fifth weather station is located 12 m north of the Oasis, on a sealed asphalt surface under an east-west treed alignment of Quercus palustris Münchh. (FAGACEAE) or Quercus rubra L. (FAGACEAE) (total height of 10 m, trunk height of 3 m, diameter crown of 9 m) (Sealed_tree). It is used as a reference for shading obtained from a mature tree alignment.

The simplified weather station compositions are presented in Table 1, with sensors both in the aerial compartment for microclimate variables, and in the soil compartment for ground Ts and soil water content measurement. Some sensors were shared among several weather stations due to their proximity: wind for instance was measured at a single location within each zone, north (Meadow and Asphalt) and south (Sealed_tree, De-sealed_tree, De-sealed_south). Rainfall was recorded at a single location. The relative humidity, Ta and globe temperature sensors height above ground level is 1.25 m to be as close as possible to the human gravity center, 1.1 m, height at which the UTCI reference environment is defined (Bröde et al., 2012), while being in the recommended range of 1.25 to 2 m for Ta measurements, as defined by international standards (World Meteorological Organization (WMO), 2023). Global radiation and wind were measured at 2 m height.

A station plan is presented in Fig. 7.

All sensors were not simultaneously installed, and the acquisitions show occasional interruptions. The installation and data acquisition timeline is presented in Appendix B. The measurement window for Asphalt is limited compared to the other weather stations as the Ts sensor was only installed in July 2023. Moreover, the Oasis is instrumented with only one Ts probe located at the de-sealed_south station. This measurement is considered representative of the Oasis as a whole. Soil TDR sensors were re-calibrated using in-situ soil sample collections followed by humidity analysis in the lab. The Mean Radiant Temperature (MRT) and Universal Thermal Climate Index (UTCI) were calculated using Huang et al. (2014) formula.

<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Left: location of the Oasis and reference weather stations on the parking lot; right: location of the two weather stations on the Oasis with the former land cover before de-sealing.</figcaption></figure>
<caption>**Table 1**</caption>

Simplified compositions of all five weather stations, with associated characteristics (height, measured variable, accuracy, unit, type) of the sensors categorized by compartment (air or soil).

<table><thead><tr><th></th><th>Height above ground</th><th>Variable</th><th>Accuracy</th><th>Unit</th><th>Sensor type</th><th>De-sealed_south</th><th>De-sealed_tree</th><th>Sealed_tree</th><th>Meadow</th><th>Asphalt</th></tr></thead><tbody><tr><td rowspan="6">Air</td><td rowspan="6">2 m</td><td>Global radiation</td><td>10 %</td><td>W/m²</td><td>RG100, Solems</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>Wind speed</td><td>0,05</td><td>m/s</td><td>2D sonic anemometer</td><td>✓</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>Wind direction</td><td>1</td><td>°</td><td>CV7, LCJ</td><td>✓</td><td>x</td><td>x</td><td>✓</td><td>x</td></tr><tr><td>Air temperature</td><td>0,2</td><td>°C</td><td>Pt1000 HMP 110, Vaisala</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>Relative humidity</td><td>1,5</td><td>%</td><td>in Davis 7714 radiation shield</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>1.25 m</td><td>Globe temperature</td><td>0,1</td><td>°C</td><td>Thermistor, Campbell Or Pt100 RTD, RS in a 15 cm black-painted copper sphere</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td rowspan="2">Soil</td><td>-2 cm</td><td>Ground surface temperature</td><td>0,1</td><td>°C</td><td>Thermistor, Campbell</td><td>✓</td><td>x</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>-20 cm</td><td>VWC</td><td>3 %</td><td>m³/m³</td><td>TDR CS655, Campbell</td><td>✓</td><td>✓</td><td>x</td><td>✓</td><td>x</td></tr></tbody></table>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** De-sealed_south weather station diagram presenting each sensor and its associated measured variable.</figcaption></figure>

### 2.2.2. Vegetation compartment

Herbaceous layer cover rate was evaluated through photographs of each of the 12 plots processed with the ImageJ software based on an adapted method of Xiong et al. (2019). Tree dimensions were measured at least once a year in January: tree height, trunk height and crown diameters based on Zhang et al. (2020) method.

### 2.2.3. Data selection procedure

Two years of monitoring were analyzed after the de-sealing: 2023 and 2024. The cumulated precipitations in 2023 and 2024 were 795.5 and 783 mm, that is to say respectively 12 % and 10 % more than the climatological normal precipitations (709.3 mm) over the 1991–2020 period (http://www.infoclimat.fr based on the Angers-Beaucouzé Météo France weather station). However, the cumulated precipitations were only 466 mm in 2022, that is to say 34 % less than the normals, which may indicate that 2023 started with a lower soil wetness than 2024. Between June and August, the monthly mean sunshine duration (time when global radiation is over 120 W/m²) was 221 h in 2023 versus 239 h in 2024, very close to the climatological normals according to Infoclimat (–3 % and + 5 % respectively compared to 228 h over the 1991–2020 period). Regarding the mean Ta, the 2023 and 2024 values were 14.1 °C and 13.2 °C (the average value over the 1991–2020 period being 12.6 °C).

