Posts Tagged: evapotranspiration
Keeping cool season vegetables cool during heat waves
Once again, we are experiencing a prolonged heat wave in the Salinas Valley. Maximum air temperature in the King City area reached 112 °F earlier in the week (Fig. 1). Recent maximum air temperatures in South Salinas have been far greater than the average temperatures recorded for the same period during previous years (2015 through 2019).
Although this heat wave will probably wane in the next several days, the central coast region will likely experience periods of record setting temperatures in the future. There are several concerns about how prolonged elevated temperatures affect cool season vegetables. Heat can cause immediate damage to plant tissue when temperatures of the plant surfaces become too high and cause cells to die (Fig. 2). In addition, sustained high temperatures can affect plant growth and development. For instance, in lettuce damage can vary from obvious burning on the edges of leaves from too much heat load (Fig. 3) to more physiological issues that result in poor head formation in iceberg (e.g. puffy heads). In broccoli, if heat damage occurs when heads are forming, it can result in uneven bead sizes when the head matures (Fig. 4). Excessive heat can result in wilting in cauliflower (Fig. 5) during high temperatures and expose curds to sunburning or cause discoloring (Fig. 6.) In the past two years, we have observed that excessive heat can stress lettuce plants and make them more susceptible to infection with Pythium Wilt (Pythium uncinulatum). That was particularly evident in the 2020 heat spells. If there is inoculum Pythium Wilt in the soil, stress caused by heat on the plant can set off infection (Fig. 7).
A previous article presented strategies for maximizing evapotranspiration rates to keep crops cool. Evapotranspiration (ET) is the process in which liquid water vaporizes from plant leaves and moist soil surfaces and is lost to the surrounding air. As liquid water vaporizes, heat is also lost from the surfaces of leaves and soil and from the surrounding air, which cools the crop. Assuring that crops have adequate soil moisture during the hottest period of the day (generally 11 am to 4 pm) can keep plants as cool as possible. Insufficient moisture to meet crop water requirements can result in stomates of the leaves closing and decrease transpiration rates. Limiting transpiration would raise leaf temperatures, potentially to temperatures greater than the surrounding air.
Hence, a good strategy to prevent heat damage to vegetable crops is to water fields that have not been recently irrigated. Also, keep in mind that during the last few days daily reference ET increased substantially due to the high air temperatures and so more water is needed than normal to replace the amount of moisture that crops transpiration. In South Salinas, for example, the CIMIS station showed that daily reference ET increased from 0.18 inches per day in late August to 0.25 inches per day during the heat wave, approximately a 40% increase in water demand (Fig. 8).
Irrigations do not need to be very long, as much as they should supply the crop with enough water to refill the soil profile to the depth of the root zone. Irrigating more frequently for less time would be a better strategy than irrigating less frequently for more time, since the soil has a limited capacity to store water in the root zone. Over-saturating the soil during high soil temperatures through heavy irrigations could worsen infections from soil-borne pathogens.
The CropManage online decision support tool can assist with determining the amount of water to apply and frequency to irrigate for most vegetable crops produced in the Salinas Valley. The software allows one to customize the recommendations for the development stage of the crop, soil type, and irrigation system characteristics.
Finally, for crops irrigated by sprinklers, short irrigations during the hottest time of the day can reduce air temperatures. This might be a good strategy for vegetables that are in a stage of development that is very susceptible for heat damage, such as cauliflower close to harvest.
How Much Water is Your Orchard Using?
Sustainable water management is one of the most challenging issues of our time, especially in the arid western U.S. Adequate water supplies are crucial to maintaining the health of communities, rivers, and wildlife, and nothing is more important to agriculture's ability to produce food for the world's growing population. Maximizing the benefits of our water supplies requires careful measurement of their availability and use. For irrigated agriculture, satellite-based estimates of evapotranspiration (ET) provide a measure of the water used to grow food — the biggest share of water consumption in most arid environments around the world. However, access to this data has been limited and expensive, keeping it out of the hands of most water users and decision-makers.
