at low elevations the main climatic snowpack predictor is temperature, but at elevations at which the zero-degree isotherm remains during most of the winter, the
variability of precipitation becomes the most important predictor for snowpack
variability (López-Moreno 2005; Morán-Tejeda et al. 2013; Sospedra-Alfonso et al.
2015). Thus, it is advisable to interpret elevation-wise, the trends observed in snow
depth and snow duration in the context of climate warming. The lack of significant
trends observed at high elevations (as observed in this work) do not mean that
warming is not affecting the accumulation of snow, but that warming at those
elevations is not yet significant enough as to affect snow accumulation. Further
warming projected by climate models will, however, make increase the elevation
threshold at which temperatures become a limiting factor for snow accumulation,
and negative trends in snowpack depth and duration will be more likely found at
increasing elevations (Morán-Tejeda et al. 2013).
13.6 Streamflow Changes
To study for hydrological changes, we selected three Pyrenean Rivers (Ésera,
Aragón and Ara Rivers) with natural flow regimes (unaffected by dams or water
extractions) and we further defined three indices that represent different moments of
the year related to the signal of snow on the hydrograph (Fig. 13.6). SP indicates
the day of the year when the streamflow pulse due to snow melting starts (see
calculation in Cayan et al. 2001); SM stands for the day of the year when the
maximum spring flow is reached; and 75M indicates the day of the year when the
75th percentile of yearly water volume is reached. After these indicators had been
computed for every year in each of the three selected rivers, we checked the
existence of trends in time using Mann–Kendall and Thiel–Sen’s slope estimator
tests (Table 13.1). The three indices show a decreasing trend for the three selected
Fig. 13.6 Example of location of hydrological indices across a yearly hydrograph
13 Changes in Climate, Snow and Water Resources in the Spanish …
315
variability of precipitation becomes the most important predictor for snowpack
variability (López-Moreno 2005; Morán-Tejeda et al. 2013; Sospedra-Alfonso et al.
2015). Thus, it is advisable to interpret elevation-wise, the trends observed in snow
depth and snow duration in the context of climate warming. The lack of significant
trends observed at high elevations (as observed in this work) do not mean that
warming is not affecting the accumulation of snow, but that warming at those
elevations is not yet significant enough as to affect snow accumulation. Further
warming projected by climate models will, however, make increase the elevation
threshold at which temperatures become a limiting factor for snow accumulation,
and negative trends in snowpack depth and duration will be more likely found at
increasing elevations (Morán-Tejeda et al. 2013).
13.6 Streamflow Changes
To study for hydrological changes, we selected three Pyrenean Rivers (Ésera,
Aragón and Ara Rivers) with natural flow regimes (unaffected by dams or water
extractions) and we further defined three indices that represent different moments of
the year related to the signal of snow on the hydrograph (Fig. 13.6). SP indicates
the day of the year when the streamflow pulse due to snow melting starts (see
calculation in Cayan et al. 2001); SM stands for the day of the year when the
maximum spring flow is reached; and 75M indicates the day of the year when the
75th percentile of yearly water volume is reached. After these indicators had been
computed for every year in each of the three selected rivers, we checked the
existence of trends in time using Mann–Kendall and Thiel–Sen’s slope estimator
tests (Table 13.1). The three indices show a decreasing trend for the three selected
Fig. 13.6 Example of location of hydrological indices across a yearly hydrograph
13 Changes in Climate, Snow and Water Resources in the Spanish …
315