Estimation of the uncertainty of hydrologic predictions in a karstic Mediterranean watershed

Assessing current and future available resources to supply urban water demands using a high-resolution SWAT model coupled with recurrent neural networks and validated through the SIMPA model in karstic Mediterranean environments

Hydrological simulation in karstic areas is a hard task due to the intrinsic intricacy of these environments and the common lack of data related to their geometry. Hydrological dynamics of karstic sites in Mediterranean semiarid regions are difficult to be modelled mathematically owing to the existence of short wet episodes and long dry periods. In this paper, the suitability of an open-source SWAT method was checked to estimate the comportment of a karstic catchment in a Mediterranean semiarid domain (southeast of Spain), which wet and dry periods were evaluated using box-whisker plots and self-developed wavelet test. A novel expression of the Nash-Sutcliffe index for arid areas (ANSE) was considered through the calibration and validation of SWAT. Both steps were completed with 20- and 10-year discharge records of stream (1996-2015 to calibrate the model as this period depicts minimum gaps and 1985-1995 to validate it). Further, SWAT assessments were made with records of groundwater discharge and relating SWAT outputs with the SIMPA method, the Spain's national hydrological tool. These methods, along with recurrent neural network algorithms, were utilised to examine current and predicted water resources available to supply urban demands considering also groundwater abstractions from aquifers and the related exploitation index. According to the results, SWAT achieved a "very good" statistical performance (with ANSE of 0.96 and 0.78 in calibration and validation). Spatial distributions of the main hydrological processes, as surface runoff, evapotranspiration and aquifer recharge, were studied with SWAT and SIMPA obtaining similar results over the period with registers (1980-2016). During this period, the decreasing trend of rainfalls, characterised by short wet periods and long dry periods, has generated a progressive reduction of groundwater recharge. According to algorithms prediction (until 2050), this declining trend will continue reducing groundwater available to meet urban demands and increasing the exploitation index of aquifers. These results offer valuable information to authorities for assessing water accessibility and to provide water demands in karstic areas.

Land-use changes and precipitation cycles to understand hydrodynamic responses in semiarid Mediterranean karstic watersheds

Non-planned agricultural land abandonment is affecting natural hydrological processes. This is especially relevant in vulnerable arid karstic watersheds, where water resources are scarce but vital for sustaining natural ecosystems and human settlements. However, studies assessing the spatiotemporal evolution of the hydrological responses considering land-use changes and precipitation cycles for long periods are rare in karstic environments. In this research, we selected a representative karstic watershed in a Mediterranean semiarid domain, since in this belt, karst environments are prone to land degradation processes due to human impacts. Geographic Information Systems-based tools and hydrological modeling considering daily time steps were combined with temporal analysis of climate variables (wavelet analysis) to demonstrate possible interactions and vulnerable responses. Observed daily flow data were used to calibrate/validate these hydrological models by applying statistic indicators such as the NSE efficiency and a self-developed index (the ANSE index). This new index could enhance goodness-of-fit measurements obtained with traditional statistics during the model optimization. We hypothesize that this is key to adding new inputs to this research line. Our results revealed that: i) changes in the type of sclerophyllous vegetation (Quercus calliprinos, ilex, rotundifolia, suber, etc.) from 81.5% during the initial stage (1990) to natural grasslands by 81.6% (2018); and, ii) decreases in agricultural areas (crops) by approximately 60% and their transformation into coniferous forests, rock outcrops, sparsely natural grasslands, etc. in the same period. Consequently, increases in the curve number (CN) rates were identified as a result of land abandonment. As a result, an increase in peak flow events jointly with a relevant decrease of the average flow rates (water scarcity) in the watershed was predicted by the HEC-HMS model and verified through the observed data. This research provides useful information about the effects of anthropogenic changes in the hydrodynamic behaviour of karstic watersheds and water resource impacts, especially key in water-scarce areas that depict important hazards for the water supply of related populations and natural ecosystems.

