IRRIGATION AND DRAINAGE
Irrig. and Drain. 62: 524–536 (2013) Published online 1 July 2013 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ird.1744
A GUIDELINE FOR DEVELOPING AN INITIAL HYDROLOGICAL MONITORING NETWORK AS A BASIS FOR WATER MANAGEMENT IN ARTIFICIALLY DRAINED WETLANDS† MARCUS FAHLE1,2*, OTTFRIED DIETRICH1 AND GUNNAR LISCHEID1,2 1
Leibniz Centre for Agricultural Landscape Research (ZALF), Institute of Landscape Hydrology, Müncheberg, Germany 2 University of Potsdam, Institute of Earth and Environmental Science, Potsdam, Germany
ABSTRACT Reliable hydrological monitoring is the basis for sound water management in drained wetlands. Since statistical methods cannot be employed for unobserved or sparsely monitored areas, the primary design (first set-up) may be arbitrary in most instances. The objective of this paper is therefore to provide a guideline for designing the initial hydrological monitoring network. A scheme is developed that handles different parts of monitoring and hydrometry in wetlands, focusing on the positioning of surface water and groundwater gauges. For placement of the former, control units are used which correspond to areas whose water levels can be regulated separately. The latter are arranged depending on hydrological response units, defined by combinations of soil type and land use, and the chosen surface water monitoring sites. A practical application of the approach is shown for an investigation area in the Spreewald region in north-east Germany. The presented scheme leaves a certain degree of freedom to its user, allowing the inclusion of expert knowledge or special concerns. Based on easily obtainable data, the developed hydrological network serves as a first step in the iterative procedure of monitoring network optimisation. Copyright © 2013 John Wiley & Sons, Ltd. key words: monitoring network; wetland; sampling locations; controlled drainage; ditch system; measurement frequency Received 26 September 2012; Revised 21 February 2013; Accepted 21 February 2013
RÉSUMÉ Un suivi hydrologique fiable est à la base de la gestion de l’eau dans les zones humides drainées. Les méthodes statistiques ne pouvant être appliquées sur des systèmes non observés ou peu observés, le dimensionnement initial d’un réseau de mesure est la plupart du temps arbitraire. Ainsi, l’objectif de cet article est de fournir un cadre méthodologique pour le dimensionnement d’un premier réseau de suivi hydrologique. Un schéma de suivi de différentes variables hydrologiques et de l’hydrométrie dans les zones humides est élaboré en se focalisant sur le positionnement des stations limnimétriques et piézométriques. Pour les eaux de surface, les stations de mesure sont positionnées sur la base des unités de contrôle correspondant aux zones dont les niveaux d’eau peuvent être régulés séparément. Les piézomètres sont placés en fonction des unités de réponse hydrologique, définies par les combinaisons de types de sol, de l’occupation du sol, et des sites sélectionnés pour la surveillance des eaux de surface. Le cadre méthodologique proposé est appliqué sur un site de recherche situé dans la région de la Spreewald, dans le Nord-Est de l’Allemagne. Le schéma présenté offre un certain degré de liberté à l’utilisateur, permettant l’intégration de connaissances expertes ou de problématiques particulières, par exemple relatives à la qualité de l’eau. Le réseau de suivi hydrologique résultant peut être développé sur la base d’un jeu de données limité et servir ainsi de premier niveau dans une procédure itérative d’optimisation du réseau de mesures. Copyright © 2013 John Wiley & Sons, Ltd. mots clés: réseau de surveillance; zones humides; points d’échantillonnage; drainage contrôlé; systèmes de fossés; fréquence de mesure
*Correspondence to: M. Fahle, ZALF, Eberswalder Str. 84, 15374 Müncheberg, Germany. Tel.: +49-33432-82258, E-mail:
[email protected] † Un cadre méthodologique pour le dimensionnement initial d’un réseau de suivi hydrologique comme base de la gestion de l’eau d’une zone humide artificiellement drainée.
Copyright © 2013 John Wiley & Sons, Ltd.
