Simulating Water Movement in Layered and Gradational Soils Using the Kirchhoff Transform

Changes in soil hydraulic properties with depth are common in field soils and need to be accounted for in simulations of field soil‐water balances. We show that this is possible using the Kirchhoff transform (K transform) when solving Richards' equation numerically, even though the K transform is not continuous across boundaries between layers with different hydraulic properties. The solution is achieved by dividing the soil into depth elements, using the K transform to compute flow within each element, and then using matric potential (which is continuous across boundaries) to couple the elements. Combination of the K transform with iterative, implicit, mass‐conserving methods of solving the flow equations results in efficient solutions that take no more than a few seconds on a personal computer to simulate infiltration. This is achieved with errors of only a few percent in the amount and distribution of soil water. Illustrative simulations using hydraulic properties that represent different soil profiles are given. These include simulation of infiltration into a layered profile consisting of tilled soil with a crust overlying an undisturbed soil, and gradational profiles with hydraulic properties of a sand varying linearly with depth to those of a clay, and conversely. Development and dissipation of surface ponding and perched water tables during simulation runs presented no difficulties. Nor did use of the Campbell formulations for the soil hydraulic properties, despite the discontinuity at the wet end.