This technical report reviews what determines water consumption by irrigation, which accounts for 80% or more of total blue water consumption. By reviewing irrigation practices and highlighting technologies that allow for water consumption monitoring and control it provides a foundation for managing water in water-stressed environments.
The pattern of water use in many countries is unsustainable: deterioration of the water-dependent environment, including both surface and groundwater systems, is ongoing and often accelerating. Understanding the drivers of this trend depends on sound water accounting, which is identifying the sources and uses of water at the appropriate scale. These accounts must distinguish clearly between consumptive and non-consumptive uses of water. Consumptive uses involve evaporation and/or transpiration of liquid water into water vapour in the atmosphere. Non-consumptive uses (domestic, hydropower and cooling for thermal generating plants) return the vast majority of water withdrawals back to the system.
Irrigated agriculture, by contrast, aims to maximise consumption of water, because transpiration is a primary determinant of yield. As farmers pursue this objective, typically through the adoption of “hi tech” irrigation technologies such as drip, local consumption increases at the expense of return flows, reducing aquifer recharge and downstream flows, which may already be productively utilised or support ecosystems. Quantifying withdrawals, consumption and return flows is thus fundamental to designing water governance programs that will result in sustainable and improved water use. Consumption of water, by transforming it from liquid to vapour, is globally by far the largest reducer of the freshwater resource.
Historically, consumption has been difficult to estimate, depending either on formulae embodying assumptions regarding climate, plant health, nutrient status, soil characteristics, etc., or on point measurements of actual consumption projected over large, heterogeneous areas. Since the mid-1990s, remotely measured data from satellites (and most recently from small drones) have offered the possibility to estimate water consumption over landscapes including irrigated areas, rainfed agriculture and natural vegetation, preferably ground-truthed by point data, and also capable of interpretation as productivity indices – ”crop per drop”.
These new technologies for measuring water consumption fundamentally enhance the basis for improved water governance. Internationally, an objective, uniformly derived set of data provides the basis for negotiation and the identification of potential common interests. Locally, the technologies can confirm that water rights are being observed (or at least can identify suspiciously high consumption rates); and at field scale, where the focus is less on water consumption and more on deriving maximum productive benefit from allocated water, when system management to maximise crop production can be improved.
