Understanding ecohydrological connectivity in savannas: a system dynamics modelling approach

Ecohydrological connectivity is a system level property that results from the linkages in the networks of water transport through ecosystems, by which feedbacks and other emergent system behaviours may be generated. We created a system dynamics model that represents primary ecohydrological networks to examine how connectivity between ecosystem components impacts ecosystem processes. Here, we focused on the savanna ecosystems, although the analyses may be expanded to other ecosystem types in the future. To create the model, a set of differential equations representing ecohydrological processes was programmed into the dynamic solver Vensim. Stocks of water storage (e.g. atmospheric and soil moisture) were linked by flows [e.g. precipitation and evapotranspiration (ET)] that were in turn dynamically controlled by the amount of water stored. Precipitation was forced stochastically, and soil moisture and potential ET controlled actual ET. The model produced extended, probabilistic time series of stocks and flows, including precipitation, soil moisture, runoff, transpiration, and groundwater recharge. It was used to describe the behaviour of several previously studied savanna ecosystems in North America and Africa. The model successfully reproduced seasonal patterns of soil moisture dynamics and ET at the California site. It also demonstrated more complex, system level behaviours, such as multiyear persistence of drought and synergistic or antagonistic responses to disconnection of system components. Future improvements to the model will focus on capturing other important aspects of long-term system behaviour, such as changes in physiology or phenology, and spatial heterogeneity, such as the patchwork nature of savannas.