Deficit irrigation: theory and practice

Deficit irrigation (DI) is an optimization strategy whereby net returns are maximized by reducing the amount of irrigation water; crops are deliberately allowed to sustain some degree of water deficit and yield reduction. This technique is not usually adopted as a practical alternative to full irrigation by either academics or practitioners. The major obstacles are that DI involves the use of precision irrigation and some risks associated with the uncertainty of the knowledge required. Furthermore, there is a certain amount of confusion regarding the DI concept. A review of about 100 papers dealing with DI recently published in major international journals has shown that only a few papers use the concept of DI in its complete sense (e.g., both the agronomic and economic aspects). A number of papers only deal with the physiological and agronomical aspects of DI or concern techniques such as Regulated Deficit Irrigation (RDI) and Partial Root Drying (PRD). The chapter includes three main parts: i) a theoretical review of the principal water management strategies under deficit conditions (e.g., conventional DI, RDI, PRD, etc.); ii) a review of the most recent case studies cited in the literature on different aspects of DI application at both farm and irrigation district level; and iii) a description of recent experimental research conducted by the authors in Sicily (Italy) that integrates agronomic, engineering and economic aspects of DI at farm level. Most of the literature on DI reviewed here show, in general, quite positive effects from DI application, mostly evidenced when the economics of DI is included in the research approach and when the application concerns planning purposes over large areas.The applications present a wide survey of the agronomic effects of DI. Generally, total fresh mass and total production is reduced under DI, whereas the effects on dry matter and product quality are positive, mainly in crops for which excessive soil water availability can cause significant reductions in fruit size, colour or composition (grapes, tomatoes, mangos, etc.). Therefore, the economic convenience of DI depends on the type of crop. As regards the experimental trial using DI on a lettuce crop in Sicily, the effect of four different irrigation levels (50, 75, 100 and 125% of the evapotranspiration rate) on the marketable yield, yield components and economic return of the crop under drip irrigation was evaluated during 2005 and 2006. The highest mean marketable yield (MTY, t ha-1) of lettuce (55.3 t ha-1 in 2005 and 51.9 t ha-1 in 2006) was recorded in plots which received 100% of ET0-PM (reference evapotranspiration by the Penman-Monteith method) applied water. In the land-limiting case, the estimated optimal economic levels were quite similar to the optimal agronomic levels (100-130% ET0-PM). In the waterlimiting case DI ranges, at least as profitable as full irrigation, were 15-44% ET0-PM and 74-94% ET0-PM in 2005 and 2006, respectively.