In California’s Mediterranean climate, with hot, dry summers and cool winters, irrigation water is essential for maintaining almond orchards. Due to recent droughts, uncertainty about future water allocations, and expanding almond markets, growers are increasingly seeking ways to improve the efficiency of their water use while maintaining or increasing kernel yields.
Increasing the organic matter content of soil can allow the soil to take up more water during rains and to store more water over time. Organic matter from sources such as incorporated wood chips plays a role in improving the soil structure so that water will infiltrate more readily in a heavy rain event or with irrigation, rather than puddling and running off. In addition, the better structure and resulting pore space will allow more of that water to be held in the soil where roots can access it over time, rather than leaching out of the root zone.
How does whole orchard recycling impact orchard water use?
UC research on whole orchard recycling (WOR) shows that, nine years after an almond orchard was recycled and incorporated into topsoil, water-use-related improvements had occurred both in soils and in the trees themselves in comparison to the typical practice of “pushing and burning” orchard trees. This research was conducted as part of a long-term WOR trial at the UC Kearney Research and Extension Center in Parlier, Fresno County.
Orchard tree water use
WOR affects water use efficiency and tree resilience to water shortage:
Water use efficiency (+20%)
Tree water stress (stem water potential) (+13%)
Stomatal conductance (+10%)
Almond yield under regular and deficit irrigation (+ up to 20%)
Soil hydraulic properties
WOR affects long-term shifts in soil hydraulic properties:
Water holding capacity (+32%)
Infiltration rate (+200%)
Soil aggregation (+19%)
This study found no significant effect of WOR on:
Tree canopy temperature
Results: Orchard water use at Kearney Agricultural Research and Extension Center
These results are from a long-term trial assessing effects of whole orchard recycling over nine years
Stem Water Potential and Stomatal Conductance
When a tree is healthy and sufficiently watered, stomata, or tiny pores, on the undersides of leaves open to allow carbon dioxide to enter the leaf and oxygen to leave. Along with this exchange of gases, large amounts of water vapor are also lost through the stomata. When this water loss exceeds the amount of water the tree’s roots can absorb from soil, the plant will begin to appear stressed. If water applications through either irrigation or rainfall are not sufficient to alleviate this stress, the stomata will close. This reduces gas exchange, photosynthesis, and production of carbohydrates, which limit the amount of energy available for vegetative growth, fruit and kernel development, and other important processes.
Research at Kearney found an increase in stomatal conductance (Figure a) and a less negative stem water potential (Figure b) in WOR plots under both regular (100% ET) and deficit (80% ET) irrigation scenarios, compared to the control treatment of burning the previous orchard. These measures indicate that stomata remain open, trees are less water-stressed, and biological processes important for almond kernel development are higher in WOR plots.
Water Use Efficiency and Almond Yield
Research at Kearney found an increase in irrigation water use efficiency of 20% in WOR plots compared to the control treatment (Table A).
Table A.Irrigation water use efficiency
Whole orchard recycled (kg kernel yield per cubic meter of water)
Burned (kg kernel yield per cubic meter of water)
Almond kernel yield was also shown to be greater in WOR plots, with a statistically significant increase of 20% under regular (100% ET) irrigation (Figure c). A smaller increase was observed in the deficit (80% ET) irrigation treatment, although this finding is not statistically significant.
Table B shows an annual comparison of yields in the WOR and control treatments at Kearney for years three through nine following the orchard turnover. Yields are greater in the WOR plots for all years, with the difference peaking in year six(2014).
Table B. Comparison of yields in WOR and control treatments at Kearney for years three through eight following orchard turnover.
Whole orchard recycling (lbs/acre)
Soil Hydraulic Properties
Wet aggregate stability refers to the ability of soil aggregates to resist disintegration under raindrop impact or water erosion. Stable aggregates also help soil to resist compaction and maintain pore space. Adding organic matter, such as wood chips, to soil feeds the soil microbes that provide the “glue” to build and keep soil aggregates intact.
WOR plots at Kearney showed an increase of wet aggregate stability of 19% over the control treatment. These plots also showed reduced soil penetrometer readings (a measure of resistance to root growth) by 14% and bulk density by 4%.
With better soil structure, WOR increased water infiltration rates by 200% (Figure d), and increased soil moisture retention by 32% (Figure e).