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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The results show a high probability of not meeting the groundwater quality standards when 25 deriving a policy from just a deterministic analysis. To increase the reliability several 26 realizations can be optimized at the same time. By using a mixed-integer stochastic 27 formulation, the desired reliability level of the strategy can be fixed in advance. The approach 28 allows deriving the trade-offs between the reliability of meeting the standard and the net 29 benefits from agricultural production. In a risk-averse decision-making, not only the reliability 30 of meeting the standards counts, but also the probability distribution of the maximum pollutant 31 concentrations.A sensitivity analysis was carried out to assess the influence of the variance of 32 the hydraulic conductivity fields on the strategies.The results have shown that larger the 33 variance, greater the range of maximum nitrate concentrations and the worst-case (or maximum 34 value) that could be reached for the same level of reliability. 35

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Abstract:The main goal of this research was to estimate the actual evapotranspiration (ET c ) of a drip-irrigated apple orchard located in the semi-arid region of Talca Valley (Chile) using a remote sensing-based soil water balance model. The methodology to estimate ET c is a modified version of the Food and Agriculture Organization of the United Nations (FAO) dual crop coefficient approach, in which the basal crop coefficient (K cb ) was derived from the soil adjusted vegetation index (SAVI) calculated from satellite images and incorporated into a daily soil water balance in the root zone. A linear relationship between the K cb and SAVI was developed for the apple orchard K cb = 1.82¨SAVI´0.07 (R 2 = 0.95). The methodology was applied during two growing seasons (2010-2011 and 2012-2013), and ET c was evaluated using latent heat fluxes (LE) from an eddy covariance system. The results indicate that the remote sensing-based soil water balance estimated ET c reasonably well over two growing seasons. The root mean square error (RMSE) between the measured and simulated ET c values during 2010-2011 and 2012-2013 were, respectively, 0.78 and 0.74 mm¨day´1, which mean a relative error of 25%. The index of agreement (d) values were, respectively, 0.73 and 0.90. In addition, the weekly ET c showed better agreement. The proposed methodology could be considered as a useful tool for scheduling irrigation and driving the estimation of water requirements over large areas for apple orchards.

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