This is from the University of California Cooperative Extension Glenn County - Bill Krueger Farm Advisor

The water budget approach is a useful tool for deciding when and how much to irrigate. Simply stated, this method involves keeping track of how much water is stored in the soil profile, subtracting water as it is used by the crop and irrigating when a predetermined amount of water bas been depleted.  

To use this system, several things must be known.  

I. Evapotranspiration (ET) is a combination of evaporation from the soil surface and transpiration from leaf surfaces. ET values from the Orland CIMIS station are available from our office. Values published in local papers have been adjusted to a specific crop and field condition. Values from the CIMIS station are reference ET's and must be adjusted by multiplying by a crop coefficient. Crop coefficients can be obtained from our office.  

2. Soil Water Holding Capacity is how much water is stored in the soil for plant use. The following table lists approximate available water holding capacity by soil texture.

3. Water application = application rate x hours operated Following are some conversion factors and formulas which should be used for calculating water applications.  

CONVERSION FACTORS  

CALCULATING THE APPLICATION RATE IN INCHES PER HOUR

4. System Efficiency -How much of the water applied is available to the plant. Following are some approximate efficiencies.

5. Root depth. What is the effective rooting depth from which water would be depleted? In most crops, 75% of maximum rooting depth is the effective rooting depth.  

6. Allowable depletion. It is not advisable to deplete all of the available moisture between irrigations. In most crops we only want to deplete 50- 75% of available moisture before irrigating.

EXAMPLES OF IRRIGATION SCHEDULING

This is a soil with a low infiltration rate and we are only able to wet to a depth of 18 inches. Although we have an allowable depletion of 3 inches ( 1.5 inches/ft x 4 ft. = 6 inches x .5 allowable depletion), we are only able to apply 2.25 inches ( 1.5 inches/ft x 1.5 ft. ). This then becomes the limiting factor and we would have to irrigate in 9 days (2.25/ .25 ET). 

Depth of penetration on most soils can be determined by using a 3/8 to 1/2 inch steel rod sharpened on one end with a " T" handle welded on the other end. In most soils, this probe can be pushed in the soil as far as the water penetrated following an irrigation. This allows for rapid, repeated checks on penetration.

Crop-Prunes 
Crop ET = .25 inches/day 
Tree spacing is 18 ft. on a diamond or 155 trees/acre. 
ET (inches/day) x 27.154 gallons/acre inch = gallons/tree/day 
                                       trees/acre 
(.25 x 27, 154)/155 = 44 gallons/tree/day

Assuming 90% efficiency, you would have to apply +49 gallons to get 44. If you have 4 one gallon/hour emitters per tree, it would be necessary to nm the system 12 hours per day.  

This method of irrigation scheduling will give you a place to start. It must then be verified with field checks using devices such as tensiometers, gypsum blocks, soil probes or a shovel  

This is from Olive Notes May 2000, University of California Cooperative Extension Tulare County

Irrigation is essential for optimal production of good quality fruit Dr. Dave Goldhamer's research found, without question, that matching olive tree water use:  

1) Provided best shoot growth. 
2) Produced the best yield (fruit load x fruit size). 
3) -caInliI1g size fruit 
4) -per acre income. 

Here are historical water use figures you can use for a fully canopied (more than 500/0 of the ground shaded) olive grove for the remainder of the season. 

Note, if less than 50% of the ground is shaded ( considered not at fun water use ), first, calculate the percent of ground shaded ( e.g. 30%), second, multiply the percent of ground shaded (converted to a decimal) by 2 ( e.g. .3 x 2 = .6), and third, multiply that answer by the water use for the period in the table. That answer is the water use for the period for your orchard .

Julian Archer

The following thoughts have been extracted from an large article that appeared in the December 1993 issue of Olivae magazine (IOOC). It dealt with the Effect of Water Quantity on Olive Oil Quality. I believe that there is more recent research on this topic as well (Bari reports etc) but this is all I have on this computer. The experiment was carried out in Italy where rain is received during the winter and where the summer is fairly dry. The young olive trees were planted five years earlier and were growing in a medium textured soil. Every ten days from June to September (December to March in Australia) the trees received 36mm of irrigation water. The irrigated trees set 33% more fruit (by number) than the non-irrigated ones. The individual fruits were 20% heavier and the total crop weight per irrigated tree was 41% greater than the non-irrigated trees. In this trial, the percentage of oil retrieved from both the irrigated and non-irrigated trees was almost identical. Secondly, both groups of trees produced high quality extra virgin oils. The acidity levels and peroxide values were almost identical. In 1996 we asked Dr Juan Caballero, director of the Olive Research Institute in Cordoba, Spain, about the difference in oil quality from dry land and irrigated trees. He stated that, "We have not found any difference in the quality of olive oil produced from irrigated and non irrigated trees".  Here comes another surprise. The polyphenols were 16% higher in the oil produced from the fully irrigated trees. These natural antioxidants also help protect the oil from oxidation during storage and thus help to extend the shelf life of the product. In light of recent postings which showed that polyphenols were reduced in irrigated oils there must be other factors which affect the levels. Maybe soil types and/or fertilizer schedules have an impact on how irrigation changes affect the polyphenol levels????

