Assessing annual and seasonal changes in the free-water reservoir surface state and turbidity conditions: implications for dam management in the Guadalquivir River Basin (Spain)

In Mediterranean areas the high seasonal and annual variability in precipitation produces large changes in the reservoir water availability. However, the state of these water bodies is not only subjected to the weather and the natural hydrology of this kind of system, but is often modified on one hand by the water demands and on the other hand, by the land uses upstream which have serious effects on the water quality of the river contributions. Remote sensing and GIS methods are currently emerging as an alternative to traditional methods like field survey (usually laborious, time consuming and expensive) to analyse the evolution of the state of these water bodies in terms of the free-water reservoir surface.

In this paper, the Guadalquivir River Basin (southwest Spain), where the gradual and intense development of large irrigated areas (which has increased in the last 50 years by more than 50%) leads to an increase in the storage capacity in the basin (which doubles during the next 40 years), was taken as a research object. The Global Surface Water (GSWE) online machine, combined with historical, hydrological, meteorological and water quality data, were used to spatially quantify free-water reservoir surface during the period 1984-2020. This allowed us, through a water balance approach on a monthly basis, the estimation of water inputs and outputs to analyse the hydrological changes in terms of seasonality, but also considering the effects of the dam operations and the changes in water quality in terms of sediments loads in those reservoirs where turbidity data series are available. 

The results show the increase of the free-water reservoir surface along the study period, which is consistent with the dramatic decrease of the contribution of the water flowing into the last receiver of the network of reservoirs (Alcalá del Río dam). This also implies the storage of associated substances such as sediments (mainly from the extensive olive groves areas located upstream) that produce the filling of the reservoirs, and turbidity episodes in flood events, which was verified with field measurements in the main control points of the basin.

This work has been funded by the project Integrated Management for the control of water inputs and sediments in reservoir systems in the Guadalquivir basin, with the economic collaboration of the European Funding for Rural Development (FEDER) and the Office for Economy, Knowledge, Enterprises and University of the Andalusian Regional Government.