Hydrological Factors and River Systems in Peninsular Spain

The waters of Spanish territory are conditioned by a variety of physical and human factors.

Factors Influencing Water Diversity in Spain

• Climate: Plays a major role, as the water in rivers, aquifers, and lakes originates from rainfall.
• Relief and Topography: Influence the organization of watersheds, the erosive capacity of rivers, and the formation of lakes and aquifers.
• Lithology (Rock Type): The type and characteristics of the rock may promote surface runoff or, conversely, facilitate filtration, as is the case with limestones.
• Vegetation: This factor acts as a shield against solar radiation and reduces evaporation.
• Human Activity: Humans are the most notable and negative factor for water resources. Consumption is often offset by the construction of infrastructure like dams and diversions.

Spanish Rivers: Influences in the Peninsula

A. Climate and River Flow Regimes

The climate generally determines the absolute flow and regime of rivers, primarily based on precipitation. River regimes are seasonal variations in flow, which depend on the distribution of precipitation. Three main regimes can be distinguished:

• Nival Regime: These rivers originate in mountains where snowfall is frequent. Peak flows occur during the spring thaw, while flows are minimal in winter.
• Pluvial (Rainfall) Regime: The maximum or minimum flow depends directly on the climate zone and the distribution of rainfall.
• Mixed Regime: These may be Nivo-pluvial or Pluvio-nival, depending on whether snowmelt or rainfall dominates the flow.

B. Relief and Topography: Organization and Erosion

Relief and topography determine the organization of hydrographic units:

• River Basin: This is the territory whose waters discharge into a main river and its tributaries. Basins are separated by watersheds, which coincide with areas of slope change.
• Catchment Area (Hydrographic Unit): This refers to the set of catchments that drain into a larger area.
• Slope and Erosion: The slope and erosive power of rivers are greater if there is a strong inclination of the terrain.

Peninsular Hydrographic Slopes

The peninsular rivers are divided into three main hydrographic slopes:

1. Cantabrian Slope

   In this area, the rivers are short, rising in mountains near the coast. They possess great erosive power. Due to the ease offered by the steep topography, reservoirs are constructed for the production of electricity.

2. Atlantic Slope

   The rivers are long, as they originate far inland (near the divide with the Mediterranean). They run through plains where they barely sink, but form ravines in areas of elevation change. Their regime is irregular: minimum flow in summer, and increased flow with autumn rains and spring thaw.

3. Mediterranean Slope

   The rivers are short, given the proximity of the sea to the mountains in which they arise. Rivers violently erode deforested hillsides. Their regime is very irregular, characterized by low flow in summer. Due to the phenomenon of the gota fría (cold drop), these rivers may suffer catastrophic floods in autumn.