Infiltration of water into soil with cracks

This paper presents the physical basis of the FRACTURE submodeled for simul ating infiltration of precipitation/irrigation water into relatively dry, c racked, fine-textured soils. The FRACTURE submodel forms part of the HYDRUS -ET variably saturated flow/transport model. Infiltration into the soil mat rix is formally divided into two components: (1) Vertical infiltration thro ugh-the soil surface; and (2) lateral infiltration via soil cracks. The fir st component is described and solved using the 1D Richards' equation. Exces s water that does not infiltrate through the soil surface is either conside red to be runoff, if no soil cracks are present, or routed into soil cracks from where it may laterally infiltrate into the soil matrix. Horizontal in filtration from soil cracks into the soil matrix is calculated using the Gr een-Ampt approach and incorporated as a positive source/sink term S-f in th e Richards' equation describing flow in the matrix. In addition to the hydr aulic properties of the soil matrix, the FRACTURE submodel requires paramet ers characterizing the soil cracks, notably the specific crack length per s urface area l(c) and the relationship between crack porosity P-o and the gr avimetric soil water content w. An example problem shows that infiltration from soil cracks can be an important process affecting the soil water regim e of cracked soils. A comparison with the more traditional approach, involv ing surface infiltration only, indicates important differences in the soil water content distribution during a rainfall/irrigation event. This extensi on of the classical approach to include crack infiltration significantly im proves the identification and prediction of the soil water regime.