Moisture and Energy Conditions during Downward Entry of Water into Moist and Layered Soils

T paper is in the nature of a second chapter to one presented earlier (i). In that study the water infiltration process was examined in laboratory packed columns of texturally uniform, air-dry soils. In the present study the same laboratory packed soils were used, but this time two different initial soil conditions were investigated. First, columns of the Yolo sandy loam and silt loam, described in the previous paper, were subjected to infiltration after having been irrigated and drained so as to raise their initial moisture contents. Second, layered columns of the two soils were used and infiltration was studied in one case with the sandy loam above and, in the other, below the silt loam. In this and the previous study the object has been to seek an explanation of the observed relationship between infiltration rate and time, based upon those conditions of soil moisture energy which can be measured below and within the infiltration zone. In the study reported earlier it was found that the soil layer wet by infiltration could be divided into three parts,, vis.,-the transmission zone, the wetting zone, and the wet front. The transmission zone occupies the upper part of the wetted soil, and, once established, it absorbs no additional moisture, serving only to conduct water from the soil surface to the wetting zone beneath. Its average moisture content represents approximately 80% pore-space saturation and its average pressure potential is —3x10* ergs/gram. As water penetrates into the soil the transmission zone lengthens. Calculations based upon water entry rates, pressure potentials measured at the top and base of this zone, and thickness of the zone have shown that its average permeability remains practically constant. The decrease in water entry rate with time is directly proportional to the decrease in average total potential gradient within this zone. The wetting zone joins the transmission zone to the wet front, and moisture within it increases as infiltration proceeds. In this zone moisture content decreases and pressure potential gradient increases with depth. The wet front is an irregular surface in soils initially air-dry. It is sharply defined by a color change, and by the very high pressure potential drop between the moist soil above and dry soil below. The moisture content of the soil immediately above the wet front plane has been found to be constant, for a given soil, at all depths of water penetration studied. It has been suggested that this moisture content represents the condition of minimum significant capillary permeability. It was concluded from these earlier measurements that the moisture potential and permeability conditions within the soil may be used to explain the observed phenomena associated with water entry. The often-observed influences upon infiltration of raindrop impact upon the soil surface, inwashing of colloids, and compression of air ahead of the wet front, all of Which were purposely excluded or minimized in this study, are considered simply to modify the specific results without altering the basic relationships between infiltration rate and soil moisture energy conditions. The present study was undertaken in order to determine whether the explanations advanced for water infiltration into dry uniform soil may be applied also to infiltration under other initial soil conditions.