OpenET provides open, easily accessible satellite-based ET data for improved water management. The OpenET collaborative includes leading national and international experts in remote sensing of ET, cloud computing, and water policy, partnered with nationally recognized web development teams and leaders in the western agriculture and water management communities.
This data site can focus in on a field or orchard using Google Earth and provide a monthly and yearly water use for that orchard. It's not the potential water use – the amount of water that could be used in the field driven by weather elements – but the actual water use, the consumptive use. If the field is under irrigated, it would show how little water is being lost there. If the field is over irrigated, it will show the potential ET.
At the moment, the data is not for daily use, although that is forecast to occur later in the year. It also has somewhat clunky navigation, but with patience, it's possible to get around to finding an individual field. It's a work in progress, but it could be a great tool soon.
It's free and you can check it out at: https://openetdata.org/about/
Atmometer Study
Efficient and precise irrigation management is critical if California producers are to maximize crop quality, conserve water, and protect the environment. The use of evapotranspiration (ET) estimates is a significant component of irrigation management. ET refers to the sum of water lost from the soil (evaporation) as well as that used by the crop (transpiration). While the California Irrigation Management Information System (CIMIS) network of weather stations derive daily ET values, there is a perception that CIMIS does not produce accurate ET estimates for all locations. This view is particularly prevalent in the canyons of Ventura County where weather conditions differ substantially compared to CIMIS locations. Since avocado and citrus thrive in these areas, it was concerning when it was determined that ET scheduling is not widely used.
That is, a Ventura County Resource Conservation District (RCD) review of California Department of Food and Agricultural State Water Efficiency and Enhancement Program (CDFA SWEEP) projects concluded that Ventura County growers substantially lagged their state-wide peers with respect to implementing ET-based irrigation scheduling (14% versus 44%).
RCD seeks to reverse the low implementation of ET-based irrigation scheduling within Ventura County by using simple, rugged on-site ET devices (atmometers) to determine on-site ET values. These on-site values will be compared to CIMIS values to determine local correction factors and develop refined ET maps for the canyon and valley areas. RCD will present these results at outreach events and provide workshops demonstrating how ET data, whether from CIMIS or on-site atmometers, can be used for irrigation management.
PHOTO: Atmometer Test/Calibration Site @ UC Hansen
atmometer calibration
Celery Irrigation Trial Field Day
USDA-ARS Spence Research farm
1572 Old Stage Rd.
(36.628792, -121.540791)
Thursday October 18th, 9 am to 10:30 am
UC Cooperative Extension will host a field day at the USDA-ARS Spence research farm to show a field trial evaluating water use in drip irrigated celery. This is an opportunity to see first- hand the effect of water management on celery growth. We will discuss using weather data for scheduling irrigations using the CropManage online tool, water application monitoring using flowmeters, soil moisture monitoring using tensiometers, and the feasibility of improving irrigation scheduling using satellite-based tools. This is a joint project among UC Cooperative Extension, CSU Monterey Bay, NASA-Ames, and the USDA-ARS. The project is funded through the USDA-Specialty Crop Grant Program, administered through CDFA. Cooperators include Dole Fresh Vegetables, Inc., and Wilbur Ellis.
Location: The trial is located in a field adjacent to Spence Rd. Enter the ranch from the west side of Old Stage Rd. Turn right at the water reservoir and drive to the end of the road.
For more information contact Michael Cahn, Irrigation and Water Resources Advisor: 831-759-7377, mdcahn@ucanr.edu
New CIMIS weather station available in the Salinas Valley
The California Irrigation and Management Information System (CIMIS) operates and maintains more than 145 weather stations throughout California. The CIMIS program is funded by the California Department of Water Resources. Most stations are located on or near agricultural land, and provide measurements of reference evapotranspiration (ETo), which can be used to estimate how much water to apply to crops. Hourly, daily, and monthly averages of data are available through the CIMIS web site (http://www.cimis.water.ca.gov). The website includes an option to automatically email data from selected stations on a daily or weekly schedule. In addition to ET data, CIMIS stations record precipitation, relative humidity, air and soil temperature, solar radiation, wind speed, and dew point. Besides irrigation management, weather data can be used for plant disease forecasts, for calculating insect and crop degree-days, and for determining wind speeds during spray operations. Water management agencies use historical reference ET data to determine pumping demands, and to estimate future ground water supplies using mathematical computer simulation models.
