5.2: Irrigation

The amount or seasonal distribution of precipitation is inadequate for reliable crop production in many locations around the world, and people in those locations must either migrate or import food or irrigate. Irrigation is the human process of delivering water to the soil, typically to promote plant growth.Irrigation is vital to human societies because irrigated land represents ~20% of cropland globally and accounts for ~40% of total crop production [11].Worldwide each year, we withdraw approximately 2,700 km 3 of water from surface and ground water sources for irrigation, and irrigation accounts for about 70% of all freshwater withdrawals [2]. To illustrate the relative importance of irrigation versus precipitation in our food production systems, the concepts of “blue water” and “green water” have been developed[12]. Blue water in this context is the water withdrawn from surface and groundwater sources and used for irrigation. Green water in this context is the water which has fallen as precipitation on agricultural land and has infiltrated the soil.Cropproduction in parts of Asia, the Middle East, and the western US relies heavily on blue water, while much of Africa, Europe, and South America rely primarily on green water.

Fig. 5-6. Regional variation in sources of water used in crop production around the world. Reproduced from Molden (2007).

Not all of the water withdrawn from surface and groundwater for irrigation is used by crops, as some is lost from the distribution systems,and some is lost to surface runoff or as deep drainage below the root zone. Of the 2,700 km³ withdrawn annually for irrigation, from 1,200 to 1,550 km_³ are used by crops, i.e.,roughly 50% [2, 12].Much effort has been devoted to increasing irrigation efficiency, and substantial progress has been made. The oldest approaches are surface irrigation methods, such as flood or furrow irrigation, which can result in 15-50% losses to surface runoff and deep drainage. More advanced sprinkler irrigation approaches, such as center pivot systems, can reduce those losses to <15%. And, drip irrigation methods can reduce losses to <10% (Fig. 5-7). However, increases in irrigation efficiency may not reduce the total water use for irrigation nor increase the water availability for other uses. Rather improved efficiencies sometimes promote increases in irrigated acreage, and the reduced water losses at the field scale may reduce the inputs to surface and groundwater resources at the watershed scale. These effects are surprising to many people and have been referred to as the “paradox of irrigation efficiency” [13].

Figure 5-7.Estimate irrigation efficiency for surface, sprinkler, and drip irrigation systems, and the water inflows and outflows at a watershed scale. Reproduced from Grafton et al. (2018).