Regional Management of Water Resources (Proceedings of a symposium held during the Sixth IAHS Scientific Assembly at Maastricht, The Netherlands, July 2001). IAHS Publ. no. 268, 2001. 231
Hydrological criteria for durable water systems
VICTOR JAN WITTER Water Authority "Hoogheemraadschap van West-Brabant", PO Box 2212, 4800 CE Breda, The Netherlands e-mail: [email protected]
TIM RAATS Water Authority "De Dommel", PO Box 10001, 5280 DA Boxtel, The Netherlands e-mail: [email protected]
Abstract In the recent past, water management has been too responsive to the needs of society. As a result, massive reconstruction of water systems took place in both urban and rural areas. For the region of West-Brabant in The Netherlands, it is demonstrated how this resulted in an imbalance of the hydrology of regional water systems. An analysis of this imbalance can serve to define hydrological criteria for durable water management. These criteria are not confined to the water compartment but have an impact on land use.
Key words water management; imbalance of hydrology of water systems; hydrological criteria
INTRODUCTION
Water management in The Netherlands by the water boards has been very responsive to the needs of society over the last 50 years. This was the period of reconstruction after World War II, sustained economic growth and rapid population increase. The result was a strong pressure to significantly increase productivity levels of agriculture. Water boards responded by a massive reconstruction of water systems in both urban and rural areas, in order to adapt them to the new circumstances. For a particular area in The Netherlands, the region of West-Brabant, this paper outlines how the water systems became out of balance due to this reconstruction. This paper deals exclusively with the hydrological balance of water systems but it will be argued that there is ample reason to believe that the balance has also been affected with respect to ecology and water quality. The analysis of the present imbalance of water systems can serve to define criteria for durable water management. These criteria are not exclusively confined to the water compartment but have an impact on land use as well. Concluding remarks are presented at the end of the paper.
THE WEST-BRABANT STUDY AREA
The region of West-Brabant (Fig. 1) is situated in the south of The Netherlands, near the Belgian border. It is mainly drained by the rivers Mark and Vliet. The Mark-Vliet basin is a transboundary basin: about 40 000 ha are situated in Belgium, whereas the 232 Victor Jan Witter & Tim Raats
remaining part (about 100 000 ha) is situated in The Netherlands. The southern part of the region consists of gentle Pleistocene slopes. At the Belgium-Netherlands border the area is about 20-25 m above sea level. The cities of Breda, Roosendaal and Bergen op Zoom (Fig. 1) are situated more or less at sea level. The northern Holocene part of the region consists of clay polders situated at or slightly below (-1 m) sea level. The region is fairly urbanized and there are strong pressures from the industrialized centres of both Antwerp and Rotterdam to create "overflow" commercial and industrial facilities within the region. Apart from urbanization, the sandy soil in the southern part of the region is used mainly for nature conservation and intensive agriculture. The clay polders in the northern part are mainly in use for large-scale cultivation of crops such as potatoes and sugar beet. Over the past 50 years the water systems have been extensively reconstructed in order to adapt them to the changing needs of urbanization and modernization of agriculture. In particular the drainage system was greatly intensified, brooks and rivers were normalized and land was reclaimed (by construction of dikes and by improvements to the runoff system) in order to make continuous land-use possible.
IMBALANCE OF WATER SYSTEMS
Originally, the southern Pleistocene part of the region was in agricultural use. The population was sparse and, apart from some scattered farmland, most of the region was covered by heath land, used mainly for some sheep grazing. This type of land use changed c. 1900, when heath land reclamation and land consolidation works were carried out in order to make permanent agriculture possible. This was, so to speak, the first round of reconstruction. After World War II, a second round of land consolidation schemes and extensive reconstruction works of water systems took place in The Netherlands. The objective was to facilitate modernization of agriculture and expansion of built-up areas. This second round was not confined to the southern, Pleistocene part of the region, but covered the northern part as well. The effects of the first round of reconstruction, which took place during 1900-1940, will be discussed in a subsequent paper. In this paper the effects of the second round of reconstruction (1950-1970) are documented.
Hydrological effects
Particularly due to the extensive introduction of tile drainage and due to other major improvements of the drainage conditions (new and larger ditches and water courses), the drainage of the region was considerably increased. Table 1 illustrates this for two major brooks in the region, the Aa of Weerijs and the Bovenmark (Fig. 1). As a consequence of the increased level of drainage, groundwater levels also fell. In Noordhuis et al. (1990) this has been documented for a part of the region. For agricultural land not subject to the influence of large-scale pumping of groundwater for use as drinking water (which already had previously resulted in lowering the water table) the drop in groundwater levels was about 15-20 cm.