The Central Coast region currently has 16 active CIMIS stations (Table 1). Stations are located in Monterey, San Benito, Santa Cruz, Santa Clara counties. The newest station is #252 (Figure 1), located near Soledad CA on the east side of the Salinas Valley. CIMIS is a cooperative program, requiring collaboration between a local entity to provide land, maintain the site, and provide periodic servicing of the station. In some cases, stations are owned by the CIMIS program, but in many situations the weather stations are purchased through grant funding obtained by a local agency. Funding for station 252 was from a proposition 84 grant administered by the Coastal Conservation and Research Inc. The installation of the station was a partnership among Monterey County Resource Conservation District, UC Cooperative Extension (Monterey County), Dole Food Company Inc., CIMIS, Monterey Bay National Marine Sanctuary, and the Central Coast Wetlands Group (CCWG). Because CIMIS stations need to be sited on well-watered grass to provide accurate estimates of reference ET, funds from this grant were also used to establish 2-acres of grass surrounding the weather station and to install an underground sprinkler system.
Table 1. CIMIS stations located in the Central Coast region.
Figure 1. CIMIS station 252 located near Camphora-Gloria Rd, Soledad CA.
Even if a ranch has a private weather station, CIMIS data can still be useful. Many private weather stations are not instrumented to monitor ET or are not sited on a well-watered reference crop. Frequently private weather stations are located near a building, parking lot, or tree that can confound micro-climatic measurements. Over time instrumentation on weather stations can malfunction and record inaccurate data. The CIMIS system uses both an automated and manual quality assurance program to flag data that appear inaccurate or outside the normal range. CIMIS staff also service and check that the instruments are working properly. Additionally, CIMIS data are archived so that historical data can be accessed by users. CIMIS weather station data also contributes to Spatial CIMIS, a hybrid ET product that uses weather station and satellite data to provide reference ET estimates at approximately a 1-mile resolution. Spatial CIMIS reference ET data are also available through the CIMIS website.
During the past decade, significant progress has been made in adding new CIMIS weather stations or revitalizing old stations with improved site maintenance on the Central Coast. In addition to the Soledad station, Station 209 was established in West Watsonville, Station 211 was installed in Gilroy, and Station 210 was located in the Carmel Valley. Permanent grass was planted at station 129 in Pajaro and also at station 214 in South Salinas. Although progress has been made to increase the number and accuracy of CIMIS stations, weather stations are still lacking in some important Central Coast growing regions. Closer to the coast, the Castroville (#19) and North Salinas (#116) Stations are no longer reporting reference ET because the sites do not have sufficient grass cover to accurate measure ET. Also, the Green Valley road station (#111), which represents a warmer zone of the Pajaro valley no longer reports reference ET data due to insufficient grass cover at the site.
Having reliable long-term weather data from the main growing regions on the Central Coast is becoming more important for our region. As water demands continue to increase on the Central Coast, the agriculture community is under increased pressure to demonstrate efficient irrigation practices. Online irrigation scheduling tools such as CropManage and the Satellite Irrigation Management Support (SIMS), use CIMIS data to help growers quickly determine crop water needs. These tools can also help growers justify water needs of their crops. To comply with the Sustainable Groundwater Management Act (SGMA), water management agencies will need accurate reference ET data for developing ground water extraction plans. My hope is that through partnerships among local and state agencies, private land owners, and grower groups, we can add new or revitalize existing CIMIS stations so that all growing regions on the Central Coast have accurate weather data.