بِسۡمِ ٱللَّهِ ٱلرَّحۡمَـٰنِ ٱلرَّحِيمِ

09/06/2026

Allah knows best, but we learn through research. We may be right or wrong, and we do our best. If we are wrong, we seek Allah’s forgiveness; if right, then Alhamdulilah (Praise be to Allah).

There are many studies on this particular topic, and It will be not correct to claim that I will be able to cover them all, as the Quran contains endless knowledge that transcends time and cultural boundaries. The depth and richness of its teachings provide invaluable insights into various aspects of life, human nature, and the universe. After we pass from this earthly existence, others will continue to discover, interpret, and teach these profound lessons to new generations. This ongoing journey of exploration and understanding is what makes the Quran truly a great miracle in its wisdom and relevance. Each verse invites further contemplation, encouraging believers and scholars alike to delve deeper into its meanings and applications in contemporary life. Allahu Akbar (God is Great).

The Hydrological Equilibrium of Surah Al-Mu’minun: A Multidisciplinary Analysis of Chapter 23 Verse 18

Exegesis and Scientific Analysis of Surah Al-Mu’minun, Verse 18

وَأَنزَلۡنَا مِنَ ٱلسَّمَآءِ مَآءَۢ بِقَدَرٍ۬ فَأَسۡكَنَّـٰهُ فِى ٱلۡأَرۡضِ‌ۖ وَإِنَّا عَلَىٰ ذَهَابِۭ بِهِۦ لَقَـٰدِرُونَ (١٨)

Translation

“And We have sent down rain from the sky in a measured amount and settled it in the earth. And indeed, We are Able to take it away.” 

This verse serves as a profound theological and teleological argument, pointing to the precision of the natural world as evidence of Divine Providence. In classical Islamic scholarship, this “measured amount” (bi-qadarin) signifies a delicate balance—sufficient to sustain life, agriculture, and the replenishment of aquifers, yet not so excessive as to cause perpetual destruction or so sparse as to lead to total desiccation.

The term fa-askannahu (and We settled it) refers to the geological and hydrological capacity of the Earth to store water in sub-surface reservoirs, ensuring a continuous supply even during dry seasons.

The Hydrological Cycle and “Measured Amount”

From a scientific perspective, the “measured amount” can be interpreted through the Global Water Budget. The Earth’s hydrological cycle is a closed system where the total mass of water remains essentially constant over geological timescales.

The precision mentioned in the verse aligns with the concept of the Steady State in hydrology. If the rate of precipitation significantly exceeded the rate of evaporation and infiltration over a long period, terrestrial life would be unsustainable due to constant flooding.

The balance of the global water cycle is represented by the Water Balance Equation:$$P=E+R+\mathrm{\Delta }S$$Where:

• $P$ is Precipitation (the water “sent down”).
• $E$ is Evapotranspiration (water returning to the atmosphere).
• $R$ is Runoff (water flowing into rivers and oceans).
• $\mathrm{\Delta }S$ is the change in storage (water “settled” in the earth).

The “measured amount” also relates to the atmospheric capacity to hold water vapor, governed by the Clausius-Clapeyron Equation, which dictates that the water-holding capacity of the air increases by approximately 7% for every 1°C rise in temperature:[5]$$\frac{d{e}_s}{dT}=\frac{L_v(T)e_s}{R_v{T}^2}$$This physical law ensures that rain does not fall all at once but is distributed according to thermodynamic limits.

“Settling” Water in the Earth: Hydrogeology

The verse highlights that water is “settled” (askannahu) in the earth. This refers to the infiltration of water into the lithosphere to form groundwater. Without the porous nature of soil and the existence of aquifers, rainwater would simply run off into the salty oceans, becoming inaccessible for human consumption and inland agriculture.

The movement and “settling” of water in the earth are governed by Darcy’s Law, which describes the flow of a fluid through a porous medium:$$Q=-KA\frac{dh}{dl}$$Where:

• $Q$ is the total discharge.
• $K$ is the hydraulic conductivity (the “settling” capacity of the soil).
• $A$ is the cross-sectional area to flow.
• $\frac{dh}{dl}$ is the hydraulic gradient.

This mechanism allows the Earth to act as a massive natural filter and storage tank. The Quranic warning that “We are Able to take it away” finds a modern parallel in the depletion of fossil aquifers and the lowering of water tables due to over-extraction or climate shifts, which can render previously “settled” water unreachable.

