Determination of deep percolation losses in loessial soils

Watershed evapotranspiration by the combination method

“…Ratio Vertical soil moisture redistribution would then need to be computed. The best current method appears to be the standard one-dimensional Darcy equation for unsaturated flow which uses moisture-tension and moistureconductivity relationships (Klute 1969, Melvin 1970.…”

“…Thus a soil moisture model would be incomplete unless this soil moisture redistribution was considered.The basic Darcy equation of soil moisture movement of unsaturated soil, both K and H are functions of the soil moisture present, and these relations must be known or determined. For the loess soils of western Iowa, work byMelvin (1970) provided relationships that were applied. The moisture-tension relationships shown infigure ^6were used as the average of the wetting and drying curves shown byMelvin (1970, p. 130).…”

“…increments for each ^-hour period and moisture allowed to flow either up or down, depending on the tension Conductivity estimates for Monona silt 60 in. depth(Melvin 1970, P» 169) 50 1 oam -l66 gravity gradients. The 6-in.…”

“…as: (l) changing the soil evaporation actual/potential relation, (2) accounting for the phenological decline of the grass during late summer, (3) slightly adjusting the assigned transpiration values of the moisture stress curves B, C, and D, and (i|) selecting the curve with lower soil moisture conductivities as presented byMelvin (1970)»Only two or three trial runs were used to make these adjustments and arrive at the results presented. Data from both years were usually used for each trial.Accumulative volumes of the principal variables that were considered for the calculations of both potential and actual ET are presented in figures $6 through 61.…”