R. Dijksma, H.A.J. van Lanen, P. Aalders
Sub-department of Water Resources, Wageningen University, Nieuwe Kanaal 11, 6709 PA Wageningen, The Netherlands
ABSTRACT
In catchments with deep groundwater tables the topographical catchment boundary is often used as a hydrological boundary. The better method, determining the catchment boundary by deep piezometers, is hampered by the scarsity of deep expensive drillings. A simple steady state groundwater model was constructed in order to derive the hydrological boundaries of the Noor catchment. This model indicates a much smaller hydrological catchment than the topographical catchment (716 and 1056 ha, respectively).
The simplified model indicated variable catchment boundaries, given the meteorological conditions. Therefore calculations have also been performed to estimate the minimum and maximum size of the catchment. Attempts to relate the NO3 load of major springs and the Noor brook to the N-input on a field scale are largely influenced by this variable catchment size.
Keywords topographical versus hydrological boundary, deep groundwater tables, NO3 load, groundwater modeling
INTRODUCTION
In the Noor catchment (The Netherlands), a small catchment with deep water tables, nitrate concentrations are increasing. Many springs have already exceeded the critical concentration of 50 mg·l-1 NO3. The biggest and therefore most important spring (Sint Brigida spring) in the catchment shows an increase from 40 mg·l-1 in the early eighties up to 85 mg·l-1 NO3 at present. The discharges of the major springs and the brook are measured continuously. In order to relate the NO3 loads to the N-input, it is important to know the size of the catchment.
Fig 1: Topography of the Dutch part of the area: South Limburg.
Common practice in determining the catchment boundary in areas with deep groundwater tables is the assumption that the hydrological boundary and the topographical boundary coincide (Querner et al., 1997).
In the Noor catchment, with little information on groundwater heads near the assumed catchment boundaries, this assumption led to a catchment of approximately 1056 ha (Dijksma and Van Lanen, 2001).
By constructing a simple steady state groundwater model of the area, with as few assumptions as possible, the catchment boundary and catchment size were calculated.
Noor Gulp
Horstergrub
METHODS
The Noor is a small tributary of the river Meuse, located in the south-east of the Netherlands and north-east of Belgium (Fig 1). The surface elevation in this catchment varies between 240 m a.m.s.l. in the south-east and 91 m a.m.s.l. at the outlet. The Noor brook starts as the Sint Brigida spring at 138 m a.m.s.l., has a length of 3 km and discharges into the Voer in Belgium.
Consolidated Upper-Carboniferous shales and sandstone, folded at the Variscan orogeny, form the impermeable base at a depth of 50-150 m below the surface (Heijde et al., 1980). In the downstream part in Belgium these Upper-Carboniferous formations have been eroded and permeable Lower-Carboniferous limestones occur, which implies that the impermeable base is at a depth of more than 800 m.
These consolidated rocks are discordantly overlain by subhorizontal Upper-Cretaceous deposits, consisting of a sedimentary series of clayey silts, interbedded with thin layers of consolidated and fractured sandstone (Vaals Formation), and soft and poorly bedded chalk (Gulpen Formation). A poorly sorted regolith is found on top of the chalk (Eindhoven Formation). These formations lead to a hydrological system with deep groundwater tables in the largest part of the area, and shallow water tables near the Noor brook and its springs.
135 137 139 141 143 145 147
Sep-79 May-82 Feb-85 Nov-87 Aug-90 May-93 Feb-96 Nov-98 Jul-01
Level (m a.m.s.l.)
Fig 2: Groundwater table in the Noor catchment (WP98).
0 20 40 60 80 100 120
May-79 Feb-82 Nov-84 Aug-87 May-90 Jan-93 Oct-95 Jul-98 Apr-01 Jan-04
NO3 (mg/l)
Sint Brigida Spring Linear (Sint Brigida Spring)
Fig 3: Nitrate concentration of the Sint Brigida Spring.
The deep groundwater tables show large fluctuations over time, due to variations in the annual rainfall surplus (Fig 2). Nitrate concentrations do not show the same fluctuation, but a gradual increase. Fig 3 shows the nitrate concentration of the Sint Brigida Spring over the last 20 years (Van Lanen and Dijksma, 1999).
MODELLING
A simple steady state groundwater model was constructed using Micro-Fem (Hemker and Nijsten, 1996).
Its boundaries were chosen at a relatively large distance from the assumed Noor catchment boundaries.
The transmissivity of the Cretaceous formations was assumed to be 100 m2 d-1 (Dijksma and Van Lanen, 2000; Peters et al, 2001). Neighbouring brooks were included in the model.
