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Impact of climate change on the hydrology and geochemistry of Crete and estimation of uncertainty

Nerantzaki Sofia

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Summary

The Mediterranean region has been identified as a “climate change hot-spot”. The specific region is projected to undergo the most significant drying among 26 regions across the world, by the end of the 21st century. Karstic springs are the exclusive source of water during dry months for most Mediterranean regions. The impact of climate change on the hydrology and water quality of karstic springs has not received proper attention in the scientific literature. In addition, uncertainty assessment of the hydrologic projections for karstic watersheds has not been studied and may reveal possible water deficits that cannot otherwise be taken into account. This study aims to (1) estimate the impact of climate change on the flow and water quality predictions of Mediterranean karstic watersheds (2) quantify the uncertainty of these predictions stemming from hydrologic model parameters, climate change scenario and internal variability, and (3) evaluate the response of different Mediterranean karstic springs to the present-day and forecasted meteorological droughts. The Soil Water Assessment Tool (SWAT) along with a karstic model (Karst-SWAT) is used to assess the composite spring and surface flow of the karstic watershed of Koiliaris along with the transferred nitrogen nitrate and sediment masses. The parameter uncertainty of both the surface and karstic flow models are estimated by combining the SUFI2 interface and the @RISK by PALISADE software. Input to the hydrologic models is provided by eleven combinations of five Regional Climate Models (RCMs) and three Representative Concentration Pathways (RCPs) of the EURO-CORDEX ensemble. Representative rainfall time series for certain of these scenarios are stochastically modeled with the LARS weather generator. Monte Carlo simulations are used to investigate the effect of input internal variability on the flow output. The meteorological drought of the present-day and future periods is translated into hydrological drought for three Mediterranean karstic springs with different properties. Specifically, the springs under study are the Stilos spring (Koiliaris River Basin), the Meskla spring and the Agia spring (Keritis River Basin). The Karst-SWAT model is used to quantify the hydrologic response of the karstic springs, which are characterized by different systems and water detention times. A non-parametric drought index is modified to estimate the future frequency, duration and intensity of meteorological and hydrological droughts in comparison to the reference period. The progress of the lowest and the highest flows in the future is analyzed. This is the first time, to our knowledge, that a combined assessment of surface and karstic flow model parameter uncertainty and internal variability is applied to a karstic Mediterranean watershed. Analysis shows that the parameter uncertainty of the hydrologic model and the internal variability of the climate change scenarios should be considered in planning water resources adaptation and mitigation measures that aim to alleviate climate change impacts in watersheds of semi-arid or arid climates, especially for the 2019-2058 period. After 2059, the climate change scenario is the most important uncertainty factor. Even under this large uncertainty, the drought status is anticipated to deteriorate after 2059, regardless of the scenario realized, for all springs. A 40-year period is offered for adaptation measures to be prepared and planned for the improved management of the springs’ water resources. The study provides a benchmark for comparative studies in other similar regions of the globe, where water needs during the summer are exclusively covered by the flow originating from karstic springs.

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