Soil moisture and geology factors implementation in enhancing water-harvesting modelling in semi-arid area: a case study in Andalusia

Rapid population growth coupled with climate change has exacerbated concerns about water scarcity globally, especially in regions such as Spain, where the population will exceed 47 million in 2024. Many countries are resorting to methods of collecting water, especially rainwater, to utilise it. During dry seasons and to avoid floods. Although there are studies on determining the locations of dams using the surface runoff or numerical curve method, which depends on the elements of rainfall (R), land use (LU), soil characteristics (SO) and slope (S), these studies neglected the effect of soil moisture (SM) and the geological factors (GF) of the region. By utilising geographical information system (GIS) and the soil conservation service curve number (SCS-CN) model, the research aims to comprehensively examine environmental influences on runoff in Andalusia, which is characterised by its unique combination of climatic conditions, topography, soil compositions, and land use patterns. The semi-arid Mediterranean climate of Andalusia, characterised by unpredictable rainfall, provides a suitable environment for studying rainwater harvesting systems. Analysing data from 2010 to 2020, the study used the SCS-CN model, which is known for its applicability in watersheds with variable rainfall, moderate slope, and limited data accessibility. Two scenarios/models were examined - one including all factors, including SM and GF, and the other excluding SM and GF. The first model includes R-LU-SO-S in addition to SO and GF. The area of the area in which these conditions are met was estimated at 8069 km2, in the Sevilla, Huelva, and Cadiz region. The most suitable areas are Sevilla, Huelva. Since the southern part of Sevilla has a high slope, wet soil, and impermeable rocks such as dolomite, limestone, flint, schist, gneiss and clayey rocks. It greatly helps surface runoff, in addition to being suitable for building dams and reservoirs. While the second model included R-LU-SO-S. The area of the preferred area was estimated at 4812 km2, in the areas of Sevilla, Huelva, Cadiz, Malaga, and Granada. The Malaga, Granada area was ignored because it is close to faults and contains dry soil and permeable rocks, which allow water to be absorbed into the groundwater. The results reveal that the combination of SM and GF significantly improved the replay model. Differences have been observed in optimal sites for rainwater harvesting with respect to location, size and coverage area. Northwest Andalusia emerges as a prime candidate for rainwater harvesting, as its runoff basin extends over an area of 77,345 km2, constituting approximately 9.5% of the total area of Andalusia in 2020. It is worth noting that impermeable rock formations dominate the primary rainwater harvesting areas in the north and northwest regions, in order to facilitate the large surface runoff model. These findings extend beyond Andalusia, providing valuable insights for alleviating water scarcity and environmental hardship in semi-arid regions around the world.