shallowgroundwater: estimated zones with shallow groundwater in the Flemish region of Belgium

General

The data source shallowgroundwater is a geospatial dataset of multipolygons that represent the estimated areas, in the Flemish region of Belgium, where the mean lowest groundwater level (MLW; Knotters & Van Walsum, 1997; Van Heesen, 1970) is less than approximately 2 m below soil surface (hence, “shallow” groundwater). We expect groundwater dependent species and communities to be present within these areas. Outside these areas we assume they are groundwater independent. We combined several data sources in order to estimate these areas.

Compilation of the data source

We compiled the dataset through an iterative process of adding specific data sources (referred in the description of attributes below), followed by validation steps based on both the actual presence of groundwater dependent habitat types or regionally important biotopes (Natura 2000 habitat map) and in situ measurements of groundwater levels (Watina+ database). The coverage of these validation data by shallowgroundwater was 96.9% and 98.6% respectively.

Most steps to compile the data source were done manually using QGIS. Final steps were done in R; see R-code in the GitHub repository 'n2khab-preprocessing' at commit 1b004e1.

Detailed properties

The data source is a GeoPackage with a single spatial multipolygon layer shallowgroundwater in the ‘Belge 1972 / Belgian Lambert 72’ coordinate reference system (EPSG-code 31370).

All attributes are boolean (true/false), each indicating if a polygon was selected from the corresponding data source by applying a set of criteria. Multiple attributes can be true for a given polygon. In order to reduce file size, polygons were dissolved by each unique combination of the values of all attributes. Hence the dataset consists of multipolygons (also multipart polygons) rather than single (part) polygons. The different attributes of this dataset reveal for each polygon (1) the data source(s) we relied on and (2) the selection criteria we applied to judge if the mean lowest groundwater level is less than approximately 2 m below soil surface. As far as possible, we reference each used data source in the description of attributes below. If one is interested in the original polygons of each datasource, selections can be made by consulting the referenced data sources and applying the specified criteria.

These are the attributes:

geomorph_wcoast:

source: Cosyns et al. (2019)

description: polygon belonging to geomorphological entities that are expected to harbour groundwater dependent types, and thus to exhibit shallow groundwater levels. It mainly concerns dune slacks, mud flats and salt marshes.

selection: "Code" IN ('ms', 'msl', 'sm', 'ss', 'ys', 'ysl', 'yfs') OR ("Code" = 't' and "Subtype" like '%vlakte%') with ms = medium old dune slack, msl = leveled medium old dune slack, sm = mud flat, ss = salt marsh, ys = young dune slack, ysl = leveled young dune slack, yfs = young frontal dune (intruded), ("Code" = 't' AND "Subtype" like '%vlakte%') = fossil beach

anthrop_gwdep:

source: soilmap_simple (10.5281/zenodo.3732903) as derived from the digital soil map of the Flemish Region (version soilmap_2017-06-20; 10.5281/zenodo.3387008); habitatmap_terr (10.5281/zenodo.3468948) as derived from the Natura 2000 habitat map of Flanders (10.5281/zenodo.3354381)

description: zones located within a 100 m buffer around (almost) everywhere groundwater dependent habitat types (or regionally important biotopes) ánd situated within zones classified as “anthropogenic” areas within the soil map. Within the zones of the soil map that are designated as “anthropogenic”, we lack information on soil characteristics. However, (almost) everywhere groundwater dependent types are present in these zones according to the Natura 2000 habitat map of Flanders, implying shallow groundwater levels. By including 100 m buffer zones around these types, restricted to the anthropogenic zones of the soil map, we consider the combined areas to have (potentially) shallow groundwater levels. So in practice, we first select the habitatmap polygons with (almost) everywhere groundwater dependent types that intersect the anthropogenic soil polygons, we buffer them, and then clip the result by the anthropogenic soil polygons.

selection: see https://github.com/inbo/n2khab-preprocessing/pull/61 for the adopted workflow

from soilmap_simple: bsm_mo_soilunitype starts with 'O'

from habitatmap_terr: (almost) everywhere groundwater dependent types only; list of this category of types is available through n2khab R-package.

