WHAT THE SCIENCE SAYS
How much is enough?
Mierau et al. (2017) present a regional water allocation method called Modified Percent of Flow (MPOF). The method is particularly applicable to the North Coast of California, a region that has experienced immense growth in agriculture (orchards, vineyards, and cannabis) and increasing demand for irrigation. The region also supports three federally listed species: steelhead, Coho and Chinook salmon. Increasing flow diversions have compromised aquatic habitat for these species, particularly during the summer and early fall low-flow seasons. MPOF differs from other instream flow methods in a couple ways. First, most percent of flow methods recommend a fixed percentage of flow for diversion. Yet, these fixed percentages are not readily defensible and, in some cases, may be arbitrary. Second, and related, POF methods generally do not link fixed diversion percentages back to meaningful ecological criteria. The MPOF method presented in Mierau et al. (2017) improves on earlier methods by defining the total amount of water available for diversion in a watershed as a percentage of a streamflow baseline and, importantly, links those cumulative diversions to aquatic habitat for fish. Specifically, the method directly links cumulative diversions to effects on stream depth referenced to the riffle crest, an important hydraulic feature for salmonids. Because the method uses daily flow data to construct a streamflow baseline, the method is also well suited for the variable hydrology that characterizes North Coast watersheds. Although developed specifically for North Coast watersheds, MPOF may also be calibrated for use in central and south coast watersheds and perhaps elsewhere.
In a second paper that was published in the same issue of Freshwater Biology, Steel et al. (2017) studied the effects of different flow regimes on stream food webs in the Central Valley’s American and Yuba River watersheds. The authors examined the effects of regulated (dammed) and unregulated (undammed) flows on downstream food webs over three years during the summer low flow period. Specifically, Steel et al. (2017) sought to understand whether the flow regimes of regulated and unregulated rivers promoted differences in biodiversity and, if so, could those differences be attributed to specific components of the flow regime?