Finding problems in the field.

From this to this, in three days.
What happened?

Is there a pattern? Is it spotty or all over? You say it's spotty, but do you see pattern in the spots?  When you see salt damage or leaf blight (often the causes are the same, one a salt accumulation, the other a fungus hitting salt/water stressed trees), you go straight to the irrigation.  Timing?  Amount?  Distribution?  If other trees are doing fine, then the amounts and times are right.  So it's the distribution?  Emitter plugging? Emitter alignment? Animal damage? 

Or is it root damage?  Are there roots?  Are there roots in the wetted zone?  What color are the roots?  It's getting cold, but there should still be some white/light colored roots.  You got a resistant rootstock, but it can still get root rot.  Maybe a shot of phosphite?  Or Orondis?

I don't like fungicides in general, because they are such a short fix, but sometimes they can help through a tough time.  I don't think it warrants Orondis at this time, though.  Not yet, until you see what is causing the problem.  Check the irrigation system and the roots first.

And this all happened on a weekend when there was wind and heat. Attention to the irrigation must be paid

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Recently I was asked why an irrigation schedule could be projected for almond and citrus in the Central Valley (Almonds:) and why the same couldn't be done for the main avocado growing areas. Here was my response:

Generating a generic irrigation schedule for avocados along the coast is very difficult and if done would be terribly misleading. Scheduling gets really hairy along the coast where avocados are grown. As you get further from the coast the water demand (ETo) increases in many months, typically increasing in the summer. This can be most pronounced in the late winter/spring when the fog along the coast really causes a contrast between coastal and inland conditions. May in Ventura, the sun comes out for about 2 hours and in Fillmore 20 miles inland it may be 90 F at 4 PM. The fog is a major determinant for irrigation demand and it varies daily, monthly and year to year from Monterey to San Diego. So fog can throw off an irrigation schedule.

The next variable to area-wide scheduling is the topography where avocados are grown, usually slopes to improve air and water drainage. Depending on the aspect and slope position, the ETo can vary tremendously depending on the sky conditions and what those conditions are depending on the time of day (such as foggy in the morning and clear in the afternoon). So west and south facing will always be higher than north and east. The top of the slope that intercepts more wind than the bottom and will have higher ETo than the bottom of the slope. And if the trees intercept more evaporative conditions midday when the sun comes out, it will be much higher than the east side in the morning when fog is dripping off the trees (zero evaporative demand). Then as you go south from Monterey to San Diego the ETo goes up, just because of latitude and sun interception. These conditions are very different from Fresno where ETo in July is 0.6 inches per day and is the same until Sept, the sky is clear most days and trees are grown on fairly flat ground.

Now throw in rainfall. Almonds are deciduous and only count on the value of rainfall as that which is stored in the rooting zone going into spring when leaves are come out. Avocados rely on winter rain for transpiration and salt leaching. In a good year a significant portion of the total yearly ETcrop can be subtracted from the irrigation demand. In a low/no rainfall year that all needs to be made up by supplemental irrigation.

An almond grower in the Valley might be able to go onto a calendar, set the clock if they have water on demand and walk away. That's never going to happen in a coastal avocado orchard. Depending on where the avocado is grown and the ETo at that site, applied water might vary from 1.5 ac-ft per acre to 3.5. This will depend on rainfall (when and how much), water quality (which determines leaching requirement) and the system delivery (system efficiency). This system issue can be further complicated by whether the delivery is on-demand or whether a certain amount will be delivered at a certain date for a certain length of time - 24 hours or 48. This makes it difficult for the grower to put on exactly what ETo and other issues the trees would demand. In this case, the delivery system determines the schedule.

So this is why there's no chart showing ET demand for coastal avocados where the bulk are grown in California.

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https://ascelibrary.org/doi/full/10.1061/(ASCE)IR.1943-4774.0001632

Evaluation of Forecast Reference Evapotranspiration for Different Microclimate Regions

in California to Enable Prospective Irrigation Scheduling

Ghaieth Ben Hamouda, Ph.D.; Daniele Zaccaria, Ph.D.; Khaled Bali, Ph.D.; Richard L. Snyder, Ph.D.

Abstract

In California, daily crop evapotranspirationis commonly estimated using near-real-time, standardized reference evapotranspiration (ET)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.

