{"id":635,"date":"2023-10-16T21:56:25","date_gmt":"2023-10-16T21:56:25","guid":{"rendered":"https:\/\/books.gw-project.org\/groundwater-in-peat-and-peatlands\/?post_type=chapter&p=635"},"modified":"2023-11-29T00:48:49","modified_gmt":"2023-11-29T00:48:49","slug":"box-4","status":"publish","type":"chapter","link":"https:\/\/books.gw-project.org\/groundwater-in-peat-and-peatlands\/chapter\/box-4\/","title":{"raw":"Box 4- Van Genuchten-Mualem (VGM) Variables and Parameters","rendered":"Box 4- Van Genuchten-Mualem (VGM) Variables and Parameters"},"content":{"raw":"

In most cases, the volumetric water content at saturation, \u03b8<\/em>s<\/sub>, is equivalent to the total porosity, \ud835\udf19<\/em>mob<\/sub> + \ud835\udf19<\/em>im<\/sub>, as discussed in Section 2.1. The residual water content, \u03b8<\/em>r<\/sub> varies between 0.01 and 0.15 and is often estimated because measurements at extreme tensions (h<\/em> > 200 cm, where h<\/em> = -<\/em>\u03c8<\/em>) are less common in peat. The residual water content, \u03b8<\/em>r<\/sub>, is a portion of the \ud835\udf19<\/em>im<\/sub> as measured by soil water retention curves. Both \u03b8<\/em>s<\/sub> and \u03b8<\/em>r<\/sub> are strongly related to bulk density and degree of decomposition, with \u03b8<\/em>s<\/sub> decreasing and \u03b8<\/em>r<\/sub> increasing with increase in bulk density.<\/p>\r\n

Similar to both \u03b8<\/em>s<\/sub> and \u03b8<\/em>r<\/sub>, the VGM parameter n<\/em> is related to bulk density, decreasing from ~0.2 to ~0.01 for bulk densities ranging from 0.02\u20130.8 g cm3<\/sup>. However, this statistical relationship is weaker than those determined by porosity measures (Liu and Lennartz, 2019). The air entry pressure (characterized by 1\/\u03b1<\/em>) is strongly correlated with both macroporosity (discussed in Section 3.1) and K<\/em>sat<\/sub> (Liu and Lennartz, 2019). Broadly, \u03b1<\/em> decreases with increasing bulk density because air entry into denser peat requires higher pressure. In sedge peat, \u03b1<\/em> ranges from ~10-3<\/sup> \u2014 ~101<\/sup> cm-1<\/sup>, and in Sphagnum<\/em> peat it ranges from 10-1<\/sup>\u2014101<\/sup> cm-1<\/sup>. Unlike both sedge and Sphagnum<\/em> peat that are constrained to relatively small ranges of \u03b1<\/em>, woody peat spans a wide range of reported values, but the limited number of reported values may bias the true range for woody peat.<\/p>\r\n

Values of K<\/em>unsat<\/sub> defined for a given tension, h<\/em>\u2014using Equation 6\u2014employ a scaling parameter, l<\/em>, related to the pore-size distribution. Typically, l<\/em> is negative in peat and ranges from -6 to 1. To date, a statistical relationship between l<\/em> and bulk density has not been defined; however, this may be due to the choice to fix l<\/em> in some peat studies, while fitting l<\/em> in other studies. Much less is known about how l<\/em> varies with other physical peat properties as compared to what is known about soil water retention model parameters, due to the paucity of studies that encompass both soil water retention and unsaturated hydraulic conductivity.<\/p>\r\n

Return to where the text linked to Box 4<\/strong><\/a><\/p>\r\n\r\n

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In most cases, the volumetric water content at saturation, \u03b8<\/em>s<\/sub>, is equivalent to the total porosity, \ud835\udf19<\/em>mob<\/sub> + \ud835\udf19<\/em>im<\/sub>, as discussed in Section 2.1. The residual water content, \u03b8<\/em>r<\/sub> varies between 0.01 and 0.15 and is often estimated because measurements at extreme tensions (h<\/em> > 200 cm, where h<\/em> = –<\/em>\u03c8<\/em>) are less common in peat. The residual water content, \u03b8<\/em>r<\/sub>, is a portion of the \ud835\udf19<\/em>im<\/sub> as measured by soil water retention curves. Both \u03b8<\/em>s<\/sub> and \u03b8<\/em>r<\/sub> are strongly related to bulk density and degree of decomposition, with \u03b8<\/em>s<\/sub> decreasing and \u03b8<\/em>r<\/sub> increasing with increase in bulk density.<\/p>\n

Similar to both \u03b8<\/em>s<\/sub> and \u03b8<\/em>r<\/sub>, the VGM parameter n<\/em> is related to bulk density, decreasing from ~0.2 to ~0.01 for bulk densities ranging from 0.02\u20130.8 g cm3<\/sup>. However, this statistical relationship is weaker than those determined by porosity measures (Liu and Lennartz, 2019). The air entry pressure (characterized by 1\/\u03b1<\/em>) is strongly correlated with both macroporosity (discussed in Section 3.1) and K<\/em>sat<\/sub> (Liu and Lennartz, 2019). Broadly, \u03b1<\/em> decreases with increasing bulk density because air entry into denser peat requires higher pressure. In sedge peat, \u03b1<\/em> ranges from ~10-3<\/sup> \u2014 ~101<\/sup> cm-1<\/sup>, and in Sphagnum<\/em> peat it ranges from 10-1<\/sup>\u2014101<\/sup> cm-1<\/sup>. Unlike both sedge and Sphagnum<\/em> peat that are constrained to relatively small ranges of \u03b1<\/em>, woody peat spans a wide range of reported values, but the limited number of reported values may bias the true range for woody peat.<\/p>\n

Values of K<\/em>unsat<\/sub> defined for a given tension, h<\/em>\u2014using Equation 6\u2014employ a scaling parameter, l<\/em>, related to the pore-size distribution. Typically, l<\/em> is negative in peat and ranges from -6 to 1. To date, a statistical relationship between l<\/em> and bulk density has not been defined; however, this may be due to the choice to fix l<\/em> in some peat studies, while fitting l<\/em> in other studies. Much less is known about how l<\/em> varies with other physical peat properties as compared to what is known about soil water retention model parameters, due to the paucity of studies that encompass both soil water retention and unsaturated hydraulic conductivity.<\/p>\n

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