Irrigation Scheduling of Grapevines
Evapotranspiration-based Scheduling Method
Evapotranspiration-based irrigation scheduling is scheduling irrigation based on evapotranspiration (ET) so that ET losses are replaced in the root zone to meet plant water requirements. In general, plant water requirements are determined from a balance of water inputs and outputs from the root zone. Evapotranspiration is the combined loss of water from soil evaporation and vine transpiration. The two terms are combined into one term because it is difficult to measure the two components separately. When there is limited leaf area on a vine, most of the water is lost through soil evaporation. However, when there is a full canopy, most of the water is lost through leaf transpiration. High ET values are associated with sunny, warm, dry, long (day length), and windy days, whereas low ET values are associated with cloudy, cool, humid, short (day length), and no too windy days.
Determining Crop Evapotranspiration
Methods for determining crop evapotranspiration include:
- Using real-time climatic data
- Using historical evapotranspiration data
- Using data from an evapotranspiration pan
- Using a Bellani Plate Atmometer
Real-time Climatic Data
Climatic data such as solar radiation, temperature, humidity, and wind speed are commonly used to calculate a reference crop evapo-transpiration. A local weather station can be consulted for the daily ETo rates and these can be summed over a 1 week period or other duration that is appropriate for the vineyard's irrigation schedule.
Historical Evapotranspiration Data
Historical ETo information is the crop water use estimated from long-term averages. For any particular day, the actual ETo may vary from the historical average, but it has been found that the actual variability is relatively small during the summer months.
One of the simplest ways to measure evaporation is with a shallow tank of water exposed to wind and sun—an evaporation pan. An example is the widely used Class A pan.
Bellani Plate Atmometer
One method to determine ETo uses a black Bellani plate atmometer. This is an inexpensive flat, black, porous plate which is kept wet. The amount of water evaporating from its surface is used to calculate ETo.
Crop Coefficient (Kc)
Crop coefficients (Kc) are used with ETo to estimate grapevine evapo-transpiration rates. Crop coefficients for grapevines vary during the growing season to reflect the development of the canopy and vine water demand. The crop coefficient is low early in the season, and as the grapevine canopy develops, it will increase. The crop coefficient is a dimensionless number that is multiplied by the ETo value to arrive at a grapevine evapo-transpiration (ETc) estimate.
Determining a Grapevine Crop Coefficient
The grapevine crop coefficient can be best determined by measuring the percent shaded area (PSA) of the vineyard floor at any time during the season. The PSA must be estimated during the solar noon hour (between 12:30 and 1:30 pm) by one of several methods:
Steps in Calculating Estimated Water Use
Steps for developing an evapotranspiration-based irrigation schedule are as follows:
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