Liquid Component of Evaporative Flow in Two Tilled Soils

Shallow tillage is generally considered to reduce evaporation by checking upward liquid flow within and across the tilled layer. To verify this contention, we partitioned the total water loss from tilled soil into loss from the tilled (5‐cm‐deep) layer per se and upward flow across the interface of the tilled layer and untilled soil. This was done to verify the occurrence and extent of liquid flow across the junction of tilled and untilled soil and within the tilled layer, and to facilitate modeling evaporation from tilled soil. The determinations were made in 10‐cm‐i.d., 95‐cm‐deep columns filled with silt loam and sandy loam soils. For 1 or 2 d after tillage, water loss from the tilled layer constituted a major portion of the total flux. However, it became negligible after 7 d in the sandy loam and 12 d in the silt loam while the flux across the interface increased. Liquid flux across the interface in 30 d of drying after tillage amounted to 72 and 42% of total loss under low and high atmospheric evaporativity, respectively, in the silt loam and 20 and 30%, respectively, in the sandy loam. The total liquid flux, taken as the difference between the experimentally determined total loss from the tilled soil and the vapor flux computed from the tilled layer by Hanks' (1958) equation, was identical in both time pattern and magnitude to that computed from the moisture profiles predicted numerically from the water‐content gradients, diffusivity D (θ), and hydraulic conductivity K (θ). Thus, the upward capillary flow was not completely cut off by tillage; it was only retarded because of the change in D (θ).