Letting water into the waterway systems during periods of insufficient precipitation helps prevent damage owing to rapidly receding ground and surface water levels. Without water intake, agricultural crops, nature conservation and infrastructure, such as peat dikes, buildings and roads, will be exposed to drought and salt stress and sustain damage as result. Apart from level management, water is also taken in for maintaining the desired water quality. In the past, brackish to salt water was frequently let in to the Dutch lowlands to dilute and flush away direct discharges from sewage or industrial effluents. Currently, flushing in West Netherlands is largely designed to prevent high concentrations of chloride from accumulating in surface water as a result of salt seepage. Flushing is by and large, a traditionally established practice, with no defined objectives or service levels. The greatest demand for low-chloride water comes from agriculture. Water is also taken in to prevent cyanobacteria blooms (formerly known as blue-green algae), to control the groundwater level as a means of counteracting peat land subsidence and for preventing dehydration of nature reserves. Flushing places a relatively high demand on water, representing approximately 20% of the total water demand nationwide.
The current practice of taking water in for the benefit of water level management and for flushing purposes is based on diverting significant quantities of river water from other European countries into the Rhine and Meuse rivers. The closure of IJsselmeer, Haringvliet and other lakes has resulted in the expansion of agricultural areas where fresh water can be taken in, for example, by way of the IJsselmeer polders and the Zuid Hollandse Eilanden. However, climate change and the increasing demand for water by different sectors present uncertainties about the availability of water of sufficient quality and quantity for agriculture. To make the right decision about this matter, it is imperative to have a working knowledge of the effectiveness and benefits of water intake. The focus in this fact sheet is on taking water in for agricultural (irrigation) purposes and thus on flushing for maintenance of the surface water quality.
Topics: Water shortage and fresh water supply, level management, irrigation, drought damage, salt damage, water quality
Water intake is a strategy employed to supply water of sufficient quality and quantity.
Part of the Netherlands' current water management policy is focused on ensuring that fresh water of good quality and sufficient quantity is available when and where it is needed. In the Dutch highlands, this is done by distributing the water that flows into our country through the Rhine and Meuse rivers and by using local and regional ground and surface water supplies. In the Dutch lowlands on the other hand, water management is aimed at preventing internal and external salinity and salt-water intrusion from the Nieuwe Waterweg as much as possible, to ensure that under normal conditions, the important fresh water intake points along the Haringvliet, Hollandsch Diep, Spui (Bernisse) and the Hollandsche IJssel remain fresh. The intake water is used for level management, nature conservation and economic purposes such as agriculture, drinking water, industry and energy. Source: National Water Plan, 2010.
Figure 2: Flushing of sub-areas on Goeree-Overflakkee. Source: Wit 1987. Note: This is an example of an ineffective and inefficient flushing design. Witteveen and Bos have meanwhile proposed better design alternatives.
Glossary of terms and definitions
Box 1: Critical indicators for water supply in areas affected by salt seepage
(quote from Visser et al., 2011; p. 49) The quality and intensity of the seepage and the quantity and chloride content of the intake water serve as the basis for determining the measures necessary to keep the system ‘fresh’. The higher the chloride concentration, the greater the quantity of water that initially needs to be taken in. Beyond a certain concentration, it is not possible to achieve the necessary (normative) concentration in the ditches. In the current situation, the concentration of chloride in the water taken in varies between 50 and 300 mg/l. In normal situations, this is sufficient to meet the demand from agriculture, i.e. the quality is sufficient as and when the demand arises.
To maintain this system it is essential to ensure that:
• the chloride concentration in the watercourses does not exceed the threshold value for the crops during the growing season;
• sufficient water can be let in to flush the watercourses in the entire area.
