How ecosystems organize their moisture storage requirement

Abstract:

The moisture storage capacity in the root zone of ecosystems acts as a buffer against climatic variability and is a critical factor controlling many physical, biogeochemical and biological processes including land-atmosphere exchanges, rainfall-runoff generation, carbon cycling and nutrient dynamics. Notwithstanding its importance this storage capacity cannot be directly observed at catchment scale.

Approaching this problem from a different angle, we can try to understand how adaptive systems cope with the variability of essential inputs through the creation of buffers. Surprisingly, there appears to be a strong correspondence between how societies and ecosystems try to safeguard their water supply. People build reservoirs to buffer against periods of water shortage; ecosystems essentially do the same by creating sufficient moisture storage in their root zone. Both try to do this at minimum expense: people by optimizing the amount of storage at minimum costs; and ecosystems by creating an optimum root zone buffer at minimum biomass investment.

A classical engineering way for designing the size of a reservoir is the Rippl (1883) diagram, where tangents to the accumulated inflow determine the required storage. It is a logical method for people to size the storage required to satisfy the long-term water demand. Using this principle, over time, many societies have tried to regulate their rivers, leveling out the natural dynamics of the system. But are people unique in trying to even out unwanted fluctuations or to bridge periods of water shortage?

Like societies, ecosystems adjust their storage buffer to climatic variability. Similar to the way in which engineers design reservoirs, we can estimate the root zone storage capacity at catchment scale on the basis of observed climate and hydrological data. This approach was proven to be remarkably accurate not only in 11 catchments of the Ping river in Thailand but also in 413 catchments across the USA, with diverse climate and land surface conditions. The results illustrate that ecosystems adjust their root zone storage to periods of drought or wetness, and that the maximum root zone storage is essentially a function of climate and land cover.