Rick Allen  AllenRic@cc.usu.edu
Professor Biological and Irrigation Engineering
Utah State University  

FAO Irrigation and Drainage Paper 33 "Yield response to water" by Doorenbos and Kassam, FAO, Rome, 1977. - qualitative information olives.

Concerning consumptive water requirements, the Spanish have been doing some quantitative studies on olive water use. One paper to read is Pastor, M. and F. Orgaz. 1994. Los programas de recorte de riego en olivar. Agricultura no. 746:768-776. (in Spanish). Drs. E. Fereres, Joaquin Berengena and others at the Centro Investigacion y Desarrollo Agrario, Apartado 4240, E-14080, Cordoba, Spain are also conducting studies on water requirements and production functions of olives under irrigation. Their fax number is 34 57 202 721/293 429. Dr. Berengena is probably most accessable. I believe that Dave Goldhamer and others at Univ. Calif. research center at Kearny, California has done some studies on olive, with results reported in a proceedings of the Int. Hort. Soc. I do not have Dave's address or Email. In the upcoming revision of the FAO-24, the Kc's suggested for mature olives having 40 to 60% ground coverage by canopy will be: Kc = 0.70 when some soil evaporation is factored in and Kc = 0.65 when the soil surface is dry. Using procedures for adjusting Kc for fraction of ground coverage that will be in the FAO revision, the Kc for olives having about 20% ground cover is predicted to be about 0.40 for a dry soil surface.

Oliver Spits/Steve Sibbett

San Rafael - Followers of the OliveOil@egroups.com discussion group have been treated to a series of panicked submissions from Manzanillo olive growers in South East Australia who have suffered very low oil yields from young trees after a wet season.  This has brought up much discussion of whether excessive irrigation will bulk up olives at the expense of oil.  Many growers in California report cutting back on irrigation several weeks before harvest to "dry the olives out" and increase the percentage yield of oil.  Presumably this doesn't affect total oil production per acre, it just decreases milling costs, which are calculated per ton.  Similarly, olives harvested late in the season are sometimes slightly wrinkled from water loss which would increase their percentage of oil per ton.  In general,  Farm advisors in California are constantly encouraging adequate irrigation and report that inadequate water is one of the main reasons for poorly growing orchards here.  Farm Advisor Paul Vossen stressed that mature trees take 30 gal/day, 3 year old trees about 5 gal/day at a recent pruning demonstration.  He felt that the roots need 80 % humidity through the dry season.  So are we to water more or less?  In speaking to Steve Sibbett, U.C. Farm Advisor, about this issue he admits that it is hard to say if it is a cultural, genetic, or process problem in Australia which is causing such low yields.  He did add these helpful comments about irrigation to the group:

Here are some comments on olive water relationships from California.  Although we are primarily a table fruit industry and our source of water is irrigation, I think most of the relationships are fundamental to oil production and apply to high rainfall areas too. Irrigation studies conducted in California with Manzanillo olive (Goldhamer et. al.) and elsewhere have conclusively shown that providing adequate water is essential to optimal olive production - fruit number and fruit size (thus total yield and value), and $ return/ac are maximized by meeting the olive tree water needs. The California studies (which used various levels of SUSTAINED deficit irrigation compared to adequate irrigation on the same trees over two growing seasons) showed that by reducing irrigations below optimal levels, in addition to the above effects, indeed increased oil percentage by weight somewhat. However, because fruit yield (tonnage per acre) was coincidently reduced almost proportionately to water deficit, total oil yield per acre was also reduced resulting in reduced revenue per acre. Subsequent studies by Goldhamer (done in case water shortages occur and one has to "parcel out" limited supplies) have identified the period from early June through mid-August here as a period when Manzanillo olive can tolerate some stress. In those studies, again for two seasons, he applied 50% of the water requirement during this period and found no impairment in yield (tonnage) but he found improved total oil yield, provided trees did not suffer for water prior to the stress and were returned to optimal irrigation following the mid-summer stress up until harvest. This REGULATED deficit irrigation (RDI) strategy should be important to oil producers as it improves oil yield (fruit % oil at the same tonnage) without compromising total yield, thus improving total oil yield (& hopefully revenue) per acre. These are the only water relationships that effect total oil QUANTITY here. Certainly, in my opinion, rain fall (or irrigation) during the maturation period is going to increase water in the olive fruits and lower their oil PERCENT. However, I don't believe that rainfall (or irrigation) before harvest is going to reduce or impair ultimate total oil yield, fruits are simply lower in percent oil because of more water, the amount of oil produced per acre has not been reduced. This point has been made before in this discussion venue. The concept of RDI discussed above however, has great utility - it saves water while maintaining yield and improves oil yield too. Note, the papers cited above will be available in the 2nd edition of California's "Olive Production Manual", currently in preparation. I hope this adds something.

Steve Sibbett 
U.C. Farm Advisor
Phone - office 559.733.6486
FAX 559.734.27086/1/00