INTRODUCTION Complex regulation systems determine the water balance in many wetlands, controlling near-surface groundwater levels by means of drainage infrastructure such as ditches or
A GUIDELINE FOR DEVELOPING AN INITIAL HYDROLOGICAL MONITORING NETWORK
drainpipes. For proper functioning of the system, the regulation structures (e.g. weirs) need a certain degree of management. Therefore system status as well as knowledge about prevailing processes are needed. Both have to be gathered by hydrological monitoring. Therefore, the design of a monitoring network is an optimisation process, where a configuration is sought that serves the objectives more efficiently than any other possible configuration (Strobl and Robillard, 2008), i.e. gaining the maximum amount of information while reducing the related costs (Harmancioglu and Alpaslan, 1992). The network design process is iterative, starting at the initial design which can subsequently be refined and adapted with information gained from measuring (Strobl and Robillard, 2008). A great variety of methods for optimisation of established hydrometric networks, based on already measured data, exist (Mishra and Coulibaly, 2009). Yet, approaches to derive the initial monitoring network are rare. Chang and Makkeasorn (2010) developed a method for gauging site selection in a watershed using various hydrology-related remote sensing data and optimised an objective function formed from these indicators. Other attempts on a catchment scale focused on embedding stakeholders via a participatory approach rather than on gauge placement (Gomani et al., 2010; Kongo et al., 2010). Considerations concerning monitoring programmes in the field of eco-hydrological river assessment and water quality monitoring are given in reviews by Brierley et al. (2010) and Strobl and Robillard (2008), respectively. A guideline for water quality monitoring was worked out by Sayed and Fahmy (2007), while recommendations for the specific case of quality monitoring in artificially drained catchments can be found in Tiemeyer et al. (2010). In the case of controlled drainage systems special approaches are necessary. Due to the high anthropogenic impact the natural rainfall–runoff processes are disturbed and monitoring considerations for mostly unregulated catchments can be only partly employed. For instance, suggesting flow measurements to be made at bifurcations and confluences (Boiten, 2008) is hardly a limitation within a network of drainage ditches. For this specific environment two information theory-based approaches were suggested by Alfonso et al. (2010a, 2010b). However, they use the simulation results of an already existing hydraulic model and are restricted to surface water gauge positioning. As a consequence, an approach to meet the requirements for an unobserved controlled drainage system seems to be missing, while comprehensive information on the hydrometry of wetlands can be found in Gilman (1994) and Gilvear and Bradley (2000). Therefore, it is the aim of this paper to provide a guideline for designing the first set-up of a hydrological monitoring network. The method focuses on the positioning of ditch Copyright © 2013 John Wiley & Sons, Ltd.
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and groundwater gauges in the specific environment of a strongly regulated ditch system in a lowland, while aspects of hydrometry in wetlands are outlined in less detail.
AN APPROACH FOR DESIGNING A HYDROLOGICAL MONITORING NETWORK Conducting the initial design of a hydrological monitoring network for a site which lacks (adequate) historical data from water gauges in an objective manner is almost impossible. By virtue of the data being unavailable, the preliminary design may be arbitrary or based on experience (Tirsch and Male, 1984), because the application of statistical methods is not possible at the beginning (Harmancioglu and Alpaslan, 1992). Therefore a common practice is to design the monitoring network based on experience first and refining it with the newly available data afterwards. To support the first draft, a scheme adapted especially for a regulated lowland area was developed. By derivation of virtually autonomous management subsystems, called control units, and specification of selection criteria the positioning of the monitoring stations is made verifiable, thus making the design less arbitrary. The basic guide through the network designing process is shown in Figure 1. It is important to mention that the sequential form of the scheme is a simplification which will not hold true due to interactions of the different steps. For instance, the constraints will have to be reconsidered constantly and the draft of the basic and temporary network will be interconnected. Generally sticking to the order (i.e. the boxes) in the illustration, a more detailed description of most steps is provided in the following sections. First of all, the aims of water management in wetlands, from which the monitoring objectives are derived, are discussed. This is followed by remarks concerning the basic as well as the temporary monitoring network and the meteorological data. Finally, fundamentals about the information system are outlined.
Objectives of water management in wetlands The main purpose of water management in wetlands is the regulation of the groundwater levels, which are generally close to the surface. The target water levels vary in time and are user-dependent; for example, agriculturists and conservationists will opt for different specifications. Hence conflicts can arise, which especially in case of fragmented land-use may be hard to settle. Generally accepted objectives have to be figured out in first place. The groundwater level is influenced by a great variety of variables and processes (Figure 2). While some are hard to manipulate, for example the groundwater fluxes over the area’s boundaries, others would necessitate a longer time Irrig. and Drain. 62: 524–536 (2013)
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Figure 1. Proposed method for developing a hydrological monitoring network. This figure is available in colour online at wileyonlinelibrary.com/journal/ird
span to do so, for example changing of plant density or plant species. The fastest and easiest way to interact with the system is modification of the ditch water levels, which can be used to drain or to irrigate the fields, ergo to control the groundwater Copyright © 2013 John Wiley & Sons, Ltd.
table. Therefore the primary target of water management in these wetlands is to adjust the regulation structures of the ditches in order to minimise the difference between actual and target groundwater levels, integrated over space and time. Irrig. and Drain. 62: 524–536 (2013)
A GUIDELINE FOR DEVELOPING AN INITIAL HYDROLOGICAL MONITORING NETWORK
P Qin
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ET S Qin
Sub-irrigation
GWlat Drainage
Qout
Qout
GWvert
Figure 2. Water balance components in a regulated wetland. For variable description refer to the text. This figure is available in colour online at wileyonlinelibrary.com/journal/ird
Monitoring objectives With regard to water management two primary tasks have to be considered: • provision of data as a basis for regulation; • understanding the processes that determine the status of the hydrological system. The former has to be served at all times, leading to a basic monitoring network, while the latter, assuming a largely inherent system, can be evaluated with the help of field campaigns, yielding an additional temporary monitoring network. Other monitoring objectives, such as determining the impact of land-use change or the interactions between hydrological conditions and the flora and fauna (Gilvear and Bradley, 2000), could also be of interest but are not considered in this article.