The Section 3.1 considers the vegetation development over the two years after the de-sealing and its impact on surface temperature and tree transmission rate (all days included), while in Section 3.2 only some specific days were used to calculate average daily cycles in 2023 and in 2024. Indeed, in order to compare the situations corresponding to the weather stations, the analysis of several days that are favorable for a high UHI was necessary. Thus, in Section 3.2, several criteria were applied to the dataset to analyze the effect of the
<figure><img src="image_11.png" /><figcaption>Fig. 8. Selected days used for average daily cycles (N = 22 in 2023 identified in green, N = 23 in 2024 identified in red), according to criteria of a typical radiative day. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

de-sealing during a typical radiative day, adapted from the Mballo et al. (2021) method: no rain, a solar energy ratio<fnref n="1" /> above 0.65, a mean wind speed measured at 2 m above ground level below 3.5 m/s, 95 % of wind speed values <3.5 m/s. For year 2023, 22 days were selected between April and August. For 2024, 23 days were selected over the same period. In relation to 2023 and 2024 meteorological conditions, most of the selected days meeting the above criteria for 2023 are in May and June, while the days selected for 2024 are mainly later in the summer, in August (Fig. 8).

Relative humidity and absolute humidity are not directly analyzed as such in this study, but are considered in the UTCI calculation.

## 2.3. Modeling with UMEP and SOLWEIG

The objective of the modeling step is twofold. First of all, while the use of weather stations alone can only evaluate a limited number of point locations, modeling enables the mapping of an entire studied site with quite fine spatio-temporal resolution. Then, it is also possible to project alternative or future scenarios. For this purpose, the “UMEP” QGIS plugin (Lindberg et al., 2018) and the SOLWEIG (SOlar and LongWave Environmental Irradiance Geometry model) model were used (Lindberg et al., 2008). SOLWEIG models the radiative exchanges between surfaces, the atmosphere and human bodies. The input data used in the present study are the altitudes through a digital surface model (from the French National Geographic Institute’s database (IGN, BD Alti®)), the land cover (identified from orthophotographs (IGN, BD Ortho®) including the buildings and the trees, and the meteorological data (global radiation, wind speed and direction, Ta, relative humidity, barometric pressure, rainfall). The input meteorological data are taken from the on-site Meadow weather station. It is indeed the one station which is the most likely to measure not only immediate local conditions but representative data at a larger scale according to international standards: in particular a flat ground, a uniform land cover and no obstacle within 20 m (World Meteorological Organization (WMO), 2023).

The chosen “transmissivity through vegetation” factor is adapted to 15 % for the new planted trees, based on shortwave radiation measurements on the sunny day of August 11th, 2023 (Fig. 12), and because the trees planted on the Oasis were rather young, with a limited development of their crown (3.1.2). This parameter is left to its default value of 3 % for the future dimensions of the trees (3.3.3). The sky is considered isotropic as we analyze radiative days where the diffuse radiation is low compared to the direct radiation. UMEP provides five possible land covers defined by default parameters: albedo, emissivity and three other parameters used to calculate the evolution of the Ts based on a linear regression between [Ts-Ta] and the sun elevation (Lindberg et al., 2016). In this study, the default land cover parameters are used (‘Grass_unmanaged’ for the Meadow and the Oasis, and ‘Dark_asphalt’ for the sealed surfaces), except for the albedo values, which were directly measured using ASTM E1918-16 (2016) standard, and implemented into the model: 0.23 for the Meadow, 0.17 for the Oasis, and 0.06 or 0.13 for the sealed surfaces, depending on the presence of white stripes on the ground in some areas (particularly around the Asphalt weather station).

Among the possible outputs, the Mean Radiant Temperature (MRT) was used to evaluate the model, and the UTCI was analyzed in this study.

It is necessary to verify at first that the model can correctly estimate experimental values, for the actual situation. For this purpose, the five weather stations (2.2.1) localizations are chosen as “points of interest” in the software, and the model performance is evaluated by comparing the modeled variables to the measured ones at these points. Then, UMEP and SOLWEIG are used to get more detailed spatial information, and to simulate various de-sealing and revegetation scenarios. To be able to compare modeled and measured data, an adjustment was made in the SOLWEIG computer code on the height of computation of MRT, changing it from 1.1 to 1.25 m, in order to match the height of the globe temperature sensor on the weather stations (Table 1). The chosen spatial resolution is 0.2 m in order to have a good resolution of the Oasis and of the shade from the tree’s crowns, which have a diameter of at least 1.3 m. Also, this is considered a relatively high resolution, as neighborhood-level studies are typically modeled with a resolution of 1 m (Lindberg and Grimmond, 2011; Lindberg et al., 2016; Gál and Kántor, 2020; Wallenberg et al., 2023).

The Root Mean Square Error (RMSE) and Mean Bias Error (MBE) were calculated using the formulas (1).

$$RMSE = \sqrt{\frac{1}{N} \sum_{i=0}^{N} (x_{sim} - x_{mes})^2} \qquad MBE = \frac{1}{N} \sum_{i=0}^{N} (x_{sim} - x_{mes}) \qquad (1)$$

<footnote n="1">¹ Ratio between the daily light integral (based on the global radiation measurement on the site) and the cumulative radiation per day above the atmosphere</footnote>
# 3. Results

## 3.1. Vegetation development and evolution of microclimate effects

### 3.1.1. Impact of development of the herbaceous layer

#### i. Cover rate

After the sowing on April 18th, 2023, the cover rate of the Oasis was evaluated on 8 dates in each of the 12 plots, based on the 1 m² survey quadrat (Fig. 9). Overall, the average cover rate raised from 33 % in June 2023 to 60 % in July but the values are heterogeneous, as shown by the error bars, due to differences in sowing between areas. At the end of 2023, the average cover rate reached 80 % and stabilized between 90 and 100 % in 2024, the values in 2024 being much more homogeneous.

