In many papers these two concepts seems to be interchangeable. However, hydrological disturbance describes the discrete pulses of high or low flow while flow variability seems to be a measure of continuous process.
What is the difference between these two concepts and are they misused in these papers?
I am not a hydrologist, but IMHO how to define where the pulse begins and a continuous process ends? On the scale of daily river flow, a flood looks like a peak/pulse, but on the scale of hours, minutes ... it looks like a steady increase in water flux. Calling it a disturbance is just a subjective judgment - water flow is of course continuous - it does not just stop and then begin again, as a real pulse would. But rainfall stops and ends, so that might be called a pulse. But would you call any bit of rainfall a disturbance, just because it has a beginning and an end?
To define a disturbance is a matter of perspective: flow variability within a certain range (normal variability) could be seen as part of the regular hydrological cycle. Organisms living in these systems are usually adapted to changes through their life cycles or specific traits, and therefore, on a long-term, will not show major changes at the community level. Events that exceeds the "normal" range (e.g. a one in 100 years flood event), may be regarded as a disturbance to the community or population (e.g. fish that experience sever floods).
However, on an organism level (e.g. aquatic insect larva or fish) a complete drying of the stream for example, although under the "normal" range, may also be seen as a disturbance (which causes mortality). Read Lake (2000) "Disturbance, Patchiness, and Diversity in Streams". doi:10.2307/1468118, for more information on the subject.
True, Yaron, sitting in very wet autumn middle Europe I ignored streams that dry out ... so flow is NOT always continuous, of course. However, usually in a stream that "normally" dries out seasonally, the inhabitants will be adapted to that, by digging into the riparian zone and/or sediments, and by wandering/flying off to another spot with water. If a stream that is inhabited by organisms that do not expect to sit dry, indeed falls dry, then this will probably be perceived as disturbance. Lake 2000 is important in that context, but also the Intermediate disturbance hypothesis by Connell, 1978. But definitely I agree: it is a matter of perspective. It will e.g. be very different for different taxa, and time and spatial scale play a huge role.
Flow variability is a numerical and often statistical measure of how flow changes over time and/or space wheras hydrological disturbance can mean a number of things such as the CHANGE in the flow regime or the EFFECT flow variability may have. Any usage should clearly explain the concept(s) and how they are to be used any any particular case.
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