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Evaporation, drainage, and changes in storage for a bare Plainfield sand were measured with a lysimeter during June, July, and August 1967, under natural rainfall conditions. Cumulative evaporation at any stage was proportional to the square root of time following each heavy rainfall. The drainage rate was found to be an exponential function of water storage. Both relations can be predicted from flow theory with knowledge of soil capillary conductivity, diffusivity, and moisture retention characteristics. Using these two relations and daily rainfall data, the water storage in the top 150 cm was predicted over the season to within 0.3 cm.

The flow equation describing the movement of water out of a soil sample in the pressure plate apparatus has been solved for a special case. It is assumed that during the outflow process (a) the capillary conductivity is approximately constant, and (b) the relation between the water content and the suction is linear. Both of these conditions can be met if the pressure increment which causes the outflow is made sufficiently small. By measuring the outflow from the sample as a function of time and using the solution of the flow equation, the capillary conductivity can be calculated for each suction interval. Conductivity measurements on three soils indicate that the conductivity decreases approximately as the inverse square to cube of the suction. By using the pressure membrane apparatus, the range of suction over which measurements were made was extended up to 15 bars.

Synopsis A mathematical theory describing water uptake by nonuniform root systems and other factors is developed. The relative distribution of roots with depth and the water retaining and transmitting properties of the soil determine the main features of the water uptake pattern. Other factors such as the total number of roots are relatively unimportant. Good agreement is obtained between the theory and an actual uptake pattern for sorghum.