#### ii. Surface temperature and Soil volumetric water content

As described in Section 2.2.1, as the Asphalt *Ts* sensor was installed later than the other *Ts* sensors, calculations including Asphalt data are based on measurements starting from July 7th, 2023. For the other sensors, summer values are calculated between June 21st and September 9th. Fig. 10 shows the evolution of the daily max *Ts* of the weather stations during the two first years after de-sealing. The Asphalt and Meadow weather stations served as references.

As expected, the Asphalt max *Ts* were always the highest for both years with daily max temperatures of up to 61 °C, because of the cumulated effects of asphalt's low albedo (0.13, see Section 2.3), and the absence of ET and shade. On the contrary, the Meadow station

<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Time evolution of the average herbaceous cover rate across the 12 plots composing the Oasis between June 2023 and July 2024, based on 8 measuring campaigns using photographs processed with ImageJ software. Error bars represent the standard deviation (N = 12).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Time evolution of daily maximum *Ts* for the De-sealed_south, Meadow, and Asphalt weather stations, calculated for each day between June 2023 and September 2024.</figcaption></figure>
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>**Fig. 11.** a. Time evolution of Volumetric Water Content in the Meadow and the Oasis measured hourly between June 1st, 2023 and August 31st, 2023, and between June 1st, 2024 and August 31st, 2024. Days with more than 5 mm of rainfall are represented by blue dots on the x-axis; b. Mean diurnal (in yellow) and mean nocturnal (in blue) Ts difference between De-sealed_south and Meadow, calculated for each diurnal and nocturnal periods between June 1st, 2023 and August 31st, 2023, and between June 1st, 2024 and August 31st, 2024. The sunlight ratio is represented as a grey area under the curve. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
had the coolest surface in 2023 (range of daily max summer temperatures from 20 to 30 °C) thanks to its higher albedo (0.23, see Section 2.3) and its ability to evapotranspirate (shown in Morel et al. (2024)). In 2023 De-sealed_south *Ts* were between those of Asphalt and Meadow (range of daily max summer temperatures from 20 to 35 °C). The differences between each of the three surfaces daily max temperatures remained the same over the first summer, despite the increasing vegetation cover rate on the Oasis (Fig. 9): on average + 3.0 °C between De-sealed_south and Meadow, on average – 18.4 °C between De-sealed_south and Asphalt. The second year, the De-sealed and Meadow weather stations had reduced differences in *Ts*: the previous difference was reduced to +0.2 °C in summer 2024, which can be considered as non-significant temperature difference considering the sensor accuracy. The difference between De-sealed_south and Asphalt in 2024 was enhanced compared to 2023: –23.0 °C. This reduction is linked to the Oasis cover rate increase between 2023 and 2024: while the average cover rate varied between 30 and 70 % in 2023, it reached almost 100 % in early summer 2024, with probably better rooting conditions (Fig. 9, Section 3.1.1i). The vegetation has also continued to grow denser and taller over time, increasing the Oasis ability to evapotranspirate.

As for the summer daily mean *Ts* (see Appendix C), the stations have similar relative behaviors – though with lower values – than for the maximum temperatures. In August for example, the daily average *Ts* of Meadow, De-sealed_south and Asphalt were respectively 23.0 °C, 25.5 °C and 44.0 °C in 2023, versus 23.3 °C, 23.4 °C and 45.1 °C in 2024. The higher value of Asphalt *Ts* in 2024 compared to 2023 can partly be explained by the synoptic differences in radiation discussed in Section 2.2.3.

More precisely, Fig. 11 shows the soil VWC of Meadow and the Oasis during the periods of interest, that is to say summer 1 and summer 2 after de-sealing, as well as the mean nocturnal and the mean diurnal daily difference between the *Ts* of De-sealed_south and that of Meadow. The VWC values in 2024 are higher than those in 2023, probably due to the very dry year that preceded it in 2022. Differences between Meadow and the Oasis VWC remained the same over the two periods of interest. In 2023, the surface of De-sealed_south had clearly higher temperatures compared to the Meadow (up to almost 6 °C higher) while in 2024, *Ts* tended to be more alike for both stations. The *Ts* difference also less depended on the daily sunlight ratio in 2024 compared to 2023.

### 3.1.2. Monitoring tree development and radiation transmission rate

The average Oasis tree crown diameter was 1.0 m at the time they were planted (shape pruning). Their average height was 4.0 m at that time. By the time the second growing season began in April 2024, their average height was 4.25 m and their average crown diameter was 1.3 m. The global radiation of the two weather stations shows the effect of the Oasis trees shading between 12:00 UTC and 15:00 UTC at the position of “De-sealed_tree” weather station (Fig. 12).

