Evaporation from High Residue No-Till versus Tilled Fallow in a Dry Summer Climate

Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture or by Washington State University. USDA and WSU are equal opportunity providers and employers. *Corresponding author ([email protected]) Farmers in the low-precipitation (<300 mm annual) region of the Inland Pacific Northwest of the USA practice summer fallow to produce winter wheat (Triticum aestivum L.) in a 2-yr rotation. No-till fallow (NTF) is ideal for wind erosion control 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-yr field study was conducted to determine if accumulations of surface residue under long-term NTF might eventually be enough to substitute for TF in conserving seed-zone water over summer. Averaged over the 6 yr, residue rates of 1500, 6000, and 10 500 kg ha⁻¹ (1×, 4×, and 7× rates, respectively) on NTF produced incrementally greater seed-zone water but were not capable of conserving as much as TF. Total root zone (0–180 cm) over-summer water loss was greatest in the 1× NTF whereas there were no significant differences in the 4× and 7× NTF versus TF. Average precipitation storage efficiency ranged from 33% for 1× 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.