Model maps shallow ground temperatures beneath ten Egyptian cities for geothermal design

Engineers designing shallow geothermal systems in Egypt have long faced a stubborn data problem: the country’s soils hold a stable thermal reservoir that could ease its enormous cooling burden, but measured subsurface temperature profiles barely exist outside a handful of localized studies. A new data paper published in Geothermal Energy on 26 October 2026 addresses that gap by presenting daily subsurface temperature profiles for ten Egyptian cities, simulated from Typical Meteorological Year (TMY) data and covering depths from the surface down to ten metres.

The study, led by Anwar Hegazy, uses a heat conduction model for a semi-infinite solid to translate hourly meteorological data into annual thermal variations underground. The dataset captures the essential physics of shallow soil heat transfer: seasonal thermal inversion, in which the temperature gradient reverses direction between summer and winter; amplitude damping, in which temperature swings shrink with depth; and phase lag, the thermal delay that pushes the underground temperature cycle progressively out of step with the seasons. According to the paper, these patterns are consistent across Egypt’s climatic zones, from the Mediterranean coast to the arid regions of Upper Egypt.

The motivation is rooted in Egypt’s energy profile. Air conditioning has become the single largest contributor to electrical peak load and overall energy consumption in many markets of the Middle East and North Africa, and the paper notes that Egypt aims to generate 42 percent of its power from clean sources by 2035. Ground Source Heat Pumps (GSHP) and Earth-Air Heat Exchangers (EAHE) exploit the fact that soil maintains a relatively stable temperature, cooler than ambient air in summer and warmer in winter, reducing the temperature difference that mechanical systems must overcome and thereby lowering energy consumption and peak demand.

Obtaining the soil temperature data that governs these systems is expensive in a country where more than 94 percent of the landscape is desert. On-site field measurements are described as prohibitively expensive and logistically demanding across Egypt’s diverse climatic zones, which is why mathematical modeling using readily available meteorological data has become the accepted standard for estimating subsurface temperatures. The paper identifies a specific practical gap: engineers currently rely on either long-term experimental measurements or unvalidated assumptions and overdesigned safety margins, leading to inefficient or cost-prohibitive systems, and no comprehensive, country-wide subsurface temperature map derived from reliable meteorological data has existed publicly.

The model itself is an analytical solution to periodic heat conduction. It assumes constant soil thermal properties, with the local sandy soil characterized by a density of 1775 kilograms per cubic metre, a specific heat capacity of 840 joules per kilogram-kelvin, and a thermal conductivity of 0.91 watts per metre-kelvin. The surface boundary condition is a sinusoidal annual cycle around a mean temperature, and the solution yields temperature as a function of depth and time given the soil’s thermal diffusivity. Three parameters anchor the calculation: the annual mean soil surface temperature, the surface temperature amplitude, and a phase shift. The paper estimates the mean by adding 1.7 degrees Celsius to the annual average ambient temperature, estimates the amplitude by adding 1.1 degrees Celsius to half the difference between average July and January air temperatures, and sets the phase lag at 36 days, derived from the theory that the surface temperature wave lags solar radiation by one-eighth of a cycle.

The model’s predictions were compared against annual-cycle borehole measurements recorded between August 2022 and July 2023 at a pilot site at Cairo University’s Faculty of Engineering. Across all evaluated depths and monthly intervals, the percentage error between simulated and measured profiles remained below 10 percent, and the Mean Absolute Percentage Error was 3 percent. The authors attribute the residual discrepancies to three factors: a spatial offset between the experimental site and the Cairo weather station supplying the meteorological inputs, the inherent measurement uncertainty of the reference study’s sensors, and a temporal mismatch, since the meteorological baseline relies on 30-year monthly averages spanning 1981 to 2010, which may not fully account for recent decadal warming or intensified urban heat island effects in Cairo.

The resulting profiles reveal a clear geographic pattern. Cities in Upper Egypt, including Asyut, Minya, Luxor, Kharga, Hurghada, and Aswan, show the highest thermal amplitudes, with peak soil surface temperatures reaching or exceeding 35 degrees Celsius. Alexandria, Arish, Ismailia, and Cairo show more attenuated ranges, with peaks remaining at or below 30 degrees Celsius. Across all ten cities, minimum subsurface temperatures occur between days 50 and 100 of the year and maxima between days 200 and 250, and the amplitude attenuation and increasing phase lag with depth follow standard thermal diffusion behavior.

Perhaps the most consequential finding for system designers concerns depth. Beyond four metres, the modeled soil temperature remains stable, with an annual fluctuation below 1.5 degrees Celsius, and throughout the ten cities the temperatures at those depths stay bounded between 20 and 30 degrees Celsius. That stability is precisely what makes the shallow subsurface attractive as a heat source and sink: at the modeled depths, the soil remains cooler than ambient air in summer and warmer in winter, creating a temperature differential that can be harnessed for space heating and cooling. The monthly vertical profiles also display the characteristic V-shaped thermal distribution, with summer months showing a negative gradient with depth and winter months a positive gradient in which subsurface temperatures exceed surface values.

The authors are explicit about the model’s limits. The formulation operates on an annual single-harmonic sinusoidal wave driven by monthly and daily average ambient temperatures, which fully dominates heat conduction for installations between 0.5 and 10 metres deep, but for ultra-shallow systems buried in the top half-metre, such as shallow horizontal ground heat exchangers or agricultural EAHE ducts, diurnal solar and temperature fluctuations introduce secondary thermal waves that the current model does not resolve. The model also assumes homogeneous, isotropic soil properties across the ten-metre column, whereas real soil stratigraphy features layers with variable moisture content and thermal conductivity. Regional hydrogeology matters too: coastal cities such as Alexandria have high saline water tables and groundwater advection dynamics that alter effective thermal conductivity compared with dry inland desert soils like those at Kharga or Aswan, and local variations in compaction, bulk density, and organic fraction will introduce deviations in phase lag and amplitude damping in field implementations. The dataset is limited to the upper ten metres, and the authors note that future work should investigate deeper geothermal gradients for vertical borehole optimization.

As a data resource rather than a performance study, the paper’s contribution is a baseline. The profiles are intended for preliminary design and feasibility assessment of shallow geothermal systems, allowing engineers to optimize pipe burial depths and design parameters for Earth-Air Heat Exchangers and Ground Source Heat Pumps, and the daily resolution permits coupling with building energy simulation tools for dynamic modeling of integrated HVAC systems. By moving Egyptian shallow geothermal research from localized case studies to a nationwide thermal baseline, the authors argue, the dataset provides the technical foundation for sizing geothermal assets in a country where cooling demand strains the grid and where, until now, the shallow subsurface has remained largely unmapped in publicly accessible, depth-specific detail.

Subject of Research: Modeled subsurface temperature profiles for shallow geothermal potential across ten Egyptian cities

Article Title: Subsurface temperature profiles across ten Egyptian cities: mapping low-enthalpy geothermal potential for regional energy adaptation

Article References: Hegazy, A. (2026). Subsurface temperature profiles across ten Egyptian cities: mapping low-enthalpy geothermal potential for regional energy adaptation. Geothermal Energy, 14(1), Article 30. https://doi.org/10.1186/s40517-026-00409-1

Image Credits: AI Generated

DOI: 10.1186/s40517-026-00409-1

Keywords: geothermal energy, Egypt, subsurface temperature, ground-source heat pump, earth-air heat exchanger, typical meteorological year, heat transfer model, shallow geothermal, arid climate, building cooling, thermal diffusivity, Cairo validation