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Seite 1 von 8 II Natural and Human influences on groundwater 2D-Modeling of flow processes in the unsaturated zone of wetlands Barbara EULER & Oliver KEMMESIES & 1) Peter-Wolfgang GRAEBER 2) 1) KP Ingenieurgesellschaft für Wasser und Boden mbH, Bahnhofstraße 37, D-91710 Gunzenhausen, Germany 2) Institute of Waste Management and Contaminated Sites, Technische Universität Dresden, Pratzschwitzer Straße 15. D-01796 Pirna, Germany ABSTRACT The exploitation of water resources, which is the base of a secure drinking water supply, is based on certain calculation rules and estimates from the past. Nevertheless climatic changes are nowadays already observed and for the future expected. One expected change is the increase of extreme precipitation events with high intensity. It is foreseeable, that under these conditions even, if the mean precipitation amount should remain constant, the groundwater recharge will decrease, because of the limited infiltration capacity of soil. A further problem is the increasing cultivation of energy crops which cause a higher evapotranspiration and remove additionally water from the soil. Taking all these different aspects into account, it can be said that the result is an enhancement of surface run-off and in the end a minimization of groundwater recharge. The creation of synthetic wetlands might be a solution to this problem. Wetlands are areas which feature less surface run-off and a high infiltration capacity. Therefore it is possible to increase the groundwater recharge in an area with lack of groundwater by creation and optimization of wetland-zones. The main features of the soil in wetlands areas are a high hydraulic conductivity as well as a high storage coefficient. Soil erosion is avoided by a vegetation cover, which features a low evpotranspiration. It is strong recommended to use a 2D-vertical-plane flow model to optimize the construction of such wetlands. For this purpose we developed the simulation program SiWaPro DSS. It is able to describe the required complex properties of soil and the interaction between vegetation and soil. The simulation program SiWaPro DSS is also coupled with a stochastic weather generator. Keywords: Richards equation, van-Genuchten parameter, wetlands, water managment Decrease of Groundwater Recharge The exploitation of water resources is the base of a secure drinking water supply. Today up to 80 % of groundwater is used as drinking water in arid areas (BFAI, 2006). According to the directive 98/83/EC of the European Union, water shall not contain any concentration of micro-organisms, parasites or any other substance which constitutes a potential human health risk. The free admission for everyone to drinking water and a secure supply of water should be ensured anytime. Today many countries with arid climate conditions have problems to meet these demands. Accelerated population growth and non-sustainable (ground) water management can have an additional negative effect. Due to the imbalance between pumping and recharge the groundwater levels decline step by step consequently. The climatic change can cause these phenomena in areas too, where today the people are still not faced with the problem of lack of water. That is why solutions have to be developed to secure the supply of groundwater used for drinking today and in the future. A sustainable water management is very important in arid and semiarid areas where geographic position and climatic conditions cause a permanent lack of water. The climatic conditions of these areas are featured by precipitation events which occur in intervals. But these precipitation events are not able to refill the groundwater storage because of the limited infiltration capacity of soils. This aspect can be increased by a lack of vegetation caused by deforestation and overgrazing. Taking all these different aspects into account, it can be said that, the result is an enhancement of surface run-off, and a reduction of groundwater recharge. Seite 2 von 8 The water problem is increased by enhanced population growth and urbanization. Moreover the