Consequences of secondary succession on water availability in Mediterranean areas: a study case in northeastern Spain
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Mountainous areas in Europe, especially in Southern regions such as the Spanish Pyrenees, have undergone major depopulation during the past century, which has lead to the abandonment of agricultural fields. These abandoned fields have been colonized by natural species which changed the hydrological characteristics of the area. Studying the hydrological effect of secondary succession in abandoned fields on water yield can be helpful to determine future reservoir management strategies. This work uses a modeling approach to investigate the effects of secondary succession on monthly inflow into the Yesa reservoir (central Spanish Pyrenees). The study focuses on three major elements. First the change in hydrological relevant parameters of the vegetation and soil due to secondary succession is simulated from 1950 until 2050. This is done by simulating the change in aboveground carbon density and organic matter fraction with a logistic Markov-chain model. This Markov-chain model has spatially distributed transition probabilities that depend on terrain characteristics in order to simulate different transition rates depending on the suitability of the terrain. Vegetation and soil parameters are related to carbon and organic matter with equations from literature. Secondly, a hydrological model is used to calculate the discharge flowing into the Yesa reservoir during different stages of the succession. These modeled discharges are analyzed to determine trends in water yield on both monthly and decadal scale. Finally, the calculated future reservoir inflow is compared with reservoir outflow. Analysis of present and future monthly residuals of reservoir in an outflow allows for a simple way of assessing changes in water stress. Results of this study indicate that secondary succession in abandoned fields below 1600 m has been responsible for a decrease in river discharge of over 4% from 1950 to 2005. By 2050, the continuing vegetation growth is expected to cause river discharges to decrease further by more than 5% compared to 1950. Decreases are only noticed from autumn to spring and are not expected to cause a major shortage of water in the reservoir.