Posts Tagged: ETo
Predicting Water Demand Along the Coast Versus in the Valley
Recently I was asked why an irrigation schedule could be projected for almond and citrus in the Central Valley (Almonds:http://cekern.ucanr.edu/Irrigation_Management/Almond_Drip_-_Microsprinkler_-
_Flood_Weekly_ET/Citrus: http://cekern.ucanr.edu/Irrigation_Management/Citrus_ET_by_age/ ) 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.
A CIMIS (CA Irrigation Management Information System) DWR weather station for calculating crop water requirement.
CIMIS station
Scheduling a Landscape Specific Drip Irrigation System
Advice from the Help Desk of the Master Gardeners of Contra Costa
Client: I've recently bought to a 10 year old home in Walnut Creek. It has an established garden. A large part of the garden consists of azaleas, camellias, and rhododendrons. They are currently watered by hand. I like these plants, but I would also like to convert that part of the garden to drip irrigation to possibly be more efficient with my water and as my time is somewhat limited by other family activities. Could you please give me some guidance on how to set up a drip system for this part of my landscape.
MGCC Help Desk Response and Advice: We are writing to you to provide some information about the watering needs for your landscape, specifically your azaleas, camellias, and rhodedendrons. With the process described below and with appropriate modifications, it could apply to setting up drip irriagion to any part of your landscape.
We've tried unsuccessfully to reach you by phone several times to obtain some additional information that would affect the water requirements. Since we've been unable to connect with you by phone, we are writing to give you some preliminary general guidance about the water needs of your plants and suggestions for how you might want to set up a new drip system. If you have further questions after reviewing the information provided below, you are welcome to contact us again.
The amount of water needed in a garden landscape is influenced by many different factors such as soil type, sun and wind exposure, the plants' “thirstiness” level, plant density in the garden, plant maturity and weather conditions.
I understand that the primary plant varieties in your landscape for consideration are azaleas, camellias and rhododendrons. Each of these plant types are classified as “moderate” water need plants (see WUCOLS link at end). I have assumed for purposes of the water need calculations that the plants are well established in your landscape (i. e. , they have not been only recently planted). I have also assumed that you do not have ground covers or other dense plantings around the shrubs that would compete with the plants for water.
Since you live in Walnut Creek where clay soils are common, I have assumed that your primary soil type is clay. I have also assumed, however, that you have reasonably good drainage around the plants. Because azaleas, camellias and rhododendrons all grow best in areas outside of direct strong sunlight (i.e. , in filtered sun areas), I have assumed that those conditions exist in your garden area. I have also assumed that the plant are in a somewhat protected area in which they would have some protection from strong winds.
If my assumptions about the growing conditions for your plants are incorrect, it would affect the water needs for the plants. In that case you may want to contact us again, so we can discuss adjusting the calculations to reflect the actual growing conditions in your garden.
If you look at the Sonoma guide, you will find a formula at the bottom of page 3 that shows how to calculate the water needs for a garden. That is the formula that I used for calculating the water needs for your shrubs. Because I did not know the total size of the landscaped area that you will be watering with your drip system, I have calculated the water needs of only a single mature shrub of the types you are growing. Before giving you the results of my calculations, let me explain a few of the terms used in the formula.
Another important variable for calculating water needs is the “Water Use Classification” of the plants you are watering (referred to as the “crop coefficient” in the Sonoma County Guide). In the case of your azaleas, camellias and rhododendrons, the plants are considered “Moderate” water use plants so the “crop coefficient” to be used in the formula is someplace between 0.4 and 0.6. For purposes of my calculations, I have used the midpoint of this range (0.5).
Based on the ETo for Walnut Creek and the “crop coefficient” of 0.5 for your shrubs and using the assumptions for your growing conditions explained above, I have calculated that each of your shrubs will need about 12.85 gallons of water during July (or roughly 3 gallons per week). Because the ETo declines in cooler months, by September. the water need would drop to about 9 gallons per plant per month (or just a little more than 2 gallons per week) and by November, the water need would decline to 3. 2 gallons per month (less than a gallon per week).
One of the questions you asked when you contacted our office was whether it would be better to use 1 GPH drippers or 2 GPH drippers. If I am correct that your predominant soil type is clay but that you have reasonably good drainage, you could use either flow rate. (If drainage is slower than I have assumed, you may be better off with the 1 GPH rate to allow the emitted water to absorb more slowly into the ground without producing run off).