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Spatial Overview - How it Works

The California Irrigation Management Information System (CIMIS) currently manages over 145 active weather stations throughout the state. Archived data is also available for 85 additional stations that have been disconnected from the network for various reasons. Most of the CIMIS stations produce estimates of reference evapotranspiration (ETo) for the station location and their immediate surroundings, often in agricultural areas. Because of California's diverse landmass and climate, many locations within the state lack a representative CIMIS station. Some counties, for example, do not have a CIMIS station and others have only one or two stations. As a result, there are significant spatial ETo data gaps, especially in urban areas. In an attempt to mitigate this problem, CIMIS initiated a study to investigate the possibility of coupling remotely sensed satellite data with point measurements from the CIMIS weather stations to generate spatially distributed ETo values (ETo maps).

A contract was awarded to the University of California Davis (UCD) remote sensing group, lead by Professor Susan Ustin, to conduct the study. The Department of Water Resources (DWR) formed an advisory committee comprised of individuals with expertise in remote sensing, GIS, modeling, and water management from DWR and UCD. The committee met, on an as needed basis to discuss new developments and plan future actions. After thorough research, the team decided to use combinations of data derived from satellites and interpolated from CIMIS station measurements to estimate ETo at a 2 kilometer (km) spatial resolution. The resulting product has been vigorously tested and has demonstrated a degree of accuracy that is acceptable for most irrigation applications. The CIMIS program will continue to evolve and expand to meet the future irrigation information needs of California. For a brief description of the methodology used to generate the ETo maps, see the Spatial Model discussion below.

Spatial Model

Daily reference evapotranspiration (ETo) at a 2 km spatial resolution are calculated statewide using the American Society of Civil Engineers version of the Penman-Monteith equation (ASCE-PM). Required input parameters for the ASCE-PM ETo equation are solar radiation, air temperature, relative humidity, and wind speed at two meters height. These parameters are estimated for each 2 km pixel using various methods.

Daily solar radiation is generated from the visible band of the National Oceanic and Atmospheric Administration's (NOAA) Geostationary Operational Environmental Satellite (GOES) using the Heliosat-II model. This model is designed to convert images acquired by the Meteosat satellite into maps of global (direct plus diffused) irradiation received at ground level. The model has also been used with other geostationary satellites such as the GOES. For details on the Heliosat-II model and its accuracy, please refer to the HelioClim web page.

Interpolation

Air temperature, relative humidity, and wind speed values at each pixel were obtained by interpolating point measurements from CIMIS stations. Originally two interpolation methods, Spline and DayMet, were selected based on accuracy of results, code availability, and computational efficiency. Spline – the method currently used – is an interpolation method that fits a surface through or near known points using a function with continuous derivatives. Two- or three-dimensional Spline is used based on which weather parameter is to be interpolated.

The accuracy of both methods has been tested using cross-validation analysis, but DayMet is no longer used. DayMet is an interpolation method that was developed at the University of Montana to generate daily surfaces of temperature, precipitation, humidity, and radiation over large regions of complex terrain. It determines the weights associated with a given weather station for each point where weather parameters are to be determined depending on the distance and density of the stations.

The accuracy of ETo values estimated from these methods depends on many factors. For example, solar radiation remotely sensed through GOES is significantly affected by such factors as cloudiness and snow cover. Therefore, mountainous areas with snow cover and coastal areas with cloud and fog are more susceptible to errors. Also, interpolation accuracy is affected by the density of the weather stations and geographic features of the region. CIMIS stations are purposely placed in irrigated, open, flat areas – usually valleys – to provide the best reference data for adjacent farmlands and other irrigated areas. As a result, interpolation in valleys between CIMIS stations may not provide accurate data for mountainous terrain. Despite these potential problems, however, we believe the ETo estimates provided will be superior to only using data from a distant weather station with a different microclimate. For CIMIS station Siting criteria click the “Siting” tab.

Further Details

For detailed descriptions of the methodology used to map daily ETo, refer to the RESOURCES navigation button https://cimis.water.ca.gov/Resources.aspx

If you would like to get ETo data created using the methods described here, you may do so by logging into your account, clicking on the SPATIAL navigation bar, then on the Spatial Report tab. If you do not have a CIMIS account, you can create one by registering with us. Please note that CIMIS data is provided free of charge and registration is required for statistical purposes only. https://cimis.water.ca.gov/Default.aspx