Operational management The water management of a ditch network relies on the water volumes involved, allowing us to answer questions like how much water is available and how much water can be diverted or redirected. Hence the water balance of the area (depicted in Figure 2) is crucial and all its components, if possible, should be measured carefully. On the basis of Dooge (1975), the water balance can be written as P þ Qin ET Qout þ GWlat þ GWvert ΔS ¼ b
(1)
where P is the precipitation, Qin the surface water flow into the area, ET the evapotranspiration, Qout the surface water flowing out of the area, GWlat the sum of all lateral groundwater fluxes over the boundaries of the area, GWvert the sum of all vertical groundwater fluxes over the boundaries and ΔS the change of the condition of the storage elements (e.g. reservoirs, ponds, groundwater, soil moisture) over the considered time. The right-hand-side term b represents Copyright © 2013 John Wiley & Sons, Ltd.
the water balance’s divergence from zero due to measurement errors in one or more components. Despite Qin and Qout, which can be measured at specific points, all the other values are spatially distributed. Their areal values have to be estimated derived from point measurements. Alternatively, remote sensing techniques could potentially be used for calculation of precipitation and evapotranspiration. Especially the groundwater fluxes over the system boundaries can only be assessed with a high amount of uncertainty, but may need to be estimated when b becomes too large. In addition to water balance aspects, groundwater and surface water levels are of major importance for management and ecological issues. Groundwater levels are essential since they are the main target variables management is adapted to. Surface water depths facilitate the deduction of flow paths, discharge and flow velocity, allowing us to evaluate the canal ecosystem. Further, if levels of groundwater and surface water are captured, flow conditions can be identified, that is to say whether water drains from the fields to the ditches or vice versa (i.e. sub-irrigation takes place; Figure 2).
Planning and science Besides the mere description of the hydrological status, knowledge of the guiding processes has to be gathered. This enables the operator to anticipate the behaviour of the system and its reactions to artificial changes. The following processes are of major importance: (1) Interactions between surface water and groundwater (i.e. between ditches and fields). Flow direction and exchange rate of the gradient-equalising adjustment between the two media are of interest; (2) Impacts of weir setting changes. Reactions to changes have to be assessable, for example if and Irrig. and Drain. 62: 524–536 (2013)
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how flow directions in the system might change, to what distance changes will affect surface water levels and how long it will take to adapt to the new (stationary) condition; (3) Impacts of technical structures. Knowledge of the effects of technical devices, mainly pumping stations, should be acquired. The reach and the required time to achieve a specific ditch level lowering as well as its possible magnitude are of importance.
BASIC MONITORING NETWORK The aim of the basic monitoring network is to quantify the water balance and to describe the current state of the wetland area by means of surface water and groundwater gauges. The design of the network follows two steps. First, a comprehensive gauge network is selected according to the following three subsections. Then the draft is reduced or adapted to the prevailing constraints, presented in the last subsection.
Areal water balance For the water balance all inflows (Qin) to and outflows (Qout) from the area should be captured. Since direct discharge measurements require expensive devices, discharge time series are often calculated from water levels of gauges in combination with rating curves (Lee et al., 2010). The latter can be derived from periodic discharge measurements or, if a weir is involved, calculated by means of its geometry and the water head over its crest, but the relationship has to be checked in the field as well (Mosley and McKerchar, 1993). Where artificial outflows, i.e. pumping stations, are used, discharge can be determined by the operating data (operation times, performance) or estimated using the change in surface water levels multiplied by the water area affected by the pumps. If lateral groundwater in- and outflows (GWlat) influence the water balance of the area, groundwater gauges at the boundaries could be used to estimate their quantities, while piezometric measurements at different depths can assess vertical groundwater movements (GWvert). Estimation of the areal precipitation and evapotranspiration is addressed later. Last, the storage ΔS has to be determined. Its most important components in wetlands are soil moisture and groundwater. Since both are interconnected, groundwater levels can be used to account for their actual states when they are combined with the respective soil physical properties (e.g. porosity, water retention curve). Because the release of soil water is quite slow and manipulation of groundwater levels is only possible indirectly by weir settings in adjacent ditches, manageable reservoirs like lakes with a regulated outlet might also be of high importance. Therefore, the water levels of larger reservoirs should be Copyright © 2013 John Wiley & Sons, Ltd.
measured in order to deduce whether or not water releases for raising ditch water levels in times of drought are possible.
