Optimization of Supplemental Irrigation
Optimal SI in rainfed areas is based on the following three criteria: (1) water is applied to a rainfed crop that would normally produce some yield without irrigation; (2) because rainfall is the principal source of water for rainfed crops, SI is applied only when rainfall fails to provide essential moisture for improved and stable production; and (3) the amount and timing of SI are scheduled not to provide moisture stress-free conditions throughout the growing season, but to ensure a minimum amount of water available during the critical stages of crop growth that would permit optimal instead of maximum yield (Oweis and Hachum 2012).
Deficit Supplemental Irrigation
Deficit irrigation is a strategy for optimizing production. Crops are deliberately allowed to sustain some degree of water deficit and yield reduction (English and Raja 1996). The adoption of deficit irrigation implies appropriate knowledge of crop water use and responses to water deficits, including the identification of critical crop growth periods, and of the economic impacts of yield reduction strategies. In a Mediterranean climate, rainwater productivity increased from 0.84 to 1.53 kg grain/m3 of irrigation water when only one-third of the full crop water requirement was applied (Figure 14.2). It further increased to 2.14 kg/m3 when two-thirds of the requirement was applied, compared to 1.06 kg/m3 at full irrigation. The results show greater water productivity at deficit than at full irrigation. Water productivity is a suitable indicator of the performance of irrigation management under deficit irrigation of cereals (Zhang and Oweis 1999), in analyzing the water saving in irrigation systems and management practices, and in comparing different irrigation systems.
There are several ways to manage deficit irrigation. The irrigator can reduce the irrigation depth, refilling only part of the root zone soil water capacity, or reduce the irrigation frequency by increasing the interval between successive irrigations. In surface irrigation, wetting furrows alternately or placing them farther apart is one way to implement deficit irrigation. However, not all crops
Water productivity of wheat under rainfed, deficit, and full SI conditions. (Adapted from Oweis, T. and Hachum, A. 2012. Supplemental irrigation, a highly efficient water-use practice. 2nd- edition. ICARDA, Aleppo, Syria. iv + 28 pp.) respond positively to deficit irrigation. This should be examined for local conditions and under different levels of water application and quality.