Based on these calculations, if you place three 1 GPH drippers around each of the shrubs, in July you would need to run the trip system for one hour per week. You could either apply all the water once a week or could spread it over two water days per week. If instead you use three 2 GPH drippers, you could run the system half as long. And if you use fewer than 3 drippers per plant, you can adjust the water time accordingly.
As temperatures drop in future months, you'll want to periodically reduce the length of time (and therefore the gallons) the system runs. Most gardeners usually make monthly changes. Most controllers have percentage settings so that , for example, if you consider (and set) July as 100%, then, again for example, come September the controller would be set to 80%.
These calculations only produce a “starting point” for setting up your system. In particularly hot spells, you may need to increase run times and in unusually cool periods, you can reduce the run time. You should also pay attention to how the plants are performing to “tweak” the controller to your specific micro climate. You might also want to invest in a water probe so that you can periodically check to see how damp the soil is in the root area of your shrubs. Based on what you are observing about the health of the plants and the moisture level in the root zone, you can adjust the watering schedule accordingly.
I hope that you find this information to be helpful. You are welcome to contact us again if you have questions or if adjustments to our assumptions need to be made to more accurately reflect the growing conditions in your landscape.
Note: Promised updated links to be substituted for those in the Sonoma guide: |
Master Gardeners of Contra Costa Help Desk
Note: The Master Gardeners of Contra Costa's Help Desk is available year-round to answer your gardening questions. Except for a few holidays, we're open every week, Monday through Thursday for walk-ins from 9:00 am to Noon at 75 Santa Barbara Road, 2d Floor, Pleasant Hill, CA 94523. We can also be reached via telephone: (925) 646-6586, email: ccmg@ucanr.edu, or on the web at http://ccmg.ucanr.edu/Ask_Us/
/table>/span>/span>ATMOMETERS FOR IRRIGATION MANAGEMENT
Efficient and precise irrigation management is becoming increasingly important inCaliforniaagriculture, both for maximizing crop quality and for conserving water. The most advanced irrigation scheduling strategy is based on local measurements of reference evapotranspiration (ETo), which is converted to crop evapotranspiration (ETc) with an appropriate crop coefficient (kc).
To be able to use this method, an irrigation manager needs to have locally accurate ETo values throughout the growing season. However, the highly variable microclimates that characterize many farming areas often make it difficult to use data from distant weather stations; therefore an accurate local measurement may often be preferable to relying on a regional value.
One inexpensive option for measuring ETo locally is to use a simple atmometer (Fig. 1). Atmometers are water-filled devices, in which the actual evaporation of water is measured over time. In their simplest form, the atmometer is outfitted with a graduated sight glass on the water supply tank which allows the user to easily measure the evaporation that occurred over a given period. In practice, this type of atmometer is most suited for making readings at multiple day intervals, for example once per week, or on days when irrigation is applied.
The performance of atmometers versus more expensive weather stations was evaluated on theCentralCoastin 2003. In this study, atmometers were placed adjacent to seven weather stations throughout the area, and weekly values for both methods were compared (Fig. 2). The results indicate that the atmometers and weather stations have very comparable ETo readings, with the atmometers indicating somewhat lower ETo values under conditions of lower evapotranspiration.
Like any technique, using atmometers has advantages and disadvantages. Advantages include their very low cost and ease of operation, with no computer or power required. Disadvantages include the potential for damage by freezing weather, the need to refill the water supply (every three to six weeks), and the need to read the gauge manually. Also, if they are installed in a large open area, birds may tend to perch on the evaporating surface and foul it with their droppings; for this reason several wires are installed on top of the device to
discourage birds from perching there. In general, atmometers function quite reliably with few problems.
Converting atmometer ETo readings to the amount of irrigation run time required to replenish the soil moisture lost to evapotranspiration is fairly straightforward. A relatively simple example for a sprinkler-irrigated field is presented below in Table 1.
Table 1. Example conversion of ETo to irrigation run times for a sprinkler irrigated field |
||
|
|
|
A. Measured atmometer ETo for one week |
2 |
inches |
B. Crop coefficient (kc) |
0.8 |
|
C. Calculated ETc for the week (=AxB) |
1.6 |
inches |
D. Sprinkler application rate |
0.13 |
in/hr |
E. Hours of irrigation required (=C/D) |
12.3 |
hours |
(Note: To convert Gallons to Inches: Gallons ÷ Area (square feet) ÷ ?0.6234 = Inches
To convert Inches to Gallons: Inches * Area (square feet) ÷ 1.604 = Gallons)
Atmometer installed on a fence post
atmometer
atmometer ET
Using Evapotranspiration (ETo) for Scheduling Irrigations: An Improvement on Guessing?