increasing cultivation of ‘thirsty’ plants, like cotton wool burdens the water supply. Additionally, the evaporation removes water from the soil. Over-irrigation and inefficient use are further aspects that endanger the water balance. Groundwater Recharge in Artificial Wetlands An innovative approach is the artificial recharge of groundwater. The artificial groundwater recharge can be done using infiltration ponds or even wetlands. In general natural wetlands are divided in coastal and inland wetlands. The inland wetlands are located along rivers, streams, margins of lakes and ponds or in depressions. Most of them are seasonal (EPA, 2004). Wetlands are a transition zone between the surface and the groundwater. They are areas which feature less surface run-off and a high infiltration capacity. Moreover the plants in wetland can soak up the chemicals from the groundwater that might contaminate it (Science Update, 2007). A high ratio of infiltration capacity and hydraulic conductivity as well as a high storage coefficient can increase the groundwater recharge. Evaporation and vegetation reduce it. Fig.1Parameters of Groundwater Recharge The construction of a artificial wetland and the following optimization of these wetlands leads to a maximum of groundwater recharge under the given environmental conditions. The useable water resources for an infiltration in wetlands are precipitation, cleaned wastewater and water of rivers, lakes and dams. The groundwater recharge depends on the features of climate, soil and vegetation in these areas. The properties of soil and vegetation have to be optimized to ensure a maximum infiltration. The advantages of optimized artificial groundwater recharge areas, that are modelled on wetlands, are: F minimum construction effort F no complex technology F low costs F low maintenance F cleaning of water by natural attenuation Seite 3 von 8 F reduced evapotranspiration deficit F water protected against contamination Flow Processes in the Unsaturated Zone The processes in the unsaturated zone are intended to identify and figured by different models. There are different approaches to describe the flow processes in the unsaturated zone. A deterministic approach is the Richards equation (1a+b). This equation represents the connection between the water flow, caused by a potential gradient, and the change in water content of a soil volume per time unit. and ⎛ ∂h p ⎞ ⎞ ∂θ ∂ ⎛⎜ k (θ) ⋅ ⎜⎜ − w0 + 1⎟⎟ ⎟ = ⎜ ⎟ ∂r ⎝ ⎝ ∂r ⎠ ⎠ ∂t (1a) ∂h p ∂θ = C(h c ) ⋅ ∂t ∂t (1b) The independent variables are time t and spatial coordinate (x,y,z). The dependent variables of equation (1) are the water pressure head hp and the water content θ. w0 is defined as the sink/source term. The capillary capacity function C(hc) is the first derivative of the hysteretic soil water retention curve. The unsaturated hydraulic conductivity k(θ) depends on the water content in the soil. Soil Retention Curve The capillary capacity function C(hc) and the unsaturated hydraulic conductivity k(θ) are implemented in the Richards equation. Both are van-Genuchten-Parameter-functions called after the model on which they base. In the following a modified version, the hysteretic parametric model by van Genuchten (1980) and Luckner et al. (1989) is given: θW = A + φ − A −B [1 + (α ⋅ h ) ] 1 n 1− n hc > 0 (2a) hc ≤ 0. (2b) c θW = φ − B The parameters of equation 2a and 2b are porosity φ, residual wetting fluid content θW,r, residual air content θA,r, scaling factor α, and slope parameter n. Figure 2 shows a typical curve set for this hysteretic function. To describe these different curves, the variable A as a function of the residual wetting fluid content A=A(θW,r), the variable B as a function of the residual air content B = B(θA,r), and the scaling factors α and α i for the drainage and imbibition branches, respectively, were introduced. A detailed description of the parametric model, which differs from the original work of van Genuchten (1980), is given by Luckner et al. (1989), Nimmo (1991), Luckner et al. (1991), Nielsen and Luckner (1992) and Kemmesies (1995). d Seite 4 von 8 capillary pressure head in cm 0 5 10 15 