Ecohydrology: it's all in the game?

✍ Scribed by Albert van Dijk

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 122 KB
• Volume: 18
• Category: Article
• ISSN: 0885-6087
• DOI: 10.1002/hyp.5762

✦ Synopsis

Reflecting the widespread merging of Earth and life sciences, recent years have seen the emergence of ecohydrology as a new branch on the hydrological tree. The term itself was coined only a few years ago (see Baird and Wilby (1999)), but its objective-to understand the role of ecosystems in hydrology and vice versa-is not new. The hydrology of land-use change has a distinguished past and the (real or perceived) understanding of it drives land management decisions world-wide. Milestones in the development of ecohydrological theory included the description of the role of the vegetation in controlling evapotranspiration (ET), in affecting the amount of rainfall intercepted and evaporated again and, conversely, the effect of soil water availability on vegetation water use. To this should be added the role of ecosystems in maintaining or altering soil structure and, thereby, its hydraulic properties, although a consistent physical theory is still lacking in this respect. Despite this good understanding of most ecohydrological processes, the application of physical deterministic models describing these processes has met with variable success. A case in point is the Penman-Monteith model, which is arguably the greatest contribution to ecohydrology so far and the basis of most current surface-vegetation-atmosphere transfer models (e.g. Sellers et al., 1997). Despite its often good performance in describing observed evaporative fluxes, it has proved difficult to obtain robust a priori estimates of a key vegetation characteristic, surface conductance. Upscaling leaf-level measurements of stomatal conductance to stand level has proved a non-trivial affair, and, in fact, variability at stand level appears to be less than at leaf level (Kelliher et al., 1995). We face similar discrepancies when describing rainfall interception, the effect of (seasonal) water shortage on ET and the surface energy balance, and the effect of land-use change on infiltration. At larger scales, relationships between ET, rainfall and radiation energy generally appear to become more straightforward, and water use by vegetation in similar climates more conservative, than may be expected on the basis of Penman-Monteith theory (Roberts, 1983; Zhang et al., 2001).

What causes this inconsistency? Probably, we are missing a physical scaling 'trick' or two, and improved scaling rules may help to overcome some of the discrepancy. However, there may be a second, more profound cause, related to the complex and self-regulating nature of ecosystems. Complex systems theory shows how individual components (cell, leaf or tree) may follow one particular set of rules,