The Atmospheric Balance and Precipitation Physics

The process of rain being “sent down” involves the condensation of water vapor onto Cloud Condensation Nuclei (CCN). The physics of droplet growth is essential for the “measured” delivery of water. If droplets did not reach a specific terminal velocity, they would remain suspended as clouds; if they were too large, they would cause physical damage upon impact.

The terminal velocity ($v_t$) of a falling raindrop is calculated by balancing the gravitational force with the drag force:$$v_t=\sqrt{\frac{4gd({\rho }_p-{\rho }_f)}{3C_d{\rho }_f}}$$Where:

• $g$ is acceleration due to gravity.
• $d$ is the diameter of the drop.
• ${\rho }_p$ is the density of the water.
• ${\rho }_f$ is the density of air.
• $C_d$ is the drag coefficient.

Theological and Ecological Implications

Classical commentators like Ibn Kathir emphasize that the “measured amount” refers to the specific needs of different geographical regions.

Modern ecological science supports this by showing how ecosystems are finely tuned to local precipitation patterns. A disruption in this “measure”—such as through anthropogenic climate change—leads to the “taking away” of water through desertification or the “excess” of water through catastrophic flooding, both of which are mentioned as warnings in the text.

The verse concludes with a reminder of Divine Power over these cycles. In environmental science, this is reflected in the fragility of the “Goldilocks” conditions that allow liquid water to exist on Earth. If the Earth were slightly closer to or further from the sun, or if the atmospheric pressure changed, the water would either boil away or freeze permanently, effectively “taking it away” from biological use.

Geological Structures and the “Settling” of Water

The Quranic term fa-askannahu (and We settled it) describes the transition of water from a dynamic, falling state to a stored, static state within the Earth’s crust. Geologically, this “settling” occurs through the process of infiltration and percolation, where water moves through the unsaturated zone (vadose zone) to reach the saturated zone, forming an aquifer.

1. The Architecture of Aquifers

For water to be “settled” effectively, the geological formation must possess two specific physical properties:

• Porosity (ϕ): The measure of void spaces in a material. It is defined as the ratio of the volume of voids ($V_v$) to the total volume ($V_t$):$$\varphi =\frac{V_v}{V_t}$$
• Permeability (k): The ability of the rock or soil to transmit fluids. Without permeability, water would be trapped in the sky or remain on the surface as stagnant pools rather than “lodging” in the earth.

2. Confined vs. Unconfined Storage

The “settling” mentioned in the verse can be categorized into two geological structures:

• Unconfined Aquifers: These are closer to the surface and are replenished directly by rainwater. The upper limit is the water table.
• Confined (Artesian) Aquifers: These are trapped between impermeable layers of rock (aquicludes). This represents a more “permanent” settling, where water can remain for thousands of years (fossil water) until it is “taken away” or extracted.

Atmospheric Distribution and the “Measured” Global Climate

The “measured amount” (bi-qadarin) is not only about the volume of water but also its spatial and temporal distribution. This is governed by the General Circulation Models (GCMs) of the atmosphere, which ensure that water vapor is transported from the oceans to the interiors of continents.

1. The Hadley Cell and Latitudinal Measure

The Earth’s atmosphere is divided into circulation cells (Hadley, Ferrel, and Polar). The Hadley Cell is responsible for the “measured” rainfall in the tropics. Warm air rises at the equator, cools, and drops moisture as heavy rain. The dry air then sinks at ${30}^{\circ }$ latitude, creating the world’s deserts.

This distribution is a physical manifestation of the “measure” described in the verse—providing abundance where tropical ecosystems require it and scarcity where desert ecosystems have adapted.

2. The Role of Latent Heat

The “sending down” of rain is a massive energy transfer process. When water evaporates, it absorbs energy (Latent Heat of Vaporization, $L_v$). When it condenses into rain, it releases that energy back into the atmosphere:$$Q=m\cdot L_v$$Where:

• $Q$ is the heat energy released.
• $m$ is the mass of the water.
• $L_v\approx 2.5\times {10}^6\text{J/kg}$.

This energy release drives atmospheric circulation, ensuring that the “measured amount” of rain is moved across the globe rather than falling only over the oceans.

