Predicting seed-zone water content for summer fallow in the Inland Pacific Northwest, USA

The Horse Heaven Hills in south-central Washington contains the world's driest rainfed wheat (Triticum aestivum L.) region. The climate is Mediterranean with average annual precipitation as low as 150mm. The cropping system is winter wheat-summer fallow. Tillage is used in the spring of the 13-month fallow to establish a dry soil mulch to help retain seed-zone water to establish winter wheat planted deep into fallow in late August. However, the Horse Heaven Hills is often so dry that even tillage-based summer fallow (TF) cannot retain adequate seed-zone water, and farmers must then wait until the onset of rains in mid October or later for planting. In such dry years, farmers would be better off practicing no-till fallow (NTF) to protect the soil from wind erosion; but no predictive tools are available to assist in these decisions. The objectives of our study were (1) to predict seed-zone water contents and water potentials in late August or early September based on soil water content measured in early April and (2) to compare seed-zone water in TF and NTF. Experiments were conducted for 5 years at each of two sites. Soil water content was measured in both early April and late August. Soil properties and residue loads were characterized to calibrate the Simultaneous Heat and Water model (SHAW). Seed-zone water was simulated in late August based on measured soil water contents in early April and measured temperature and precipitation from April to August. The SHAW model correctly predicted seed-zone water content 80% of the time. The amount and timing of rainfall occurring in April, May, and June was the most important factor controlling the seed-zone water content in late August, suggesting that farmers should delay their decision on whether to practice TF or NTF until late in the spring.