Quite frankly, in a county where water is costing $700 to $1000 per acre foot, we though this practice would have been a common practice. Added to this is the increasing pressure to reduce nitrate leaching into creeks and ground water, where there is a serious problem developing. The natural response when water prices are high is to reduce water use, but we have seen groves where even a 10% reduction in water reduces the yield by 50%, and we have also seen quite a few growers irrigating too much with the belief that a couple of extra feet of water per acre will more than pay the cost of water in increased yield. Clearly we need to apply enough water to make the trees produce a profitable yield, How does a farmer accomplish this?
I believe every grower should be using tensiometers or some other kind of soil moisture monitoring equipment to determine when to water, and using CIMIS to determine how much to water. There, just simply, is no an easier, or a better method.
Some growers said that tensiometers don’t work. Well, they work just fine if they are installed correctly and serviced periodically. If the soil gets too dry (the reading goes above 80 cb) the device breaks suction from the soil, and they don’t work until they are removed, filled, pumped and re-installed. As for gypsum blocks, they work just fine also, but are not very accurate under wet conditions. Both work a lot better than just guessing. There are newer electronic devices that work very well if calibrated with the soil moisture, but they don’t work very well in rocky soil (rocks don’t hold water).
Using CIMIS
This assignment is to help you figure out the water use in your grove. The following is a step by step procedure that is not difficult. Several of our grove managers use this on a weekly basis to calculate the water requirement in each of their groves. We have one grower who has this task assigned to his child in the third grade…Really, this is not that difficult!
This assignment will demonstrate how to use CIMIS to calculate the irrigation requirement for an avocado grove in Escondido. ETo is called the reference evapotranspiration (defined as the water use for eight inch tall grass), and all crops in California are related to this water use by adjusting ETo with a “crop coefficient”. In this example you will see that the crop coefficient for avocado in November is 0.55. ETo data is gathered from the automated weather stations that are part of the CIMIS network in California. The irrigation calculator you will be using multiplies the ETo number by the crop coefficient and gives you Etc, the water use by the crop in question. This comes from the station in “inches” of water loss, and the calculator changes this into gallons per tree per day. The calculator then tells you how much water to apply to the avocados to replace the water they used during the last seven days.
Go the website www.avocado.org
Click on California Industry (on the top right side of the page)
Click on Growers
Click on Water
Click on Irrigation Calculator
Start with Evapotranspiration (ETo).
Click on Go To CIMIS
Use the drop down box and Click on San Diego
Click on Submit
Choose Escondido
Click on Daily Data
- “Select a Time Period”, in this example we will select the previous week; select November 15 through November 21
- In “Select Variables”, leave everything selected with the green checkmark.
- Leave “English Units” selected.
- Click “Retrieve Data”
Write down ETo for the last week. In this case it will be: 0.12, 0.11, 0.11, 0.10, 0.12, 0.12 and 0.10.
Add these up, and you get 0.78 (this is your ETo for the past week). Minimize this window.
You are now back to the Irrigation Calculator on the Avocado website.
- Evapotranspiration, delete the 0.22 and fill in your 0.78
- Under “Crop Coefficient”, just click on November in the drop down box.
- Leave “Distribution Uniformity” at 0.85.
- Leave trees at 109 per acre.
- Leave sprinkler output at 17 gal/hr. (of course, you can change this to match your sprinkler output, but for the sake of this example, leave this at 17).
- Click on Calculate.
You should get 138 gallons (this is the amount of water used by one tree in the last seven days) and a watering run time of 8 hrs and 8 minutes.
As I mentioned earlier, you should have tensiometers (soil moisture meters) set at the 8 inch depth (avocado) or 12 inch depth (citrus) to tell you “when” to water. In avocados, I like to irrigate when the shallow tensiometer reads 20-25 cb, and in citrus when the tensiometer reads 35 – 40 cb. You cannot rely on irrigating every seven days because the tensiometer may tell you the soil is getting dry by the fourth day. This often happens in the summer.
To review, CIMIS tells you how much to water, the tensiometer tells you when to water. Now, in actual use, you may find that, in a windy area or on the south side of a slope, your trees may need more water. Merely add a 10% increase to the run time, and keep making minor adjustments until you get this right for your grove. Or, if you have root rot, you may want to water 10% to 30% less water.
By the way, if you are using this calculator for citrus, merely put 0.65 into the crop coefficient for each month, and you can use the same calculator. Some people believe the crop coefficient in the avocado calculator might be too low. Both Ben Faber and I believe the coefficient should be 0.80, but we don’t exactly have good data to support this…just experience. At any rate, the calculator will put you in the ballpark…and it is a lot better than “guessing”.
