Effect of water level on bird habitat at lake Maggiore
Lake Maggiore and the Ticino River are water bodies shared by
Italy and Switzerland: they are important resources for drinking
water, irrigation and hydroelectricity generation as well as for
tourism and biodiversity. The cross-border character and the
often conflicting needs of the different users make the shared
management of this resource very complex, but of great importance.
The `‘Parchi Verbano Ticino´’ project, funded by Regione
Lombardia / EU – INTERREG Italia Svizzera 2014/2020, aims
to study the effects of water levels of the lake on various environmental
components with a particular focus on protected
natural areas. The level of the lake is regulated by a dam located
at the southern shore of the lake. In this framework, this
study aims to analyse the effect of water level on bird migration
by: 1) Calculate the inundated bird habitat using a simulation
based on measured water level; 2) calculate the inundated habitat
from Sentinel-1 remote sensing imagery 3) Use the flooded
area derived from S1 as ground truth to validate the previous
simulation 1).
The study area is centerend around Bolle di Magadino (Switzerland,
8°51’56.90”E, 46°9’42.17”N, a protected wetland located
on the north shore of lake Maggiore at the confluence with the
Ticino river. The area is a recognized nesting and stopover
site for birds, listed as a Ramsar Wetland of International Importance
and as Important Bird and Biodiversity Area (IBA).
We defined the habitats of interest using
a vegetation map provided by Fondazione Bolle di Magadino.
The vegetation types collected from a phytosociological field
study were aggregated into ten land cover classes that described
the habitat types and land use. The final habitat map covers
an extent of 6.7 km², including the 1500 ha of wetland called
Bolle di Magadino. Daily passage of migrant birds have been
recorded at Magadino ringing station and since 2019, traditional
net captures were coupled with an Avian Vertical-looking Radar
In this study we focus on the following periods, during which
bird monitoring systems were both deployed: P1: 2019-05-01–
2019-06-20; P2 2019-10-01–2020-02-20 and P3 2021-02-01–
2021-07-20.
The lake level measured at the hydrological station of Locarno
(CH) was used to determine the inundated area in GRASS GIS
using ther.fill module (GRASS Development Team, 2022) and
a DTM that included the lake bathymetry (cell size 0.5 m). The
lake level fluctuated between 192.3 and 194.9 m.a.s.l. over the
study period, with a minimum in April and May, when the waters
are used to irrigate the rice fields downstream, and a maximum
in late autumn.
We used the Google Earth Engine Platform (GEE) (Gorelick et
al., 2017) to extract Sentinel-1 Synthetic Aperture Radar (SAR)
images (ESA, 2021) as they are suited for surface water mapping
and not affected by cloud coverage (Ovakoglou et al., 2021).
We used Edge Otsu Algorithm with terrain correction (Markert
et al., 2020) to estimate the inundated areas of the collection of
a total of 236 images for the three time periods when bird migration
was also monitored. The calculation was implemented
GEE with the approach described by Gorelick et al. (2017) calibrating
the threshold for our study area, adapting the code provided
by Open Geo Blog (2021). The inundated areas were then
overlapped with the land use map in order to estimate the extent
of the submerged vegetation over the three time period defined.
The resolution of all maps was 10 m, except the DTM that has
0.5 m, the CRS used in this work were WGS84 in GEE and the
local CRS GCS CH1903 for all the other analysis.
The area covered with water, according to S1, varied between
103.9 and 471.7 ha (210+- 83.6), some of the surface is a permanent
wetland so it is never completely dry. Each habitat was
affected differently by the flooding: when the water was at its
highest, croplands were completely inundated, grasslands and
reeds were submerged for 80% of their extent whereas urban
areas and infrastructures were not affected (less than 1% underwater).
The flooded area calculated by filling the terrain model
at the level of the ranged from 130.4 to 248.90 ha (208+- 48.6).
The correlation between inundated areas obtained using r.fill in
GRASS and S1 on the same dates was fair for P1 and P2, but
not for P3, when the water of the lake were taken for irrigation,
but the habitats were flooded by rainfall. An interpolation of the
flooded from S1 is a more efficient way to obtain an estimate of
the flooded habitat on a daily basis, that is necessary to study
its effect on migratory birds.
The results presented here will contribute to the definition sustainable
management tools of water management of lake Maggiore
taking into account the effect of lake level on biodiversity
in general and on bird habitat in particular.
