The influence of land cover and geological structure on the natural regulation of river runoff

Abstract

Background. In arid and semi-arid regions like the Karabakh region of Azerbaijan, the efficient management of scarce water resources is critically dependent on the natural regulation of river runoff. This process, which ensures a more balanced intra-annual distribution of water, is traditionally understood to be influenced by climate and surface cover. However, the specific role of underlying geological structures has often been assessed only indirectly, presenting a significant research challenge. This study investigates the impact of both geological formations and surface cover characteristics on the natural runoff regulation of rivers from 1999 to 2024, a period marked by climatic changes and significant landscape alterations.

Methods. An integrated approach combining hydrometeorological data, remote sensing, and GIS technologies was employed. The study utilized long-term climate data (precipitation, temperature, etc.), satellite imagery (Landsat), and a digital elevation model. Key methodologies included the application of the NRCS-CN (Natural Resources Conservation Service Curve Number) method and the calculation of a "natural regulation ratio" (NRR), defined as the ratio of baseflow (Qu) to surface runoff (Qs). Hydrologic Soil Groups (HSGs) and various indices (NDVI for vegetation cover, BSI for bare soil) were analyzed to characterize land use/land cover (LULC) and its changes. The analysis focused on comparing river basins with similar climatic and morphometric characteristics but differing geologies.

Results. The study confirmed that geological structure is a key factor in the rivers feeding. Rivers flowing through the highly water permeable, fractured volcanic rocks of the Karabakh Plateau exhibited high NRR values (often above 1.0), with baseflow (groundwater flow shares) exceeding 50%. In contrast, rivers in the south, underlain by impermeable rocks, showed very low NRR, with surface runoff dominating (70–90% of rivers total flow). Overall, the regional average NRR was 0.97, indicating a near-equal balance between surface and subsurface flow sources. However, a negative trend was observed: over the 25-year period, landscape changes driven by aridification and reduced vegetation cover led to surface consolidation. This caused a 11.4% decrease in initial abstraction parameter (flow-forming potential of precipitation) and a 4.79% decline in the regional NRR, signaling a reduction in natural regulatory capacity.

Conclusions. The findings conclusively demonstrate that the geological foundation is a decisive controller on the natural regulation of river flow in arid regions. The presence of fractured, permeable rocks facilitates high infiltration and substantial subsurface flow, creating a natural reservoir that ensures water availability during dry periods. Consequently, assessing the geological context is essential for effective water resource management planning. The identified decline in the natural regulation ratio underscores the vulnerability of these water systems to ongoing climate change and land degradation. For rivers with inherently low regulation, artificial measures like reservoir construction remain necessary. This study provides a critical framework for prioritizing water management strategies based on a basin's specific hydro-geological characteristics.