234 Victor Jan Witter & Tim Raats
Table 1 Mean water levels in the Mark-Vliet water system before and after the reconstruction works carried out during 1950-1970 (from: Raats, 1996).
Mean water level (m a.s.l.): Before After Bovenmark: BlauweKamer(l)* 0.97 0.79 Duivelsbrug (2)* 0.26 0.10 Aa of Weerijs: Oranjeboombrug (3)* 0.26 0.14 * Numbers in parentheses refer to locations shown in Fig. 1.
The increased drainage resulted also in a process of adjustment of the water balance, in which the ratio of runoff to rainfall steadily decreases (Fig. 2) until a new equilibrium is reached. As can be judged from the data points for the last 3 years in this figure, there are indications that a new groundwater equilibrium has been reached. The increase in drainage also resulted in an imbalance of the runoff system: low discharges becoming even lower and peak discharges becoming even higher (Table 2). The data in the top rows of Table 2 have been derived as follows. Annual discharge, expressed in m3 s"1, has been plotted against annual rainfall and a straight regression line has been fitted to the data. This regression line was subsequently used to derive mean discharges for low (700 mm) and high (950 mm) values of annual rainfall. Also on a daily basis, peak discharges have increased significantly. For the Bovenmark and Aa of Weerijs the daily discharges with a 1-year recurrence interval were 21.2 m3 s"1 and 16.4 m3 s"1, respectively (Anonymous, 1964). At present these quantities are 28.1 m3 s"1 and 26.9 m3 s"1, respectively. Also very extreme daily peak discharges have increased: before the reconstruction works the daily discharge with a recurrence interval of 3000 years just downstream of Breda was estimated as 122 m3 s"1 (Anonymous, 1965). For the period after the reconstruction works this figure should be definitely higher as can be deduced from the estimate of the 7-day discharge for a 2000-year recurrence interval for this period, 121 m3 s"1 (Anonymous, 1996). In the last two rows of Table 2, an accompanying effect is shown—the increase of maximum water levels. This is done by comparing historical maximum water levels
0.6 ; ; I 0.5 4• A 0.4 . . 1 r - - • " " \ o 5;o.3 4 • • 4 y • o 0.2 _ J, > ' J 4 y= -0.0059X+12095 ë o-i FT" = 0.1315 0.0 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000
Titre
Fig. 2 Ratio of runoff to rainfall (on a yearly basis) for the Mark-Vliet basin. Hydrological criteria for durable water systems 235
Table 2 Mean annual discharges as a function of annual rainfall and maximum water levels before and after the reconstruction works carried out during 1950-1970.
Mean annual discharge (m3 s"1): Before After Bovenmark (1)* annual rainfall 700 mm 3.22 2.47 annual rainfall 950 mm 4.09 4.70 Aa of Weerijs (3)* annual rainfall 700 mm 2.36 2.16 annual rainfall 950 mm 3.13 3.87 Maximum water level (m a.s.l.): Before After Bovenmark (2)* 2.41 (23 Nov. 1930) 2.70 Aa of Weerijs (3)* 2.20(1 Jan. 1926) 2.50 * Numbers in parentheses refer to locations shown in Fig. 1.
(for an approximately 50-year period) before the reconstruction with simulated maximum water levels (for a 50-year recurrence interval) after the reconstruction works. The increases of peak discharges and of maximum water levels were mainly due to a faster hydrological response, because of the introduction of tile drainage and the normalization of brooks and rivers. The increase of maximum water levels was also due to the reduction of the physical space available for the water compartment. About 2000 ha of valleys and wetlands, which used to be frequently flooded, were reclaimed due to the expansion of intensive agriculture. Under present-day conditions, the effect of a 260-ha retention area close to Breda on maximum water levels in Breda has been estimated as about 5 cm (Anonymous, 1998).
