How people and ecosystems organize their storage requirements

Abstract:

At the start of the Anthropocene, one of the first things human society undertook was to tap water from the natural system: designing wells, diverting river water, harvesting rainwater, tapping groundwater by underground tunnels (qanats), and building canals and aquaducts to convey the water to where it was needed. Although sometimes highly complex engineering works, this was only a first step towards manipulating the natural system. In guaranteeing access to water, people soon realized that it was necessary to create sufficient storage to offset the high variability of hydrological fluxes in the natural system. The building of reservoirs dates back to 3000 BC, when the first reservoir was built in the Middle East, not surprisingly in an area with high hydrological variability.

A classical engineering way for designing the size of a reservoir is the Rippl (1883) diagramme, where tangents to the accumulated inflow determine the required storage. It is a logical method for people to size the storage required to satisfy long-term water demand. Using this principle, many societies have regulated their rivers, trying to level out the natural variability of the climate. But are people unique in tempting to even out climatic fluctuations or to bridge periods of drought? The hypothesis is that ecosystems do the same.

In contrast to a mechanistic model of the hydrological reality, where the moisture storage capacity in the root zone is simulated by a fixed parameter, we should realize the root zone is actually part of a living ecosystem, which is able to adjust itself to climatic variability. This maximum root storage capacity is a crucial parameter in all hydrological models, regulating not only the moisture available for transpiration by vegetation, but also the threshold above which runoff is generated. In contrast to what is generally assumed, this crucial hydrological parameter is alive! The hypothesis is that ecosystems adjust their root zone gradually to periods of drought or wetness, and that the maximum root zone storage is essentially a function of climate and land cover independent of soil characteristics such as porosity or permeability. Using a Ripple diagram approach for the dimensioning of root zone storage appears to yield an estimate of the required storage that a surviving ecosystem must have created to overcome a critical periods of drought.

This hypothesis has been tested in nine sub-catchments of the Ping river in Thailand and has been validated in 420 catchments across the USA, and proven to be remarkably accurate. The method presented here is a completely new and independent way of estimating a key hydrological parameter on the basis of climatic information.