In delivering workshops on irrigation (and drought) management, there are always a few questions on how to estimate canopy coverage without the use of special equipment. Not being content with the typical answer of “Use your best guess,” I began to review the research, and found that an estimate of photosynthetically active radiation (PAR) (also known as mid-day light interception) can be calculated from orchard yields.
This question was asked because in a water short year, the available water needs to be spread out evenly as the respective % of evapotranspiration. This following will help determine how much water your trees are using.
Work by Bruce Lampinen and colleagues has found that yield is directly related to the percentage of light intercepted by the orchard’s canopy. As canopy coverage increases, so does the yield potential. This relationship is shown in figure 1, and shows that for every 1% of light intercepted, the orchard’s yield potential increases by 50 lbs. In other words, if the orchard is intercepting 50% of the photosynthetically active radiation (PAR), it has the potential to yield 2500 lbs/acre. This can be demonstrated by individual data points from several orchards, which when followed for multiple years will tend to alternate above and below this line in production.
This same relationship can be used to estimate the PAR of an orchard. By taking the average of the two highest consecutive years of production for a mature orchard block, and dividing it by 50, we can calculate an estimated PAR. For example, if a block has yielded 2900 lbs/acre and 3300 lbs/acre for 2011 and 2012, respectively, the average is 3100 lbs/acre which puts the estimated PAR at 62%.
Once the percentage of PAR is estimated, it can be multiplied by 1.6 to determine the percentage of almond ETc to apply. Using our example from above, a 62% PAR would indicate that we need to apply 99% of almond ETc to meet the orchard’s water demand, while an orchard at 50% PAR would need 80% of almond ETc.
For developing orchards, the data is a little less clear. Orchard yields are based on canopy developed in the previous year. Since young orchards are rapidly developing new canopy, estimating PAR and ETc from yield would lead to a under-estimation of water needs. A basic guideline that has been gleaned from the data suggests that first leaf trees start off around 5% almond ETc and may reach up to 20% of ETc by the end of the first year of growth. In the subsequent years, ETc will increase by about 15% annually. This of course, is based on how vigorous the trees are growing.
Knowing the percentage of canopy coverage is crucial for determining orchard water needs. Once calculated and the total water supply for the orchard is known, previously discussed drought irrigation strategies can be followed and the amount of water to apply throughout the season can be determined. It is important to note, however, that if not reaching peak production, more water may be needed to develop the canopy to support the future crop.
Text in blue was added as edits on 5-31-2014.