• Water quality of the surface water system, which is determined by:
- Extent of internal salinity (salt load)
- Water quality at inlet points (Cl content and cyanobacteria)
• Increased fresh water demand for irrigation purposes
• Available volume of inlet water from main water system
• Capacity of the water system [end of quote from Visser (2011), p. 49]
Remarks with regard to box 1
"To maintain this system it is essential to ensure that the concentration of chloride in the watercourses does not exceed the threshold value for the crops during the growing season"
Threshold value is a term derived from the PAWN study conducted in 1982 (see fact sheet on salt-tolerant crops). It is the chloride (Cl) concentration above which the yield is assumed to decrease linearly as the concentration increases. An example: the threshold value for fodder maize is 217 mg Cl/l and the yield decreases by 3.43% per each additional 100 mg Cl/l. This example indicates that strict adherence to the specified threshold values is not always necessary. Stuyt et al., (2011) therefore advocate abandoning previously accepted salt standards:
'sufficient water can be supplied to flush the watercourses in the entire area.'
Flushing occurs when the water in the watercourses is replaced by intake water. The flushing process is most efficient when it takes place in a straight waterway without distributaries, with an intake on one side and an outlet on the other, and a uniform gradient in the direction of the outlet. This could not be more different from the situation in polders with multiple distributaries, variable geometry and soil gradient, and blind ditches (see Figure 2). Recent measurements using gadolinium as a tracer show that intake water does not penetrate small waterways as deeply as previously assumed (Figure 4).
For the Knowledge for Climate Programme [Kennis voor Klimaat Programma], the water supply need for Southwest Delta is calculated (Vries et al., 2009) using the Netherlands Hydrological Modelling Instrument (NHI) and different and larger units than applied in the ICW study. The results are also presented in a different manner (Figure 6), in that the respective figures do not readily lend themselves to comparison. However, it is clear that the quantity of water (intake) for flushing purposes is many times greater than the quantity of water for irrigation: about 30 times as much for Voorne Putten and slightly less than 20 times as much for Goeree Overflakkee, equalling an approximate flushing efficiency of 3% for VP and 5% for GO. This means a loss of efficiency compared to the efficiency level reported in the ICW study of 1983 (16% for both VP and GO). The increase in internal salinity arising from the rise in sea level was also expected to increase the required quantity of water intake by 40%, which currently has been adjusted down to 25% (Delta Programme, 2011).
Figure 6 Components of the water balance for the summer period, situation in 2003 Source: Vries et al. (2009)
Flushing and irrigation in practice
Recent surveys find that large differences exist between the water boards in terms of the extent to which salt is regarded as a problem and in the approach used to solve it. (Stuyt et al., 2011; Witteveen and Bos, 2011).
Both the effectiveness and efficiency of water intake are found to vary significantly from one location to another, posing problems when users demand a certain water quality when and where they need it. A demand that water managers cannot possibly meet due to the inadequacy of the infrastructure and the lack of sufficient water intake in some periods. This situation does not occur in current practice: of all agricultural land in the Netherlands, 19% is under irrigation, with 14% in Southwest Delta (The State, 2011). It also poses less of a problem when water managers and users want and are able to come to an agreement about the locations that will be provided with water of sufficient quality and quantity in the next growing season. This requires consultation and flexibility on both sides, as well as a high degree of solidarity among the users to reach agreement on a skewed distribution of costs and benefits, as some locations require much more water and/or much more effort is needed to deliver a sufficient quantity of water to some locations than others. It also requires regional and crop-specific knowledge for developing workable solutions. There are also significant differences between the users in the irrigated area, the extent of irrigation and the required water quality, making it virtually impossible to provide an accurate quantification of the costs incurred by the water manager in respect of each individual user.
Accessible fresh water supply monitoring tool
The €ureyeopener is an accessible tool that provides fast and interactive access to information about the current fresh water supply for a region or (part of a) catchment area and offers different approaches for distributing fresh water in times of water scarcity. Following development of the €ureyeopener version 1.0 for the management area of the Rijnland Water Board (Stuyt et al., 2012), the tool was further developed for Southwest Delta and the Rijnmond Drechtsteden regions (Schipper et al., 2014) in support of policy development for the sub-programme Delta Fresh water Programme. Water managers hope to gain a better understanding of the effects of the current fresh water supply on the benefits in agriculture and identify appropriate and cost-effective measures to make the fresh water supply future-proof. The tool provides useful and clear answers to knowledge questions about the regional fresh water supply. The added value of €ureyeopener 2.0 is its ability to provide fast and interactive access to information about the effectiveness of potentially attractive options for the fresh water intake system by calculating the water and salt balance, translating the effects on agriculture into yield changes and making the effects of the measures immediately transparent by creating an accurate breakdown of costs and benefits. Additional location-specific figures have been collected for calculating the costs of a number of measures.
