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In California, daily crop evapotranspiration (ETc) is commonly estimated using near-real-time, standardized reference evapotranspiration (ETo) calculated from ground-based meteorological data and crop coefficient (Kc) values that adjust for the difference between ETo and ETc. However, using ETo calculated from forecast rather than near-real-time meteorological data provides more timely information to growers, farm managers, urban water users, irrigation practitioners, and water purveyors to anticipate water demand for the upcoming days. This is particularly important for water allocation and delivery planning and for irrigation scheduling decisions. Forecast ETo is also relevant for scientific research for developing methods and tools enabling more resource-efficient water use for agricultural production and urban landscapes. Verifying the reliability of ETo forecast models for various climatic conditions is crucial for promoting the broad adoption and use of ETo predictions for weather-based adaptive water management and prospective irrigation scheduling. The US National Weather Service (NWS) has released a product called forecast reference evapotranspiration (FRET) that provides ETo forecasts at a 2.5-km grid resolution for the entire continental US. In this study, comparisons were made between ETo forecasts from FRET and observed ETo values from the California Irrigation Management Information System (CIMIS) for a total of 78 days during the peak irrigation water demand period of 2019, which corresponded to the midseason period for major crops. FRET ETo forecasts and CIMIS ETo observations (calculated from measured weather parameters) were compared for 15 CIMIS station locations to represent a variety of weather conditions of some major agricultural production and urban areas in California. Air temperature, dew point temperature, wind speed, and solar radiation data were collected from NWS and CIMIS and analyzed to assess the accuracy of predicted weather variables and FRET ETo forecasts. The FRET data consisted of 1, 3, 5, and 7-day forecasts of weather parameters and calculated ETo values; these were compared with the corresponding observed daily weather and ETo data from CIMIS in order to statistically evaluate FRET performance. The comparison among forecast and measured weather variables revealed a good match for maximum air temperature (R2 between 0.98 and 1.00), minimum air temperature (R2>0.91), dew point temperature (R2 between 0.7 and 1.00), and wind speed (R2 between 0.66 and 0.99); less accurate results were obtained for solar radiation (R2 between 0.21 and 0.87). The analysis also showed a good correlation between FRET ETo forecast data and observed CIMIS ETo data (R2 between 0.93 and 1.00 and root mean square error (RMSE) was less than 1 mm·day−1) for the majority of the selected stations, with some differences that were likely due to the climatic conditions of specific locations. This suggests that FRET ETo forecasts provide reliable information for predicting near-future water demand and improving irrigation water management in California. The comparisons showed a good match for most of the 15 CIMIS stations, and the FRET model may potentially provide accurate ETo forecasts in similar semiarid and subhumid areas elsewhere. Weekly irrigation schedule examples are provided in the Appendix to illustrate the use of FRET ETo forecasts in combination with site-specific information on soil hydraulic properties and irrigation system performance for prospective irrigation scheduling of some specialty crops and urban landscapes. The information provided in this article can help improve the water management efficiency of high-frequency agricultural irrigation systems and urban sprinkler systems in California and other locations with similar climates. It also offers a less expensive method to obtain ETo data in countries that do not have ETo station networks, because it offers an alternative to installing and maintaining ETo stations in agricultural and urban areas where it is difficult to find acceptable surfaces for developing reliable ETo information. [ABSTRACT FROM AUTHOR]

A deterministic-stochastic combined model named HydroGEN was developed, as described in a companion paper (Part I: Model development), to enable the simulation of demanded daily volumes and hourly flow rates during peak periods in pressurized irrigation delivery networks. The model was applied to a pilot large-scale irrigation system located in southern Italy for calibration and for testing its reliability in analyzing the operation of large-scale pressurized delivery systems through the simulated flow configurations. Daily input data on rainfall, temperature, solar radiation, wind speed and relative humidity were gathered from a meteorological station located within the study area, whereas information on local irrigation management practices were collected through interviews with farmers and from extension specialists. The model was tested at different management levels, from district to sector and hydrants. The model testing was supported by the use of high-resolution remote-sensing imagery acquired on a single overpass date in 2006 and then classified and recoded following a ground-truthing campaign conducted during the same year. Simulations were performed to identify the 10-day peak-demand period and to generate the hydrographs of daily volumes and of hourly flow rates. Results from the different simulations were compared with historical datasets of irrigation volumes and discharges recorded during the 2008 and 2009 seasons at the upstream end of the irrigation network under study, at a sector level during the 2007 season and at selected delivery hydrants during the 2005 season. Some discrepancies between simulated and recorded data were noted that can be related to small errors in estimating crop and soil parameters, application efficiency at field level, as well as to large variability in irrigation management practices followed by local farmers. Overall, the results from testing showed that the model is capable of forecasting with good accuracy the timing of peak-demand periods, the irrigation volumes demanded during the season, as well as the hydrographs of daily volumes and hourly flow rates withdrawn by farmers during these peak-demand periods, especially when it is applied to large multi-cropped command areas. [ABSTRACT FROM AUTHOR]

• A user-friendly simulator for estimating tree PAR interception under different latitudes, canopy dimensions, leaf densities, tree spacing, and row orientations. • Predicting the solar radiation interception by mechanically pruned citrus trees. • PAR interception by citrus trees based on the canopy geometry and its shading patterns. • To inform irrigation planning and management decisions for different site-specific conditions. Diurnal and seasonal patterns of photosynthetically active radiation (PAR) intercepted by tree canopies in production orchards have an important role for eco-physiological processes that determine tree water use and productivity. Under well-watered conditions, PAR intercepted by trees is a reliable predictor of crop consumptive water use, which is regulated by tree spacing, tree row orientation, and by the tree canopy dimensions and geometry. Despite its advantages, only rare attempts have been made by the scientific and fruit production communities to develop canopy PAR interception simulators that rely on analytical principles to inform irrigation planning and management decisions. In this article, a novel model is presented that simulates PAR interception by citrus trees based on the canopy geometry and its shading patterns. The article also illustrates preliminary field testing and validation of the model with data collected in commercial citrus production orchards located in the San Joaquin Valley of California. The testing and validation could enable citrus growers and orchard managers to predict the net radiation for mature, micro-irrigated Navel orange and Page mandarin orchards grown with north–south and east–west tree row orientations and different tree densities. Results from the simulations and field validation showed that the canopy PAR light interception contributed 58.8 (± 0.02) and 71.1 (± 1.60) percent to the overall net radiation for the Navel orange orchards during the periods around summer solstice and autumnal equinox, respectively, whereas for the Page mandarin orchards, PAR light interception contributed 63.0 (± 5.60) and 85.7 (± 18.87) percent to the overall net radiation during the same time periods. Good correlations were found between daily values of PAR and net radiation for these citrus production orchards grown with the two alternative row orientations, which suggest that the proposed model could be used to inform irrigation planning and management decisions for different site-specific citrus orchard features and growing conditions. [ABSTRACT FROM AUTHOR]