Systems of the Duero River Basin Cega-Eresma-Adaja and Tordesillas-Toro (Spain) Water resources

Recursos hídricos, usos, infraestructuras e impactos

The natural water resources estimated by the CHD (2021) for CEA and T (1940-2005) are 644.6 hm³/year and 1,000.8 hm³/year. The water is lightly mineralized, with a predominance of calcium bicarbonate salts. The CEA system has 12 water reservoirs (see location in Figure 3.1a); the main one is "Las Cogotas" on the Adaja River, which supplies water to irrigation areas, municipalities, and industries. The second is "Pontón Alto," which supplies water to the main cities of Segovia and Ávila. In 2027, it is planned to use the "Ciguiñuela" and "Carboneros" reservoirs on the Eresma River and "Lastras" on the Cega River to allocate water to new irrigation areas intended to replace the use of groundwater in "Los Arenales" with surface waters.

The T system encompasses four water reservoirs: San Román, Villalcampo, Castro, and San José (see location in Figure 3.2b). The first three are dedicated to hydroelectric production (San Román and Pereruela); the second supplies water to the San José and Toro-Zamora irrigation areas.

The quantity and quality of the water in the reservoirs are monitored through several discharge and quality control points. Surface and groundwater bodies are presented in the Figures.

 
In CEA, most rivers are directly connected to aquifers, but groundwater depletion due to overexploitation has sometimes disconnected the flow between riverbeds and aquifers. This situation is aggravated during dry periods when most rivers have very low flows. The water table level in CEA is between 100-300 m deep. In T, the water table level ranges between 80-120 m and has dropped nearly 16 m since 1975, not only due to dry hydrological years but also because of illegal extractions (CHD, 2015). In the CEA, the irrigation area "El Carracillo" and the "Cubeta de Santiuste" use groundwater supplied through Managed Aquifer Recharge (MAR). In the first, water is diverted from the Cega River, 30 km from the irrigable area, and in the second, from the Voltoya River (taken from the Pinarejos dam). Both areas are associated with the "Los Arenales" groundwater body, which is in poor quantitative condition according to the Water Framework Directive. The "El Carracillo" irrigation district can divert water from the Cega River between December 1 and April 30, up to a maximum of 14.2 hm³/year; the maximum volume must not exceed 5 hm³/month. The diversion of water from the Voltoya River is restricted from December to May with a maximum volume of 0.48 hm³/year. In "El Carracillo," the growing water demand has deteriorated water quality and threatened the sustainability of the basin and its economic activities. This issue may be worsening, as the Duero Basin Plan (2015-2021) has approved an increase in irrigated area (equivalent to 7,000 ha) by 2027, despite the existing water gap (Rivas-Tabares et al., 2019). In 1998, hydraulic infrastructures for water supply, flood control, wastewater treatment, and irrigation were designed for the MAR project, declared of general interest by Royal Decree (R.D. Law 9/1998). The decree aims to recover and preserve the aquifer and provide water to meet the agricultural needs of the area (3,000 ha). The technology includes ponds that infiltrate water at various points (see Figure 3.3). The EU-funded MARSOL project (2013-2016) conducted studies in the area to demonstrate that managed aquifer recharge techniques can ensure the "excess" water and store it in the soil.

Artificial recharge of managed aquifers in "El Carracillo" (CEA): (a) control head; (b) settling pond; and (c) infiltration canal.

The main use of water is for irrigation (96% in T and 79% in CEA), followed by human consumption (3% in T and 18% in CEA) and industry (0.5% in T and 1.9% in CEA). The CEA is a surface water body that shows a low IPS (Specific Pollution Sensitivity Index), a biological indicator of water quality related to benthic organisms. The economic development of the El Carracillo region (CEA) is due to the overexploitation of the aquifer, which has prompted the CHD to address associated water management issues. The actual water consumption of crops is expected to cause significant or severe water stress levels for two to five months per year. The anticipated expansion of irrigation in the coming years could complicate water management. In the CEA, in the "Los Arenales" area, the quantitative and qualitative status of water is rated as "poor"; its exploitation index is above 0.8 (0.86), mainly due to the drop in the water table level caused by agricultural activities. Currently, it is stabilized, although the accumulated decline remains significant. The water is in poor condition; the average nitrate concentration exceeds 50 mg/l in 10 of the 31 monitoring stations. Additionally, ammonium content has also been high, although it is currently rated in good condition (CHD, 2015). Similarly, in T, the "Tordesillas" area, the quantitative and qualitative status of water is rated as "poor" (exploitation index = 1.03), similar to that of the CEA and caused by the same factors. Its poor quality conditions are associated with high ammonium content detected in two of the nine monitoring stations. Furthermore, the diversity of bird species has decreased, resulting in a reduction in insect control.

Authors: Leonor Rodriguez Sinobas, Universidad Politécnica de Madrid, Spain Illiasou Naroua, Universidad Politécnica de Madrid, Spain

To cite this document: Rodríguez Sinobas, L., Naroua, I. 2022. Document 2 – Systems of the Duero River Basin Cega-Eresma-Adaja and Tordesillas-Toro (Spain). NEXUS Water-Energy-Environment: fair and sustainable agri-food systems. Delivery D3.1. H2020 PRIMA NEXUS-NESS Project. https://prima-nexus-ness.org