soil texture and hydraulics: Soil texture and hydraulics

Description

Functions soil.psi2thetaSX and soil.theta2psiSX calculate water potentials (psi) and water contents (theta) using texture data the formulae of Saxton et al. (1986) or Saxton & Rawls (2006) depending on whether organic matter is available. Functions codesoil.psi2thetaVG and soil.theta2psiVG to the same calculations as before, but using the Van Genuchten - Mualem equations (W<U+00F6>sten & van Genuchten 1988). Function soil.USDAType returns the USDA type for a given texture. Function soil.vanGenuchtenParamsCarsel gives parameters for van Genuchten-Mualem equations for a given texture type (Leij et al. 1996), whereas function soil.vanGenuchtenParamsToth gives parameters for van Genuchten-Mualem equations for a given texture, organic matter and bulk density (Toth et al. 2015). Correspondingly, functions soil.waterFC and soil.thetaFC calculate the water volume (in mm or as percent of soil volume) of each soil layer at field capacity, according to a given water retention model. Functions soil.waterWP and soil.thetaWP calculate the water volume (in mm or as percent of soil volume) of each soil layer at wilting point (-1.5 MPa), and functions soil.waterSAT, soil.thetaSATSX and soil.thetaSAT calculate the saturated water volume (in mm or as percent of soil volume) of each soil layer. Functions soil.psi and soil.theta return the current water potential and water content of the soil object, according to a given water retention model.

Usage

soil.psi2thetaSX(clay, sand, psi, om = NA)
soil.psi2thetaVG(n, alpha, theta_res, theta_sat, psi)
soil.theta2psiSX(clay, sand, theta, om = NA)
soil.theta2psiVG(n, alpha, theta_res, theta_sat, theta)
soil.USDAType(clay, sand)
soil.vanGenuchtenParamsCarsel(soilType)
soil.vanGenuchtenParamsToth(clay, sand, om, bd, topsoil)
soil.psi(soil, model="SX")
soil.theta(soil, model="SX")
soil.waterFC(soil, model="SX")
soil.waterWP(soil, model="SX")
soil.waterSAT(soil, model="SX")
soil.thetaFC(soil, model="SX")
soil.thetaWP(soil, model="SX")
soil.thetaSAT(soil, model="SX")
soil.thetaSATSX(clay, sand, om = NA)
soil.waterTableDepth(soil, model="SX")

Arguments

clay

Percentage of clay (in percent weight).

sand

Percentage of sand (in percent weight).

n, alpha, theta_res, theta_sat

Parameters of the Van Genuchten-Mualem model (m = 1 - 1/n).

psi

Water potential (in MPa).

theta

Relative water content (in percent volume).

Percentage of organic matter (optional, in percent weight).

Bulk density (in g/cm3).

topsoil

A boolean flag to indicate topsoil layer.

soilType

A string indicating the soil type.

soil

Soil object (returned by function soil).

model

Either 'SX' or 'VG' for Saxton's or Van Genuchten's water retention models.

Value

Functions soil.psi2thetaSX and soil.psi2thetaVG return the soil water potential (in MPa) from soil volumetric water content, and functions soil.theta2psiSX and soil.theta2psiVG do the reverse calculation returning water potential in MPa. Function soil.USDAType returns a string. Function soil.vanGenuchtenParamsToth and soil.vanGenuchtenParamsCarsel return a vector with four parameter values (alpha, n, theta_res and theta_sat, where alpha is in MPa-1). Function soil.waterTableDepth returns water table depth in mm from surface.

References

Leij, F.J., Alves, W.J., Genuchten, M.T. Van, Williams, J.R., 1996. The UNSODA Unsaturated Soil Hydraulic Database User<U+2019>s Manual Version 1.0.

Saxton, K.E., Rawls, W.J., Romberger, J.S., Papendick, R.I., 1986. Estimating generalized soil-water characteristics from texture. Soil Sci. Soc. Am. J. 50, 1031<U+2013>1036.

Saxton, K.E., Rawls, W.J., 2006. Soil water characteristic estimates by texture and organic matter for hydrologic solutions. Soil Sci. Soc. Am. J. 70, 1569. doi:10.2136/sssaj2005.0117

W<U+00F6>sten, J.H.M., & van Genuchten, M.T. 1988. Using texture and other soil properties to predict the unsaturated soil hydraulic functions. Soil Science Society of America Journal 52: 1762<U+2013>1770.

T<U+00F3>th, B., Weynants, M., Nemes, A., Mak<U+00F3>, A., Bilas, G., & T<U+00F3>th, G. 2015. New generation of hydraulic pedotransfer functions for Europe. European Journal of Soil Science 66: 226<U+2013>238.

Examples

Run this code

# NOT RUN {
# Plot Saxton's water retention curve
psi = seq(0, -6.0, by=-0.01)
plot(-psi, lapply(as.list(psi), FUN=soil.psi2thetaSX, clay=40, sand=10, om = 1), 
     type="l", ylim=c(0,0.6),ylab="Water content (prop. volume)", 
     xlab = "Soil water potential (-MPa)")

#Determine USDA soil texture type
type = soil.USDAType(clay=40, sand=10)
type

#Van Genuchten's params (bulk density = 1.3 g/cm)
vg = soil.vanGenuchtenParamsToth(40,10,1,1.3,TRUE)
vg

#Add Van Genuchten water retention curve 
lines(-psi, lapply(as.list(psi), FUN=soil.psi2thetaVG, alpha=vg[1], n = vg[2], 
      theta_res = vg[3], theta_sat = vg[4]), lty=2) 

legend("topright", legend=c("Saxton", "Van Genuchten"), lty=c(1,2), bty="n")
# }