Two water worlds: implications for water resources sustainability

Understanding soil water partitioning is fundamental to predicting the sustainability of water resources.
Two water worlds: implications for water resources sustainability

Soil water partitioning sits at the heart of sustainable crop production, silviculture, and forest management1. Over the last decade, however, a growing number of studies2-9 suggested that soil water partitioning was more segregated than most hydrological models had considered. Phenomenological at best, these studies had come to be known otherwise as the two water worlds. That is, there appears to be a segregation between more-mobile soil water that ends up in streams, and less-mobile soil water that plants use10. And by extension, a segregation of solutes (e.g. nitrate) dissolved in respective more- and less-mobile waters11.

But this segregation has yet to be explicitly represented in hydrological models where more-mobile water – linked to flow [of water] and transport [of pollutants, nutrients, etc.] – is often the focus. Barring absence of evidence at the pore and grain scales, there is not a dearth of evidence in support of the two water worlds from mesocosm to global scales (see figure below).

The case for controlled experiments. Multiple lines of evidence exist in support of the two-water-worlds, from global and continental to mesocosm scales, but high degree of control to reveal processes at the cm to µm scales are lacking. Refs represent the scale-pertinent published studies; see References.

Now how might the two water worlds be relevant to water resources sustainability? Lest I fall into the single-profile trap, I reckoned it might be prudent instead to share what some peer reviewers thought of the two water worlds as a concept, and its broader implications for water resources sustainability.

Reviewer #1: “This [two water worlds] idea has wide ranging and significant implications for crop irrigation, fertilization, pesticide application, drought recovery, and flooding. The implication of the two water worlds is that pesticides, fertilizers and solutes can move through soil into adjacent watercourses potentially without interacting with bound soil water. This would provide an increased risk of pollution due to these sources, something that would need to be accounted for in sustainable agricultural practices and could have impact on society.”

Reviewer #2: “The implications of the two water worlds for a more appropriate management of soil water and nutrient resources are a crucial issue especially for worldwide regions with arid climatic conditions and where plants are frequently subjected to drought conditions.”

Moreover, in a recent modeling exercise12, it was suggested that: 

 “…a two water worlds [model] would result in the interpretation of lower total nitrification for the portion of water that supplies streamflow…substantially alter[ing]…how we would expect reactive transport to occur for many solutes and pollutants.”

These hopeful comments notwithstanding, the mechanistic underpinnings of the two water worlds remain unresolved. As one thoughtfully critical peer reviewer put it:

“While I can see that this area of hydrological-plant research may be moving into uncharted (perhaps exciting) territory, it is in need of theoretical grounding and better data before anymore arm-waving about the existence of two water worlds beneath our feet is a believable (and testable) idea or not.” 

Indeed, studies that allow for high degree of control at sufficiently small spatial scales hold great promise to unraveling the mechanisms of the two water worlds (see figure). State-of-the-art imaging techniques of soils, roots, and water may get us closer to understanding soil water partitioning with less arm-waving than necessary. Perhaps, only then can we optimally actualize the potential of the two water worlds and its implications for water resources sustainability.

[1] McDonnell, J. J. et al. Nat. Sustain. 1, 378–379 (2018).
[2] Evaristo, J. et al. Nature 525, 91–94 (2015).
[3] Evaristo, J. & McDonnell, J. J. Sci. Rep. 7, 4110 (2017).
[4] Good, S. P., Noone, D. & Bowen, G. Science. 349, 175–177 (2015).
[5] Evaristo, J. et al. Hydrol. Process. 30, 3210–3227 (2016).
[6] Evaristo, J. et al. Hydrol. Process. 31, 3750–3758 (2017).
[7] Zhang, Z. Q. et al. Hydrol. Process. 31, 1196–1201 (2017).
[8] Brooks, J.R. et al. Nature Geoscience 3, 100–104 (2010).
[9] Evaristo, J. et al. Water Resour. Res. 55, 3307–3327 (2019).
[10] Bowen, G. Nature 525, 43–44 (2015).
[11] Hall, S. J. et al. Oecologia 1–11 (2016).
[12] Cain, M. R. Hydrol. Process. 33, 2658–2675 (2019).

Poster image credit: Dr. Sim M. Reaney (Durham University)

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