Climate change poses increasing challenges to urban environments, with rising temperatures, sea-level rise, and altered precipitation patterns contributing to both flooding and drought. While much research has been dedicated to flood mitigation, urban drought has received compara
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Climate change poses increasing challenges to urban environments, with rising temperatures, sea-level rise, and altered precipitation patterns contributing to both flooding and drought. While much research has been dedicated to flood mitigation, urban drought has received comparatively less attention, despite its significant impact on vegetation, water availability, and overall ecosystem stability. The Paris Agreement of 2016 aims to limit global temperature increases, yet urban areas continue to experience extreme weather variations, necessitating a deeper understanding of their hydrological implications.
This thesis investigated the influence of urban environmental conditions on soil moisture dynamics, particularly during drought periods. The study aims to identify the relationship between green spaces in urban areas and soil moisture dynamics in relation to drought. It specifically examines how factors such as sun, shadow, vegetation type, and soil texture impact soil moisture retention and depletion. To achieve this research goal, the following three research questions are determined:
\begin{enumerate}[noitemsep]
\item How do urban environmental conditions affect soil moisture content in green spaces?
\item What is the effect of soil texture on soil moisture dynamics?
\item How do initial soil moisture conditions influence soil moisture dynamics during a drought period, as modelled using HYDRUS-1D?
\end{enumerate}
The study employs a combination of field measurements and HYDRUS-1D modelling, to analyse the soil moisture behaviour under different urban environmental conditions. Data collection took place at the campus of Delft University of Technology, where five distinct environmental conditions were selected as measurement points, sun, shadow, grass, trees and paved areas.
Soil moisture data was collected using the Profile Probe PR1 sensor (in combination with the HH2 Moisture Meter) at depths of 100mm, 200mm, 300mm, 600mm and 1000mm. Measurements were conducted manually over 10 weeks, 3 to 4 times a week, during the summer of 2024, with a total of 33 recorded sessions. Soil texture analysis was conducted using soil samples taken from depths of 150mm, 300mm, 500mm and 800mm. These samples were dried at 105$\degree$C for 24 hours and then sieved through six mesh sizes to determine the proportion of sand, silt and clay in each sample. The soil texture provided understanding of changes in soil moisture dynamics and was used for the critical input data for the HYDRUS-1D simulations.
HYDRUS-1D was employed to model the impact of initial soil moisture conditions and urban environmental conditions on the soil moisture dynamics during longer periods of drought. The model setup included defining soil hydraulic properties using the van Genuchten-Mualem equation and incorporating root water uptake parameters based on the Feddes approach. Monte Carlo simulations were conducted to optimize model performance, using 10.000 iterations to determine the best fitting parameters. The model was then applied to simulate soil moisture depletion during the extreme drought of 2018, to determine the effects of extremely wet or dry initial soil moisture conditions.
The measured soil moisture content results demonstrate significant variations in soil moisture retention among different urban environment conditions. Areas covered with vegetation exhibited higher moisture retention compared to paved surfaces. Within vegetated areas, shaded locations retained moisture for longer periods due to reduced evaporation rates. In contrast, measurement points in sunny areas experienced rapid soil moisture depletion, particularly in the upper 300mm of the soil.
Soil moisture at shallower depths, 100mm to 300mm, was highly responsive to precipitation events, while the deeper layers, 600mm and 1000mm, exhibited more stable moisture levels, indicating a stronger influence of soil texture and groundwater interactions. Statistical analysis using Kendall's and Spearman's correlation coefficients showed strong correlations between shaded grass-covered areas and tree-covered sites up to 300mm depth, but significant divergence beyond that, likely due to differences in root water uptake and soil texture.
Additionally, paved areas consistently maintained lower soil moisture levels across all depths due to limited infiltration, with only minor variations observed at the access tube edges where small gaps allowed some water entry. The modeling results confirmed that urban design choices, such as increasing permeable surfaces and optimizing vegetation placement, can significantly enhance urban resilience to drought.
Soil texture played a crucial role in soil moisture availability, with finer-textured soils (such as silt and clay) demonstrating superior retention capabilities compared to sandy soils, which facilitated rapid drainage.
Modeling results from HYDRUS-1D reinforced these findings, showing that initial soil moisture conditions had a notable influence on drought resilience. Wetter initial conditions resulted in prolonged moisture availability, while extremely dry conditions led to rapid soil desiccation. The impact of extreme initial soil moisture states was most pronounced in shaded grass areas and tree-covered locations, where higher retention capacity helped delay drought effects.
The study's findings align with existing literature on urban drought dynamics, confirming that meteorological, agricultural and hydrological drought factors interact to shape soil moisture behaviour. Meteorological drought, characterized by prolonged low precipitation, was found to be a primary driver of soil moisture deficits, with its effects compounded by urban environmental conditions. The research underscores the critical role of green infrastructure in mitigating the adverse effects of drought by preserving soil moisture.
The limitations of the study are acknowledged, including the constraints of the HYDRUS-1D model, which assumes only vertical water flow and does not account for lateral redistribution. Additionally, the generic settings of the Profile Probe PR1 resulted in soil moisture values not specifically related to the soil type. Finally, only one drought event has been evaluated in the model, limiting the generalizability of the findings.
This research provides valuable insight into the interplay between urban environmental conditions and soil moisture dynamics during drought periods. The findings highlight the importance of green spaces in urban resilience to drought, demonstrating that shaded and vegetated areas significantly enhance moisture retention compared to exposed, paved environments. Soil texture and initial moisture conditions were identified as key determinants of drought response, with finer-textured soils and wetter initial states offering greater resilience.
The study emphasizes the necessity of integrating soil moisture considerations into urban planning and climate change adaptation strategies. Enhancing green infrastructure, optimizing soil management practices, and incorporating soil moisture monitoring into urban water management can help mitigate the impacts of future drought events. Future research should focus on expanding the dataset across multiple urban locations and improving model accuracy by integrating groundwater dynamics and lateral water flow processes.