Positioning of surface water gauges The surface water gauges for measuring the water balance are normally located at the boundaries of the study site, leaving a knowledge gap about the interior area. Hence additional gauges should supplement the ditch water level information. For their placement an approach based on control units is proposed. As a control unit we define a subsection of the system, which is enclosed by regulation structures, for example weirs, that can control the surface water and accordingly the groundwater levels as well as the water storage of this particular area. Their regulation is to a certain degree isolated from the adjacent system but is of course dependent on inflows and the possibility of discharging water. The extent of the control units thus depends on the location of the regulation structures, the ditch system and the relief. Accordingly, the site is divided into quasi-autonomous regions which serve as a basis for the selection of suitable gauges. The starting point for the derivation of the control units is a map of the ditch system and its inherent regulation structures. First, the present flow directions should be clarified using the relief as well as information from field trips and reference measurements. Then, the sphere of influence has to be determined for each regulation structure. It starts with the weir under consideration and extends upstream to the next one (Figure 3a). This gives the maximum impact, but in reality the reach of the weir’s backwater can be much shorter depending on the prevailing topography. On the sides the control unit stretches orthogonally from the ditch to the middle of the distance to the next, separately controlled, ditch (Figure 3b). This is obviously another approximation, since the affected area is a function of several variables like soil type or distance between the drainage elements and can vary with regulation changes. When a weir is located upstream of a junction (Figure 3c), the assumption is that all waterways have an equal impact on the groundwater, resulting in a control unit borderline departing at a 45 angle from the regulation structure. Since the water levels upstream and downstream of the weir differ, the influence of the ditches before and behind the weir may differ as well. But the assumption of an 45 angle is only valid if both water levels are identical. Yet, since the soil water flow that results from different ditch water levels cannot be described easily, the assumption of a 45 angle should be reasonable, respectively is the most likely one. For a weir behind a junction the same procedure applies accordingly (Figure 3d). After drafting the controlled domain for every regulation structure, more complex systems relying on several weirs can be identified (Figures 3e and f). In the case of a large Irrig. and Drain. 62: 524–536 (2013)
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Figure 3. Examples of derivation of control units.This figure is available in colour online at wileyonlinelibrary.com/journal/ird
number of resulting control units, neighbouring districts should be merged to reduce their total number. The final map of control units is then used to select representative gauges for each domain. Regulation structures at main ditches (e.g. collector drains) or at junctions are favourable due to their higher impact. Since this approach can generate a lot of possible monitoring stations, several additional factors should be kept in mind: • Accessibility. Easy access is helpful to install the gauge. But manual metering instead of automatic data registration is an option in these cases, too. In contrast, important but poorly accessible gauges are best metered automatically; • Availability of geodetic points. To reference the monitoring stations with one another, the water level should be measured as an absolute elevation. Existence of levelled staff gauges or datum heights nearby can thus reduce effort. But, if necessary, the gauges can also be calibrated relative to each other when elevation references are missing; • Construction structures. Placement of monitoring gauges at artificial structures secures a permanent installation while a gauge mounted on a post in a ditch can be subject to ice-heave in winter (Gilman, 1994).
Positioning of groundwater gauges The design of groundwater monitoring stations should serve two purposes. First, it should be distributed over the entire investigation area to give an overall insight into the achievement of management. Second, it should cover zones which are characteristic of a wide share of the area. For the latter, hydrological response units (HRUs) can be used, given that they are also likely to form the basis for later modelling. In lowlands these are given by the intersection of soil type and land-use maps (e.g. Schmalz et al., 2008; Piniewski and Okruszko, 2011). Due to its nature as a plain neither the Copyright © 2013 John Wiley & Sons, Ltd.
elevation nor the slope, which are influential in hilly or mountainous regions, should have a significant impact and are often not taken into account. However, the classification has to be extended when deep and shallow groundwater sections occur (Becker and Braun, 1999). After the derivation of the HRU, the number of the most common combinations that should be monitored depends on several factors like their areal share (i.e. minor fractions may only have a restricted influence on the system) or their distribution (i.e. combinations which are continuous rather than scattered may be more influential). Besides the spatial variation of soil type and land use attention should be paid to the following aspects: • Problem areas. Since water management is judged especially on how it deals with extremes, areas prone to drought (e.g. elevated areas) or to waterlogging (e.g. zones with peat degradation and concomitant subsidence) should be considered first; • Proximity to surface water gauges. Placement in the vicinity of surface water gauges enables characterisation of the interaction between surface and groundwater, i.e. whether flow conditions are influent or effluent; • Positioning in relation to ditches. At the field scale the groundwater gauge should be put in a central position, given that the dependence of groundwater on the surrounding ditch water levels diminishes with distance. This position also minimises interference with ditch maintenance or agricultural work.
Adaption to constraints The resulting draft of the basic monitoring network will generally be excessive because the availability of a greater amount of data will always be preferred. Nonetheless, financial resources and hence equipment are limited. Also the available time for measuring and data processing may be restricted. The handling of huge amounts of data may Irrig. and Drain. 62: 524–536 (2013)
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create problems as well. Hence selection of the most important and appropriate locations is inevitable. Therefore differentiation into two categories of stations as a function of their importance is required. First, the most relevant stations form the basis of subsequent management by providing highly frequent measurements. Therefore they are equipped with automatic data registration (data loggers) and, if possible, remote data transmission. Second, manually metered stations can complete the information about the system status or serve as model validation points. Besides its importance, a key factor driving this classification is accessibility, since manually measured gauges should be easily accessible to minimise time and work effort.