The Warning: “We are Able to Take it Away”

The verse concludes with a warning of the water’s removal. Scientifically, this occurs through several mechanisms:

• Lowering of the Potentiometric Surface: If extraction exceeds the “measured” recharge rate ($R$), the water table drops, effectively “taking away” the water from human reach.
• Salinization: In coastal areas, if the pressure of the “settled” fresh water decreases, salt water intrudes into the aquifer, rendering the water unusable.
• Climatic Shifts: Changes in the Earth’s orbital parameters (Milankovitch Cycles) or atmospheric composition can shift rain belts, turning fertile “settled” lands into arid wastes.

Fossil Aquifers and the Natural Purification of Subsurface Water

The “settling” of water into the earth, as described in Surah Al-Mu’minun (23:18), involves two remarkable phenomena: the long-term storage of water in “fossil aquifers” and the intricate chemical and physical filtration that occurs as water migrates through the lithosphere. These processes ensure that water is not only stored but also rendered safe for consumption over millennia.

Fossil Aquifers: Water Settled Through Geological Time

A “fossil aquifer” (or paleowater) is a large underground reservoir of water that was “settled” or trapped during past geological ages, often under climatic conditions significantly different from those of today.

These aquifers are typically found in arid regions, such as the Sahara Desert or the Arabian Peninsula, where current rainfall is insufficient to recharge the groundwater.

The water in these aquifers is often tens of thousands of years old. For example, the Nubian Sandstone Aquifer System in Northeast Africa contains water that was deposited during the Pleistocene epoch, when the region was a lush, rainy savannah.

Because these aquifers receive little to no modern recharge, they are considered non-renewable resources.

The age of this “settled” water is determined using Isotope Hydrology, specifically Carbon-14 (${}^{14}C$) dating or Chlorine-36 (${}^{36}Cl$) for much older samples. The decay of the radioactive isotope follows the first-order kinetic equation:$$N(t)=N_0{e}^{-\lambda t}$$Where:

• $N(t)$ is the remaining amount of the isotope.
• $N_0$ is the initial amount.
• $\lambda$ is the decay constant.
• $t$ is the time since the water was “settled” and isolated from the atmosphere.

The Chemical and Physical Purification of “Settled” Water

As water moves from the surface into the earth, it undergoes a process known as Soil-Aquifer Treatment (SAT). This natural filtration system utilizes physical, chemical, and biological mechanisms to purify the water as it “lodges” in the soil.

1. Physical Filtration and Mechanical Straining

As water percolates through the soil, the pore spaces between sand and silt particles act as a natural sieve. Larger suspended solids, pathogens, and organic debris are trapped. This is governed by the Straining Ratio, where particles larger than approximately 5% of the soil grain diameter are physically removed.

2. Chemical Adsorption and Cation Exchange

Soil particles, particularly clays and organic matter, often possess a negative surface charge. This allows them to attract and bind to positively charged ions (cations) such as heavy metals (lead, mercury) and certain toxins. This process is described by the Freundlich Adsorption Isotherm:$$q_e=K_f{C}_e^{1/n}$$Where:

• $q_e$ is the amount of solute adsorbed per unit mass of soil.
• $C_e$ is the equilibrium concentration of the solute in the water.
• $K_f$ and $n$ are constants specific to the soil type.[6]

3. Biological Degradation

The upper layers of the soil (the vadose zone) are teeming with aerobic bacteria. As water passes through, these microbes break down organic pollutants and nitrogenous compounds. This “settling” process effectively transforms harmful waste into harmless minerals through oxidation-reduction (redox) reactions.

The Fragility of “Settled” Water: The Warning of Depletion

The Quranic verse warns, “And indeed, We are Able to take it away.” In modern hydrogeology, this “taking away” is observed through Groundwater Mining. When the rate of extraction ($Q_{out}$) exceeds the rate of natural recharge ($Q_{in}$), the water table drops, and the aquifer may eventually collapse or become contaminated by saltwater intrusion.

The depletion of a fossil aquifer can be modeled by the change in hydraulic head ($h$) over time ($t$) in a confined aquifer:$$S\frac{\partial h}{\partial t}=T{\nabla }^{2}h-Q$$Where:

• $S$ is the storativity (the volume of water the aquifer releases).
• $T$ is the transmissivity.
• $Q$ is the pumping rate.

If $Q$ is too high, the “settled” water is effectively “taken away,” leaving the land parched and the soil prone to subsidence.