The Kleine Nete River, Belgium

The Kleine Nete basin is located in Belgium, approximately 60 km northeast of Brussels, in the province of Antwerp (Figure 1). This basin is part of the larger Nete basin, which includes both the Kleine and Grote Nete sub-basins. The Nete basin itself is a sub-catchment of the Scheldt basin, draining towards the North Sea. Average annual precipitation in the study area is approximately 840 mm. Soils are predominantly sandy, although loamy sand, sandy loam, and sandy clay occur locally within the valley bottoms.

The Kleine Nete basin is characterized by a gently rolling landscape dissected by the Kleine Nete River, the Aa River, and their tributaries. This results in elongated ridges, slightly elevated interfluves, and broad, swampy valleys. The basin is relatively flat, with elevations ranging from 3 to 48 m above sea level and an average elevation of about 24 m.

Figure 1 Province of Antwerp (red/yellow), catchment area of the Kleine Nete river (purple) and the kleine nete (light-blue) with its tributaries (dark-blue). Municipilaties that border the Kleine Nete are labeled in black.

Geologically, the study area is part of the Campine Basin, a subsiding sedimentary basin located north of the Brabant Massif. The aquifer systems of the Kleine Nete catchment are composed entirely of Cenozoic deposits. During the Oligocene epoch, a thick clay layer known as the Boom Clay was deposited across the basin. Owing to its very low vertical hydraulic conductivity, this clay layer effectively acts as an impermeable base to the overlying aquifer system, strongly constraining vertical groundwater flow and separating the shallow sandy aquifers from deeper geological units.

Above this clay substratum, the landscape is dominated by thick, laterally extensive sandy deposits that form the central Campine river and dune district. These sands are highly permeable and allow large volumes of precipitation to infiltrate deeply into the subsurface. In the Kleine Nete basin, groundwater recharge primarily occurs on the higher-elevation sandy ridges and dune complexes, while groundwater discharge takes place in the lower-lying valley floors, where seepage and springs are common. The movement of groundwater is driven by gravity and controlled by subtle variations in topography, resulting in a characteristic pattern of infiltration on elevated areas and upward groundwater flow in depressions and valleys (Figure 2).

Figure 2 Schematic overview of groundwaterflows. Sandy ridges in brown, temporary wet landscape depressions in green and permantly wet valleys in blue. Depending on where rainwater is allowed to seep into the soil, it can take days/months/years for it to arrive into the valleys.

These groundwater dynamics have played a key role in shaping the geomorphology of the basin. The interaction between permeable sandy massifs, shallow groundwater tables, and limited relief has led to the development of elongated sandy ridges, dune fields, and poorly drained depressions. In the lower parts of the landscape, persistent groundwater discharge has promoted the formation of wet valleys and swampy areas, while the higher, better-drained zones remained dry and sandy. Together, these processes define a geomorphological landscape characterized by infiltration and seepage within a porous sandy system, which underlies both the present-day hydrology and the spatial organization of dunes, valleys, and depressions in the Kleine Nete basin.

The present land use of the Kleine Nete basin reflects a strongly diversified landscape shaped by both natural conditions and long-term human intervention. The largest urban centre within the basin is Turnhout, with approximately 40,000 inhabitants, while several smaller towns and villages such as Beerse, Arendonk, Retie, Kasterlee, Lichtaart, Tielen, Gierle, Lille, Vosselaar and Vorselaar are dispersed throughout the catchment. Forested areas are most prominent along the Campine ridge and in the northern and eastern parts of the basin, where coniferous forests dominate, interspersed with smaller patches of deciduous woodland.

Agriculture plays a central role in land use across the basin. Dairy and livestock farming are predominant, resulting in extensive areas of agricultural grassland and a high proportion of maize cultivation. Land use consists mainly of grassland, crop or mixed farming systems, urban areas, and forest, complemented by smaller proportions of industrial and infrastructure zones, open water bodies, bogs and marshes, and remnants of heathland (Figure 3)

Figure 3 Current land use is dominated by forests and agriculture (grasslands and arable crops).

Historically, the study area was characterized by wide river valleys and a dense network of slowly flowing watercourses. Many landscape depressions were largely disconnected from the main river channels, allowing surface water to remain in place for extended periods and to infiltrate gradually into the subsurface. This setting gave rise to well-developed infiltration and seepage patterns, which, in combination with a humid maritime climate with moderate annual precipitation and mild winters and summers, created highly favourable conditions for wetland development. As a result, wetlands once covered a substantial part of the basin (Figure 4).

Figure 4 also shows, however, that these wet landscapes were extensively altered. Parts of the wet valley floors were converted into hay meadows, peatlands were drained and exploited for peat and later for iron extraction, and, particularly after the Second World War, large-scale drainage intensified rapidly. Technological advances enabled the economically viable drainage of increasingly larger areas, leading to the construction of an extensive network of ditches in the valleys. This drainage system efficiently lowered groundwater levels, promoted the mineralisation of organic soils, and resulted in nutrient-rich agricultural land. At the same time, dry heathlands were largely replaced by forest plantations or transformed into productive agricultural land through irrigation and fertilisation.