Surface water gauges. In the case of surface water gauges the water balance related stations are more relevant than those describing the control units. On account of the importance of the in- and outflows for management, the gauges which are planned to cover the areal water balance should only be excluded at locations that are insignificant for the system, for example storage reservoirs of small extent or negligible inflows. For measuring stations meant to describe the system status, the basis of the selection is the aforementioned criteria like accessibility and available geodetic points. Further, it seems plausible to spread the monitoring stations over the whole area rather than concentrating on specific domains. Groundwater gauges. For groundwater gauges, accessibility and geodetic points as well as dispersion over the investigation area are accordingly of interest. In contrast to surface water gauges, landowners and land users will have a far more pronounced influence. They need to be informed and their permission asked for before installation. The inclusion of stakeholders is a recommended approach in order to clarify disputes, adjust the network based on their knowledge and gain acceptance for the project (Gomani et al., 2010; Kongo et al., 2010). Measurement frequency. Time intervals of 15 min should be acceptable for automatically registering gauges (Boiten, 2008). For manual measurements, taking into account the effort by the operator, a weekly or biweekly interval seems appropriate, since it is sufficient to provide an insight into the site’s natural and anthropogenic influences (Gilman, 1994). For the former the intervals needed are mainly influenced by the reaction of the system for example to precipitation events. Generally, as long as no additional costs or limitations occur, a smaller time step is preferred since it can help to isolate specific processes (Gilvear and Bradley, 2000) and the data can still be accumulated afterwards. Even a high-frequency groundwater Copyright © 2013 John Wiley & Sons, Ltd.
measurement is beneficial, in order to measure fast reactions to changes in boundary conditions. This results in a pronounced diurnal variation which can be used for the estimation of evapotranspiration, a method first proposed by White (1932) and later modified by several reseachers (see review by Gribovszki et al., 2010). For this purpose a sampling interval of 10 min or less is actually desirable (Gribovszki et al., 2008). The metering interval for manually read stations depends apparently on the available resources, since a broader basis of data always improves characterisation of the domain.
Discharge measurement. Besides the gauge metering, discharge measurements have to be taken to establish rating curves to calculate volume fluxes. The priority of discharge measurements is set to the in- and outflows for determination of the water balance, but eventually rating curves for all surface water gauges are desirable. At weir discharges, the position of the weir crest and water head are interdependent and should be noted together. TEMPORARY MONITORING NETWORK In order to achieve the monitoring goals related to process analysis additional measures should complement the basic monitoring network. Temporary high-frequency measurement at gauges not equipped with data loggers can help to determine relationships between groundwater, surface water tables and the prevailing meteorological conditions (Gilvear and Bradley, 2000). Apart from that, the impacts of regulation changes (e.g. change of gate settings) are easier to identify for a small observation space than for the whole drainage system. Hence the temporary use of additional gauges and conducting well-defined experiments are suggested here, based on a twostep approach.
Selection of sections First the domains for the temporary monitoring have to be chosen. It is advisable to select subareas that are characteristic of large parts of the catchment. Therefore one can revert to the HRU used for the positioning of the groundwater gauges. Additionally, for regulation experiments, domains should be used that possess a manageable ditch system, having welldefined and regulable in- and outflows and are of limited extent, in order to reduce the time needed for conducting the experiments. Hence, one or two of the control units defined for the basic monitoring network can be used.
Possible types of investigation • Intensification of measurements. More in-depth information can be generated by placing additional gauges. Irrig. and Drain. 62: 524–536 (2013)
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This can include the usage of a greater variety of groundwater gauges, allowing for example quantification of variations at field scale. Also transects, i.e. monitoring stations along a line, can be used. In the case of surface waters a transect can serve to investigate water level changes in a reach. This leads to the derivation of the influence of a weir, for example the extent of its backwater, or the determination of the propagation of water level lowering due to pumping stations. For groundwater transects, normally set perpendicular to adjacent ditches, inference about flow direction and discharge/recharge rates between groundwater and surface water can be drawn; • Field experiments. Controlled changes in the management of the subarea (e.g. changes in weir settings or in the pumping regime) can be used, for instance, to assess (i) interactions between groundwater and surface water, i.e. the reaction time and extent of the groundwater level to changing ditch water levels, (ii) reaction time of ditch water levels to reset damming heights, i.e. how much time is needed to achieve a new stationary state, as well as (iii) impact of the management on flow directions in the system examined. Before conducting the field experiments a concept has to be developed, including which management changes can be made at what time and what consequences are expected. This draft has to be discussed with the stakeholders and a suitable time frame has to be found, which can be difficult since the reaction is not known beforehand, and nor are the driving forces like weather conditions.
