A soil water-balance ‘bucket’ model for paleoclimatic purposes 2. Model application to late Holocene climate

Abstract

The paper describes the application of the Soil Water-Balance Bucket Model (SWBBM), version B, to outline potential Holocene climate conditions leading to water fluctuations at the Lagoni peat-bog (Mercurago, Northern Italy). These are reconstructed from a series of radiocarbon-dated peat cores taken along present-day shores. Attention focuses on an event occurring during the late Sub-Boreal (4000-3000 years before present [BP]), during which peat recedes and subsequently expands, indicating reduced and increased water availability, respectively. This period is associated with a major change in human settlement patterns, both at local and regional scales, the appearance and disappearance of Bronze age lake-dwelling sites in Northern Italy; settlement patterns shift from the low plains of the Po valley to the upper plain and hills of the pre-Alpine system. Palynological data indicate no shift in mesic regional vegetation while changes are documented at the local scale, with an increase in hygrophilous vegetation as a response to raised lake levels. SWBBM/B, using a limited number of input climate parameters (temperature, precipitation and insolation), quantifies changes in soil hydrology parameters (evapotranspiration, soil water, percolation and runoff). Modern climate at the site is perturbed with insolation, temperature and precipitation altered to obtain variations in percolation and runoff, without varying evapotranspiration. Results show that 10% seasonal differences in total precipitation, with only a minor temperature change < 1° C, lead to 30% changes in percolation and runoff but with no change in evapotranspiration. It appears that for this minor climatic shift in the late Sub-Boreal, palynological evidence does not seem to be a suitable proxy-climate indicator at regional scales. This results from sensitivity of lake levels and vegetation to different components of the hydrological cycle.