Surface Water Management: Rivers, Reservoirs, and Quality

Surface Water Resources and Management

Surface water bodies are the most important source of water supply. A prominent feature of these resources is their variability, which is highly dependent on irregular rainfall patterns.

Rivers: Characteristics and Flow Regime

The river network in Spain (approximately 70,000 km) often carries a relatively small volume of water, and its flow rate is highly irregular. Furthermore, rivers receive large volumes of spills, resulting in many contaminated sections.

Factors Characterizing River Flow

The river regime is characterized by several factors:

1. Flowrate: The amount of water flowing through a river at one specific point. It is measured in m³/s.

Flow = A · P · v

Irregularity: The ratio between the average annual flow and the maximum and minimum flows recorded. Floods and Droughts: Measured data relating water volume versus dates, frequency, and probability. Seasonal Variations: The ratio of the average monthly flow to the annual average flow rate. Materials Towed: The elements (debris) dragged by the river, measured in Kg/cm².

The characteristics of the flow regime differ greatly from one river to another, and even between different sections of the same river.

River Catchment Conditions

• Water collection should preferably occur upstream, but the collection point must be located at a minimum distance away from population centers.
• Water intake can take two forms:
  • Direct: Via a submerged channel.
  • Indirect: Via an infiltration gallery.

Reservoirs: Definition and Importance

Definition: An artificial lake that holds water in a river's unidirectional flow.

Reservoirs are used to regulate watershed runoff and adapt the flow to meet demand needs. In Spain, without reservoirs, only 10% of water resources would be utilized; with them, utilization reaches 30%.

Reservoir Catchment and Water Quality

Catchment conditions must account for water quality variations at different depths:

• Ideally, the intake point (tap/area) should be selected carefully due to varying water quality.
• The surface layer often contains large amounts of algae, plankton, and floating materials (leaves, branches).
• Deep water often suffers from low O₂ levels, black sludge, putrefaction reactions, and foul odors (sulfides). Intake points are often located near the surface or near the bottom (30–35 m depth) to avoid mid-level issues, but must be managed carefully.
• Outlets must be positioned away from the banks.

Managing Eutrophication in Reservoirs

Eutrophication is the main factor (problem) to be considered in a reservoir. It differs depending on the reservoir's situation. The best reservoirs are often those located in the upper reaches of the river, characterized by rich forest cover, mild summers, and major renovation (turnover) of the water.

Total Phosphorus Concentration and Control Measures

Measures to address eutrophication:

• Use manure instead of inorganic fertilizers.
• Use detergents with reduced phosphate content.
• Eliminate wastewater discharges.
• Use of algaecides (e.g., CuSO₄).
• Use of predatory organisms or organisms toxic to algae (careful dosage is required).
• Reduce water illumination by creating shadow areas, staining the water, or adding suspended particles.
• Mechanical harvesting of algae.
• Dredging the bottom of the reservoir.
• Remove nitrogen using microorganisms.
• Hydraulic control to modify the rates of water removal to control the dam's output and tailor it to water demand.