20 25 30 35 40 45 50 0.40 porosity φ = 0.36 Be=0.144 wetting fluid content 0.35 0.30 residual non wetting fluid content Bd=Bf-g=θNW,r=0.10 0.25 0.20 f 0.15 Ad=0.113 0.10 0.05 e d c d c e f g g PDC - Primary Drainage Curve SWC - Scanning Wetting Curve SDC - Scanning Drainage Curve MWC - Main Wetting Curve MDC - Main Drainage Curve residual wetting fluid content Ac = Ae-g = θW,r= 0.05 0.00 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 capillary pressure in Pa Fig. 2 Example of a Hysteretic Soil Water Retention Curve Relative Permeability Function The parameter function of the unsaturated hydraulic conductivity k(θ) was modeled by Mualem (1976) and Luckner et al. (1989) with ⎛ S ⎞ k (θ) = k 0 ⋅ ⎜⎜ ⎟⎟ ⎝ S0 ⎠ λ m 1 ⎤ ⎡ ⎛ m ⎞ ⎢1 − ⎜ 1 − S ⎟ ⎥ ⎠ ⎥ ⋅⎢ ⎝ m 1 ⎥ ⎢ ⎛ m ⎞ ⎢1 − ⎜ 1 − S 0 ⎟ ⎥ ⎠ ⎦ ⎣ ⎝ 2 3) Parameters of eq. 3 are the hydraulic conductivity k0(θ0) at a matching point (0) with a known degree of wetting fluid mobility S 0 =(θ0-θW,r)/(φ- θW,r), the connectivity parameter λ and the transformation parameter m. An example of the function is shown in figure 3. Seite 5 von 8 mobility degree 0.00 0.25 0.50 0.75 1.00 relative permeability 1.0 0.8 – S0 = 1.0 – S0 = 0.86 0.6 0.4 0.2 0.0 0.00 0.06 0.12 0.18 0.24 0.30 0.36 wetting fluid content Fig. 3 Example of a Relative Permeability Function Water mangament Traditionally water resources management till now has been practically performed for surface water flows. It has been realized mainly by reservoir operation for both ordinary water use and flood prevention. The management task is expressed in an optimized hierarchical distribution of water volumes between different water users for an annual or seasonal time interval. During the last two decades the problem of integrated water management has reached a certain level of development. Groundwater management for instance is very important because of observed trends of water supply deficiencies and of possibilities to use water for irrigation purposes more economically. Regulating groundwater table contributes to the solution of diverse ecological problems. The general concept of groundwater management is based on controlled drainage according to available water resources in the region. The classical drainage facilities are ditches, open channels, drainage pipes, mole drainage and vertical drainage wells, providing mainly subsurface drainage. Water management systems are set up in various configurations. The drainage facilities should be operated alternately for the management purposes. Investigations on water management systems useable for subsurface irrigation have been carried out in different countries: Slovakia, Russia, the Netherlands, Poland, Bulgaria, Germany, the USA, the Ukraine and elsewhere. In most cases the attention has been focused on drainage equipment operation and on possibilities of maintaining a certain groundwater level, or on determining the drainage water amount under constant operation conditions. The results of investigations and practical applications clearly confirm the possibility of regulating the groundwater level by means of drainage equipment. These results are only the basis for new research work on a complex of problems depending on requirements for control systems operating under dynamically changing conditions and still meeting the needs of water users. Seite 6 von 8 Modeling of flow processes using SiWaPro DSS A computer based Decision Support System including computer simulation tools with active graphical user interface is the basis for the modern water management. Since the unsaturated zone is considered the use of 2D-vertical-plane models are recommended. To obtain a maximum groundwater recharge or rather to optimize an artificial wetland, the processes in the unsaturated zone have to be calculated and described by these models. The simulation program SiWaPro DSS is able to compute the two dimensional vertical plane and rotationally symmetric flow and transportation processes including the degradation and sorption in the un/saturated zone. 2. Modeling of Flow and Transport Processes Transfer of Measurement 2 3 1 3. Optimization of Groundwater Recharge 