Water: rivers, basins, aquifers and water resources management
Domain 3 treats water as a system: where runoff is generated, how regimes behave, who allocates the resource and what limits its quality.
Basins and regimes
- The Duero is the axis: about 897 km, roughly 97 000 km² of basin (some 79 000 km² in Spain), Atlantic drainage, rising in the Picos de Urbión and reaching the sea at Porto. Tributaries: Pisuerga, Esla, Órbigo, Tera, Cea, Valderaduey, Adaja, Tormes, Águeda, Duratón and Riaza.
- Regime: pluvio-nival, maximum in February–March (rain plus snowmelt from the Cantabrian and Iberian headwaters), minimum in August–September, with strong interannual variability and drought sequences (1992–95, 2005, 2017, 2022).
- Other basins touching the region: Ebro (Burgos), Miño-Sil (León) and the Cantabrian catchments.
- Aquifers: detrital fill of the Duero basin - páramo limestone, rañas and terraces - locally overexploited and affected by diffuse nitrate pollution.
Management
- The Confederación Hidrográfica del Duero (1927) is the oldest basin authority; it drafts the Plan Hidrológico de cuenca (third cycle, 2022–2027) and runs the Sistema de Explotación.
- Storage: Riaño, Barrios de Luna, Ricobayo, Villalcampo, Castro, Almendra and Saucelle - headwater dams for regulation and the Saltos del Duero hydroelectric chain.
- Uses: irrigation dominates consumptive demand (roughly 80–85 %), followed by urban supply and industry; hydropower is non-consumptive.
- Quality is assessed through the Water Framework Directive's ecological status, with nitrate-vulnerable zones and wastewater treatment as the main instruments.
Worked example
A 1 200 km² basin receives 650 mm of precipitation and loses 470 mm to actual evapotranspiration. Since 1 mm over 1 km² equals 1 000 m³, precipitation is 780 hm³ and ET is 564 hm³. Runoff ≈ 216 hm³, i.e. 180 mm, which over a year is about 6.8 m³/s, or a specific discharge of roughly 5.7 L/s/km² - a moderate yield typical of a continental basin.
Traps
- Confusing caudal (flow) with aporte (volume).
- Mixing mm and hm³ without converting.
- Calling the Duero regime nival: snowmelt is secondary to rainfall.
- Assuming hydropower consumes water.
- Forgetting the Portuguese section of the river.