Omvattennormale

Omvattennormale is a term used in hydrology and environmental science to denote the nominal water state of a system, representing a baseline or reference condition for water availability, level, and storage over a specified reference period. It is not a physical constant, but a statistical construct designed to support comparison across seasons and years. The term is used in some Scandinavian hydrological literature and may be defined differently by institutions, but generally refers to the long-term average state of the system when external drivers are within typical ranges.

Etymology: The word combines om- (about/around or surrounding), vatten (water) and normale (norm/normal). It is akin to the concept of climate normals, but applied to hydrological variables.

Definition and scope: An omvattennormale can cover variables such as water level in rivers and lakes, volumetric storage in reservoirs, soil moisture, or groundwater storage. It is derived from historical records or model-derived climatologies, often using a multi-year smoothing window to filter out short-term fluctuations. Purpose: it provides a reference for assessing anomalies, planning water resources, drought and flood risk, and ecological assessments.

Relationship to other concepts: It complements but is distinct from extreme value analyses, hydrological norms, and the water balance. It should not be confused with legal standards of minimum or maximum levels; rather, it serves as a baseline for evaluation.

See also: water balance, climate normals, hydrology, drought assessment, flood risk.