Hydrological classification of non-perennial Mediterranean rivers and
streams: a new insight for their management within the WFD
Abstract
Classification of natural flow regimes of non-perennial rivers and
streams (NPRS) is an incipient field of research. NPRS represent
approximately 70% of the total Mediterranean rivers and are expected to
increase in the next decades as a result of climate change. Due to the
ecological importance of NPRS and the need to improve national
ecological assessment methods within the scope of the Water Framework
Directive (WFD), this paper aims to classify the hydrological regime of
69 non-regulated streams, testing several hydrological indices related
to the frequency, duration, timing, and rate of change in periods of
flow cessation. Using daily flow records, a total of 315 indices were
calculated and their relationships were examined with Principal
Component Analysis (PCA) for different thresholds used to define
zero-flow condition set at 0, 1, 2, and 5 l/s. Redundancy analysis
identified five indices that better describe the patterns of
hydrological variability in Mediterranean NPRS: number of days per year
without flow, annual percentage of months without flow, mean daily of
annual flows, coefficient of variation of Julian date of the annual
start of zero flow and annual rise rate. Using these indices, a
self-organizing map (SOM) was trained in order to categorize the NPRS
into three groups with similar hydrological features. The results of
this study provide a statistically-based hydrological classification of
NPRS in Mediterranean environments. We expect that this classification
will provide useful insights to water authorities to improve the
assessments of the ecological status in this set of water bodies.