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Integrated modeling framework of hydrologic, water quality and sediment transport in temporary river basins

Gamvroudis Christos

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Summary

Understanding the complex behaviour of the integrated surface and groundwater system is very important for the regional water resources management and requires an integrated modeling approach. Predicting the spatial patterns and intensity of hydrology, groundwater and sediment transport for large river basins can be problematic in areas where few reliable field data are available so other approaches that facilitate model simulations must be applied.For the purposes of this study a comprehensive modelling framework for integrating a hydrologic (SWAT) and a groundwater (PTC) model was conceptualized, developed and tested in a large Mediterranean watershed (Evrotas River Basin).The objective of this research was to study the spatial distribution of runoff, sediment transport and better understand the dynamics of surface-groundwater (SW-GW) interactions in a large Mediterranean watershed (Evrotas River Basin) consisting of temporary flow tributaries and high mountain areas and springs by focusing on the collection and use of a variety of data to constrain the model parameters and characterize hydrologic and geophysical processes at various scales and to improve the seasonal forecasting of a potential hydrological drought under future climate change scenarios. Evrotas River Basin has a drainage area of 1348 km2 and is a complex hydrological system consisting of intermittent flow tributaries, high relief areas and springs which are the main contributors to base-flow. It is located in the southeast part of Peloponnesus, Greece and drains into Laconikos Gulf.Both monthly and daily discharge data (2004-2011) and monthly sediment concentration data (2010-2011) from an extended monitoring network of 8 sites were used to calibrate and validate the Soil and Water Assessment Tool (SWAT) model. In addition flow desiccation maps showing wet and dry aquatic states obtained during a dry year were used to calibrate the simulation of low flows. Annual measurements of sediment accumulation in two reaches were used to further calibrate the sediment simulation. Model simulation of hydrology and sediment transport was in good agreement with field observations as indicated by a variety of statistical measures used to evaluate the goodness of fit. A water balance was constructed using a 12 year long (2000‐2011) simulation. The average precipitation of the basin for this period was estimated to be 903 mm yr-1. The actual evapotranspiration was 46.9% (424 mm yr-1), and the total water yield was 13.4% (121 mm yr-1). The remaining 33.4% (302 mm yr-1) was the amount of water that was lost through the deep groundwater of Taygetos and Parnonas Mountains to areas outside the watershed and for drinking water demands (6.3%). The results suggest that the catchment has on average significant water surplus to cover drinking water and irrigation demands. However, the situation is different during the dry years, where the majority of the reaches (85% of the river network are perennial and temporary) completely dry up as a result of the limited rainfall and the substantial water abstraction for irrigation purposes. There is a large variability in the sediment yield within the catchment with the highest annual sediment yield (3.5 t ha-1 yr-1) to be generated from the western part of the watershed. The study of the surface-groundwater (SW-GW) interactions is achieved by integrating the quasi-distributed watershed Soil and Water Assessment Tool (SWAT) model with the three-dimensional groundwater flow Princeton Transport Code (PTC) model. The combined models are applied to the alluvial plain (300 km2) of Evrotas watershed by considering the interaction between the stream network and the aquifer to better spatially represent feedback fluxes within the surface and groundwater domains. The SWAT–PTC model was calibrated using field data for the 2007-2011 period. Model simulation was in good agreement with field observations demonstrating that this integrated modeling approach provides a more realistic representation of the water exchanges between surface and subsurface domains and constrains more the calibration with the use of both surface and subsurface observed data. Finally the SWAT and SWAT-PTC models were used to study the impact of future climate change on surface and ground water resources of the area under three different climate change scenarios. The climate scenario results suggest that the Evrotas catchment area will be facing a different climate in the future. All three scenarios gave consistent results, predicting significant decreases in annual precipitation, actual ET and runoff after 2030. The results suggest a 10% decrease in precipitation, 4% decrease in ET and 19% decrease in flow in 2030‐2050 compared to 2010‐2020. The KNMI-RACMO-ECHAM5 scenario presents the highest percentage of drought occurrence (21.1 %) during the 2050-2060 period while for the SMHI-RCA-ECHAM5 and MPI-REMO-ECHAM5 scenarios this is 20.0% and 19.0%, respectively. The results indicate that the upstream reaches displayed a loss of surface water to underlying groundwater systems whereas downstream the main river received recharge from groundwater as the water table approached the surface topography. The low flow characterization for the current situation showed that a large part of the stream network will be too dry to accommodate the development of a viable aquatic ecological community throughout the year. In dry periods (2043-2055) the amount of water that is supplied to the aquifer is 40.9% less than the amount of water that was supplied under current climate conditions highlighting the need for new management strategies that must be implemented in order to avoid setbacks in the allocation of water resources in the future.The results indicate also that the study area is very sensitive to potential future climate change. The developed methodology facilitated the simulation of hydrology, groundwater and sediment transport of the catchment providing consistent results and suggesting its usefulness as a tool for temporary rivers management.

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