Publicación: EL caudal mínimo medioambiental del tramo inferior del río Ebro
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2009-05-01
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Atribución-NoComercial-SinDerivadas 4.0 Internacional
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Universidad Nacional de Educación a Distancia (España). Centro Asociado de Tortosa
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La idea de mantener unos caudales mínimos en los ríos no resulta absolutamente nueva. En el siglo XIX, por ejemplo, se llegó a fijar por tramos la anchura normal de las aguas medias de los ríos del norte de Alemania, continuando por la regulación de las aguas bajas, por lo que se recomendaba referir las profundidades y las anchuras de las aguas bajas que se tenían que respetar al nivel del estiaje medio, es decir, a la media de los niveles más bajos del verano observados durante un dilatado periodo de tiempo. En subsuelo de roca, las secciones transversales normales resultaban de la profundidad y de la anchura del cauce navegable, pero en los tramos de río móviles formados por materiales sueltos, o bien cuando se intentaba variar la sección transversal para mejorar las condiciones del río, tenían que fijarse aquellos parámetros de modo que, además de la profundidad deseada, se obtuviera también la anchura exigible. Posteriormente, se establece el principio de que parte del agua disponible se tiene que destinar a asegurar el mantenimiento de un cierto “caudal ecológico mínimo” en los ríos, que asegure la permanencia de la biota preexistente, sin deterioro ambiental. Esto significa, por un lado, que las obras o infraestructuras de regulación y derivación de caudales tendrán que garantizar un caudal remanente en el río aguas abajo de las mismas, y por otro lado, que una parte de este remanente no tendrá otra utilización y constituirá por sí mismo una demanda de agua. O sea, que la parte de esta demanda ambiental que no esté incluida en las demandas existentes o consolidadas se deberá cubrir adicionalmente en base a las prioridades de uso que, en nuestro caso, el Plan hidrológico de la cuenca del Ebro señale para este tramo del río. En el caso del delta del Ebro, espacio natural de extraordinario interés ecológico nacional e internacional, el denominado "caudal de compensación" previsto en el Plan Hidrológico de la Cuenca del Ebro de 100 m3/seg., resulta insuficiente y se fijó en su día de un modo aleatorio e injustificado. Tal como ya se señala en el estudio que presentamos, que emplea una metodología novedosa, este caudal no debería ser inferior a los 273 m3/seg. en caudal ficticio continuo aguas abajo del azud de Xerta-Tivenys o bien 326 m3/seg. aguas arriba de dicho punto, distribuyéndose a lo largo del año de acuerdo al hidrograma natural de los caudales.
The idea of maintaining minimum flow rates in rivers is not absolutely new. In the 19th century, for example, the normal width of water flow for the northern German rivers were fixed by stretches in their middle sections, regulated in downstream sections, and the depth and width of the final section ought to be under the medium summer level, a volume of water obtained as the average of the lower values within a longer period of time. With rocky sub soils the normal transversal sections were worked out from the depth and the width of the navigable bed river, but in sections with either a soil of loose materials or a need to improve the river conditions, values should to be adjusted in order to get, at the same time, the wanted depth and width of water. Later on, the criterion that part of the available water has to be destined to guarantee a “minimum ecological volume” in rivers to secure the stable presence of the environmental biota was established as a principle. This means, on one hand, that the infrastructures for the regulation and derivation of flow rates will have to guarantee a residual flow of downstream water and, on the other, that a part of this surplus will not have any other use and it will constitute a demand for water by itself. In the case of the Ebro river, this would imply that the part of this environmental demand not included in existing or consolidated demands will have to be additionally met following the priorities of use established by the Ebro Basin Law (Plan) for this section of the river. In the case of the Ebro delta, a natural reserve of extraordinary national and international ecological interest, the so called “compensation volume” of 100 m3/sec. foreseen in the Ebro Basin Law (Plan) is insufficient, and was fixed in an at random and unjustified way. As we indicate in our study, using a new methodology, that volume should never be lower than 273 m3/sec. in a fictitious continuous flow rate, downstream to the Xerta-Tivenys irrigation dam, or 326 m3/sec. upstream of this place, and with a distribution round the year according to the natural hydrograph of flow rates.
The idea of maintaining minimum flow rates in rivers is not absolutely new. In the 19th century, for example, the normal width of water flow for the northern German rivers were fixed by stretches in their middle sections, regulated in downstream sections, and the depth and width of the final section ought to be under the medium summer level, a volume of water obtained as the average of the lower values within a longer period of time. With rocky sub soils the normal transversal sections were worked out from the depth and the width of the navigable bed river, but in sections with either a soil of loose materials or a need to improve the river conditions, values should to be adjusted in order to get, at the same time, the wanted depth and width of water. Later on, the criterion that part of the available water has to be destined to guarantee a “minimum ecological volume” in rivers to secure the stable presence of the environmental biota was established as a principle. This means, on one hand, that the infrastructures for the regulation and derivation of flow rates will have to guarantee a residual flow of downstream water and, on the other, that a part of this surplus will not have any other use and it will constitute a demand for water by itself. In the case of the Ebro river, this would imply that the part of this environmental demand not included in existing or consolidated demands will have to be additionally met following the priorities of use established by the Ebro Basin Law (Plan) for this section of the river. In the case of the Ebro delta, a natural reserve of extraordinary national and international ecological interest, the so called “compensation volume” of 100 m3/sec. foreseen in the Ebro Basin Law (Plan) is insufficient, and was fixed in an at random and unjustified way. As we indicate in our study, using a new methodology, that volume should never be lower than 273 m3/sec. in a fictitious continuous flow rate, downstream to the Xerta-Tivenys irrigation dam, or 326 m3/sec. upstream of this place, and with a distribution round the year according to the natural hydrograph of flow rates.
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