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Root Water Extraction and Limiting Soil Hydraulic Conditions Estimated by Numerical Simulation

^a Exact Sciences Dep., Esalq-Univ. of São Paulo, 13418-900 Piracicaba (SP), Brazil, currently at Dep. of Environmental Sciences, Wageningen Univ., Wageningen, the Netherlands
^b Dep. of Environmental Sciences, Wageningen Univ., Nieuwe Kanaal 11, 6709 PA Wageningen, the Netherlands

View larger version (15K): [in a new window]   Fig. 1. Mean water content () relative to field capacity (fc) and the permanent wilting point (pwp), * = ( – pwp)/(fc – pwp), at first occurrence of limiting hydraulic conditions, as a function of potential transpiration rate for four crop groups, as defined by Doorenbos and Kassam (1986).

View larger version (20K): [in a new window]   Fig. 2. (a) Soil water content, (b) hydraulic conductivity, and (c) matric flux potential as a function of pressure head for sand, clay, and loam soils.

View larger version (22K): [in a new window]   Fig. 3. Schematic representation of root and axial segments as used for the simulations; rm is the half-distance between roots, r0 is the root radius, dr,i is the segment size for segment i, and ri is the distance from the axial center for segment i.

View larger version (22K): [in a new window]   Fig. 4. Simulated pressure head at the root surface (hroot), mean pressure head (hmean) and relative transpiration as a function of time for low and high potential transpiration rates and low, medium, and high root density in a clay soil.

View larger version (20K): [in a new window]   Fig. 5. Pressure head as a function of distance from the axial center (hr) and mean pressure head (hmean) for low and high potential transpiration rates (Tp) for simulations with low, medium, and high root density, at the moment of first occurrence of limiting root water potential; rm is the radius of the root extraction zone or the half-distance between roots.

View larger version (19K): [in a new window]   Fig. 6. Mean pressure head at first occurrence of limiting hydraulic conditions (hmean,lim) as a function of potential transpiration rate (Tp) for low, medium, and high root density in a clay soil simulated with and without daily transpiration amplitude (for lines identified with "A", mean transpiration equals Tp and amplitude equals Tp; for lines identified with "B", mean transpiration equals Tp and amplitude is zero; for lines identified with "C", mean transpiration equals 2Tp and amplitude is zero).

View larger version (14K): [in a new window]   Fig. 7. Mean pressure head at first occurrence of limiting hydraulic conditions (hmean,lim) as a function of potential transpiration rate (Tp) for low, medium, and high root density in the sand, loam and clay soils (dotted lines refer to minimum root pressure head h = –150 m).

View larger version (15K): [in a new window]   Fig. 8. Mean water content () relative to field capacity (fc) and the permanent wilting point (pwp), * = ( – pwp)/(fc – pwp), at first occurrence of limiting hydraulic conditions (*mean,lim) as a function of potential transpiration rate (Tp) for low, medium, and high root density in the sand, loam, and clay soils.

View larger version (17K): [in a new window]   Fig. 9. Hydraulic conductivity at first occurrence of limiting hydraulic conditions (Kmean,lim) as a function of potential transpiration rate (Tp) for low, medium, and high root density in the sand, loam and clay soils. (Kpwp is the hydraulic conductivity at the permanent wilting point for the respective soil).

View larger version (15K): [in a new window]   Fig. 10. Matric flux potential at first occurrence of limiting hydraulic conditions (Mmean,lim) as a function of potential transpiration rate (Tp) for low, medium, and high root density. Lines for all three soils are coincident.

View larger version (8K): [in a new window]   Fig. 11. Ratio between mean matric flux potential at first occurrence of limiting hydraulic conditions and potential transpiration rate (Mmean,lim/Tp) as a function of half-distance between roots (rm).

View larger version (20K): [in a new window]   Fig. 12. Nomogram for the determination of pressure head at first occurrence of limiting hydraulic conditions (hmean,lim) and relative water content at first occurrence of limiting hydraulic conditions (mean,lim) for three soils from transpiration and root density data. Dotted arrow lines illustrate nomogram use for a high transpiration rate (6 mm d–1) and a medium root density (0.1 m m–3) for the sand soil.

View larger version (21K): [in a new window]   Fig. 13. Matric flux potential (M) at first occurrence of limiting hydraulic conditions as a function of distance from axial center for low, medium, and high root density at low and high potential transpiration (lines for three soils coincide at this resolution); rm is the radius of the root extraction zone or the half-distance between roots.

View larger version (29K): [in a new window]   Fig. 14. (a) Pressure head and (b) water content as a function of distance from axial center at several times after start of water extraction (clay soil, medium root density, high transpiration rate). Dotted lines indicate pressure head or water content profile at the moment of first occurrence of limiting hydraulic conditions at the root surface.

View larger version (10K): [in a new window]   Fig. 15. Pressure head at the first occurrence of limiting root water potential (hmean,lim) calculated from analytical and numerical matric flux potential (Manalytical and Mnumerical, respectively) for the three soils at low root density and high transpiration rate.