CRITERIA FOR DURABLE WATER MANAGEMENT
Basically, durable water management implies restoring the equilibrium of the water system. This means choosing a historical reference for the morphology of the water system as a basis for the restoration of the water system and adopting estimated means and variances of water levels and discharges of the water system at the time of the historical reference as criteria or target values. In the region of West-Brabant the historical reference for the morphology of a particular water system might be chosen as the morphology that existed before the above-mentioned "second round" of land consolidation schemes and extensive reconstruction works. Apart from the above-mentioned criteria, a durable water system should also meet the following criteria: (1) "vertical" hydrological equilibrium: no mining of groundwater; (2) morphological equilibrium, including admittance of "normal" morphological dynamics, for instance meandering; and (3) water quality equilibria: normal (background) levels and fluctuations of levels of pollutants. These equilibria should be dynamic in the sense that after a perturbation the system returns to its original position. Water systems that have this capacity are sometimes referred to as robust water systems. Heterogeneity of water systems and the linking of water systems 236 Victor Jan Witter & Tim Raats favour robustness (Remmelzwaal & Vroon, 2000). Theoretically, natural (ranges in) values of hydrological parameters such as water levels, discharges, groundwater flows etc., do not exist because, from time to time, catastrophic events occur. Allowing this, of course, cannot be the goal of durable water management. Table 3 sets out the different steps to be taken in the application of the above-mentioned criteria. As an illustration, consider the present water management plan for the region of West-Brabant (Anonymous, 2000a). In this plan references have been chosen for each of the water systems within the region. Quite often these references refer to the present-day situation within only minor adaptations in order to increase the hydrological and ecological robustness of the water system or to neutralize the effects of diffuse pollution (mainly caused by agriculture). For some valuable water systems (some brooks and creeks, former wetlands bordering the River Mark, some peat areas) the historical reference for the morphology goes back to 1950-1965 or even earlier.
Table 3 Criteria for durable water systems: the water compartment.
Step Description 1 Choose historical reference for morphology 2 Estimate target values for means and variances of hydrological parameters 3 Estimate target value for safe groundwater yield ("vertical equilibrium") 4 Estimate admissible, "normal" morphological dynamics 5 Estimate target values for normal and background levels of pollution
CONCLUDING REMARKS
Durable water management is motivated in particular by the notion that water systems are out of balance. To restore this balance, a morphological reference should be chosen and target values for hydrological, ecological and water quality parameters should be estimated (Table 3). Although this paper focuses on the hydrological equilibria of water systems, there is reason to believe that also the equilibria with respect to ecology and water quality have been affected by the land consolidation schemes and reconstruction of water systems which took place after World War II. The reclamation of valleys along brooks and flood plains of the River Mark resulted in loss of ecological gradients and put an end to yearly flooding of these areas, which up till then caused some natural purification of the surface water and had beneficial effects on the ecology. The lowering of groundwater levels resulted in extensive water shortages in natural reserves throughout the region and in subsequent loss of ecological values. The increased peak discharges in many of the "improved" brooks resulted in washing out of the aquatic ecosystems during flood periods. The criteria for durable water systems also have an impact on land use. Urban expansion, for instance, is not compatible with the water system of a peat area or with the water system of water-logged wetlands surrounding a river. This means that durable water management sets criteria for land use, specifically for agriculture, nature conservation and urban expansion. Also—in order to account for the loss of space available to the water system and to anticipate the effects of climatological change— Hydrological criteria for durable water systems 237 areas should be designated which can be used to store water during extreme hydrological events. For the region of West-Brabant this is done in an accompanying report (Anonymous, 2000b) to the water management plan.
REFERENCES
Anonymous (1964) Verband Tussen Piekajvoer en Herhalingstijd voor de Hoofdslromen van Het Stroomgebied Boven Breda (Relationship between peak discharges and recurrence time upstream of Breda). DHV, Amersfoort. Anonymous (1965) Rapport Waterbezwaar 1965 (Report on the inundations during 1965). DHV, Amersfoort. Anonymous (1996) Hoogwaterlijnen Mark en Vliet (Maximum water levels for the Mark-Vliet basin). "Hoogheemraadschap van West-Brabant" Water Authority, Breda. Anonymous (1998) Maatregelen Hoogwater Stedelijk Gebied Breda (Measures for flood-protection in the urban area of Breda). Arcadis, Arnhem. Anonymous (2000a) Integraal Waterbeheersplan West-Brabant (Integrated water management plan West-Brabant). "Hoogheemraadschap van West-Brabant" Water Authority, Breda. Anonymous (2000b) Water op de Kaart (Water mapped). "Hoogheemraadschap van West-Brabant" Water Authority, Breda. Noordhuis, M. van Bakel, P. J. T. & van Hoist, A. F. (1990) Onderzoek naar de Verandering van de Freatische Grondwaterstanden op Enkele Kaartbladen in de Provincie Noord-Brabant eds Gevolg van Veranderingen in de Landbouwwaterhuishouding (Study to changes in groundwater levels in the province of Noord-Brabant). Staring Centrum Publ. no. 72, Wageningen. Raats, T. (1996) De Invloed van de Normalisatie op de Waterhuishouding (The effects of land reclamation on regional hydrology). "Hoogheemraadschap van West-Brabant" Water Authority, Breda. Remmelzwaal, A. & Vroon, J. (2000) Veerkrachtige watersystemen? (Robust water systems?). HzO 3,13-15.