Human activities both aggravate and alleviate streamflow drought. Here we show that aggravation is dominant in contrasting cases around the world analysed with a consistent methodology. Our 28 cases included different combinations of human-water interactions. We found that water abstraction aggravated all drought characteristics, with increases of 20 to 305% in total time in drought found across the case studies, and increases in total deficit of up to almost 3000%. Water transfers reduced drought time and deficit by up to 97%. In cases with both abstraction and water transfers into the catchment or augmenting streamflow from groundwater, the water inputs could not compensate for the aggravation of droughts due to abstraction and only shift the effects in space or time. Reservoir releases for downstream water use alleviated droughts in the dry season, but also led to deficits in the wet season by changing flow seasonality. This led to minor changes in average drought duration (-26 to +38%) and moderate changes in average drought deficit (-86 to +369%). Land use showed a smaller impact on streamflow drought, also with both increases and decreases observed (-48 to +98%). Sewage return flows and pipe leakage possibly counteracted the effects of increased imperviousness in urban areas; however, untangling the effects of land use change on streamflow drought is challenging. This synthesis of diverse global cases highlights the complexity of the human influence on streamflow drought and the added value of empirical comparative studies. Results indicate both intended and unintended consequences of water management and infrastructure on downstream society and ecosystems.
<p>The Pyrenees are the "Water Towers" of several key river basins in France, Andorra and Spain, being the Adour-Garonne and the Ebro the largest ones. The water of these basins is used by agricultural and industrial economic sectors which have a significant socioeconomic impact. Furthermore, the water of these rivers also sustains ecosystems which have an intrinsic value and provide ecosystem services to society. For this reason, an assessment of the past and future evolution of the water resources of the Pyrenees is necessary. Until now, these assessments have often been done at the basin or at the national level, but never the water resources of the Pyrenees were assessed as a whole. This is the main aim of the PIRAGUA project, within which we develop our research.</p><p>In order to simulate the continental water cycle of the Pyrenees we have used the SASER (SAFRAN-SURFEX-Eaudyss&#233;e-RAPID) modeling chain. SAFRAN is a meteorological analysis system, that allows us to create a gridded dataset of all the variables needed by the SURFEX land-surface model. SURFEX&#8217;s outflows (runoff and drainage) are used by Eaudyss&#233;e and RAPID to calculate streamflow.</p><p>Until now there were two separate implementation of SAFRAN in France (8 km resolution) and Spain (5 km resolution). For this project we have taken the climatic zone level SAFRAN data of both countries and interpolated it to a new common grid at a resolution of 2.5 km. The dataset covers a domain that includes the Adour-Garonne, the Ebro and all other Pyrenean river basins, its time period is 1979/80-2014/15 (which will be extended to 2016/17). The RAPID river routing scheme has been implemented in the simulation domain using HydroSheds to describe the river network.</p><p>In order to simulate the future evolution of the continental water cycle we use the Pyrenean climate scenarios developed within the CLIMPY project. These include precipitation and maximum and minimum temperature. SURFEX needs other variables too, such as wind speed, relative humidity and radiation. We solve this problem using an analog based approach similar to Clemins et al (2019).</p><p>The simulated streamflow is compared to observed streamflow of natural basins. The results show that 18 (out of 38) non influenced stations present a KGE of daily streamflow larger than 0.5. For monthly streamflow, KGE is larger than 0.5 on 22 stations (out of 38).</p><p>The next steps of our research are to quantify the improvement due to the increased resolution (comparing to a lower resolution simulation), calculate trends of relevant variables at the sub-bassin scale and compared them to the observed ones in the past, and analyze future trends of these variables. Finally, we will assess the impacts of these changes on water resources.</p><p>This research is funded by the EFA210/16-PIRAGUA project, within the INTERREG V-A Espa&#241;a-Francia-Andorra POCTEFA2014-2020 program.</p>
<p>The SASER (Safran-Surfex-Eaudysee-Rapid) hydrological modeling chain is a physically-based and distributed hydrological model that has been implemented over two domains: Iberia and the Pyrenees. Currently, it is used for drought studies (HUMID project) and water resources analysis (PIRAGUA project).</p><p>In this modeling chain, SAFRAN provides the meteorological forcing, SURFEX is the LSM that performs the water and energy balances and Eaudyss&#233;e-RAPID simulates daily streamflow. SAFRAN and SURFEX are run at a spatial resolution of 5 km for the Iberian implementation and 2.5 km for the Pyrenean one. Daily streamflow is calculated by the RAPID river routing scheme at a spatial resolution of 1 km in both cases. SAFRAN analyzes daily observed precipitation, which is then interpolated to the hourly scale. For precipitation, relative humidity is currently used to hourly distribute the daily precipitation.</p><p>SASER is able to simulate adequate streamflow on the Ebro basin (KGE>0.5 on 62% of near-natural gauging stations when the LSM is run at 2.5 km of spatial resolution). However, due to the lack of a hydrogeological model, low flows are often poorly reproduced by this scheme. Furthermore, peak flows could also be improved.</p><p>This work aims at improving high and lows by correcting the distribution of hourly precipitation and adding linear reservoirs to improve low flows.</p><p>The increase of the spatial resolution from 5 to 2.5 km has caused a relevant improvement of peak flows. However, most of the peak flows are still underestimated. One way of improving simulated streamflow is improving the hourly distribution of the precipitation, as SAFRAN distributes precipitation through the day with unrealistic low hourly intensities. This will impact runoff generation and, thus, peak flow. We have used two ERA-Interim driven RCM simulations from the CORDEX project to improve the hourly distribution of precipitation. As a result, we now produce more realistic temporal patterns of hourly precipitation.</p><p>The current SASER implementation is not able to sustain low flows. A physical-based solution (hydrogeological model) would be desirable, but as it is difficult to implement we chose to introduce a linear reservoir, following the steps of Artinyan et al (2008) and Getinara et al. (2014). The reservoir is able to improve low flows in most near-natural subbasins. The challenge now is how to set its parameters in non-natural basins.</p>
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