PROGRESS WITH MEASURING AND UTILIZING CROP EVAPOTRANSPIRATION (ETc) IN WALNUT

Transcription

1 PROGRESS WITH MEASURING AND UTILIZING CROP EVAPOTRANSPIRATION (ETc) IN WALNUT Allan Fulton, Cayle Little, Richard Snyder, Richard Buchner, Bruce Lampinen, and Sam Metcalf ABSTRACT Since 1982 when the California Irrigation Management Information System (CIMIS) was established, water budgets have been promoted as a simple and effective way to assist growers with making management decisions about irrigation frequency and duration. A water budget relates estimates of crop evapotranspiration (ETc) to the specific design features of an irrigation system to assess when to turn on an irrigation system and how long to run it. CIMIS consists of a statewide network of standardized weather stations that predicts daily rates of grass reference evapotranspiration (ETo) for different regions of California by using the research-based Penman- Monteith equation. Predicted rates of ETo are then multiplied by crop coefficients (Kc) to estimate daily rates of ETc for a specific crop at a specific stage of growth and development. Accuracy and representation of the Kc values used with ETo to estimate ETc for a specific crop are frequently questioned. In walnut, at least two sources of crop coefficients have been used with CIMIS ETo predictions to provide real-time estimates of ETc to growers and consultants. One source of walnut Kc values originates from research sponsored by the California Walnut Research Board in the late 1980 s and early 1990 s. A second source dates back to other research from the 1970 s and 80 s sponsored by the California Department of Water Resources in collaboration with the University of California. This research focused on developing crop coefficients for various crops including deciduous tree fruit and nuts for different growing regions of California and was not exclusive to walnut. This report describes ongoing research initiated in 2011 in the northern Sacramento Valley with new techniques of measuring real-time ETc in walnut and to re-evaluate Kc values for modern cultivars, orchard designs, and cultural practices. Prospects of how the approach to measuring ETc in walnut may evolve in the future is also discussed. OBJECTIVES 1) Gain experience with Energy Balance theory and methods of measuring real-time ETc in walnuts. These methods are less expensive than traditional research methods using weighing lysimeters. 2) Determine how well Kc values developed from research sponsored about 30 years ago represent modern walnut cultivars and orchard cultural practices in the Northern Sacramento Valley. Increase understanding of how measurement of ETc in walnut may evolve and be applied in on-farm water management in the future. SIGNIFICANT FINDINGS Energy balance theory and field measurement techniques offer an alternative for measuring real-time ETc in walnut. This method is more adaptable to production field settings than conventional weighing lysimeters that are generally limited to research facilities. California Walnut Board 105 Walnut Research Reports 2013

2 Preliminary results from research conducted in 2011 and 2012 suggest refinements to the existing walnut Kc values may improve real-time estimates of ETc in walnut. Given the rapid pace of hardware and software development that is used with energy balance methods of measuring real-time ETc, it is possible that in the next 5 to 10 years that ETc will more commonly be measured directly in crops including walnut. The need and utility of Kc values to couple with ETo and then predict ETc for specific crops such as walnut may diminish. Irrespective of how ETc for walnut is measured or estimated, there is continued need for development of other plant- and soil-based irrigation management tools. Irrigation water is not the only source of water available to meet ETc. Plant and soil-based management tools are also needed to discern site specific conditions related to effective crop root zone, water table influences, and contributions of winter and in-season rainfall towards meeting ETc. PROCEDURES Figure 1 provides a series of photos showing the instrumentation that is needed to measure realtime ETc using energy balance theory and measurement techniques. A scaffolding system is used to install the instruments above the orchard canopy. A net radiometer, a sonic anemometer, and a thermocouple are placed 1.0 meter (3.3 feet) above the walnut orchard canopy. At least 30 meters of fetch is required for placement of the sensors. Ground sensor packages are buried just below the soil surface (2 inches) along with a soil moisture sensor. A datalogger and program are used to collect and synthesize continuous, high frequency data (10X per second). A solar panel and battery are used to power the instruments and datalogger. The basic theory of the energy balance technique is that radiation and convection are the two sources and methods of heat transfer in an orchard and a balance is maintained between incoming and outgoing energy sources. The net radiometer is used to measure the incoming and outgoing radiation from the orchard canopy. The sonic anemometer and the thermocouple are used to quantify the sensible heat from convection that is being transferred through the orchard by turbulence. The soil heat flux plates and thermometers buried just below the soil surface of the orchard floor detect the heat exchange from the orchard floor. A datalogger continuously records the rapidly changing energy dynamics in the orchard and complex research-based algorithms are used to calculate the residual energy in the orchard referred to as latent heat. Latent heat is the energy that is transferred through the conversion of water to water vapor or more commonly recognized as evapotranspiration (ETc) in irrigated crop landscapes. Energy balance stations have been installed in two commercial walnut orchards in Tehama County. Both orchards are mature and approaching their maximum yield potential, typically yielding between the 4500 to 6000 lbs dry inshell walnuts per acre. One orchard is growing on the east side of Tehama County in deep alluvial loam soil and the second orchard is growing on relatively shallow, terrace soils on the west side of Tehama County. The orchard designs are distinctly different. The east side orchard has 95 trees per acre while the west side orchard has much higher density with 180 trees per acre. Both orchards are irrigated with similar Nelson R5 minisprinklers. A mobile light bar has been used annually to measure the photosynthetically active radiation (PAR) and characterize the orchard canopy in each orchard in August. Walnut yield has been measured annually in the corresponding rows where PAR has been measured. California Walnut Board 106 Walnut Research Reports 2013