METEOROLOGICAL DATA Besides the monitoring of water bodies, meteorological data have to be obtained to describe the hydrological processes. First of all, precipitation P is of utmost importance, as it is one of the major inputs of drained areas, if not the only one in headwater catchments or bogs. Moreover evapotranspiration (ET), which can be more influential on water levels in summer than precipitation (Gilman, 1994), completes the areal water balance. For the determination of ET a great variety of methods exist. Models and micrometrological methods for the specific case of wetlands are for instance discussed by Drexler et al. (2004) and by Gilvear and Bradley (2000). In most cases ET estimation will be based on empirical or physically based equations, demanding different kinds and detail of data. To meet the demands for as many different approaches as possible, it is suggested that global radiation and air temperature are measured as well as wind speed and humidity. Copyright © 2013 John Wiley & Sons, Ltd.
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The procedure for meteorological monitoring consists of three steps. First, the closest existing weather stations and precipitation gauges have to be defined. It has to be checked if their data are accessible and if the possibility for remote data transfer, which would be necessary in the case of operational water management, exists. Second, the present data have to be analysed to see if they meet the requirements regarding measured values and measuring frequency, and if they are representative of the area. Third, if the available data do not fulfil the demands and resources are available, new stations have to be located. While weather stations would be favourably located in the centre of the area and are representative of large areas, precipitation gauges, due to the spatial heterogeneity of this meteorological variable, may have to be placed with higher density.
Information system After the installation and launch of the monitoring network the incoming data have to be stored, checked, analysed and made available for water managers (and their models), stakeholders and other users. To deal with these tasks a database and suitable tools have to be set up, forming an information system. In the first place the incorporation of all measured data into the database is necessary. Depending on the type of measurement (manual or automatic) and on the gauges, different information can be collected. Table I gives an overview of the most important information different monitoring stations provide. Some of the mentioned elements are optional or just valid if data loggers are used, as indicated in the table. For meteorological monitoring stations the same routine applies. Starting from the raw data of the measurements, several methods can be used to check and process the data. First of all the data have to be tested for plausibility as well as sudden changes and values that lie outside an expected range have to be verified. Missing data can be completed using gauges nearby. For gauges equipped with data loggers comparisons with manually checked measurements should be made to prove whether the data registration works properly. Furthermore, rating curves can be modelled from discharge and water level measurements (e.g. Mosley and McKerchar, 1993; Lee et al., 2010; Lambie, 1978). At regulation structures the dependence on damming height could also be used to calculate the discharge (e.g. Boiten, 2008). The operation time record should enable discharge estimation at pumping stations. The processed water level and discharge data, which have to be stored separately from the raw data to enable later reconsideration, is then used as a basis for water management, no matter whether or not it is model based. For a model the Irrig. and Drain. 62: 524–536 (2013)
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Table I. Potential database elements Gauges at regulation structures
Stand-alone surface water gauges
Groundwater gauges
Pumping stations (with gauge)
x x x x
x x x x
x x
x x x x
(auto) x
(auto)
Name Description Name of water body Location Elevation of ground level Logger ID Damming height Groundwater level ! Manual check measurement Water level upstream ! Manual check measurement Water level downstream ! Manual check measurement Discharge Rating curve Hours of operation
x x (auto)
(auto)
x (auto) x (auto) (opt) (auto) (opt) (opt)
x (auto)
x (auto) (opt) (auto) x
(opt) (opt)
x
(auto): necessary for gauges equipped with data loggers; (opt): optional information.
data have to be provided in numerical form, while water managers may prefer to use visualised data in the form of diagrams or graphs or displayed on maps. Finally, the data can be analysed by means of statistical methods to reveal the underlying processes that drive the system and that have to be considered in modelling and management.
APPLICATION EXAMPLE Study site The study area (Figure 4) of approximately 9.5 km2 is located 90 km south-east of Berlin (latitude 51 49’, longitude 14 02’).
It is part of the Spreewald wetland, an inland delta of the river Spree. According to the German Meteorological Service (DWD), the average temperature and precipitation for the period from 1961 to 1990 at the weather station in Cottbus, located 20 km to the east, were 8.9 C and 563 mm, respectively. At the study site peats are predominant (58%), in addition sands (35%) and peats over sand (7%) are found. Due to its wetland characteristics almost the entire domain is used as grassland (82%), but minor portions of agriculture (8%), forest (9%) and water bodies (1%) exist. A complex ditch network was built in the 1970s to make intensive farming possible. It includes over 40 regulation structures as well as two pumping stations. Since water
Figure 4. The study area.This figure is available in colour online at wileyonlinelibrary.com/journal/ird Copyright © 2013 John Wiley & Sons, Ltd.
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management efforts were cut back after German reunification due to a change of agricultural policy, the area nowadays has to deal with problems of water shortage in summer and excessive waterlogging especially in winter and spring. A research project was started in 2008 to investigate ways to reduce the occurring problems by means of a model-based operational water management. Therefore a monitoring network had to be designed first, for which 15 data loggers with remote data transfer were available.