narrowanthrop_gwdep:

source: soilmap_simple (10.5281/zenodo.3732903) as derived from the digital soil map of the Flemish Region (version soilmap_2017-06-20; 10.5281/zenodo.3387008); habitatmap_terr (10.5281/zenodo.3468948) as derived from the Natura 2000 habitat map of Flanders (10.5281/zenodo.3354381)

description: narrow zones classified as “anthropogenic” areas within the soil map that include (almost) everywhere groundwater dependent habitat types (or regionally important biotopes). Regarding the anthropogenic soil type polygons, it appears that the narrow ones containing (almost) everywhere groundwater dependent types are interesting to include as a whole as zones with supposed shallow groundwater levels. Hence we select them as a whole instead of selecting buffers around the polygons with (almost) everywhere groundwater dependent types (cfr. anthrop_gwdep and dunes_gwdep). An appropriate algorithm selects meaningful polygons, based on a “thinness” criterion and the fraction of (almost) everywhere groundwater dependent types that are present within the polygons.

selection: see https://github.com/inbo/n2khab-preprocessing/pull/61

drainage:

source: soilmap_simple (10.5281/zenodo.3732903) as derived from the digital soil map of the Flemish Region (version soilmap_2017-06-20; 10.5281/zenodo.3387008)

description: Drainage classification is based on a combination of groundwater depth, soil permeability, presence of impermeable layers, soil depth and topography (see Van Ranst & Sys, 2000). 

selection: bsm_mo_drain in ('c-d', 'd', 'e', 'f', 'g', 'h', 'i', 'e-f', 'e-i', 'h-i'); these are soils that are at least moderately gleyic or wet. 

dunes_gwdep:

source: soilmap_simple (10.5281/zenodo.3732903) as derived from the digital soil map of the Flemish Region (version soilmap_2017-06-20; 10.5281/zenodo.3387008); habitatmap_terr (10.5281/zenodo.3468948) as derived from the Natura 2000 habitat map of Flanders (10.5281/zenodo.3354381)

description: zones located within a 100 m buffer around (almost) everywhere groundwater dependent habitat types (or regionally important biotopes) ánd situated within zones classified as “dunes” areas within the soil map. Within the zones of the Belgian soil map that are designated as “dunes”, we lack information on soil characteristics. However, (almost) everywhere groundwater dependent types are present in these zones according to the Natura 2000 habitat map of Flanders, implying shallow groundwater levels. By including 100 m buffer zones around these types, restricted to the “dunes” of the soil map, we consider the combined areas to have shallow groundwater levels. So in practice, we first select the habitatmap polygons with (almost) everywhere groundwater dependent types that intersect the “dunes” polygons, we buffer them, and then clip the result by the “dunes” polygons.

selection: see https://github.com/inbo/n2khab-preprocessing/pull/61

from soilmap_simple: bsm_mo_soilunitype = 'X'

from habitatmap_terr: (almost) everywhere groundwater dependent types only; list of this category of types available through n2khab R-package.

peat_profile:

source: soilmap_simple (10.5281/zenodo.3732903) as derived from the digital soil map of the Flemish Region (version soilmap_2017-06-20; 10.5281/zenodo.3387008)

description: variant of the soil profile indicates a superficial peaty cover, mostly on gleyic or permanently water saturated soil with or without profile development (‘(v)’), eventually combined with strong anthropogenic influence (‘(o)’)

selection: bsm_mo_profvar in ('(o)(v)', '(v)')

peat_substr:

source: soilmap_simple (10.5281/zenodo.3732903) as derived from the digital soil map of the Flemish Region (version soilmap_2017-06-20; 10.5281/zenodo.3387008)

description: soil substrate (layer underlying superficial layer, and lithologically diverging from it) consists of peat material starting at small (less than 75 cm; ‘v’) or moderate depths (75-125 cm; ‘(v)’), or a combination of the previous (‘v-’)

selection: bsm_mo_substr in ('(v)', 'v', 'v-')

peat_parentmat:

source: soilmap_simple (10.5281/zenodo.3732903) as derived from the digital soil map of the Flemish Region (version soilmap_2017-06-20; 10.5281/zenodo.3387008)

description: parent material contains a mixture of at least 30% of peaty material

selection: bsm_mo_parentmat = 'v'

peat_texture:

source: soilmap_simple (10.5281/zenodo.3732903) as derived from the digital soil map of the Flemish Region (version soilmap_2017-06-20; 10.5281/zenodo.3387008)

description: soil consists of plain peat material 

selection: bsm_mo_tex in ('V-E', 'V')

phys_system:

source: Fysische systeemkaart - Gegeneraliseerde bodemkaart voor Vlaanderen (here available at geopunt.be). Lhermitte & Honnay (1994).

