Vzj50061 628..640

As compared with mineral soils, there are few in situ measurements of the unsaturated hydraulic properties of peat soils available. We used parameter estimation (inverse) methods to estimate the water retention and hydraulic conductivity functions of drained peat soils from both laboratory and field data. The laboratory data were obtained on small cores using the traditional evaporation method, while the field data were obtained by means of evaporation experiments using groundwater lysimeters with and without vegetation. The field experiments without vegetation produced highly uncertain parameters and were limited to a relatively small pressure head range. Better results were obtained for the lysimeter with a grass cover that caused the peat soil to dry out more. However, a physically realistic minimum in the objective function for the plant-coveredlysimetercouldbefoundonlywhenpriorinformationabout severalparameterswasincludedintheoptimization.Goodagreementwas obtained between the laboratory and field measurements. The hydraulic functions were subsequently tested by comparing forward simulations with independently measured pressure heads and water contents of an additional lysimeter experiment under grass. The dynamics of the drying process was described well using the optimized soil hydraulic properties. UNDERSTANDING the processes that control the retention and flow of water in peat soils is critical to effective management of such soils from both agricultural and ecological perspectives. In contrast to mineral soils, much less is known about the unsaturated soil hydraulic properties of peat soils, especially the hydraulic conductivity. This is in part due to the unique nature of the physical and hydraulic properties of peat soils, such as volume changes during dewatering (Schwärzel et al., 2002; Price, 2003). Only a relatively few measurements of the unsaturated hydraulic conductivity of peat soils have been reported in the literature (e.g., Rijtema, 1965; Renger et al., 1976; Loxham and Burghardt, 1986; Schouwenaars and Vink, 1992; Baird, 1997; Silins and Rothwell, 1998; Schlotzhauer and Price, 1999; Schindler et al., 2003; Naasz et al., 2005). Most of these studies involved direct laboratory measurements of the unsaturated hydraulic conductivity using steady-state methods on small core samples. Many laboratory and field methods are currently available for direct determination of the soil water retention and unsaturated hydraulic conductivity functions (Dane and Topp, 2002). In addition to these direct methods, inverse solution techniques are now also increasingly used to determine the hydraulic properties from transient field or laboratory measurements (e.g., Dane and Hruska, 1983; Parker et al., 1985; Eching and Hopmans, 1993; Šimůnek et al., 1998b; Abbaspour et al., 2000; Jacques et al., 2002; Sonnleitner et al., 2003). The application of inverse methods assumes a certain parametric model for the hydraulic properties with as yet unknown parameters. A flow experiment is then typically conducted to estimate these parameters by minimization of deviations betweenmeasured and predicted flow variables (Kool and Parker, 1987). An excellent review of the principles and advantages of inverse methods was given by Hopmans et al. (2002). In this study we use inverse procedures to determine the unsaturated hydraulic properties of peat soils. In general, laboratory measurements are quick and precise, but often lead to soil hydraulic properties that are not representative of field conditions. Differences between laboratory and fieldmeasurements have long been noted for mineral soils (e.g., Sonnleitner et al., 2003), but such differences may be especially important for peat soils because of their unique physical properties. Moreover, standard field methods such as the instantaneous profile method (Vachaud and Dane, 2002) or the planeof-zero-flux method (Arya, 2002) are generally not applicable to peat soils, which are usually situated in areas with relatively shallow water tables. While the use of lysimeters may overcome some of these limitations, their installation andoperation is oftenvery expensive. In contrast to conventional lysimeters, we developed a relatively low-cost lysimeter that can be installed more easily, and with minimal technical effort (Schwärzel and Bohl, 2003). Field lysimeters of this type, without vegetation, were used in the Schwärzel and Bohl (2003) study to determine the unsaturated soil hydraulic functions of peat soils using traditional direct methods. Results showed that direct estimation of the hydraulic conductivities from the lysimeter data was very tedious, uncertain (due to very small pressure head gradients), and limited to a relatively small pressure head interval. In contrast, measurements of the hydraulic properties in the laboratory were quicker and more accurate over a much wider range of water contents. Still, questions arose as to what extent the relatively small size of the laboratory samples and soil disturbance during sample collection affected the hydraulic properties of the peat soils. The objectives of this study hence were to use parameter inverse estimation methods to determine the K. Schwärzel, Institute of Soil Science and Site Ecology, Univ. of Technology, Dresden, Germany; J. Šimůnek, Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521 USA; H. Stoffregen and G. Wessolek, Institute of Ecology, Dep. of Soil Science and Soil Protection, Technical Univ. of Berlin, Germany; M.Th. van Genuchten, USDA-ARS, George E. Brown, Jr. Salinity Lab., Riverside, CA 92521, USA.Received 10May 2005. *Corresponding author (Kai.Schwaerzel@ Forst.TU-Dresden.de). Published in Vadose Zone Journal 5:628–640 (2006). Original Research doi:10.2136/vzj2005.0061 a Soil Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: VGM, van Genuchten–Mualem; WRC, water retention curve. R e p ro d u c e d fr o m V a d o s e Z o n e J o u rn a l. P u b lis h e d b y S o il S c ie n c e S o c ie ty o f A m e ri c a . A ll c o p y ri g h ts re s e rv e d . 628 Published online May 26, 2006