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Finally, for positioning of the groundwater gauges the hydrological response units (HRUs) were derived by intersection of spatial land-use and soil type data. Resulting from the dominance of grasslands in the area, its combinations with peat (50.1%), sand (25.1%) and peat over sand (6.3%) are the three most important HRUs. The groundwater monitoring locations were chosen to represent these types. Five gauges belonged to the top category (GW3, GW4, GW5, GW8, GW10), three to the second (GW1, GW6, GW7), while one (GW2) lay in the transition between peat and sand soil,
Drafting the basic monitoring network Concerning the areal water balance, all surface water in- and outflows as well as the only significant storage reservoir, a pond (weir Q018), were equipped with monitoring stations (Figure 5). All four inflows were regulated by weirs (R001, R005, Q017, Q020), but one creek flows directly into the domain without the possibility of diversion (Q017). There are two outflows (Q001, Q025) via pumping stations, while one of them (Q001) additionally drains the area by gravity via an inverted syphon. Groundwater inflow can be expected from the south-west, where groundwater levels are recovering after being lowered for coal mining purposes. However, the railway embankment crossing the drainage area (see Figure 4) is believed to hinder groundwater flow. Hence, the only affected part would be control unit VII (Figure 6). As an autonomous subsystem, which is only artificially drainable, its management options are limited to the pumping regime and quantification of groundwater inflow was considered to be of minor importance. The north and west boundaries of the area are defined by large canals and lateral groundwater fluxes were assumed to be negligible there. The same was supposed for the east, where the adjacent areas are drained by two creeks. However, should the emerging water balance error term be too large, expansion of the monitoring network with additional groundwater gauges outside the research area might be necessary. To locate additional surface water gauges, the control units were derived. After aggregation, nine control units were defined (Figure 6), of which three (VII, VIII, IX) were already characterised by water balance gauges at pumping stations (Q025, Q001) and the reservoir outlet (Q018), respectively. In addition three units (I, IV, V) were monitored at weirs which were located at junctions with their main ditches (R010, Q012, Q026). The three stations were accessible via roads and equipped with levelled staff gauges, thus providing elevation information. Control units II and VI were left out due to their lack of staff gauges and geodetic points. In the case of control unit III, the inflow monitoring at Q020 did not describe the subsystem, but since two other weirs in the unit were equipped with staff gauges and placed on a main road, they were considered for manual measurement in the first place. Copyright © 2013 John Wiley & Sons, Ltd.
Figure 5. Design of basic monitoring network. GW9 was not installed because permission from the land user was not forthcoming.This figure is available in colour online at wileyonlinelibrary.com/journal/ird
Figure 6. Control units derived for the ditch network. Background colours are merely used for better differentiation.This figure is available in colour online at wileyonlinelibrary.com/journal/ird Irrig. and Drain. 62: 524–536 (2013)
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characterising the third HRU. The stations were spread over the entire domain. Only control unit IX was left out, since its groundwater levels are not subject to management concerns. Ten locations were selected, five of them (GW6 to GW10), which were easily accessible (at a distance between 150 and 200 m from the nearest road) and regarded as less important, were not equipped with data loggers. Automatically measured gauges (GW1 to GW5) were preferably located close to surface water gauges with data registration (R010, Q012, Q026, Q025, Q001) to account for the interactions of the two water entities. The final basic monitoring network is shown in Figure 5. Meteorological data consisting of precipitation, air temperature, global radiation, humidity and wind speed were available in high temporal resolutions from a weather station located 5 km north of the study area.
Drafting the temporary monitoring network For temporary intensified observations two control units were selected (Figure 7), targeting different objectives. Control unit VII was chosen to determine the impact of pumping stations, since the pumping station is its only means of outflow. Three ditch gauges (GP4, GP5, GP6) were drafted at increasing distances from the pumping station (approx. 200 m, 500 m and 1100 m) to determine the effect of pumping on ditch water levels in varying distances. Control unit V was selected to clarify questions about impacts of regulation changes, since it represents a precisely delimited system regulated by four weirs, one determining the inflow (Q013) while the other three are possible outflows (Q010, Q011, Q026). The prevailing combination of grassland and peat is also the most common HRU and therefore representative of a great share of the entire domain. In addition to the two gauges designated for basic monitoring
Figure 7. Temporary monitoring network. See Figure 6 for location.This figure is available in colour online at wileyonlinelibrary.com/journal/ird Copyright © 2013 John Wiley & Sons, Ltd.
(GW3, Q026), each regulation structure was equipped with gauges, and a groundwater transect (GW11, GW3, GW12) as well as a transect in one ditch (Q011, GP1, GP2) were intended to give comprehensive insight into the processes affected by regulation. The supplementary installed gauges were equipped with data loggers just during specific campaigns.
