Rainfall, Soil Moisture Deficit & Subsidence

Rainfall, Soil Moisture Deficit & Subsidence: Insights from London’s Recent Rainfall

Introduction

At Geobear, we specialise in ground engineering solutions to address subsidence, a common issue in regions like London where fluctuating soil moisture levels can impact building stability. This week, London experienced approximately 40 mm of rainfall according to NW3 weather (http://nw3weather.co.uk/wx12.php) prompting us to explore how this affects Soil Moisture Deficit (SMD) and, consequently, subsidence risk. Using a comprehensive dataset from January 2018 to July 2025, we analysed the relationship between weekly rainfall and SMD to provide insights into soil behaviour and its implications for property owners.

What is Soil Moisture Deficit & Its Link to Subsidence?

Soil Moisture Deficit (SMD) measures the amount of water (in millimetres) needed to restore soil to its field capacity, where it can hold no more water without drainage. SMD is a critical factor in subsidence, as dry soils (high SMD) can shrink, particularly in clay-rich areas like London, causing ground settlement and structural damage to buildings. Conversely, significant rainfall can reduce SMD, rehydrating the soil and potentially stabilising or even swelling it, which may affect foundations differently. Understanding this relationship helps Geobear assess and mitigate subsidence risks effectively.

The dataset we analysed represents SMD for deciduous grass, a standard output from the MORECS (Meteorological Office Rainfall and Evaporation Calculation System) model. We obtain weekly rainfall and SMD data, allowing us to evaluate how this week’s 40 mm rainfall impacts soil conditions and subsidence potential.

The Dataset

The dataset we used spans January 2018 to July 2025, with columns for:

Week Number: The week of the year

Week Ending: The date the week ends

Total Rainfall (mm): Weekly rainfall in millimetres

SMD_DEC_H (mm): Soil Moisture Deficit for deciduous grass

The latest data point (Week 30, ending July 15, 2025) shows 0.4 mm of rainfall and an SMD of 315.5 mm, indicating very dry soil typical of mid-summer in London. With this week’s 40 mm rainfall, we aim to predict the resulting SMD and its implications for subsidence.

Analysing the Rainfall-SMD Pattern

To understand how rainfall affects SMD and, by extension, subsidence risk, we conducted the following analysis:

Calculated SMD Changes (ΔSMD): For each week (except the first), we computed ΔSMD = SMD_current_week – SMD_previous_week. A negative ΔSMD indicates wetter soil, potentially reducing subsidence risk in clay soils, while a positive ΔSMD suggests drier conditions that may increase it

Correlation Analysis: We calculated the Pearson correlation coefficient between weekly rainfall and ΔSMD to quantify their relationship

Historical Comparison: We examined weeks with rainfall between 35–45 mm to estimate the typical SMD change for 40 mm of rainfall

Seasonal Context: Since SMD and subsidence risk vary seasonally, we focused on mid-July data to reflect summer conditions

Correlation Between Rainfall & SMD

Our analysis revealed a Pearson correlation coefficient of approximately -0.58 between weekly rainfall and ΔSMD. This moderate negative correlation indicates that higher rainfall generally reduces SMD, adding water to the soil and potentially alleviating subsidence risk by reducing soil shrinkage. However, factors like evapotranspiration, soil type, and runoff influence the extent of this reduction, particularly in summer when evaporation rates are high.

Historical Cases with ~40 mm Rainfall

To predict the impact of 40 mm of rainfall, we reviewed weeks in the dataset with rainfall between 35–45 mm. Key examples include:

Week 14, 2018 (49.0 mm): SMD dropped from 131.7 mm to 87.8 mm (ΔSMD = -43.9 mm)

Week 22, 2018 (54.4 mm): SMD dropped from 153.0 mm to 120.6 mm (ΔSMD = -32.4 mm)

Week 42, 2018 (48.3 mm): SMD dropped from 328.7 mm to 297.0 mm (ΔSMD = -31.7 mm)

Week 40, 2019 (43.3 mm): SMD dropped from 310.5 mm to 286.4 mm (ΔSMD = -24.1 mm)

Week 52, 2019 (42.2 mm): SMD dropped from 131.0 mm to 91.0 mm (ΔSMD = -40.0 mm)

Week 25, 2021 (45.8 mm): SMD dropped from 105.8 mm to 78.1 mm (ΔSMD = -27.7 mm)

Week 40, 2021 (45.9 mm): SMD dropped from 188.1 mm to 158.1 mm (ΔSMD = -30.0 mm)

Week 43, 2023 (54.4 mm): SMD dropped from 278.9 mm to 237.1 mm (ΔSMD = -41.8 mm)

Week 2, 2025 (45.2 mm): SMD dropped from 102.1 mm to 58.0 mm (ΔSMD = -44.1 mm)

The average ΔSMD for these cases is approximately -33.7 mm, with a range of -24 to -44 mm. This suggests that 40 mm of rainfall typically reduces SMD by 30–40 mm, which could decrease subsidence risk by stabilising soil volume in clay-rich areas.

Seasonal Context & Subsidence

The current week (around July 22, 2025) is in mid-summer, when SMD is typically high due to elevated evapotranspiration. Historical mid-July data shows:

Week 30, 2024 (0.7 mm rainfall): SMD = 122.0 mm

Week 29, 2024 (27.5 mm rainfall): SMD = 95.5 mm

Week 30, 2023 (12.7 mm rainfall): SMD = 206.8 mm

Week 30, 2022 (4.0 mm rainfall): SMD = 323.4 mm

The current SMD of 315.5 mm reflects very dry soil, increasing the risk of subsidence in clay-heavy regions of London. Significant rainfall, like this week’s 40 mm, can mitigate this risk by reducing SMD and rehydrating the soil.

Estimating the Impact of 40 mm Rainfall on SMD & Subsidence

With a starting SMD of 315.5 mm and 40 mm of rainfall:

Historical data suggests an average SMD reduction of ~33.7 mm, with a range of 24–44 mm

Given the high starting SMD and summer conditions, some water may be lost to runoff or evaporation, but a significant reduction is still expected

We estimate the SMD will decrease to approximately 281.8 mm (315.5 mm – 33.7 mm), with a possible range of 271.5–291.5 mm

This reduction in SMD suggests that the soil will become less dry, potentially reducing subsidence risk by minimising soil shrinkage. However, the soil remains in a significant deficit, indicating that subsidence risks may persist, particularly in areas with clay soils prone to volume changes.

Implications for Subsidence & Geobear’s Solutions

The reduction in SMD from 315.5 mm to approximately 281.8 mm (range: 271.5–291.5 mm) suggests that this week’s rainfall will rehydrate the soil, potentially reducing subsidence risk by minimising soil shrinkage in clay-rich areas of London. However, the soil remains in a significant deficit, indicating that subsidence risks may persist, particularly for properties built on shrinkable clay soils.

By understanding soil moisture dynamics, we can better predict when and where subsidence risks are highest and tailor our solutions to protect properties. This analysis underscores the importance of monitoring weather patterns and SMD to proactively manage subsidence risks.

Conclusion

Based on historical patterns from 2018–2025, London’s recent 40 mm rainfall is expected to reduce the Soil Moisture Deficit from 315.5 mm to approximately 281.8 mm – we will find out when the latest Met Office data is released this week. This reduction can help mitigate subsidence risk by stabilising soil volume, but the high summer SMD suggests ongoing vigilance is needed. At Geobear, we’re committed to leveraging data-driven insights to protect properties from subsidence. If you’re concerned about subsidence in your area, contact us to learn how our innovative solutions can safeguard your home or business.