description: polygons designated as seepage areas where groundwater is supposed to gather after having infiltrated elsewhere (infiltration areas) and being transported through the landscape (passage areas). The classification of infiltration/passage/seepage areas is entirely based on the soil map polygons of Flanders (10.5281/zenodo.3387008). Each polygon is attributed to one of these classes by considering soil, topographical and hydrological characteristics. In the seepage areas, we consider the groundwater to be shallow below topography.

selection: INKWEL = 3

zwin:

source: Open Street Map, consulted 2021-10-06 by QGIS plugin QuickOSM

description: the contour of the Zwin Nature Reserve in the most eastern part of the Flemish coastal area, as mapped in the Open Streep Map on 2021-10-06

selection: leisure = nature_reserve

habitat_1130:

source: Natura 2000 habitat map of Flanders (10.5281/zenodo.3354381)

description: polygons located within estuaries (habitat type 1130); these areas are located outside dike infrastructure and harbour several types that rely on tidal dynamics or brackish water, and at the same time are considered (almost) everywhere groundwater dependent, a.o. brackish pioneer habitats, salt marshes and meadows (habitat types 1310, 1320, 1330 resp.), reed beds (6430_mr, rbbmr) and riverine Salix forests and shrubs (91E0_sf, rbbsf).

selection: Hab1 = '1130' or Hab2 = '1130' or Hab3 = '1130' or Hab4 = '1130' or Hab5 = '1130'

gwdepth_coast:

source: interpolated groundwater level of point measurements along the Flemish coast, taken from the WATINA database of the Research Institute for Nature and Forest (INBO)

description: locations with estimated average lowest groundwater table less than 2.5 m below soil surface (shallow groundwater), based on interpolation of measured groundwater levels in areas along the Flemish coast with sufficient gauge densities; in these coarse sandy areas, groundwater levels are supposed to behave independently of topography, allowing simple interpolation among point measurements

selection:

calculation of mean yearly spring groundwater level at each gauge (GVG; i.e. average of three groundwater head measurements in meter Ostend height, EPSG:5710, closest to 1st April, but with a minimum time lapse of 14 days, averaged over years)

calculation of mean yearly amplitude of groundwater level at each gauge (i.e. difference between min and max water level per hydrological year, averaged over years)

inverse distance weighted interpolation of both these datasets (IDW; factor 3, resolution 5 m) for areas with sufficient gauge densities

calculation of relative GVG (depth below soil surface) by subtracting soil surface level (Flemish DTM) from GVG, both in meter Ostend height (EPSG:5710)

subtract average amplitude from relative GVG as an estimate of lowest groundwater level (GLG)

select locations with estimated GLG >= -2.5 m located within the coastal dunes ecodistrict (see Flemish ecoregions and ecodistricts)

gwdepth_local:

source: Batelaan et al. (2012)

description: mean lowest groundwater level less than 2 m below soil surface (MLW) for the present day situation. The modelled area is a large military training site for which no information is available in the soil map of Flanders.

selection: GLG <= 2 m

seepage:

source: Batelaan & De Smedt (1994)

description: area with modelled seepage fluxes of at least 0.8 mm/day. In these areas we inherently consider the groundwater level to be shallow. The extent of the regional groundwater model encompasses the valleys of the rivers Demer, Dijle en Nete in the central part of eastern Flanders

selection: gridcode >= 3

peat_survey:

source: peat pricks executed or compiled by the Research Institute for Nature and Forest (INBO)

layer with interpreted presence of peaty zones evaluated by simple measurements of the depth of plain and superficial peaty soil layers at regular intervals (about 20 m), sampled by manually stabbing the peaty layers with a simple stick until the underlying mineral layer is reached (i.e. resistance is felt). These measurements originate from localised inventories and are thus sporadically available only.

duneslack:

source: Provoost et al. (2020)

description: polygons with dune slack vegetations along the Flemish coast that typically imply shallow groundwater levels. 

selection: see source

A reading function to return shallowgroundwater in a standardized way into the R environment is provided by the R-package n2khab.