The amount of lost wetlands in the area is estimated to be around 1.100 hectares (over 84% of wetlands since 1950), which has had profound hydrological consequences. The reduction in wetland area led to a marked decline in the natural buffering capacity of the landscape. Precipitation is now drained more rapidly, limiting opportunities for delayed infiltration and groundwater recharge. During dry periods, weakened infiltration and seepage processes result in insufficient shallow groundwater availability to sustain baseflows in rivers, while during wet periods downstream flood risks have increased. Consequently, the Kleine Nete basin has become highly sensitive to both drought and water excess, reflecting the cumulative effects of long-term land use change on the basin’s hydrological functioning.

Figure 4 Overview of historic wetlands in 1950 (above) and current wetland areas in 2000 (below). Source: Decleer, K., Wouters, J., Jacobs, S., Staes, J., Spanhove, T., Meire, P., & van Diggelen, R. (2016). Mapping wetland loss and restoration potential in Flanders (Belgium): an ecosystem service perspective. Ecology and Society, 21(4), Article 46. https://doi.org/10.5751/es-08964-210446

The hydrological functioning of the Kleine Nete basin can be understood as that of a naturally well recharging but structurally weakened system. The basin is underlain by thick, highly permeable sandy deposits that allow substantial infiltration and groundwater storage, particularly on the higher ridges and dune complexes. These elevated zones act as recharge areas, while groundwater naturally discharges in the lower valley floors as seepage, historically sustaining wetlands and providing a stable baseflow to the river network. In principle, this makes the basin relatively resilient to drought. In practice, however, land use change, drainage and increasing groundwater abstractions have significantly reduced this buffering capacity. 

Under current land use and average climatic conditions, roughly one third of annual precipitation infiltrates into the shallow subsurface. For the study area, this corresponds to an infiltration volume on the order of 170 million m³ per year. The remainder of the rainfall is largely lost through evapotranspiration, with a smaller but hydrologically important fraction converted into surface runoff. Infiltration is spatially uneven: it is highest on higher, sandy agricultural and forested areas where groundwater tables are deeper and storage capacity is large, while it is much lower in valley bottoms where groundwater levels are shallow and soils saturate quickly (Figure 5). As a result, rainfall in valleys is more readily translated into runoff or evaporation rather than recharge.

Figure 5 Volume (m³/ha*year) of rainwater that percolates in the soil (infiltration/recharge) modelling based on current land use scenario and yearly precipitation of 800mm. Darker areas allow for higher volumes of recharge.

This recharge potential is further constrained by land use. Impervious surfaces associated with housing, roads and industry prevent infiltration altogether and route water rapidly towards ditches and streams, increasing peak discharges (Figure 6). In addition, soil degradation in agricultural areas—through surface crusting and compaction caused by intensive machinery use—reduces soil porosity and infiltration capacity, even on sandy soils. These processes accelerate runoff, reduce groundwater recharge and increase evaporative losses from ponded water. Together, sealing and soil compaction have made the system more sensitive to both short term flooding and longer dry periods.

Figure 6 Above: sealed soils due to buildings (red) or pavement (grey and black), with recharge potential as a backdrop (dark brown areas are sandy ridges with high potential for ground water recharge). Urban developments are often located on drier, sandy ridges, limiting recharge potential and increasing runoff. Below: amount of rainwater (in mm) that is lost through runoff, modelled based on current land use and yearly precipitation of 800mm.

The dominant loss of water from the “hydrological battery” is drainage. The Kleine Nete valleys are densely intersected by ditches and, locally, subsurface drainage systems that were historically installed to make wet soils suitable for agriculture and other land uses (Figure 8). These drainage networks intercept shallow groundwater and rapidly convey it out of the system. Under current drainage practices, we estimate that around 60% of the naturally available shallow groundwater stock is lost (Figure 7). The natural shallow groundwater supply of the landscape—defined as the volume of water present up to one meter below ground level—is reduced from about 140 million m³ to roughly 60 million m³. This represents a fundamental weakening of the system’s capacity to buffer drought and to sustain river baseflows.

Figure 7 Above: naturally available shallow groundwater stock. Below: the same shallow groundwater stock reduced by drainage. The darker blue areas show where water naturally remains near the surface and has an important function in the direct supply of water to support baseflow of the rivers.

Figure 8 Overview of the ditches (red) present in the area.

The consequences are clearly visible in river discharge dynamics. Reduced shallow storage means that the Kleine Nete reacts more rapidly to rainfall, with sharper but shorter lived discharge peaks, while during dry periods the river level drops quickly due to insufficient groundwater supply. In summer, when precipitation is low and evapotranspiration is high, the river increasingly depends on treated wastewater effluent to maintain flow. During dry periods, effluent can account for a very large share of the river discharge, indicating a structurally weakened natural baseflow and limiting flexibility for water reuse. Groundwater abstractions add further pressure to this already compromised system. The Kleine Nete basin is an important groundwater resource due to its permeable sandy aquifers, and permitted abstractions have increased strongly over the past decades (Figure 9). The total permitted annual abstraction is now on the order of 65 million m³, with particularly strong growth in permitted daily abstraction rates. Drinking water production dominates annual volumes, while agriculture accounts for a large share of peak daily abstractions, typically during warm and dry periods when groundwater levels are already low. In relation to current infiltration volumes, these abstractions represent a substantial claim on the system and further reduce drought resilience.

Figure 9 Permitted groundwater abstraction volume as compared relative to volumes in 2000. Blue: total yearly permitted volumes. Red: total daily permitted volumes.