The daily mean interception rate of the trees, between 12:00 UTC and 15:00 UTC over 2023 and 2024 shows the evolution of the Oasis tree shading over time at the position of De-sealed_tree station (Fig. 13). To ensure that this reduction is indeed due to the trees, only days with a solar energy ratio above 0.65 are plotted: 67 days between April 8th, 2023 and August 8th, 2024. The other criteria used for data selection procedure described in Section 2.2.3 (rain and wind criteria) are not considered here because they have no direct effect on interception rate. It can be considered that this graph represents the effectiveness of the new planted trees, in terms of

<figure><img src="image_16.png" /><figcaption>**Fig. 12.** Global radiation measured every 10 min on the Oasis at both De-sealed_south (in pink) and De-sealed_tree (in violet) stations, at a height of 2 m on August 11th, 2024. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Daily mean interception rate of the trees at the position of De-sealed_tree station [(incident radiation - transmitted radiation)/incident radiation] between 12:00 UTC and 15:00 UTC, calculated for sunny days between April 8th, 2023 and August 8th, 2024 presenting a solar energy ratio above 0.65. ("Incident radiation": De-sealed_south station, always exposed to the sun; "transmitted radiation": De-sealed_tree station).</figcaption></figure>

shading. The tree growth between 2023 and 2024 allows the mean interception rate to increase over the two years, to reach values above 40 % the second year. This value remained quite low (e.g compared to the maximum interception rate of 0.9 visible on Fig. 12 on August 11th, 2024) because the chosen window is larger than the shading period.

Also, the daily duration of an interception rate value above 35 % between 12:00 UTC and 15:00 UTC was calculated for the same selected days. When the average tree daily shading duration is 60 min between April and September 2023, it is 100 min between April and September 2024.

## 3.2. Daily cycles in radiative conditions – summers 2023 and 2024

The following figures shows the typical behavior during a mean radiative day (N = 22 days for 2023; N = 23 days for 2024, days selected between April and August) for the available weather stations during the first and second years after the de-sealing, based on the method explained in Section 2.2.3.

### 3.2.1. Air temperature

Mean Ta variations and differences between each weather station and Asphalt were calculated and plotted over 24 h (Fig. 14). De-sealed_south and De-sealed_tree Ta are hardly distinguishable, owing to their proximity (2 m distance only between the two stations, and both in the de-sealed Oasis). If we consider Asphalt as a reference before the de-sealing, this operation leads to a small diurnal reduction in Ta (for De-sealed_south: mean reduction between 8:00 UTC and 17:00 UTC of 0.7 °C, max reduction of 1.0 °C in 2023; mean reduction between 8:00 UTC and 17:00 UTC of 0.7 °C, max reduction of 1.1 °C in 2024). This difference may appear low, but it is still significant compared to the sensor accuracy (0.2 °C). Results are very close for both years, with little differences. Overall, temperature amplitude is slightly lower in 2024 than in 2023. This can be due to the fact that the radiative days for 2023 and 2024 do not exactly occur at the same time of the year (see Section 2.2.3, Fig. 8), owing to the usual change of synoptic conditions between years. However, conclusions about the microclimate effect of the Oasis can still be drawn, since our approach focuses on differences between stations acquiring data synchronously. The Meadow station appears to experience an even greater drop in minimum temperatures compared with the other weather stations, which may also be explained by the higher soil moisture in 2024 (21 % on average between
<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>**Fig. 14.** Mean Ta variations over an average radiative day for 2023 (N = 22, a) and 2024 (N = 23, b), for all five weather stations, and with 10 min time resolution; Mean Ta differences for De-sealed_tree, De-sealed_south, Meadow and Sealed_tree compared to Asphalt over an average radiative day for 2023 (N = 22, c) and 2024 (N = 23, d). Negative (resp. positive) values indicate that the weather station is experiencing lower (resp. higher) Ta compared to the Asphalt station.</figcaption></figure>

May and August 2024 versus 17 % on average between May and August 2023). The nocturnal behavior of the Oasis is very close to Asphalt. Indeed, the large sealed surface around the Oasis may prevent it to cool down during the night. A small evolution is visible from year one to year two: the De-sealed_south station is on average 0.4 °C higher than Asphalt between 22:00 UTC and 04:00 UTC in 2023, but 0.3 °C lower in 2024 on the same timeslot.

### 3.2.2. Radiative load

The radiative load corresponds to the difference between MRT (Mean Radiant Temperature) and Ta (Air Temperature). Occasionally, the De-sealed_tree station's radiative load reaches low values during the day, even lower (−4 °C) than the Sealed_tree station at some time of the day around 14:00 UTC thanks to the shading of the trees (Fig. 15). In 2024, De-sealed_tree radiative load can be down to 7 °C lower than Sealed_tree station during the same short period around 14:00 UTC). This means that the Oasis trees provide a more efficient shading effect in 2024 compared to 2023. This effect is also longer: 100 min in 2024 versus 20 min in 2023.

At night, the Oasis trees are too small to generate a radiative trapping effect, contrary to the treed alignment for Sealed_tree.
<figure><img src="image_22.png" /><figcaption>**Fig. 15.** Mean radiative load evolution over an average radiative day for 2023 (N = 22, left) and 2024 (N = 23, right), for all five weather stations, and with 10 min time resolution.</figcaption></figure>