4 1. Measurement of Soil Parameters On/Off Enter 2 3 1 4 O n/O ff Enter Artificial Groundwater Recharge Area Leachate Groundwater Unsaturated Zone Saturated Zone . Fig. 4 Modeling and Optimization The title of the program is the german synonym for Sickerwasserprognose (leachate prognosis) Decision Support System. The software was developed by the Technical University Dresden in cooperation with the KP Society of Engineers for Water and Soil mbH. It serves as a tool for risk assessment. The Richards Equation and the van-Genuchten-parameter functions are implemented in the simulation program SiWaPro DSS. The program based on the commonly used simulation code SWMS_2D (Šimunek et al., 1994) and solves the Richards-equation using the Finite-Elements method (Kemmesies, 1995). Furthermore the software has the ability to simulate solute transport in the unsaturated zone based on the convection-dispersion equation. Therefore the risk of a possible contamination of groundwater can be estimated and a contamination avoided. For this reason a contaminant database is implemented. Seite 7 von 8 An implemented weather generator simulates the time series of precipitation, solar radiation and the parameter evapotranspiration (Nitsch et al., 2007). A soil database is implemented in SiWaPro DSS, too. In figure 5 a simulation of the flow and transport processes with SiWaPro DSS is shown. The simulation program SiWaPro DSS is able to calculate and describe the existing complex properties of soil and the interaction between vegetation and soil in wetlands. Therefore the program can calculate and optimize the water processes in a groundwater recharge area. Fig.5 Simulation run Literature BFAI (2006): Regionale Engpässe in der Wasserversorgung Syriens. Bundesagentur für Außenwirtschaft. – Servicestelle des Bundesministeriums für Wirtschaft und Technologie (Germany), http://www.bfai.de/ EPA (2004):Wetlands Overview. Office of Water. http://www.epa.gov/owow/wetlands/pdf/overview.pdf Kemmesies, O. (1995): Prozeßmodellierung und Parameteridentifikation von Mehrphasenströmungsprozessen in porösen Medien. Dissertation. Fakultät für Geowissenschaften, Geotechnik und Bergbau der TU Bergakademie Freiberg. Proceedings des Dresdner Grundwasserforschungszentrum e.V.. Heft 7. Dresden. ISSN 1430-0176. Seite 8 von 8 Luckner L., van Genuchten, M.Th., Nielsen, D.R. (1989): A Consistent Set of Parametric Models for the Two-Phase Flow of Immiscible Fluids in the Subsurface. Water Resour. Res. 25(10): p. 2187-2193. Luckner L., van Genuchten, M.Th., Nielsen, D.R. (1991): Reply to NIMMO's "Comment on the Treatment ....". Water Resour. Res. 27(4): p. 661-662. Mualem, Y. (1976): A New Model for Predicting the Hydraulic Conductivity of Unsaturated Porous Media. Water Resour. Res. 12: p. 513-522. Nimmo, J.R. (1991): Comment on the Treatment of Residual Water Content in "A Consistent Set of Parametric Models for the Two-Phase Flow of Immiscible Fluids in the Subsurface" by L. Luckner et al.. Water Resour. Res. 27(4): p. 661-662. Nitsch, B., Gräber, P.-W., Kemmesies, O. (2007): Anwendung synthetischer Niederschlagszeitreihen bei der Strömungssimulation in der ungesättigten Bodenzone. TU Dresden, Dresden, Workshop 2007-Simulationen in Umwelt-und Geowissenschaften, Shaker Verlag Nielsen, D. R., Luckner, L (1992): Theoretical aspects to estimate reasonable initial parameters and range limits in indentification procedures for soil hydraulic properties. In, Proc. Intl. Workshop on Indirect Methods for Estimating the Hydraulic Properties of Unsaturated Soils, edited by M. Th van Genuchten, F. J. Leij, and L. J. Lund, University of California, Riverside, pp. 147-160. Science Update (2007): http://www.sciencenetlinks.com/sci_update.cfm?DocID=262 Šimunek, Vogel, J.T., van Genuchten, M. Th.; (1994): The SWMS_2D code for simulating water flow and solute transport in two-dimensional variably saturated media, Version 1.1., Research Report No.132, U. S. Salinity Laboratory, USDA, ARS, Riverside, CA. van Genuchten, M.Th. (1980): A Closed-form Equation for Predicting the Hydraulic Conductivity of Unsaturated Soils. Soil Sci. Soc. Am. J. 44: p. 892-898.