The experiences gathered during the past 30 years support the operational use of irrigation scheduling based on frequent multi-spectral image data. Currently, the operational use of dense time series of multispectral imagery at high spatial resolution makes monitoring of crop biophysical parameters feasible, capturing crop water use across the growing season, with suitable temporal and spatial resolutions. These achievements, and the availability of accurate forecasting of meteorological data, allow for precise predictions of crop water requirements with unprecedented spatial resolution. This information is greatly appreciated by the end users, i.e., professional farmers or decision-makers, and can be provided in an easy-to-use manner and in near-real-time by using the improvements achieved in web-GIS methodologies (Geographic Information Systems based on web technologies). This paper reviews the most operational and explored methods based on optical remote sensing for the assessment of crop water requirements, identifying strengths and weaknesses and proposing alternatives to advance towards full operational application of this methodology. In addition, we provide a general overview of the tools, which facilitates co-creation and collaboration with stakeholders, paying special attention to these approaches based on web-GIS tools.

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Mapping irrigated surfaces and the crops growing on these surfaces by using Remote Sensing is a well known first relevant step to contribute to water governance at different scales ranging from farm, irrigation scheme and, by scaling-up, to the whole river basin. These maps provide a first estimation of the spatially distributed water flows about evapotranspiration and irrigation water requirements based on the cropping agronomic knowledge. During the last 20 years, annual maps of crops and irrigated surfaces elaborated by using time series of multispectral satellite imagery have been in the basis of a successful water management of a big groundwater body placed in the Southern Spain. Threatened by over-exploitation, this aquifer extends over around 10,000 km 2 of land surface, in the middle Júcar river basin, supporting around 100,000 ha of irrigated crops, and providing drinking water for 150,000 inhabitants, with competing uses from downstream users. This paper describes the main learned lessons. In addition, the paper tackles a necessary further step in the context of the current requirements for water governance of this aquifer: the direct remote sensing-based water accounting, by quantifying agricultural water flows (e.g. rainfall, irrigation, evapotranspiration, drainage and recharge, at practical spatial and temporal scales for water governance purposes). This RS-based WA approach relies on dense time series of multispectral imagery acquired by the multisensor constellation formed by Landsat 8 and Sentinel-2, jointly with meteorological data. By this, we discuss the technical and non-technical feasibility to rely monitoring water abstraction on this RS-based WA approach, providing the basis for a hybrid system.