Field-scale water flow and solute transport : SWAP model concepts, parameter estimation and case studies = [Waterstroming en transport van opgeloste stoffen op veldschaal]

Research output: Thesisinternal PhD, WU

Abstract

<p>Water flow and solute transport in top soils are important elements in many environmental studies. The agro- and ecohydrological model SWAP (Soil-Water-Plant-Atmosphere) has been developed to simulate simultaneously water flow, solute transport, heat flow and crop growth at field scale level. The main features and theoretical concepts of SWAP are described.</p><p>A serious limitation of many model applications is the availability of accurate input parameters. With the rapid increase of processor calculation speed and development of effective optimization algorithms, the optimization of input parameters by inverse modeling has become an attractive option. Typical and verifiable examples of the inverse modeling technique, are the laboratory One-step and Multi-step outflow experiments, which are used to determine the soil hydraulic functions. It is shown that in the One-step method the cumulative outflow data with time are insufficient to derive unique parameter estimates, and should be supplemented with retention data. In Multi-step experiments, where the air pressure is increased in several steps rather than one large step, the cumulative outflow data proved to be sufficient to derive unique and reliable soil hydraulic parameters.</p><p>The accuracy of field scale model predictions will increase if the inverse problem is also applied to measurements at field scale level. A prior method is presented to select the type of measurements and timing of observations that result in well-posed inverse problems. This method was applied to a theoretical field experiment at a drained loamy soil, cultivated with maize. SWAP in combination with the optimization shell PEST (Parameter ESTimation) were used to perform the prior inverse analysis. At least four selected parameters could be optimized uniquely, using ordinary, generated observations augmented with a random observation error. The prior analysis showed that inclusion of a crop, a tracer as well as measurements at periods with extreme and rapidly changing hydrological conditions improved the accuracy of the optimized parameters significantly.</p><p>An accurate and efficient numerical solution of Richards' water flow equation is presented. Special attention is given to proper selection of the top boundary condition during the iterative solution of Richards' equation. The stability of the scheme is shown for extreme events of infiltration, soil evaporation and rapidly fluctuating, shallow groundwater levels in two strongly non-linear soils. It is shown that in case of nodal distances of 1 cm and arithmetic spatial averages of the hydraulic conductivity, errors due to numerical discretization are small compared to errors due to hysteresis and horizontal spatial variability of the soil hydraulic functions.</p><p>Concepts for hysteresis and mobile-immobile regions due to water repellency have been incorporated in SWAP. Both concepts were applied to data sets from two locations with hysteretic and water repellent soils. In general hysteresis retards soil water movement, while preferential flow enhances soil water movement. Application of the hysteresis and mobile-immobile concept improved the correspondence between measured and simulated water and bromide contents.</p><p>Also an extended model concept for water and solute movement in cracked clay soils is discussed and applied to a field experiment. Inclusion of this concept in SWAP improved considerably the simulation of soil water contents and bromide leaching to the groundwater. The bromide amounts leached were especially sensitive to the saturated hydraulic conductivity of the top layer, the solute transfer from the soil matrix to crack water flow and the mean residence time of rapid drainage.</p><p>We may expect that in the coming years SWAP will be useful to explore new flow and transport concepts for agro- and ecohydrology, to analyse laboratory and field experiments, to select viable management options, to perform regional studies employing geographical information systems, and to illustrate transport processes for education and extension.</p><p><em>Additional index words</em> : clay cracks, crop growth, heat flow, hysteresis, inverse modeling, outflow experiments, Richards' equation, salinization, transpiration, unsaturated zone, water repellency</p>
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
Supervisors/Advisors
  • Feddes, R.A., Promotor, External person
Award date15 Sep 2000
Place of PublicationS.l.
Publisher
Print ISBNs9789058082565
Publication statusPublished - 2000

Keywords

  • soil water
  • infiltration
  • seepage
  • hydraulic conductivity
  • models
  • ecohydrology

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