Principles Governing the Food-Energy-Water Nexus as Revealed by Accurate Prediction of the Water Balance

Abstract:

Food, water, and energy (FEW) are key resources to sustainability. Accordingly, understanding their concepts and interconnections, their nexus, has recently become an active area of research. Here, we focus on percolation theory and its potential applications to integrate sub- aerial and sub-surface characteristics of the FEW nexus. Sub-surface inputs are solute transport to chemical weathering and carbon cycling, and water uptake from roots; corresponding above-surface processes are carbon uptake, evaporation, transpiration and precipitation. Combined above and below ground carbon cycling is tracked using net primary productivity, NPP. Applying percolation scaling relationships for solute transport in 3D yields verified predictions for chemical weathering over the entire range of Earth’s climates on time scales up to at least 6 Myr and soil formation to 130 Myr. This success owes to the limits placed on reactions from fluxes of reaction products in non-Gaussian solute transport that are incompatible with the common form of the advective-dispersion equation (ADE), and proportionality to the relevant water flow rate (i.e., infiltration or subsurface run-off). Applying related scaling results for the optimal paths exponent in 2D, using transpiration fluxes, predicts vegetation growth rates over the range of earthly climates and on time scales from minutes to 100 kyr. Relevance to climate (change) is enhanced by coupling these particular fluxes with the global carbon cycle. Using the verified hypothesis that the ecosystem NPP is proportional simultaneously to the soil depth and to the growing season transpiration depth to a power equal to the fractal dimensionality, d_f, of plant root systems, allows for accurate prediction of the global water balance when the theoretical value for d_f is applied, and its local variability when actual variability of measured d_f values are used. This result follows from optimizing ecosystem productivity with respect to the water partitioning, placing this research topic in the realm of the food-energy-water nexus. It is thus shown that assessing edaphic limitations to soil-plant synergy allows evaluation of the optimal drawdown of atmospheric carbon and capture of atmospheric water under conditions of available solar energy.