Assessment of Streamflow from EURO-CORDEX Regional Climate Simulations in Semi-Arid Catchments Using the SWAT Model

This research studies the effect of climate change on the hydrological behavior of two semi-arid basins. For this purpose, the Soil and Water Assessment Tool (SWAT) model was used with the simulation of two future climate change scenarios, one Representative Concentration Pathway moderate (RCP 4.5) and the other extreme (RCP 8.5). Three future periods were considered: close (2019–2040), medium (2041–2070), and distant (2071–2100). In addition, several climatic projections of the EURO-CORDEX model were selected, to which different bias correction methods were applied before incorporation into the SWAT model. The statistical indices for the monthly flow simulations showed a very good fit in the calibration and validation phases in the Upper Mula stream (NS = 0.79–0.87; PBIAS = −4.00–0.70%; RSR = 0.44–0.46) and the ephemeral Algeciras stream (NS = 0.78–0.82; PBIAS = −8.10–−8.20%; RSR = 0.4–0.42). Subsequently, the impact of climate change in both basins was evaluated by comparing future flows with those of the historical period. In the RCP 4.5 and RCP 8.5 scenarios, by the end of the 2071–2100 period, the flows of the Upper Mula stream and the ephemeral Algeciras stream will have decreased by between 46.3% and 52.4% and between 46.6% and 55.8%, respectively.

A Multi-Disciplinary Approach to Understand Hydrologic and Geochemical Processes at Koiliaris Critical Zone Observatory

Koiliaris CZO is a European Critical Zone Observatory (CZO) typical of the Mediterranean karstic geomorphology, which represents watersheds affected by humans over the centuries. This study aims to provide information that underpins the hydrologic and geochemical processes functioning at Koiliaris CZO. Linking geomorphologic and tectonic analysis improved the delineation of a karstic area which extends outside of the Koiliaris watershed and identified how structural elements influence the regional hydrology. The fluctuation in the river flow represents processes occurring in the karst and the periodic signal is related to Earth tide stressing of the karstic reservoirs. The conceptualization of a two-reservoir, well-mixed karstic system is confirmed by both the geomorphologic and tidal analysis. The hydrologic response is fast and it is manifested especially during extreme events where 70% of the precipitation becomes surface runoff, creating major flood events. The different sampling sites in the Koiliaris CZO were geochemically clustered and the quantification of the weathering fluxes showed that 25 mm/1000 years and 39 mm/1000 years of carbonate were removed by chemical weathering for the Keramianos ephemeral river and the springs, respectively. These studies illustrate the importance of critical zone science and transdisciplinary studies on water and soil management.

Climate change impact on karstic aquifer hydrodynamics in southern Europe semi-arid region using the KAGIS model

Nowadays, there are many urban settlements in arid and semiarid areas supplied by groundwater from adjacent small aquifers. Climate projections with expected decreases in averages precipitation values jointly with increases in the frequency of heavy rainfall events does not show a clear pattern to how water resources in karstic aquifers are going to evolve. This paper, focused in the province of Alicante (Southeast of Spain), assesses the behaviour of a small karstic aquifer, the Mela aquifer, whose resources supply urban water consumption for close municipalities. We assess the hydrogeological response of the aquifer, through the KAGIS black-box GIS-based model, for the present climate conditions and for the long period analysing the four scenarios provided by the International Panel of Climate Change. Main results prove that, if we do not diminish the greenhouse gas emissions, the climate change impact on the hydrological response of the study aquifer shows a decrease in the flow rate from its unique spring and will be non-existent during the summer months. So, it will be necessary to design supply strategies for these municipalities and to carry out them, meeting budget restrictions and avoiding potential water shortages.

Vision-Based Decision-Making Methodology for Riparian Forest Restoration and Flood Protection Using Nature-Based Solutions

Nature-based solutions (NBS) are actions that use natural processes in a resource efficient manner to solve societal challenges. The lack of supportive legislature, and financial, communication and social barriers complicate the process of NBS implementation. It is an urgent need to develop approaches to design and implement NBS that would act as drivers to overcome potential barriers and enhance the social acceptability of the project. The vision-based decision-making methodology and participatory process created in this study has been carried out in the Koiliaris Critical Zone Observatory in Crete to design erosion and flood protection NBS and restore the riparian forest. The methodology consists of four distinct steps as follows: i) develop a vision of the area, ii) conduct a baseline assessment study, iii) NBS design and co-design, and iv) procurement and implementation. The methodology overcame multiple barriers because of the effective stakeholder engagement and the vision “drove” the project and created the necessary consensus that is necessary to achieve the objective of converting privately owned prime agricultural land to riparian forest. It offers an exemplar of a functional ecosystem restoration project that protects the river in a sustainable way, improves its biodiversity and water quality and improves the quality of life and social cohesion.