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Modeling Water Movement in Heterogeneous Water-Repellent Soil: 2. A Conceptual Numerical Simulation

^a Sustainable Land Use Team, HortResearch, Tennent Dr., P.O. Box 11030, Palmerston North, New Zealand
^b Institute of Soil Science, Univ. of Hannover, Herrenhäuserstr. 2, 30419 Hannover, Germany

View larger version (87K): [in this window] [in a new window]   FIG. 1. The (A, B, C, D) soil properties, (E) root density distribution, and (F) initial condition of the simulations. (A) Distribution of scaling factors for the water characteristic function. (B) Distribution of scaling factors for the hydraulic conductivity function. (C) Distribution of the minimum contact angle; the mean contact angle is 5 ± 1.5°. (D) Distribution of the maximum contact angle; the mean contact angle is 90 ± 20°. (E) Distribution of the root density factor b. (F) Distribution of matric heads as an initial condition for the simulation; the averaged matric pressure head of the soil profile is 60 hPa.

View larger version (45K): [in this window] [in a new window]   FIG. 2. The distribution of volumetric water contents at three times during the simulated drying–rewetting cycle (B, D, E; right-hand side) with and (A, C, E; left-hand side) without considering water repellency. Locations where the actual contact angle is above the minimum contact angle (reduced wettability) are marked by dots. (A, B) End of the drying phase: after 20 d without precipitation and at a constant potential evapotranspiration rate of 0.42 cm d–1. (C, D) Start of the rewetting phase: after 2 d with a constant precipitation rate of 2 cm d–1. (E, F) End of the rewetting phase: after 7 d with a constant precipitation rate of 2 cm d–1.

View larger version (36K): [in this window] [in a new window]   FIG. 3. The impact of water repellency on the simulated behavior of two locations (grid node 1003 at 75-cm horizontal and 30-cm vertical distance, and grid node 1089 at 87.5-cm horizontal and 32.5-cm vertical distance) during the rewetting phase. The time 0 is the start of the rewetting phase. Water repellency is not considered for A–B and is considered for C–H. Simulation without considering water repellency: (A) Matric heads and actual contact angles (dashed line) at node 1003. (B) Volumetric water contents at node 1003. Simulation with considering water repellency: The pore surface properties are on average max 90° ± 20° and min 5° ± 1.5°; at node 1003 they are max = 93.6° and min = 5.27°; at node 1089 they are max = 67.1° and min = 3.28°. Node 1003 is outside of fingers and node 1089 is inside a finger. (C) Matric heads and actual contact angles (dashed line) at node 1003. (D) Volumetric water contents and actual contact angles (dashed line) at node 1003. (E) The values of the parameter vG during the simulation and the actual contact angles (dashed line) at node 1003. (F) The values of the parameter nvG during the simulation and the actual contact angles (dashed line) at node 1003. (G) Matric heads and actual contact angles (dashed line) at node 1089. (H) Volumetric water contents and actual contact angles (dashed line) at node 1089.

View larger version (59K): [in this window] [in a new window]   FIG. 4. The impact of a different parameterization of water repellency on the distribution of volumetric water contents at the start of the rewetting phase (after 3 d of precipitation). All simulations use the parameter set with l2= 0.1, min = 5° ± 1.5°, max = 90° ± 20°, crit = 0.015, t0 = 10 d, and FK at 60 hPa. Only one of these parameters is modified for each of the simulations: (A) max = 70° ± 20°, (B) max = 90° ± 20°, (C) max = 130° ± 20°, (D) crit = 0.03, l2 (E) = 0.05, (F) t0 = 20 d.

View larger version (49K): [in this window] [in a new window]   FIG. 5. The impact of a different parameterization of water repellency on (A–E) the dynamics of water flow and repellency in the root zone and (F) the drainage rates out of the soil profile. All simulations use the parameter set with l2 = 0.1, min = 5° ± 1.5°, max = 90° ± 20°, crit = 0.015, t0 = 10 d, FK at 60 hPa. Only one of these parameters is modified for each simulation set: 0: max = min, 1: max = 70° ± 20°, 2: max = 90° ± 20°, 3: max = 130° ± 20°, 4: t0 = 20 d, 5: crit = 0.03, 6: FK at 20 hPa. Entire drying–rewetting cycle (vertical line denotes the start of the rewetting phase); the averages presented here are the arithmetic average of all nodes within the root zone (Fig. 1E): (A) Average volumetric water contents of the root zone. (B) Average matric heads of the root zone. (C) Average actual contact angles of the root zone. (D) Relative areas of the entire root zone where the actual contact angle >min. Rewetting phase only: (E) Average infiltration rate along a transect at the top of the root zone. The transect extends in 12.5-cm depth from 75- to 125-cm horizontal distance. The rate is in percentage of the precipitation rate; each symbol denotes 24 h. (F) Average drainage rate at the bottom of the soil profile in percentage of the precipitation rate; each symbol denotes 24 h.