What could cause a decrease in water level in an uncoated concrete cistern?

An uncoated concrete cistern can experience a decrease in water level primarily due to its porous nature, which allows water to seep into the material itself. Concrete, particularly when uncoated, has tiny pores and voids that can absorb water, leading to a gradual loss of water from the cistern. This infiltration contributes significantly to the overall decrease in water volume, as the water is not merely lost to the atmosphere or through evaporation but is absorbed directly into the structure of the concrete.

While other options might also lead to a reduction in water level, they are less likely to be as significant as the factors related to the inherent properties of uncoated concrete. For example, evaporation of surface water can occur, but it is generally a slower process compared to the water loss caused by the absorption of water into the porous concrete. Leakage due to insufficient seals typically refers to physical breaches that might not affect uncoated cisterns unless there are obvious cracks or gaps present. Similarly, local wildlife consumption could result in some water loss, but is often not as substantial as the water absorbed by porous materials like concrete. Therefore, the porosity of uncoated concrete is the primary reason for the water level decrease in a more pronounced manner.