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Water has been labelled �blue gold�, and �blue gold� is destined to be the critical issue of the 21st Century. Globally, irrigation is responsible for 80% of the world-wide spending of �blue gold�.
Next, we outline future research needs. This includes a requirement to infer better the matric potential at the soil�root boundary and its control on plant transpiration. We suggest that the role of reverse flow and specification of the root resistance also needs more researching. Further, linking the functioning of water uptake with the form of the root system will not be achievable until we know more about root resistance. Canopy area and architecture are critical for controlling transpiration, yet they are tiresome to measure. Improved measurement techniques, preferably remote, would enhance our ability to predict crop water-use and to assess more accurately the need for irrigation [Wesseling, J.G., Feddes, R.A., 2006. Assessing crop water productivity from field to regional scale. Agric. Water Manage. 86, 30�39].
Finally, we demonstrate how our scientific knowledge can be used to develop sustainable irrigation practices.
Development of sustainable irrigation practices will require that we understand better the biophysical processes of root-water uptake in soil, and transpiration from plant canopies.
Our review paper is divided into four parts: models, measurements, knowledge gaps and policy.
First, we present a retrospective on what has been done with root-water uptake models since the pioneering scheme of Wilford R. Gardner in 1960. His solution for water movement to a plant root was analytical. Since then, nearly all the models calculate water flow using numerical solutions of the Richards� equation. These schema include a water-uptake term specifically for the distributed uptake of water from soil by the root system. These models fall into two groups based on how the uptake term is handled. The most common formulations, called Type I, have evolved from the work of Gardner [Gardner, W.R., 1960. Dynamic aspects of water availability to plants. Soil Sci. 89, 63�73] and describe the microscale physics of water flow from the soil to, and through, the plant roots. The second form, Type II, comprises macroscopic, empirical functions that describe uptake based on responses to water potential. We discuss the merits and potential of these schemes. Yet, models are data hungry. Effective modeling requires apposite parameterisation to be effective. This can require substantive empiricism.
Second, we present new data on the functioning of root-water uptake and transpiration by kiwifruit vines. We describe new observations in the root zone, obtained using arrays of time domain reflectometry (TDR) sensors. As well, we present results obtained with new methods of sap-flow measurement inside the kiwifruit vine's roots. These reveal the uptake dynamics during partial root zone drying (PRD), a technique oft-touted to reduce irrigation volumes.
