A Management System for the Conjunctive Use of Rainfall and Limited Irrigation of Graded Furrows

A new system was developed and tested for efficient use of irrigation and rainfall for furrow irrigation of grain sorghum (Sorghum bicolorL. Moench). The design uses a limited water supply to irrigate a larger area than could be fully irrigated and thus reduces the area in dryland sorghum where a farmer produces both. A 600‐m long field was divided into three water management sections. The upper half of the field was managed as fully irrigated. The next one‐fourth was a “tail‐water‐runoff” section that utilized furrow runoff from the fully irrigated section. The lower one‐fourth was designated a “dryland” section, retaining and utilizing any runoff generated either by irrigation or rainfall from the wetter, fully irrigated, or tailwater‐runoff sections. Furrow dams were placed about every 3 to 4 m throughout the length of the field after planting. However, these were washed out by the irrigation water over the distance the water advanced down the furrows. The remaining furrow dams on the lower part of the field stopped rainfall runoff and prevented any water from leaving the field. As the profile soil water was depleted on the drier lower field sections, shrinkage cracks developed that helped to absorb rainfall and prevent rainfall runoff. Seeding rates were reduced down the field from 6 kg/ha on the fully irrigated section to 3 kg/ha on the tailwater section, to 1.5 kg/ha on the dryland section. Nitrogen applications were also reduced on the tailwater and dryland sections to correspond with anticipated yield levels. The three irrigation treatments were 5‐cm water applied to every furrow; 3.75‐cm water applied to alternate furrows; and 2.5‐cm water applied to every third furrow between crop rows in a two‐row planted, and one row left out, skip‐row system. These treatments were compared with conventional furrow irrigation without dams involving tail‐water runoff. Five irrigations were applied at 14‐day intervals, regardless of seasonal rainfall. The extent that the field was irrigated depended on the rainfall. The fixed 14‐day irrigation schedule and 12‐hour sets reduced management requirements and facilitated the potential for automation of graded furrow water application, without the farmer having to be concerned about tailwater runoff from the end of the field. For 1979, a year with near average rainfall, the system successfully prevented runoff from both irrigation and rainfall.