Give this a try, and Good Luck!
Irrigation Calculator developed by Reuben Hofshi, Shanti Hofshi and Ben Faber.
citrus irrigation
Got weather data? Spatial CIMIS may improve accuracy of ET estimates
I have heard growers complain that the nearest CIMIS station is too far away from their ranch to provide accurate reference evapotranspiration (ETo) estimates. The CIMIS staff have recently improved the spatially accuracy of ETo estimates. CIMIS ETo estimates can now incorporate solar radiation data from the Geostationary Operational Environmental Satellite (GOES). GOES is the satellite that monitors ocean temperatures (think El Niño). The data (Figure 1) has a spatial resolution of 2 km (1.25 mi) so local effects of cloud cover and fog can be factored into ETo estimates. Relative humidity, air temperature, and wind speed data which are also required for the ET estimates are estimated by triangulating from stations closest to the location of interest. The more CIMIS stations operating in your region, the more accurately CIMIS can estimate ET for your field. Finally, like “MyCIMIS,” the spatial CIMIS reports can be emailed to you daily or weekly and in multiple file formats.
You can access spatial CIMIS ETo data from the CIMIS website (wwwcimis.water.ca.gov)
- Log on to MyCIMIS (you may need to set up a user account which is free).
- Go to the spatial CIMIS tab.
- Click on the link “Map Reports.”
- Choose the option “Map Coordinates” to bring up Google Mapping tool (Figure 2).
- Select the fields for which you would like to have spatial CIMIS ETo estimates (Figure 3).
- Select if you would want the spatial CIMIS report emailed daily or weekly or not emailed (Figure 4).
- Select the units for the data (english/metric), start and end dates, and file format. Note that csv format can be imported into spreadsheet programs like excel (Figure 5).
Figure 1. Map of solar radiation (right) and daily reference ET estimates from GOES data for California on May 10 , 2010. Solar radiation is expressed in units of MegaJoules/m2 . ET estimates are expressed in millimeters.
Figure 2. Selecting “map coordinate” option brings up the google map screen.
Figure 3. Fields of interest can be selected by zooming in on the Google Map screen. Longitudes and latitudes of locations are displayed below the map.
Figure 4. After selecting fields of interest, the user chooses email, unit, date, and format options, and submits the query.
Point | Lat | Long | Date | CIMIS ETo (in/day) | Sol Rad (Ly/day) |
1 |
36.82 |
-121.78 |
5/5/2010 |
0.16 |
640.93 |
1 |
36.82 |
-121.78 |
5/6/2010 |
0.18 |
637.74 |
1 |
36.82 |
-121.78 |
5/7/2010 |
0.17 |
654.02 |
1 |
36.82 |
-121.78 |
5/8/2010 |
0.016 |
650.08 |
1 |
36.82 |
-121.78 |
5/9/2010 |
0.13 |
514.43 |
1 |
36.82 |
-121.78 |
5/10/2010 |
0.1 |
353.03 |
1 |
36.82 |
-121.78 |
5/11/2010 |
0.15 |
655.33 |
|
|
||||
2 |
36.86 |
-121.7 |
5/5/2010 |
0.17 |
661.39 |
2 |
36.86 |
-121.7 |
5/6/2010 |
0.19 |
656.88 |
2 |
36.86 |
-121.7 |
5/7/2010 |
0.18 |
662.51 |
2 |
36.86 |
-121.7 |
5/8/2010 |
0.16 |
657.43 |
2 |
36.86 |
-121.7 |
5/9/2010 |
0.13 |
481.99 |
2 |
36.86 |
-121.7 |
5/10/2010 |
0.09 |
274.04 |
2 |
36.86 |
-121.7 |
5/11/2010 |
0.16 |
671.71 |
|
|||||
3 |
36.93 |
-121.7 |
5/5/2010 |
0.17 |
663.62 |
3 |
36.93 |
-121.7 |
5/6/2010 |
0.19 |
668.97 |
3 |
36.93 |
-121.7 |
5/7/2010 |
0.18 |
655.7 |
3 |
36.93 |
-121.7 |
5/8/2010 |
0.17 |
662.72 |
3 |
36.93 |
-121.7 |
5/9/2010 |
0.12 |
426.91 |
3 |
36.93 |
-121.7 |
5/10/2010 |
0.1 |
304.96 |
3 |
36.93 |
-121.7 |
5/11/2010 |
0.16 |
672.15 |
Figure 5. Example of Spatial ETo data imported into excel from an emailed CIMIS report. First column refers to field number.