Validated global estimates of environmental flow requirements

Validated global estimates of environmental flow requirements

Environmental demands for freshwater have received limited consideration in assessments of global water availability. Until now.

Original Paper:
Pastor, A.V. Ludwig, F., Biemans, H., Hoff, H., and Kabat, P. "Accounting For Environmental Flow Requirements in Global Water Assessments." Hydrology and Earth System Sciences. 2014 Volume 18. 5041-5059. DOI: http://dx.doi.org/10.5194/hess-18-5041-2014

Throughout history humans have disproportionately located their cities and towns adjacent to bodies of freshwater. Aquatic ecosystems are appreciated across the world for their beauty and the recreational opportunities that they provide. Freshwater also provides a resource that sustains domestic, agricultural, and industrial systems. In regions and seasons of low water flow, this diversity of uses can lead to competition between humans and ecological systems for scarce water resources. The amount of water required to sustain natural systems is termed environmental flow requirement (EFR). EFR provides habitat for aquatic animals and moisture needs of aquatic plants. Historically, EFRs have been mostly determined at a local level for individual rivers or basins. Consequently, past global water assessments have largely omitted consideration of EFRs as they seek to determine the balance between supply-and-demand for water.
 
A new article, written by leading scientists from universities and research institutes throughout Northern Europe and published in the journal Hydrology and Earth System Sciences, tests the performance of five methods capable of incorporating EFRs into global water assessments. Three of the methods were pulled from existing literature, while the authors develop the two other methods. In their paper, the results of model-based EFR estimates are validated against local EFR measurements in 11 river basins throughout the world. All of the models tested are hydrological in nature, which means that they are based on low-flow thresholds for each basin. All five methods differentiate seasonal flow requirements, with greater proportions of in-stream water being needed to sustain aquatic habitats during dry seasons.
 
All five global EFR methods perform well, exhibiting high correlation with values produced by detailed, local EFR estimates.  Correlation (R2) values are between 0.86 and 0.91 (with 1 being the maximum value). Two of the methods — known as the Tessmann method and variable monthly flow (VMF) — outperformed the others. Overall, across all basins and estimation methods it was found that an average of 37 percent of the mean available renewable freshwater should be reserved to meet the needs of aquatic ecosystems. During dry seasons an average of 46 to 71 percent of freshwater was needed to meet EFRs. Specific river basins and methods indicate even higher environmental demand for water resources. The Tessmann method, for example, assumes that 100 percent of mean monthly flow should be left to fulfill ecological needs during low-flow months. These results indicate likely competition for freshwater resources between human and environmental users.
 
Globally, there are many classifications of riverine aquatic habitats. And the tested EFR estimation methods perform differently depending on the type of river under consideration. Understanding the variability in model performance will help managers to appropriately interpret the result of EFR studies and make appropriate management decisions. For example, all of the hydrological models estimated lower EFRs for dry freshwater systems than the local EFR tests used for validation. The EFR of polar freshwater systems on the other hand tended to be overestimated using global EFR methods. Importantly, only the Tessmann and VMF methods were able to capture the monthly variability in rivers with seasonal or inconsistent flow. Due to the difficulty that any single method faced with predicting EFR in all river types, the authors suggest using a combination of the EFR methods to develop an understanding of the uncertainty that exists. This will be especially valuable in regions that currently lack local EFR studies.
 
There are still a number of challenges to overcome in the modeling of EFRs. How to appropriately consider and allocate water in periods of low-flow remains uncertain. The VMF method, for example, allocates 60 percent of flow to the environment during dry periods, while the Tessmann method allocates 100 percent. Also at this time, there is a relatively limited understanding of the difference between freshwater ecosystem types in their sensitivity to low environmental flows. While the numbers are far from perfect, they have the potential to vastly improve global freshwater assessments bringing environmental needs within the frame of the analysis. As competition for scarce water resources increases over the coming decades, we are going to need all of the information that we can to make smart water management decisions considerate of both people and the environment.

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