Field-Level Irrigation

Abstract

Field irrigation is the largest consumer of freshwater in the world covering 63 million hectares in the 1900s to 300 million hectares in the early 2000s to provide a multitude of benefits and ecosystem services to people around the globe, such as consistent food supply, higher crop productivity, and shared resource collectivism. Field irrigation intensifies land use mostly in the arid and the semiarid regions where precipitation cannot fully satisfy human and crop water demands. Climate change impacts the distribution and the timing of water availability into humid regions as well through increases in drought frequency and intensity, further augmenting the demand for irrigation. However, designing and operating irrigation infrastructure and scheduling practices for an agricultural region requires sound contemporary and historical knowledge of the local circumstances vis-à-vis humans, crops, soils, hydrology, and climate. Sub-Saharan Africa stands as a large-scale narrative of poorly performing field irrigation against decades of investments due to designs exclusive to the socioeconomic ecosystems. Optimal water allocation in the water–energy–environment–food nexus to achieve the greatest social and economic benefit for the region invariably a task of continuous cocreation between many actors. Therefore, field irrigation remains a challenging project and most of the agricultural water use worldwide—both from groundwater and surface water—remains suboptimal in terms of design, water allocation, and monitoring for farmers, communities, and regulators. Many diversions of surface water for irrigation in both economically developed and developing countries are small-scale temporary infrastructures in and outside official plans and permits, which altogether results in severe aquifer depletion worldwide with negative impacts on food safety, economy, environment, and society. Traditional surface flooding is the dominant mode of irrigation globally and mostly applied on new agricultural fields, whereas water-saving irrigation methods are practiced on fewer and older fields. Water-saving technologies involve either scheduling of regulated deficit irrigation or local water storage to optimize crop water supply, which may be combined with drip irrigation, biodegradable soil amendments to retain soil water, and plastic mulches to minimize evaporation, whereas the use of partial root zone drying and biochar mostly remain at the experimental stage. Global analyses over the late 20th and early 21st century find no water saving by water-saving technologies at field scale because increased return flow from newly irrigated fields surpasses the reduced soil evaporation from old, irrigated fields, whereas regionally, return flow to fresh aquifers is a benefit rather than a loss, which results in some water savings. At the same time, increased crop transpiration exceeds regional water savings, which explains the paradox between the wide application of water-saving technologies and more severe regional water shortage. With nonscientific decisions on when and where to irrigate practiced by most farmers worldwide, scheduling remains the top priority task in field irrigation, as both too little and too much water leads to yield decreases and loss of nutrients to the environment. Where water is abundant, scheduling aims to keep crop transpiration and yield at a maximum with minimum use of irrigation water. In dryer areas, this luxury can rarely be sustained unless the irrigation area and therefore production is reduced. Instead, regulated deficit irrigation may be practiced on drought-tolerant crops and cultivars. Regulated deficit irrigation seeks to limit crop transpiration to a fraction of the maximum during less drought-sensitive growth stages. In this way, crop water use efficiency increases, and yield per m3 of water rather than m2 of land is maximized. Remote sensing of soil and crops through satellite and aerial multispectral and thermal products have the potential to enhance irrigation scheduling by precisely quantifying and distinguishing crop transpiration (beneficial water consumption) from soil evaporation (nonbeneficial water loss) in space and time and to facilitate regulated deficit irrigation and other water-saving measures. However, irrigation management significantly falls behind in adapting state of the art information and communication technologies. With a global rise in frequency and duration of droughts, the lack of irrigation infrastructure in humid regions and of water availability in semiarid regions induces enormous losses in agricultural production and social well-being and unveils an urgent need for a macrolevel drought governance approach in order to strengthen multisectoral water management and mitigate climate change damage to human and natural assets.