Impact of climate variability and human activities on water resources.

Abstract

This PhD Thesis assess the extent to which the global driving forces (climate change, groundwater dynamics, etc.) and human activities (land-use/land-cover changes, dam construction, etc.) interact and affect the hydrological regime in some selected basins in the semi-arid region of southeastern Spain. To conduct a comprehensive causal analysis, this PhD Thesis used the Soil and Water Assessment Tool (SWAT) hydrological model as the main tool and combined it with the Interbasin Groundwater Infiltration (IHA) method and the Chloride Mass Balance (CMB) method to make calculations of specifics water balance components. Climate change and land-use and land-cover changes (LULC) caused by human activities are significant contributors to ecosystem degradation and the availability of a basin's water resources. Climate change forecasts for the Mediterranean region this century reveal that rising temperatures, combined with less precipitation, have resulted in a 20% drop in water resources. Human activities, such as agricultural irrigation expansion and urbanization alter runoff regimes and have an impact on the availability of water resources. Significant LULC has occurred in southeastern Spain since the 1970s, as a result of the increasing abandonment of dryland farming and the implementation of reforestation programs to prevent ecoservices degradation in watersheds. It is critical to understand how these activities impact the quantity of water resources in the headwater of large basins in southeastern Spain like the Segura River basin. The SWAT model was used to investigate the aforementioned impacts. SWAT is the most widely used model for simulating the quality and quantity of surface water and groundwater balance components at different basin spatial scales, predicting the effects of climate change on the water balance components, and extrapolating the influence of anthropogenic activities on water resources. However, when it comes to modelling groundwater flow and storage, SWAT has substantial conceptual constraints, and other specificts techniques are needed. Because the earth's surface water resources can no longer supply the needs of living things, which is especially evident in drylands, groundwater resources have become the focal point of water resource development. As the most important form of freshwater on land, groundwater and surface water are transformed into each other on a spatiotemporal distribution as an indivisible whole. The laws and simulations of surface water and groundwater have not been carried out jointly due to the complexity of the hydrological cycle and the varied modes of occurrence and movement. The interaction between regional surface water and groundwater has become more and more common as a result of global changes and human activities, particularly large-scale groundwater extraction and inter-basin water transfer projects, especially in arid and semi-arid watersheds where water resources are scarce, surface water-groundwater exchange is more intense, and surface water and groundwater conversion is a factor that must be taken into account in the analysis of surface water and groundwater conversion. This study attempted to consider groundwater and surface water as a whole to provide a more trustworthy and accurate basis for water resource planning and management in order to better reflect the hydrological cycle process. Although the SWAT model considers shallow and deep groundwater in the structure, it is solely used to calculate watershed water balance; there is no dynamic mechanism to simulate and output the net groundwater resource. The Chloride Mass Balance (CMB) is a robust method to estimate net groundwater. SWAT model and CMB method were coupled in order to accurately replicate the specific groundwater situation of the Castril River basin (CRB) in the Guadalquivir River basin. This basin was chosen to assess the reliability of combining the SWAT model and the CMB method to improve streamflow modeling in high–permeability bedrock basins receiving Interbasin Groundwater Flow (IGF), in this case from the Segura River basin. Dam construction is also seen as having a significant impact on hydrological regime and ecosystems in the CRB. The impact of human activities has continued to change the hydrological regime of rivers and dependent ecosystems. The hydrological situation is the basic attribute and important driving force of the river ecosystem, and changes in the morphology of rivers and riparian zones, flow patterns, water quality, flora and fauna, and riparian vegetation, which affect the normal structure and function of the river ecosystem, and even an irreversible ecological crisis, will result from intra- and inter-annual variation. We provide a framework for assessing the effects of the El Portillo dam construction on streamflow regime and river ecosystem downstream. The streamflow regime data was compared with and without dam impact. IAHRIS (Indicators of Hydrologic Alteration in RIverS) which is a free Spanish software based on IHA method to assess the hydrologic alteration in rivers was used in this calculation. The introduced methodology and the conducted analysis of results make this PhD Thesis valuable to design water resources management strategies aimed to combat the negative effects of climate change and human activities in the headwaters of large river basins in semi-arid southeastern Spain, including the relevance of interbasin groundwater flow in streamflow and the effect of dam construction on river ecosystem downstream.