Evaporation from High Residue No-till Versus Tilled Fallow in a Dry Summer Climate.

Growers in the low-precipitation (< 300 mm annual) region of the Inland Pacific Northwest practice summer fallow to produce winter wheat (Triticum aestivum L.) in a 2-year rotation. No-till fallow (NTF) is ideal for control of wind erosion but is not widely practiced because of seed-zone soil drying during the summer, whereas adequate seed-zone water for germination and emergence of deep-sown winter wheat can generally be retained with tilled fallow (TF). Successful establishment of winter wheat from late August – early September planting is critical for optimum grain yield potential. A 6-year field study was conducted at Lind, WA (242 mm annual precipitation) to determine if accumulations of surface residue under long-term NTF might eventually be enough to substitute for TF in preserving seed-zone water over summer. Averaged over the six years, residue rates of 1500, 6000, and 10,500 kg/ha (1x, 4x, and 7x rates, respectively) on NTF produced incrementally greater seed-zone water but were not capable of retaining as much as TF. Total root zone (0 to 180 cm) over-summer water loss was greatest in the 1x NTF whereas there were no significant differences in the 4x and 7x NTF versus TF. Average precipitation storage efficiency ranged from 33% for 1x NTF to 40% for TF. We conclude that for the low-precipitation winter wheat-summer fallow region of the Inland Pacific Northwest: (i) Cumulative water loss during the summer from NTF generally exceeds that of TF; (ii) there is more extensive and deeper over-summer drying of the seed-zone layer with NTF than with TF; (iii) increased quantities of surface residue in NTF slow the rate of evaporative loss from late-summer rains, and (iv) large quantities of surface residue from April through August will marginally enhance total-profile and seed-zone water in NTF, but will not retain adequate seed-zone water for early establishment of winter wheat except sometimes during years of exceptionally high precipitation or when substantial rain occurs in mid-to-late August.