Degree Type


Date of Award


Degree Name

Doctor of Philosophy




A mathematical model was developed to simulate water uptake of the soybean roots as a function of rooting characteristics. Parameters for the model were obtained from field experiments conducted during the 1975 to 1977 and 1979 growing seasons at the Western Iowa Research Center, Castana, Iowa;A theoretical analysis of the set of water uptake equations, based on Ohm's analogy, gave a solution defining the "effective resistance" and the "effective water potential" for the rooting zone in terms of the resistances to water flow. Two different row spacings with rows 25 and 100 cm apart were chosen to compare the rooting and water extraction patterns with plant growth. Determination of the parameters was undertaken focusing on the functional relations with respect to time;The water consumption pattern showed that soybeans used more of the available water in the early growing season of dry years in a 25-cm row spacing than in a 100-cm row spacing. This greater water use had no effect on yields. In a wet year, the yield of soybeans in a 25-cm row spacing was greater than that in a 100-cm row spacing. It is not known why the soybeans grown in the 100-cm row spacing (with their higher LAI, greater dry mass and greater height) did not yield more in dry years than those grown in the 25-cm row spacing. The root length density and dry mass was generally greater for plants in the 25-cm than in the 100-cm row spacing. Root length density and water uptake rate was shifted downward as available water of the upper layers was depleted. The root conductivity, K(,rs), showed a large variability ranging from 0.7 to 5.6 x 10('-5) cm/day; however, it decreased by a factor of 8 as the plants aged. The potential drop longitudinally in the root system, calculated from Poiseuille's equation, was roughly 0.06 bar per cm of length per cm of transpiration rate. Plant water potentials measured on fully exposed leaves and on leaves covered with aluminum foil at the sunset of the previous day decreased as soil water potential decreased;Simulation results based on the numerical solution of the flow equation by a differential equation solver, DGEAR (a package implemented in IMSL) agreed well with the measured data in the field; however, some disparities were observed due to possible inaccuracies in magnitude of the parameters necessary in the model. The model showed that the "effective soil water potential" might be more realistic than the "average soil water potential" (calculated from the arithmetic mean after weighting for the thickness of the soil layer) especially in the cases where large differences in the soil-water potential were expected among the layers. This model needs further development with more precise values for the parameters governing water uptake.



Digital Repository @ Iowa State University,

Copyright Owner

Yeong-Sang Jung



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192 pages