Description du sujet de thèse

Spatio-temporal monitoring of urban drought islands (UDI) in contrasting climatic contexts: a comparative study between France and Brazil using dense measurement networks

Context and Research Problem :
In the face of increasing drought episodes linked to climate change and the continuous expansion of urban areas, cities are increasingly exposed to tensions regarding water resources and the ecological functioning of their environments. Urban drought islands (UDIs), defined as areas exhibiting significantly reduced water availability compared to their rural or peri-urban surroundings, remain understudied compared to urban heat islands (UHIs). This PhD research aims to characterize the spatio-temporal dynamics of UDIs by comparing two contrasting climatic contexts—temperate (France) and tropical (Brazil)—using dense microclimatic measurement networks including sensors for temperature, humidity, evapotranspiration, and plant water stress.
The main objective is to develop a methodology for fine-scale and real-time monitoring of UDIs by integrating data from ground-based sensor networks and satellite observations. The study will pay particular attention to the influence of urban morphology, vegetation, and land use. By comparing observed dynamics in Rennes (France) and Presidente Prudente (Brazil), the thesis will contribute to proposing locally adapted climate strategies and urban resilience frameworks based on objective and generalizable data.
Scientific Challenges and Detailed Research Problem :
While urban heat islands (UHIs) have been extensively studied since the 1990s, their hydrological counterpart—urban drought islands (UDIs)—remain an emerging, poorly conceptualized issue, with insufficient documentation of their spatio-temporal dimensions. UDIs are characterized by urban areas where soil moisture, vegetation water content, or evapotranspiration are significantly lower than in surrounding zones. They result from a complex combination of climatic (precipitation, temperature, wind), anthropogenic (urbanization, impervious surfaces, water management), and biophysical (soil type, vegetation cover) factors. Their identification and monitoring face several key scientific challenges:
 Lack of an operational definition of UDIs: Unlike UHIs, there is no scientific consensus yet on detection criteria, relevant indicators (NDWI, VHI, ETa, etc.), or thresholds for characterizing urban drought islands.
 Challenges in high-resolution spatio-temporal observation: available satellite data (Landsat, MODIS, Sentinel) offer regular coverage but often lack the resolution required to capture intra-urban hydric micro-variations. Ground-based sensor networks remain underdeveloped, especially in the Global South.
 Scarce comparative studies across climatic and urban contexts: Few studies have attempted to compare UDI dynamics across contrasting climatic and socio-economic contexts. Comparing temperate French cities (Rennes) with tropical Brazilian cities (Presidente Prudente, Sinop) would help identify structural and functional specificities of UDIs by latitude.
 Insufficient consideration of urban uses and social practices: Watering routines, green space management, and water access are rarely integrated into urban drought models, though they significantly shape neighborhood vulnerability.
 Absence of local and adaptive predictive models: Current forecasting models for urban droughts lack robustness, being generally designed at regional or national scales, with insufficient integration of fine-scale urban dynamics (housing types, vegetation, shading, reflective surfaces, etc.).
Specific Objectives of the Thesis :
1. Design and test a protocol for high-resolution spatio-temporal detection and monitoring of UDIs;
2. Analyze spatial and seasonal differences in UDIs between the two countries;
3. Assess the influence of environmental factors on UDI intensity;
4. Propose decision-support tools for urban climate adaptation policies.

Data and Methodology :
The thesis aims to address current knowledge gaps on UDIs by developing an integrated methodology for monitoring, analyzing, and modeling UDIs using dense networks of microclimatic sensors installed in Rennes (Rennes Urban Network: https://run.letg.cnrs.fr/) and Presidente Prudente, as part of the SCO-ALTELYS project: https://www.spaceclimateobservatory.org/fr/altelys-lutter-contre-les-ilots-de-chaleur-urbains .
Key methodological steps include:
• Installation and calibration of micro-sensor networks in various urban typologies (residential, commercial, green spaces);
• Analysis of the frequency and intensity of UDIs as a function of climate context;
• Use of satellite data (Landsat, Sentinel-2, MODIS) for hydric stress indices analysis (NDWI, VHI, etc.);
• Application of geostatistical models to identify spatial patterns and predictors of UDIs

Contexte de travail

This three-year PhD is funded under the TELKANTE LAB joint laboratory (LabCom) established between the Rennes site of UMR LETG and the Alkante company. Both entities have previously collaborated on urban climate monitoring within the SCO-ALTELYS project, of which this thesis is a continuation.
The UMR LETG is a joint research unit spread across three university sites (Brest, Nantes and Rennes). It is under the supervision of the CNRS and the universities of Brest, Nantes and Rennes 2. The unit brings together around 140 members, including 64 permanent staff (7 research directors, 5 research associates, 15 university professors, 25 lecturers, and 12 engineers and technicians), as well as about 70 contract staff (around 50 doctoral students, 2 post-doctoral fellows and 20 contract engineers). LETG's scientific work falls within the field of environmental geography in all its dimensions (physical, human, etc.), with particular attention paid to informational issues throughout the life cycle of geographic data (production, structuring, processing, dissemination and uses). The research activity is structured around three main areas: coastal environments, continental environments, and remote sensing-geomatics.
The doctoral student will also collaborate regularly with the company ALKANTE. The project is part of the TELKANTE LAB, a joint laboratory funded by the French National Research Agency (ANR), bringing together the LETG research unit and ALKANTE. This laboratory's mission is to develop technical and software solutions to facilitate the integration of remote sensing data into geographic data infrastructures (GDIs). The complementary nature of LETG and ALKANTE allows for scientific and technological advances to facilitate the use of spatial information in various application areas (environment, risk management, etc.). ALKANTE has extensive experience in implementing GDIs, particularly through the development of the interdepartmental open-source platform PRODIGE, which enables the promotion of geographic information for institutional and private users.
The thesis project includes co-supervision with UNESP Presidente Prudente (Brazil), a long-standing partner of LETG-Rennes, notably in the recent co-management of the CAPES-COFECUB CICLAMEN project (https://run.letg.cnrs.fr/ciclamen.html). UNESP has also developed extensive expertise in monitoring heat islands using ground-based measurements and remote sensing data.

Contraintes et risques

The PhD candidate will be enrolled at the ESC Doctoral School of the University of Rennes 2 and will be hosted at LETG-Rennes and occasionally at Alkante. Field missions in Brazil are planned.
Applicants should hold a Master's degree in Environmental Geography with a specialization in remote sensing, GIS, and modeling. Proficiency in programming tools (R, Python) is essential.
Fluency in scientific English is required. However, most of the work and discussions will take place in French and Portuguese. Therefore, a strong command of these two languages is essential, particularly for fieldwork.