### 3.2.3. UTCI

The de-sealing leads to a mean *UTCI* reduction of 2 to 3 °C during the daytime between 8:00 UTC and 17:00 UTC for a radiative day (Fig. 16), taking into account the herbaceous layer only (De-sealed South station compared to Asphalt station). The Oasis diurnal lower *UTCI* can be attributed to the Oasis evapotranspiration and the lower *Ts* (Fig. 10). However, the *UTCI* reduction is smaller during the nighttime, with an average *UTCI* reduction of only 1 °C between 19:00 UTC and 5:00 UTC. *UTCI* trends are very close to the radiative load's trends, reflecting the sensitivity of *UTCI* to radiation. The de-sealing with the planting of the trees (considering De-sealed_tree compared to Asphalt, at the time of the day where the Oasis trees provide shading to the station) can lead to a *UTCI* reduction that can be as much as 8 °C (7 °C for 2024 dataset). To compare with other situations, the Sealed_tree weather station's *UTCI* can be up to 12 °C lower than Asphalt (9 °C for 2024 dataset). As for Meadow, the *UTCI* is indeed lower than Asphalt and the Oasis during the nighttime, but it is actually slightly higher during the daytime, which can be explained by the lower inertia and by the higher albedo (0.23, see Section 2.3). Between 2023 and 2024, the nighttime overheating seems to be lowered on the Oasis: the average difference between Meadow and De-sealed_south between 22:00 UTC and 04:00 UTC is 1 °C in 2023 versus 0.2 °C in 2024.

<figure><img src="image_23.png" /><figcaption>**Fig. 16.** Mean UTCI evolution over an average radiative day for 2023 (N = 22, left) and 2024 (N = 23, right), for all five weather stations, and with 10 min time resolution.</figcaption></figure>
### 3.3. Contribution of modeling to benefit evaluation

#### 3.3.1. Model evaluation with experimental data

MRT-Ta modeled with UMEP/SOLWEIG for July 19th, 2024, radiative day of 2024 summer, was analyzed and plotted (Fig. 17), in order to evaluate the model performance. The tree dimensions are the ones measured closer to this date.

Except the MRT of Asphalt and Sealed_tree during daytime, which exhibit MBE values close to zero (respectively 1.1 °C and 0.7 °C), the model seems to underestimate all other values with a MBE error comprised between -6.8 °C and + 1.1 °C (Table 2).

The MRT-Ta RMSE varies between 3.4 and 9.2 °C, depending on the station and on the day period (Table 2). The overall dynamics between the weather stations are logical, and shading is clearly visible, even though its effect on radiative load is underestimated. The mean calculated RMSE value of 6 °C for a specific radiative day is in line with errors reported in the literature (MRT RMSE of 6.8 °C for Lindberg and Grimmond (2011); 6.0 °C for Briegel et al. (2023)) on SOLWEIG, which is a recommended model for microclimate evaluation (Bouzouidja et al., 2021a). This value is further discussed in a dedicated section of the discussion.

#### 3.3.2. Scenarios with de-sealing but no tree plantation

Two scenarios were estimated with variation in land cover on the patch: before de-sealing, and after de-sealing with herbaceous cover and no tree. The input meteorological data are the same date to set this variable: July 19th, 2024. For each of the situations, the MRT is mapped around the Oasis at the solar noon, 12:00 UTC. The exact estimated MRT value is provided for six points of interest (the actual location of the weather stations and three points located 1, 3, and 5 m north of the Oasis, to see the extension of the tree shade at that time of the day), as well as for the average of the Oasis (see Fig. 18).

The MRT located under the Sealed_tree alignment at the north, is the same for each scenario (45 °C), which is expected given that the land cover change is more than 10 m away from the point of interest and that convective effects are not represented in the model. The shading effect of the mature trees is dominant here. Overall, replacing the initial Asphalt surface of the Oasis by a grass surface results in a MRT reduction of only 1 °C on the Oasis according to the estimations, way below the calculated RMSE of the model (6 °C). These results indicate that land cover has almost no effect on MRT, which is consistent with the work of Lindberg et al. (2016). Also, the points of interest located north of the Oasis have the exact same MRT values across the two scenarios, showing that land cover only has a local effect of MRT with this model.

#### 3.3.3. Scenarios with variation in trees dimensions

As the trees were only about seven years old at the time they were planted, their dimensions are quite modest on the previous simulations. Based on nurseries information, considering a height growth rate of 30 cm per year and a crow diameter growth rate of 25 cm per year, the trees are expected to reach 9 m height with a crown diameter of 5.3 m 16 years after the 2024's dimensions, that is to

<figure><img src="image_24.png" /><figcaption>Fig. 17. Modeled (left) and measured (right) radiative load on July 19th, 2024, calculated for each of the five weather stations with a 10-min time step.</figcaption></figure>

**Table 2**

RMSE (°C) and MBE (°C) calculated for the nighttime and daytime periods for each of the five weather stations, as well as on average, on July 19th, 2024.

<table><thead><tr><th></th><th></th><th>De-sealed_south</th><th>De-sealed_tree</th><th>Meadow</th><th>Asphalt</th><th>Sealed_tree</th><th>Average</th></tr></thead><tbody><tr><td rowspan="2">MRT-Ta RMSE (°C)</td><td>Night</td><td>5</td><td>5,9</td><td>5,2</td><td>7,4</td><td>3,4</td><td><strong>5,4</strong></td></tr><tr><td>Day</td><td>9,2</td><td>9</td><td>5,5</td><td>4,5</td><td>4,1</td><td><strong>6,5</strong></td></tr><tr><td rowspan="2">MRT-Ta MBE (°C)</td><td>Night</td><td>-4,3</td><td>-5,3</td><td>-4,6</td><td>-6,8</td><td>-2,1</td><td><strong>-4,6</strong></td></tr><tr><td>Day</td><td>-4,5</td><td>-4,8</td><td>-2,7</td><td>1,1</td><td>0,7</td><td><strong>-2,0</strong></td></tr></tbody></table>
<figure><img src="image_25.png" /><figcaption>**Fig. 18.** MRT estimation maps of the Oasis and its surroundings based on the SOLWEIG model, using weather data from July 19th, 2024, at solar noon, with no trees on the Oasis. MRT values were computed at 1.25 m above the ground with a spatial resolution of 0.2 m. a. Situation with no de-sealing b. Oasis land cover is grass.</figcaption></figure>

