Authors: C. Michael Barton, Isaac IT Ullah, Nicolas Gauthier, Nari Miller, Grant Snitker, Irene Esteban, Joan Bernabeu, and Arjun Heimsath
Validation is key to assessing the usefulness of a model. Ironically, the richness and sophistication of spatially-explicit agent based or cellular models of past socio-ecological systems makes them particularly difficult to validate. This is because such models commonly produce results in the form of high-resolution, multidimensional, digital landscapes that can include locations of simulated land-use, human settlement, vegetation communities, soils, topography, and more. But we cannot directly observe past human and natural system processes to validate these models, and instead must rely on proxy data to infer the processes represented. Proxies for past socio-ecological systems include samples of sediments and soils, plant micro- and maco-fossils, discarded artifacts, and cosmogenic radionuclides. Such proxy records tend to be incomplete, sparsely preserved, collected from a limited number of points on the landscape, and often at multiple depths below the surface. Even converted to digital form, the great differences between empirical proxies and model results make direct comparisons impossible, creating important challenges for model validation.
To address the incommensurability between our models and the empirical data available to validate them, the Mediterranean Landscape Dynamics Project (MedLanD) has developed a validation instrument that creates a ‘digital proxy’ record based on model results. The digital proxy is analogous to extracting a digital core at specified points in the gridded, digital landscape. It simulates the accumulation over time of a proxy-like record for modeled human land-use, vegetation, landscape fire, and surface processes. Digital proxy ‘cores’ can be extracted from any point in the simulated world and compared directly with empirical samples taken from analogous points in real world landscapes, improving our ability to validate complex models. We present a brief overview of our digital proxy modeling method and provide a test case of comparing digital and empirical data from locales in Mediterranean Spain.
An earlier version of this paper was presented at the European Archaeological Association meetings in Barcelona, Spain (7 September 2018)
The MedLanD model is available at https://www.comses.net/codebases/4609/releases/1.1.0/