Stakeholder involvement, installation and information system The first draft of the monitoring network was presented to stakeholders, i.e. the concerned water and environmental authorities as well as agriculturists, discussed and adjusted by placing groundwater gauge GW10 in an as yet uncovered area with waterlogging problems. Afterwards farmers who cultivate fields where potential groundwater monitoring stations were located were asked for permission, while the owners of the land were informed. We had the experience that contacting the agriculturists by telephone was much more promising than communication via mail. Agreements were reached in all but one case, where the gauge (GW9) was finally cancelled. Since the adjusted position of GW10 was close to GW9 (Figure 5), the implications of the absence of the latter were considered negligible. Meetings were arranged to determine the exact gauge position in the field in order to prevent any conflicts. Installation of the gauges and data loggers started subsequently. A program was set up for data processing, which converted the raw data and additionally was able to create HTML files for each monitoring station, which were made available for stakeholders, providing measurement information of the last 30 days.
DISCUSSION A guideline to develop an initial draft of a hydrological monitoring network in wetlands has been presented. Its aim is to enable the user to place monitoring locations in areas as yet unobserved or with insufficient hydrological data in an objective manner employing only basic information. The scheme drawn leads to a monitoring concept that is user-specific. Nevertheless it is more reproducible than designing the network just based on experience or completely arbitrarily, which is normally the case when no data are available (Tirsch and Male, 1984). The designed network will in most cases be suboptimal, since, for example, the extent of spatial variability is normally unknown until measurements are made (Gilvear and Bradley, 2000). However, it gives a starting point for the iterative process of monitoring network optimisation. There are certain constraints to the proposed methodology, most of all its already mentioned subjectivity. This could have been circumvented by application of a fuzzy logic approach, as has been done for monitoring network design in fields of Irrig. and Drain. 62: 524–536 (2013)
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water quality in catchments (Strobl et al., 2006a, 2006b) and water distribution systems (Francisque et al., 2009) before. This would have led to higher reproducibility, but since the fuzzification (i.e. the selection of threshold factors) and the choice of the weighting factors are also subjective, this method was not used here. As a further restriction, the suggested method of surface water gauge positioning via definition of control units may also lead to problems when bypasses exist, i.e. water is able to get behind a specific weir unimpeded. This situation will be found in natural rather than in artificially drained areas and can be rectified by expanding the considered control unit accordingly. Additionally, the assumption that the control units represent a relatively autonomously adjustable subsystem is made just to a certain degree. For instance, lowering the water level in one control unit requires the restriction of inflow, which influences the upstream units, or an increased discharge to the downstream units, which may be not possible without extending the discharge of these subsystems as well. Another drawback is the neglect of water quality consideration. But since the user has a high amount of freedom in his decisions, adaptations can be made, which would for example mean placing monitoring stations close to disposals or tributaries constituting significant quality changes. A possible alternative for the initial monitoring design in regulated wetlands is the use of models as shown by Alfonso et al. (2010a, 2010b). However, this approach has two disadvantages. First, the set-up of a reliable model requires a good deal of data, for example water level information for calibration, which contradicts the assumption of an unobserved system. And also if some data for this purpose are available, there is a second shortcoming: the model merely reflects the supposed behaviour of the system. It involves various assumptions and simplifications for generating its results, which build the basis for the network design. This can lead to misjudgement. Whenever possible, network design and analysis of the guiding processes should rather be based on measured data. The usage of model results, if available, is nevertheless a viable alternative to the procedure proposed here. Altogether it will hardly be possible to evaluate the performance of a monitoring network universally, since the measured data are just a sample out of an unknown population. Consequently the question ‘Can the measurements represent the spatial heterogeneity of the area as well as its singularities?’ cannot be answered conclusively, since that would require an all-embracing knowledge of the system. So the optimal layout, which would probably also vary in time, may never be reached. Also it cannot be determined whether an installed monitoring network satisfies the needs the best. Notwithstanding, the layout can be assessed on the basis of the measured data, once available, and accordingly adaptations could be made, but this is beyond the scope of this work. Copyright © 2013 John Wiley & Sons, Ltd.
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CONCLUSION The aim of this paper was to provide a guideline that makes the positioning of surface water and groundwater monitoring stations for a strongly regulated wetland less arbitrary. The suggested method supports the person in charge to draft the network when few or no measurements are available, notwithstanding the outcome is still subjective. The resulting draft presents the first step in the iterative procedure of monitoring network optimisation, which encompasses questions about necessary spatial and temporal measurement frequencies. After installation and start of operation, the measured data can be used to analyse the information by diverse statistical approaches, thereby optimising the efficiency or ‘informativeness’ (Harmancioglu et al., 1994) of the monitoring step by step.
ACKNOWLEDGEMENTS The authors would like to thank the German Federal Ministry of Education and Research for funding the research project INKA BB (FKZ: 01LR0803A) as part of the funding activity KLIMZUG. Further, we want to point out that Ophélie Fovet did a great job in translating the abstract into French––merci beaucoup! We would also like to acknowledge the contribution of Steven Böttcher, who pointed out the drawbacks of using models in monitoring network design, and thank Björn Thomas, Tobias Hohenbrink and the reviewer for their detailed comments on the manuscript.
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