Cross-drought interactions show a complex behaviour in hydrological and ecological systems in the Spanish Pyrenees

In the frame of the CROSSDRO (Cross-sectoral impact assessment of droughts in complex European basins) project it has been analysed the influence of the interannual variability of climatic drought on ecological and hydrological droughts for a basin in the central Spanish Pyrenees using variables derived from climatic observations and hydro-ecological simulation. For this purpose, the Regional Hydro-Ecologic Simulation System (RHESSys) was used to model the eco-hydrological processes in the basin from 1970 to 2020. RHESSys couples an ecosystem carbon cycling model with a spatially distributed hydrology model to simulate integrated water, carbon and nutrient cycling and transport over complex terrain at small to medium scales.

Using different climatic, ecological and hydrological standardized drought indices, the results show the greater role of meteorological droughts in hydrological systems than in ecological systems, and the small influence of vegetation activity and growth in explaining the interannual variability of water resources in the basin. By contrast, hydrological droughts are strongly affected by precipitation variability with relationships characterized by seasonal differences and the role of different time-scales in the standardized drought metrics. As expected, the effect of meteorological drought variability in the upper Aragón basin is much stronger in hydrological systems than in ecological systems. Nevertheless, it has been a surprise to find little evidence for a strong role of vegetation in influencing hydrological drought variability in the basin at interannual timescales. Although long-term changes in vegetation plays a very important role in determining trends in water resources, the role of interannual variability in plant conditions seems to be small as our results suggest.

This means that years characterized by high vegetation activity and growth do not have a negative effect on water availability in the basin. This is likely linked to the similar influence of climate variability on ecological and hydrological droughts, as higher precipitation causes positive anomalies in both metrics. Thus, although in humid years vegetation growth would be higher and evapotranspiration would increase given higher photosynthesis, water availability would be sufficient to maintain positive anomalies in the surface water resources in the basin. The connection between different hydrological drought metrics in the basin is very strong and modulated by the seasonal response to precipitation and the reservoir management. These findings are highly relevant for evaluating how ecological and hydrological droughts are related in complex hydrological basins.

Location of the study area, topography and hydrological network, including the Yesa reservoir.

Example of the analysis developed in which monthly partial correlations between the basin Standardized Outflows in the basin and the variables that may have a role on it (EDDI Evaporative Demand Drought Index, SPI Standardized Precipitation Index, SSMI Standardized Soil Moisture Index, SSI Standardized Streamflow Index, LAI Leaf Area Index, NPP Net Primary Production and Standardized Reservoir Storages). Dotted lines frame months and time-scales in which the correlations are statistically significant.

The figure illustrates how outflows are mostly controlled by reservoir storages throughout the year, especially in summer months, but outflows are more affected by streamflow than by reservoir storage in some winter months. In addition, outflows are strongly determined by precipitation recorded over long time- scales. Nevertheless, the influence of vegetation metrics that inform of high/lower annual activity and growth do not show a relevant influence on the water generation of the basin.