Safeguarding Mediterranean Agroecosystems Under Climate Change: Ex-Parcel Runoff as Hydrologic Buffer for Viticulture and Oliviculture

Global climate change is intensifying water scarcity in Mediterranean agroecosystems, demanding a transition from rainfed to irrigated management for high-value crops like vineyards and olive groves. This study introduces a novel hydrologic framework to assess field-scale rainwater harvesting potential across nearly 60,000 individual vineyard and olive grove parcels in continental Portugal. Unlike conventional valley-focused models that delineate catchments at drainage junctions, our approach uses high-resolution digital elevation models and open-source spatial libraries (Python’s Fiona, Rasterio, Whitebox) to link every agricultural pixel to its unique upstream hillslope catchment. We quantify and compare “in-parcel” resources (direct precipitation, Vp) with “ex-parcel” resources (upstream runoff, Vup) under historical (1981–2010) and future (2041–2070) climate scenarios (CMIP6; SSP1-2.6, SSP3-7.0, and SSP5-8.5). A central contribution of this study is the evaluation of water security, defined here as the relative safety buffer between harvested water and the biological irrigation requirements (Vip) prescribed for both cultures in each of seven agroclimatic zones defined across the country. Security categories are based on the ratio (Vp − Vip)/Vip for in-parcel resources and (Vup − Vip)/Vip for ex-parcel resources, where values above zero indicate a sustainable surplus, and negative values signify a state of insecurity. Results demonstrate that ex-parcel resources are significantly more substantial, offering 2.5 to 35 times the potential of in-parcel counterparts. While vineyards currently exhibit high security nationwide, southern olive groves face a critical degradation from “secure” to “insecure” status by 2070 under fossil-fueled pathways (SSP5-8.5), with security indices dropping as low as −33.2 in the southern Alentejo region. This highlights ex-parcel runoff as a vital, underutilized hydrologic buffer that can safeguard Mediterranean agriculture against projected climate-induced deficits.