3 Applied irrigation water was measured weekly with in-line flow meters in both orchards. Midday stem water potential was measured weekly to characterize the level of crop stress in each orchard during the course of the season. Irrigation was managed so that both orchards were consistently maintained within low to moderate levels of crop stress (-4 to -8 bars tension). Soil moisture was also monitored weekly to depth of ten feet in the deep alluvial soil at the east side orchard and to a depth of six feet in the terrace soil of the west side orchard to better understand the depth of the effective root zone and extent of the soil storage contribution. RESULTS AND DISCUSSION Hardware and software advancements with energy balance techniques for measuring real-time ETc in walnuts has been rapid since the installation of these two stations in 2010/11. However, it has come at the expense of missing data due to experimentation with different sensors, changes to the dataloggers, and modifications to the compiling and analytical programs. Some of the key improvements have been the development of a calibration technique for the thermocouple so that it can be used as the primary sensor for detecting sensible heat from convection. This breakthrough gives potential to eliminate the sonic anemometer from the suite of instruments required to measure ETc and reduces the cost of a station by about $3500 or 23 percent. More experience with the various sensors has been acquired to improve selection of sensors to withstand damage. For example, a net radiometer that is not as prone to damage from birds and and UV light is now compatible with the datalogger and compilation program. A memory card reader previously associated with the datalogger has been eliminated so it is no longer necessary to download large data files with a laptop in the field. The compiling and analytical program has been modified so that large data files created by high frequency sensing (as often as 1/10 of a second) are condensed down to 30 minute averages which lend themselves to much easier retrieval and analysis. Phone modems are now operational in the stations to enable remote connections to check that sensors are working correctly and to minimize trips to the field for station maintenance. Progress is being made towards remote retrieval of the actual data but more development is needed. Improvements have been made with the power systems to reduce power failures and prevent the loss of data. Table 1 provides published Kc values for walnut and deciduous trees from two different references. It also gives preliminary Kc values calculated from these recent studies using energy balance methods to measure ETc. By definition, Kc values are calculated as the ratio of ETo to ETc. In this research, ETo measurements were queried from nearby CIMIS station #8 and compared to the real-time ETc measured in these two commercial orchards to calculate Kc values. The preliminary results from 2011 through 2013 suggest there may be potential to refine the Kc values for walnut. The Kc values from mid July through August for walnut that have been published in the UC Walnut Production Manual are higher than those measured in this study. In contrast, the Kc values from May 15 through July 15 that were published for deciduous tree fruit and nut crops in Leaflet are lower than observed in this study. Light bar measurements in these orchards indicate that the east side walnut orchard averaged 77 percent PAR while the west side orchard averaged 89 percent PAR. Sufficient data has not been collected to compare these two different orchards and assess the effect of the different orchard designs and canopy light interception on ETc and Kc values. California Walnut Board 107 Walnut Research Reports 2013

4 Figure 2 illustrates the fluctuation in daily Kc values measured at the east side orchard in From leaf-out in April through June, the daily Kc value fluctuated as much as 0.3 units. From July through August the Kc values did not fluctuate as much. Then the daily Kc values fluctuated more in September and into October. This emphasizes the challenges with defining an average Kc curve in semi-monthly increments that accurately accounts for the inherent variability in ETc. As the development of hardware and software continues to advance, it may be possible to use energy balance methods of measuring ETc in walnut directly as an alternative to using ETo (grass reference) with Kc values (crop coefficients). For smaller stature annual and perennial crops, energy balance methods may eventually have on-farm utility. However, larger stature tree crops like walnut pose more challenges and may require scaffolding systems to properly install and maintain the necessary instrumentation. The on-farm application in these crops may be limited unless it is supported by an external entity much like the CIMIS network is currently supported by the California Department of Water Resources. Figure 3 shows how the seasonal ETc was supplied to each of the orchards in this study. Measurements of real-time ETc and estimates of Kc values suggested the crop water consumption was similar in both orchards. However, the applied irrigation water was substantially lower in the east side orchard growing on the deep alluvial soils. Only 51 percent of the seasonal ETc was supplied by irrigation in this orchard which was located about ½ mile from the Sacramento River. The balance was supplied by soil storage from winter rainfall and capillary movement of water into the root zone from a relatively shallow water table. In contrast, a much larger proportion, 81 percent, of the seasonal ETc was supplied by irrigation at the west side orchard where it is growing on shallow terrace soils and is not influenced by capillary movement of water from a water table. This emphasizes that while investment in climate based methods to measure ETc are useful for both land use planning purposes and on-farm irrigation management, development of other plant- and soil-based irrigation management tools remains important to discern site specific conditions and irrigation needs. California Walnut Board 108 Walnut Research Reports 2013

5 Figure 1. Instrumentation needed to measure real-time ETc with energy balance method. Net Radiometer Scaffolding Sonic Anemometer Soil Heat flux plates and thermometers Datalogger Solar Panel California Walnut Board 109 Walnut Research Reports 2013

6 Table 1. Comparison of crop coefficients (Kc) for walnut. Two published sources from 1998 and 1989 are provided along with preliminary results from three years of data collection using the energy balance method to measure ETc. Date 1998 UC Walnut Production Manual Historic Kc 1989 UC Leaflet Historic Deciduous Tree Kc 2011 East Side Tehama County 2012 East Side Tehama County 2013 West Side Tehama County Apr Apr May May June June July July Aug Aug Sept Sept Oct Oct Nov California Walnut Board 110 Walnut Research Reports 2013

7 Figure 2. Actual crop coefficients (Kc) measured in east side walnut orchard in 2012 using energy balance method of measurement. Figure 3. Comparison of how seasonal ETc was supplied to the east side and west side walnut orchards in this study. East Side Walnut Orchard West Side Walnut Orchard California Walnut Board 111 Walnut Research Reports 2013