Agent-based simulation of holocene monsoon precipitation patterns and hunter-gatherer population dynamics in semi-arid environments

Based on archaeological evidence from Kutch-Saurashtra (N Gujarat, NW India), we use agent-based modelling (ABM) to explore the persistence of hunter-gatherer (HG) groups in semi-arid environments in the mid and late Holocene. Agents interact within a realistic semi-arid environment dominated by the monsoon. Precipitation trends are modelled from instrumental records (1871-2008) calibrated with existing models for the Asian monsoon in the Holocene (c. 12 ka-present). Experiments aim at exploring dependencies between population dynamics and climate-driven environmental change (in terms of resource availability) for precipitation patterns at the local, regional and continental scales. Resources are distributed across a simplified ground model. Average yearly precipitation (AYP, i.e. mean) and variance in yearly precipitation (VYP, i.e. standard deviation) are the main parameters affecting resource availability in the simulations. We assess the effects of environmental change on HG populations at different timescales: (1) patterns of seasonal (inter-annual) resource availability, (2) effects of changes in mean precipitation trends over the long (Pleistocene-Holocene) and the mid (Holocene, millennial) periods, and (3) effects of intra-annual precipitation variability, i.e. changes in standard deviation from mean precipitation trends over the short period (annual to decadal). Simulations show that (1) strong seasonality is coherent with the persistence of HG populations in India, independently of the geographical scale of the precipitation models, (2) changes in AYP over the mid period (Holocene) are not sufficient to explain the disappearance of HG populations in Kutch-Saurashtra (K-S) 4 ka and (3) precipitation variability (VYP) over the short period (annual to decadal) is the main parameter affecting population performance and overall ecosystem dynamics. To date, sufficiently refined palaeoclimatic records do not exist for the study area, but higher VYP values 4 ka do not exclude the possibility that other factors may have driven the disappearance of HG populations in Kutch-Saurashtra.