<figure><img src="image_26.png" /><figcaption>**Fig. 19.** MRT estimation maps of the Oasis and its surroundings based on the SOLWEIG model, using weather data from July 19th, 2024. MRT values were computed at 1.25 m above the ground with a spatial resolution of 0.2 m. The subplots illustrate different tree dimensions and times of the day: a. 2024 tree dimensions, solar noon; b. 2040 tree dimensions, solar noon; c. 2024 tree dimensions, 15:00 UTC; d. 2040 tree dimensions, 15:00 UTC.</figcaption></figure>

say in 2040. Based on this estimation, Fig. 19 shows MRT maps on the studied site with actual and future dimensions implemented with the same input meteorological data (for July 19th, 2024), at solar noon (a. and b.) and at 15:00 UTC (c. and d.). The Oasis land cover is grass in both cases.

Since the transmission rate was changed from 15 to 3 % for 2040 simulation (see 2.3), the MRT below the northernmost point of interest (below Sealed tree) slightly changes from 45 °C to 41 °C. At the solar noon, the trees development leads to a lower average MRT (to 56 °C to 44 °C) on the Oasis: microclimate benefits are expected to increase over time on the Oasis. Locally, some areas experience significant cooling (–16 °C for the De-sealed_tree station for instance) due to direct tree shading, even outside of the Oasis limits.
<figure><img src="image_27.png" /><figcaption>**Fig. 20.** Modeled daily UTCI variations, calculated with a 10-min time step for three scenarios using weather data from July 19th, 2024. The solid red line represents Asphalt evolution with 2024 tree dimensions; solid purple line represents De-sealed_tree evolution with 2024 tree dimensions; dashed purple line represents De-sealed_tree evolution with 2040 tree dimensions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

Indeed, the point of interest located 1 m above the northern limit of the Oasis shows a MRT reduction of 18 °C at the solar noon. In the afternoon, at 15:00 UTC, the shade of the trees reduces the MRT way beyond the Oasis limits. At 15:00 UTC for instance, the MRT is reduced by 21 °C at a distance of more than 5 m east of the Oasis. The daytime differences of MRT induced by the trees and calculated by the model can be considered as significant as the values are well above the RMSE of the model. However, the average Oasis MRT at midnight is 14 °C with 2024 trees dimensions versus 19 °C with 2040 trees dimensions (not shown here) because of the trees-induced radiative trapping. It is worth noting that in both cases, the corresponding UTCI values on the Oasis indicate ‘no thermal stress’ (Bröde et al., 2012).

An extended period of lower UTCI values<fnref n="2" /> is expected (Fig. 20). Indeed, for a radiative day with meteorological conditions identical to that of July 19th, 2024, the trees benefits duration is expected to increase from 1 h and 10 min with 2024’s trees dimensions up to 9 h and 40 min with 2040’s trees dimensions for De-sealed_tree station, with maximum UTCI reduction of 7 °C. In addition, the station De-sealed_south is also expected to benefit of the Oasis trees with larger tree dimensions, with a benefit duration of 5 h 50 min expected in 2040. As for the nighttime, as seen for the MRT, the UTCI is expected to be a bit higher with larger tree dimensions.

# 4. Discussion

## 4.1. Vegetation development following the de-sealing operation

After the sowing in April 2023, the cover rate on the Oasis progressively raised 60 % in July. From the beginning of the second growing season, in April 2024, this cover rate was above 90 % with much less variation among the 12 plots composing the patch. As for the four planted trees, their crown diameters increased by 30 % between the first and second growing seasons, which led to an increase in both the interception rate and the duration of the shading effect – 60 min during the first summer compared to 100 min during the second summer, as recorded by the De-sealed_tree weather station.

## 4.2. Evolution of surface temperature

The vegetation development on the Oasis has a clear impact on Ts. The measured differences between the Oasis maximum Ts and the Asphalt reference are high and strengthen between the two year (–18.4 °C in 2023 and – 23.0 °C in 2024). These values seem a little bit higher, but of the same order as Bouzouidja et al. (2021a, 2021b), who found a maximum Ts (measured at a depth of 3 cm) difference of slightly less than 15 °C between an asphalted parking surface and a control grassed zone during the warmer period of summer 2019 in northern France. Also, the Oasis Ts trend was strongly influenced by the sunlight in 2023, which was no longer the case in 2024. Supporting the hypothesis that the vegetation growth would reduce the Ts, Takebayashi and Moriyama (2009) showed a relation between the green cover ratio of a parking lot and its Ts (evaluated with IR camera). The lower long wavelength emissions

<footnote n="2">² The considered threshold for the UTCI difference compared to De-sealed_south in 2024 situation is 3 °C</footnote>
were assumed from the lower *Ts*: they found a mean decrease of the sensible heat flux during the nighttime (−50 W/m²) and the daytime (−150 W/m²) above grassed surfaces compared to sealed surfaces in July and August 2005 in Japan.

The following two sections refer to average daily cycles calculated from the selection of radiative days.