Fig 4: Reference groundwater model of the Noor and neighbouring brooks, depicting flow direction and velocity.
Fig 4 shows the result using the long-term average of 269 mm·year-1 as rainfall surplus. The small arrows indicate the flow direction and velocity. The darker area is the resulting Noor catchment. The long tail of the catchment is the result of inaccuracy at the south-east model boundary. In the calculations, this tail is ignored.
This reference model indicates that the catchment extends over only 716 ha, and is thus much smaller than the topography based catchment of 1056 ha. On all sides, the catchment boundary is shifted towards the brook.
Since the catchment is certainly not at steady state with fluctuations in rainfall surplus and accompanied fluctuations of the deep groundwater tables, calculations were also made under wet and dry conditions.
The wet condition was represented by a doubled rainfall surplus, the dry condition by half the rainfall surplus (Table 1).
Table 1: Calculated Noor catchment size; maximum variation.
Rainfall surplus (mm·year-1)
Catchment size (ha)
Reference 269 716
Double (x 2) 538 1016
Half (x 0.5) 135 437
Fig 5: The Noor catchment, with rainfall surplus x 2 and x 0,5 respectively.
These results indicate that the catchment size is strongly dependent on the climatic conditions. As shown in Fig 2, the wet and dry periods only last a few years. Then the conditions change again. Equilibrium in groundwater levels, discharges and therefore catchment boundaries is never reached. Thus, the catchment boundaries in this system are fluctuating over time, but not as much as indicated in Table 1.
It was estimated that, given the duration of the wet and dry periods, it would be more realistic to use less extreme values for rainfall surplus. Another indication that the catchment size fluctuation should be less than that reported in Table 1 was that the calculated groundwater heads (max. and min.) were never reached.
A correction factor was derived, using the calculated and real groundwater levels. Table 2 shows the results of these calculations.
Table 2: Calculated Noor catchment size; realistic variation.
Catchment size (ha)
Reference 716
Wet conditions 781
Dry conditions 644
Another possible check on the most likely catchment size is to calculate the water balance. However, in areas with deep groundwater tables a large quantity of water can be stored in the unsaturated zone. It would be necessary to calculate a water balance over tens of years to eliminate the storage uncertainty.
1016 ha 437 ha
CONCLUSIONS
In areas with deep groundwater tables the topographical boundary is often used as the hydrological catchment boundary. Piezometers to validate this assumption are often scarce. Water balance calculations to derive the catchment size are hampered by the large storage capacity of the unsaturated zone.
In the Noor catchment, the most likely hydrological catchment area is only 68% of the topographical catchment area. The catchment size is not constant over time, but is related to variations in rainfall surplus (± 10 %). The correlation of Nitrate loads in springs and brooks, and N-input on the field scale can contain large errors (> 25%) because of this phenomenon.
ACKNOWLEDGEMENT
The research was carried out as part of the programme of the Wageningen Institute for Environment and Climate Research (WIMEK/SENSE).
REFERENCES
Dijksma, R., van Lanen, H.A.J. (2000) Monitoring and modelling of springflow in the Noor catchment (the Netherlands) In: Catchment hydrological and biochemical processes in the changing environment; IHP 2000, 31-36.
Dijksma, R., Lanen, H.A.J. van (2001) De afvoer van de Noor (Zuid-Limburg); Periode 1992 – 2000 (in Dutch). Subdepartment Water Resources, Wageningen University.
Heijde, P.K.M. van der, Kuyl, O.S., van Rooijen, P. (1980) Grondwaterkaart van Nederland; Kaartblad Maastricht 61 en Heerlen 62 West (in Dutch), Dienst grondwaterverkenning TNO, Delft, the Netherlands.
Hemker, C.J., Nijsten, G.J. (1996) Groundwater Flow Modelling using Micro-Fem; Version 3, Free University Amsterdam, the Netherlands.
Lanen, H.A.J van, Dijksma, R. (1999) Water flow and nitrate transport to a groundwater-fed stream in the Belgian-Dutch chalk region. Hydrological Processes, 13, 295-307.
Peters, E., van Lanen, H.A.J., Alvarez, J., Bradford, R.B.B. (2001) Groundwater droughts; Evaluation of temporal variability of recharge in three groundwater catchments. ARIDE Technical report no. 11, Wageningen.
Querner, E.P., Tallaksen, L.M., Kašpárek, L. van Lanen, H.A.J. (1997) Impact of land-use, climate change and groundwater abstraction on stream flow droughts, using physically based models. FRIEND
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