LAUSR

Agricultural production in California faces weather and hydrologic extremes of both floods and drought, both of which are detrimental to crop yield. Californian producers of high-valued perennial crops, thus, increasingly rely on groundwater extraction to sustain and even expand production (Matios and Burney 2015). The extreme consecutive droughts that have recently occurred significantly impacted California’s ground water resources. At current levels of groundwater pumping, the long-term sustainability of groundwater resources is in jeopardy (Richey et al. 2015). As a result, the state and its agricultural community have recognized the need to develop long-term water management strategies. Operatively, California enacted the Sustainable Groundwater Management Act in 2014, which mandates development of management plans for critical basins within California. Achieving such long-term water use sustainability in an economically viable way will almost certainly require more efficient irrigation management to successfully address future water shortages. Currently, the irrigation management decisions for many California crops are based on a combination of in situ observations of plant available water via root zone soil moisture measurements, using crop coefficients with reference evapotranspiration (ETo) from a nearby weather station, or estimates of actual evapotranspiration (ETa) via micrometeorological techniques that are assumed to represent crop water status of whole fields. For obtaining spatially distributed information on crop water use and condition, some have adopted a simple remote sensing-based estimate of normalized difference vegetation index (NDVI) for quantifying fractional canopy cover in conjunction with the Food and Agriculture Organization of the United Nations (FAO) crop model with crop coefficients that have been tuned for specific crops (Allen et al. 1998). None of these methods can provide spatially distributed information of actual crop ET and plant water status, and are not sufficiently robust for strongly clumped and highly structured canopies such as vineyards and orchards. Moreover, none are capable of separating row crop water use from the interrow soil and/or cover crop water use, and the crop coefficients are not well defined for stressed conditions, particularly for perennial crops (e.g., Ting et al. 2016). Producers of wine grapes—a California crop valued at nearly $6 billion annually—have actively sought tools to better monitor crop water status and manage water use. Along those lines, E. & J. Gallo Winery contacted scientists with the US. Department of Agriculture-Agricultural Research Service (USDA-ARS) Hydrology and Remote Sensing Laboratory (HRSL) seeking advice on robust yet practical methods for applying earth observations to guide irrigation decisions. It is well established that in wine grape production, there are vine and berry development stages where the timing and amount of irrigation are critical for optimal quality and yield. Generally, these include (1) when to initiate irrigation in the spring; (2) the timing and amount of water to apply early in the growing season that balances vine growth and berry development; and (3) in later stages during berry ripening when there are carefully timed periods of mild stress imposed (deficit irrigation) to improve berry quality for wine production. Spatial information on variations in vine water status across vineyard blocks is necessary to ensure best quality yield through judicious application of water only where it is needed. E. & J. Gallo Winery researchers realized that accurate maps of ETa at daily to weekly increments and at subfield spatial resolutions could significantly reduce water use and Communicated by Melania Ruiz.