Crop Coefficients - Paso Panel

Using the Paso Panel to quickly measure the canopy shaded area and estimate vineyard irrigation crop coefficients

Introduction

The Paso Panel is a simple device which is used to measure the midday canopy shaded area of trellised grapevines. This measurement is of interest because it can be used to calculate the irrigation crop coefficient (Kc). The midday canopy shaded area can also be useful as a quick non-destructive measure of general vine vegetative growth, for example in evaluating the relative growth due to different fertilizer treatments or rootstocks.

Operation

The theory behind the operation of the Paso Panel is that the electrical current produced by the solar panel is proportional to the amount of direct sunlight striking the panel; thus, as the solar panel surface area is increasingly shaded by the vine leaf canopy, it produces proportionally less current.

There are several important requirements for using the Paso Panel correctly:

1. The measurements can only be taken with clear sky conditions. Clouds can be present elsewhere in the sky, but not directly between the solar panel and the sun; any obstructions of the direct sunshine to the solar panel will lead to erratic readings and significant errors. Note that airline contrails disrupt the sunlight in the same way that clouds do.

2. Measurements should only be taken near solar noon; in California, solar noon in the summer occurs at approximately 1:00 pm (the shift in time from noon is due to Daylight Savings Time). It is suggested to only take measurements between 12:30 and 1:30. East-west oriented rows will be less affected by changes in the shaded area due to straying outside these time constraints as compared to north-south oriented rows.

3. The device needs to be held uniformly level, and the solar panel surface needs to be kept free of dust and debris.

4. Position the panel as close as possible to the underside of the canopy; the farther away the canopy is from the panel, the greater the possible error.

5. Any inadvertent shading of the panel needs to be avoided, such as that caused by the shadow of the person operating the panel. Orient the panel to avoid any such shading.

6. The switch which completes the electrical circuit should only be engaged very briefly (about 1 second per reading). This switch short-circuits the solar panel; if it is engaged for excessively long periods in bright sun, the solar panel wiring will burn out, as the solar panel was not designed to be short-circuited like this. Please heed this warning to avoid having to buy an expensive replacement solar panel.

7. Under windy conditions, the canopy shaded area will change somewhat as shoots and leaves move around. This will cause the current readout on the digital multi-meter to fluctuate accordingly.

Videos

Example measurement in the field

Construction

The Paso Panel consists of three main components:

1. A homemade frame with carrying handles

2. A PowerFilm™ brand solar panel

3. A common digital multi-meter

There are many possible ways to construct a device similar to the Paso Panel; my current design made the best use of my available materials. Your materials, tools and construction skills may make a different design more practical. Photographs of the device under construction are available below in the "Downloads" section.

The lightweight frame is made from common extruded aluminum available at most hardware stores. For the large sized Paso Panel, the overall length is 8 feet (2.4 m). The frame is 1 foot (0.3 m) wide for all versions. The device can be made in different sizes, to fit different canopies and row spacings; in the photograph below the large sized device is on the left, while a smaller sized device for narrow rows is on the right. The two handles are oriented to place the device at a comfortable height directly underneath the canopy when held by the user.  The frame also incorporates a bubble level, to help the operator ensure that the device is held uniformly level for all readings.

The rollable solar panel I used is the PowerFilm™ brand, model number R15-1200 for the larger sized device. The more recent versions of the PowerFilm™ rollable solar panels have slightly different dimensions than the versions I used when developing the original Paso Panel; your design will therefore have to be adjusted accordingly based on which panel you use. For the most current information on these panels, see the manufacturer’s website here.

The digital multi-meter is a commonly available instrument; it is important that the device have the ability to read electrical current of sufficient range. I use a model rated up to 10 amps. The simple wiring diagram of the device is shown below. The two output wires from the solar panel are connected directly to the digital multi-meter, with a momentary pushbutton switch that completes the circuit only when the switch is depressed. This switch is located in the rear carrying handle. I use a switch from Napa Auto Parts, part number ECH STB6301.

Downloads:

Spreadsheet for calculating Kc values

Spreadsheet for estimating the number of measurements to make per site

Example data - crop coefficient over season

Example data - canopy size for different rootstocks

Photographs of the device under construction

Field measurement protocol

Step 1: Take a “Full Sun” reading

Hold the Paso Panel with the same orientation and at the same height as will be used when taking the shaded area readings in the vineyard. Press the switch momentarily and view the electrical current value on the screen of the multi-meter. Record this value.

Step 2: Take the “Shaded” readings

Immediately after recording the “Full Sun” value, begin taking the “Shaded” readings. Hold the panel level and perpendicular to the vine row, directly beneath the leaf canopy, and press the switch to view the electrical current value on the screen of the multi-meter. Take as many readings as necessary to accurately characterize the area of interest. These readings should be done quickly over a period of only a few minutes; if your needs require readings over a longer time period, break the measurement into separate short sequences, each with its own separate “Full Sun” reading preceding the corresponding “Shaded” readings. It is very important to do this, because the “Full Sun” reading is used as a reference value in the calculations, and the magnitude of the “Full Sun” reading will change throughout the day.

To facilitate rapid readings when working alone, use a digital voice recorder with clip-on microphone to record all readings verbally; after finishing the field measurements, play back the recordings while seated at a computer and type the values directly into the spreadsheet which will do the calculations.

Calculations

The calculations are most easily explained in several steps. In practice, the computer spreadsheet listed above will handle all of the calculations.

1. Determine the shaded percentage of the solar panel

• Shaded percentage of solar panel = [1 – (Shaded reading/Full Sun reading)] * 100%

• (Note that the “Shaded” reading above is the average of all shaded readings for the plot)

2. Determine the shaded percentage of the field

• Shaded percentage of field = Shaded percentage of panel * (Panel length/Row spacing)

3. Calculate the crop coefficient

• Kc = (0.017 * Shaded percentage of field) – 0.008

Example calculation

Assume that the “Full Sun” reading was 1.7 Amps, and that the ten “Shaded” readings were 1.0, 1.1, 1.1, 1.2, 1.0, 0.9, 0.8, 1.0, 1.1, and 0.9 Amps. The solar panel length is 5.75 ft, and the row spacing is 10 ft. What is the Kc value?

Step 1:

• Average of the ten shade readings = 1.01 Amps

• Shaded percentage of the solar panel = [1 – (1.01 Amps / 1.7 Amps)] * 100%

• Therefore, shaded percentage of the solar panel = 41%

Step 2:

• Shaded percentage of field = 41% * (5.75 / 10)

• Shaded percentage of field = 23.6%

Step 3:

• Kc = 0.017 * 23.6% - 0.008

• Kc = 0.39

An important note when attempting to measure crop coefficients in the field using this method:

By definition the crop coefficient value represents the relative water use of a crop which is fully supplied with water. If measurements of the midday canopy shaded area are taken in a vineyard which is not fully supplied with water, then the shaded area will likely be smaller than it would otherwise be, and the calculated crop coefficient value will likewise be smaller. This discrepancy can be reduced by taking measurements during periods when the leaf canopy has reached full size and while vine water stress is minimal; in many winegrape vineyards this occurs in the late spring around the period of bloom and fruit set. Later measurements after deficit irrigation regimes have been applied may lead to different values which are less representative of the true crop coefficient value for that vineyard.

For further information on the theory of using canopy shaded area for estimating the crop coefficient, see the following journal article:

Williams, L.E. and J.E. Ayars. 2005. Grapevine water use and the crop coefficient are linear functions of the shaded area measured beneath the canopy. Agricultural and Forest Meteorology, 132 (3-4) p. 201-211

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