### 4.3. Evolution of air temperature

The reduction of *Ts* can help explaining the results about air measurements. During the daytime, the de-sealing led to a noticeable average reduction in *Ta* compared to Asphalt (−0.7 °C), although this reduction might seem limited by the small size of the patch (64 m²), being itself surrounded by sealed surfaces. As a comparison, Yan et al. (2018) measured *Ta* difference between some points in an urban park (Beijing, China) and the surrounding areas during the afternoon of a clear windless summer day of 2010, in open areas with direct solar radiation. The authors did not systematically measure cooler *Ta* in the park depending on the ground cover around the measurement point, showing that lower temperatures in vegetated areas is not that obvious. Yet, the average *Ta* reduction within the park was −0.6 °C, very much in line with what was measured between the Oasis and the sealed reference. As for the nighttime, the slight *Ta* improvement of Oasis nocturnal values in 2024 compared to 2023 is attributed to the growth of herbaceous vegetation that reduced *Ts*. Additionally, the winds were stronger and more frequently coming from the North in 2023 than in 2024 during the selected radiative days (see Appendix A). Thus, to a larger extent in 2023 than in 2024, the Oasis nocturnal *Ta* of the Oasis may have been warmed up by the surrounding asphalted surfaces, while that of Asphalt station may have been cooled down by the Meadow, located just 40 m north of Asphalt station (see Section 2.2.1). Most authors suggest a significant nocturnal cooling effect from large vegetated areas (Yokobori and Ohta, 2009; Yan et al., 2018; Meili et al., 2021), comparable to the noticeable cooling observed in the Meadow. This reinforces the idea that the Oasis may be too small to produce a similar effect. It is also likely that both Meadow and the Oasis would evapotranspire more if they were irrigated (as the VWC graphs show the soil moisture contents might be limiting factor of ET, especially in the summer), as further reduction of thermal stress was observed with irrigation by Cheung et al. (2022) when soil moisture is a limiting factor of ET.

### 4.4. Thermal comfort

Concerning the *UTCI*, the lower values on the Oasis compared to Asphalt's, even when the weather stations do not directly benefit from the shade, can be attributed to herbaceous layer leading to lower *Ts* (see Section 3.1.1, Fig. 11) together with the local shading on the ground provided by the young trees of the Oasis. This would also explain why the Meadow *UTCI* remains higher than the Oasis during the daytime. Thus, from the first year, the lower *Ts* thanks to the herbaceous layer might indeed have reduced the long wavelengths radiations on the Oasis, reducing the Oasis *Ta* and *UTCI* compared to Asphalt during the day. However, there is no clear evidence of improvement in daytime *Ta* or *UTCI* between the first and second years in relation to the development of low vegetation.

In addition, the radiative load and *UTCI* on the Oasis can be significantly reduced – down to 8 °C *UTCI* (maximum value during an average radiative day, at the time of the day when the trees cast shadow on the weather station) - compared to Asphalt via the reduction of short wavelengths thanks to the tree shading. The further reduction of the radiative load between 2023 and 2024 is due to the Oasis trees growth: both the interception rate and the duration of the shading effect increased, resulting in measurable improved benefits from one year to the next. Yet, if De-sealed_south is considered as the reference instead of Sealed_tree or Asphalt, no clear improvement of De-sealed tree (in lowered radiative load and *UTCI* duration and intensity) was observed between year one and year two after de-sealing. Additional years of monitoring might thus be interesting to further study the effect of the tree crowns development. Indeed, no study measuring the tree growth over time and its effect on microclimate was found, as already suggested by Zhao et al. (2023). Yet these very authors as well as Middel et al. (2015) simulated the effects of tree dimensions increase development on microclimate.