A complicating factor is the coexistence of public and private water supplies, with vastly different service levels and rates. Another complicating factor is the uncertainty over the quantity and quality delivered, which is reinforced by recent investments by farmers banking on having access to water of sufficient quality when and where they need it.
Flushing in the Netherlands was applied as of the late 1900s as a means to improve the water quality of canals in the cities. Flushing for purposes of salinity control in agricultural lands occurred later. Van den Noort (2003) shows that costly studies needed to be conducted in respect of both practices before a decision could be made on building facilities such as the ‘Ververschingskanaal’ [drainage canal] (1888) and the Brielse Meer pipeline (1988). The quantity of water required and the effectiveness of the measurewas a continuously recurring issue of controversy.
While there is extensive international literature available on flushing salt from irrigated agricultural lands in dry regions (see e.g. http://www.alterra.wur.nl/NL/publicaties+Alterra/ILRI-publicaties/Downloadable+publications/), there is no known international literature on the flushing of waterway systems that are salified as a result of salt seepage; however, in recent years we have started building a nationwide file on the interaction of fresh and saline water in an interconnected ground-surface water system. The file explicitly cites the doctoral dissertation of Joost Delsman (Deltares, Knowledge for Climate Programme, 2010-2014), who illustrates the impact of flushing on salt concentrations in the ditch water in the southern part of the Haarlemmermeer polder, for example (Delsman et al., 2013; 2014). The surface water system recharges at a rapid rate (Figure 7).
Figure 7: Chloride concentration measurements (in terms of Electrical Conductivity, EC) in a fixed drainage level area of the Haarlemmermeer polder: the flush water reaches a much smaller section of the area than expected. The inlet in the southeast consists of (blue) fresh water, however, just a few hundred metres away, the fresh water taken in has become too saline for irrigation due to the salt wells in the surface water. A diffuse rendering of the spatial image does not reflect the actual situation, but the salinity of the surface water varies from ditch to ditch.
On a larger scale, we studied the flushing/salt damage relationship in the Rijnland management area. The study found significant spatial differences in flushing effectiveness: flush water enters only a small section of the area, quickly increasing the salt concentration as it recharges while moving through the ditches (through salt wells). Much of the same water is sometimes used repeatedly. Whether flushing actually increases the demand for water depends primarily on the distance to the inlet. Thus, we advocate integrating ‘salt’ and 'flushing' into local processes: it is time to use 'salt' as a guiding principle! This will lead to tailor-made solutions at both local and regional scales: differentiated requirements/standards/expectations regarding accumulated salt concentrations that may vary throughout the year for agriculture and nature, thereby allowing:
A recent study by Deltares concludes that:
'We were unable to obtain a clear picture of the exact efficiency of the present fresh water supply system due to the lack of access to verified water balances of all sub-areas in the Netherlands, nor were we able to establish the cost-effectiveness of the current policy.' (Klijn et al., 2010). Verification of water balances requires accessibility to measurement data on all intake points. Figure 8 shows that this is currently not the case.
Figure 8 Measured and unmeasured intake points in Rivierenland. Source: van Boekel 2011
Key action perspective of regional water manager: "Screen the flushing policy for the areas by conducting an effective area analysis". Consequences:
What can water managers do based on current knowledge?
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The knowledge and diagnostic methods presented in this publication are based on the latest insights in the professional field(s) concerned. However, if applied, any results derived therefrom must be critically reviewed. The author(s) and STOWA cannot be held liable for any damage caused by application of the ideas presented in this publication.