### 4.5. Ability of modeling to predict next year's benefits

The de-sealing and revegetation led to an average Oasis *MRT* reduction from 59 °C to 56 °C at solar noon during an average summer radiative day. This reduction may appear low when compared with the calculated *RMSE* of the model (6 °C). However, this reduction refers to a spatial average and *MRT* may vary locally: some areas remain unaffected, while others experience significant cooling due to direct tree shading. When looking at the projected dimensions of the trees (height of 9 m, crown diameters 5.3 m) and increase in light interception of the tree crown after 16 more years, this average Oasis *MRT* is expected to decrease by an additional 12 °C (for the same meteorological conditions and the same time of the day), value way above the calculated *RMSE* of the model. Wang and Akbari (2016) evaluated with ENVI-met the *MRT* reduction induced after the planting of a double treed alignment in Montreal (tree height of 10 m, crown diameters of 9 m), with no overlapping tree crowns and no land cover change. During a typical summer day, at the solar noon, the *MRT* was reduced by about 10 °C, which is within the same range (though slightly lower) than what was found in this study with lower trees dimensions. Teshnehdel et al. (2020) also used ENVI-met to simulate the reduction in *MRT* by comparing two models: a baseline with 17 needle trees per hectare and an improved version with 60 deciduous trees per hectare (tree height of 7 m, crown diameter of 3 m), in Iran. This greening simulation showed a reduction in average daily *MRT* from 60 °C to 39 °C, which is a higher value than the maximum decrease evaluated in this study (19 °C). Finally, although the Oasis appears to have a limited impact on its immediate surroundings except for direct areas shaded thanks to the trees (especially in the morning and in the afternoon), research suggests that the extent of the vegetated area determines its capacity to influence regions beyond its boundaries (Feyisa et al., 2014). Fan et al. (2019) even evaluated the optimal vegetation patch size for urban cooling in hot-humid Asian cities.
The errors obtained for modeled MRT are within the same range as those already reported in the literature for SOLWEIG (Lindberg and Grimmond, 2011). Gál and Kántor (2020) mainly attributed MRT errors to (i) brief artifact due to shadow mismatch in the model, in relation to coarseness of the spatial and temporal resolution; (ii) calculation schemes for MRT. In our case, the largest error is obtained during daytime, on the Oasis, where there is a shadowing effect from the trees, which tends to confirm this issue of shadow mismatch. For the calculation schemes of MRT, the deviation from the measurements can partly be explained by the fluctuations of the measured data, which are not reproduced by the model, because the MRT is not calculated in the same way in the model, where it is calculated from the radiative fluxes, and in the experimental data, where it is calculated from the globe temperature with a correction for convective loss (Huang et al., 2014). This correction is based on the wind speed, whose fluctuations are then propagated to the MRT. The deviation of the models from the measurements is also due to the model's limitations regarding the 'ground surface' temperatures, which are actually more likely to represent vegetation Ts, while our measurements are taken at 2 cm depth in the ground. Underestimation of the model was found for all stations, and has also been reported for grass land cover (Lindberg et al., 2016), probably because grass surface used for the model constructions is different. Also, the numerical results presented in this study do not consider evapotranspiration, resulting in only a minimal and localized effect of land cover on thermal comfort in the simulations. Also, the five default parameters defining each type of ground cover are used by the model to calculate ground temperatures empirically: the model was calibrated with specific surfaces. Another strong hypothesis made by the model is the equality between the Ts and the Ta during the nighttime: there is no actual thermal resolution of the ground Ts in the model. This hypothesis of the model relies on the fact that the increase of Ts during daytime is limited thanks to evapotranspiration, and that the surface has a low thermal inertia and thus cools down rapidly at night with the decrease of Ta. This can be valid for a vegetated surface, such as Meadow, but is more questionable for an asphalt surface, which has a higher thermal inertia and cannot evapotranspirate. For Asphalt especially, which experiences the highest Ts, the model certainly underestimates Ts and MRT at night. This corroborates the underestimation that we observed. The role of Ts in the underestimation of MRT has also been highlighted by Gál and Kántor (2020).

## 5. Conclusion

De-sealing of a small patch in an urban area, where space is highly constrained, showed promising results on the site microclimate for improving thermal comfort. Experimental monitoring was realized using a negative (Asphalt) and a positive (Meadow) control, over two consecutive years. Studying the evolution of benefits over time was valuable, as microclimatic variables changed significantly in the short term with vegetation development. Revegetation was beneficial, with plant species carefully selected based on predefined criteria to ensure plant survival and development.

The benefits on the Oasis were observed at first on Ts, with the growth of the herbaceous cover rate leading to visible progress: maximum Ts reduced by 18.4 °C year one and by 23.0 °C year two compared to sealed reference. Herbaceous layer and trees both had a positive impact on the UTCI during the day (average value of -2 °C to -3 °C). Among these two strata, the tree shading effect was more efficient at lowering thermal stress (up to -8 °C), although this effect was limited in time to the shaded period of the day, as already shown by Armson et al. (2012). An extension of this shading period was shown from year one (60 min) to year two (100 min). Additionally, no clear effect of de-sealing on nocturnal Ta was observed, although some evolutions were noted between the first and second years.

Finally, SOLWEIG model appears to be a powerful tool to evaluate future benefits on MRT or UTCI indicators using future trees dimensions. With tree growth over time, significant UTCI improvement is expected during the day on the Oasis and its immediate surroundings thanks to direct tree shading, while radiative trapping is anticipated during the night.

## Declaration of generative AI and AI-assisted technologies in the writing process

During the preparation of this work the author(s) used DeepL and ChatGPT in order to improve the clarity and fluency of the English language in this manuscript. After using these tools, the author(s) reviewed and edited the content as needed and take(s) full responsibility for the content of the publication.

## CRediT authorship contribution statement

**Antoine Morel:** Writing – original draft, Visualization, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.
**Laure Vidal-Beaudet:** Writing – review & editing, Supervision, Funding acquisition, Conceptualization.
**Lydia Brialix:** Writing – review & editing, Software, Resources, Investigation.
**Dominique Lemesle:** Software, Resources, Investigation.
**Adeline Bulot:** Writing – review & editing, Supervision, Funding acquisition, Conceptualization.
**Sophie Herpin:** Writing – review & editing, Validation, Supervision, Software, Methodology, Investigation, Funding acquisition, Formal analysis, Conceptualization.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Antoine Morel reports financial support was provided by idverde company for his PhD scholarship. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
## Acknowledgments

- The research is based on a PhD program carried out at **Institut Agro Rennes-Angers** and benefited from financial support of **idverde** compagny. The authors would like to thank **RATP Dev Angers** and **Angers Loire Métropole** for their support and collaboration regarding the site management.

- The authors also wish to thank **Martin Hendel**, associate professor at ESIEE Paris (France), for his help in measuring the surfaces albedo at the experimental site; **Christophe Ducommun**, pedologist from EPHOR research unit, for his help with soil description; and **Yvette Barraud-Roussel** and **Lucas Gilberton**, both lab technicians from EPHOR research unit, for the laboratory soil analysis.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.uclim.2025.102371.

## Data availability

Data will be made available after a 2-year period following publication, enabling to complete the analysis of the entire dataset.

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