Measurement of Potential Tree Water Stress Effects on Almond Tree Growth and Yield

Introduction

The 20-acre Marine Avenue almond planting was established in 1990 to compare the response of 4 almond varieties (Butte, Monterey, Nonpareil and Carmel) to irrigation by various microirrigation systems (surface drip, subsurface drip, and microsprinklers). A key component of the comparison has been to apply the same amount of water with each of the different microirrigation systems. Thus, what is being compared is the response of the trees to the various microirrigation methods; no differences in the amount of applied water.

Previous years of investigation indicated that there was a yield and tree growth advantage associated withy use of microsprinklers. To investigate a possible cause of this difference - differential water stress among trees irrigated with different microirrigation methods - a project, funded by the California Almond Board, was undertaken during the summer of 1998.

Experimental Procedures

To determine if there were different levels of water stress occurring between trees irrigated with different microirrigation systems, we monitored Nonpareil almond trees irrigated with surface drip (18 trees), subsurface (12 trees), microsprinklers (18 trees), and microsprinklers 1.2ET (9 trees). The same trees were monitored weekly from July through October for midday leaf stem water potential using a pressure bomb device. Three trees in the same Nonpareil row in each monitored plot were measured. In addition, a flow meter was installed in each of the lateral lines being monitored and flow meter readings, to determine applied irrigation water, were taken when leaf water potential was measured.

Results

Almond Growth and Yield

Tables 1 and 2 summarize the almond tree growth measured as trunk diameter and the yields at the Marine Ave. orchard. Trunk diameter measurements for 1998 are still being analyzed. It appears that there continues to be an advantage of use of microsprinklers in this orchard.

The soils at the Marine Ave. orchard are gravely, low water-holding capacity soils which are relatively shallow (3-4 feet to a restricting clay layer). Backhoe investigations and extensive soil moisture monitoring using neutron probes, tensiometers, and gypsum blocks show that by midsummer the active root zone is restricted to the soil volume wetted by the irrigation system. For the microsprinklers, this is a circle, 14 ft. in diameter, centered between trees in the row. For the drip system, this is a wetted "trench", 4-6 ft. wide, along the tree row below the drip lateral line. It is strongly suspected that the different wetted soil volumes associated with the different microirrigation methods is a cause of the differential tree growth and yield. In addition, it is suspected that the different wetted soil volumes affect tree water stress during the growing season.

Irrigation and Leaf Water Potentials

The seasonal water applications for each of the microirrigation systems along with the estimated almond evapotranspiration (ET) were monitored. Nearly the same amount of water was applied via the surface drip and the microsprinkler systems. As expected, the 1.2 ET microsprinkler treatment applied approximately 20% more water. All three of these microirrigation systems applied less water than the estimated ET demands of the almonds. Soil moisture reserves would make up some of this deficit early in the season. Some deficit irrigation was intentional later in the season to aid in hull split. Irrigations also had to be cut back at the end of the season for harvest.

The subsurface drip irrigated trees received only 70% of the applied water as compared to the surface drip and microsprinkler irrigated trees. Following excavation and lateral line flushing investigations, it was determined that there was root intrusion occurring in the subsurface drip lines. Remedial actions are now being investigated to solve this problem.

Seasonal midday leaf water potential measurement, applied irrigation water, and almond ET for each of the microirrigation treatments were monitored throughout the summer.

Analyzing the results of the summer’s work, the following observations can be made:

1. All trees monitored experienced water stress during the season with the greatest water stress occurring during August – a period of peak water demand. This, in conjunction with the water application/ET information, indicates that the trees were under-irrigated during this time.
2. Slightly greater water stress was measured in the microsprinkler treatments than in the surface drip irrigated treatment. This is attributed to the less frequent microsprinkler irrigation as compared to the drip irrigations. During peak water demand periods, the drip irrigated trees received water on nearly a daily basis while the microsprinkler trees were irrigated approximately every 3 days. The microsprinkler trees were monitored just prior to irrigation on two occasions in August (8/5 and 8/19) and the leaf water potential indicated the greatest water stress recorded during the summer’s monitoring. Even this level of water stress though would not be considered a severe stress level which would significantly impact yield and growth.
3. Water stress occurs even under the frequent irrigation regimes of microirrigation. This is particularly true of microsprinkler irrigated trees where there may be 2-3 days between irrigation during midseason. It is very possible that the soil conditions at the Marine Ave. orchard (shallow soils with low waterholding capacity) contribute to this condition. An orchard with better soil conditions (greater soil moisture reserves) may not exhibit as great a water stress swing between irrigations.

Table 1

Mean almond tree trunk cross sectional area (square inches) by irrigation treatment and almond variety for 1994-1998. Statistical comparison of mean trunk diameters done by variety and by year. Numbers followed by the same letter are not significantly different at the 5% level.

Irrigation Treatment       Carmel        Butte     Nonpareil      Monterey

 1994

Surface drip                       17.7 b         22.2 b          20.3 c           21.2 a

Microsprinklers                    19.2 a         24.3 a         24.3 a           21.7 a

Subsurface drip                    17.1 a         22.2 b         21.3 b           21.4 a

1995

Surface drip                       23.0 b         33.8 b           29.8 b          28.7 b

Microsprinklers                   26.0 a         37.5 a           31.8 a           35.6 a

Subsurface drip                  24.2 b         35.3 b           29.8 b           29.4 b

1996

Surface drip                       30.9 b         44.6 b             40.0 b         36.2 b

Microsprinklers                   35.6 a         52.9 a             43.4 a          48.9 a

Subsurface drip                  30.4 b         46.7 b             38.1 b          36.7 b

1997

Surface drip                      35.9 a          52.0 b            47.2 b          41.6 b

Microsprinklers                 38.2 a          55.9 a            49.7 a          53.4 a

Subsurface drip                 36.7 a           48.4 c            45.8 b          39.7 b

1998

Surface drip

Microsprinklers

Subsurface drip

Table 2

Almond Yield (lbs/acre)

1994

Surface drip                            1047 b             1053 c

Microsprinklers                       1543 a             1532 a

Subsurface drip                       1235 b            1234 b

1995

Surface drip                              752 a               745 a               920 a              1293 a

Microsprinklers                         715 a              726 a                983 a              1332 a

Subsurface drip                         873 a              701 a                639 b             1241 a

1996

Surface drip                            1777 a             1924 a               2362 a           2492 ab

Microsprinklers                       1748 a             2276 b               2708 a           2884 a

Subsurface drip                       1673 a            1845 a                2350 a           2231 ab

1997

Surface drip                            2002 a            2468 a               1991 a             1948 ab

Microsprinklers                       1888 a            2513 a                2179 a             2252 a

Subsurface drip                       1829 a            2422 a               1846 a              1714 b

1998

Surface drip                            1727             2822                   2419                2243

Microsprinklers                       1891             2984                   2736                2295

Subsurface drip                       1525             2505                   2332                1847

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Copyright © 1999 Colusa County Cooperative Extension, Univ. of California
Last modified: September 01, 2000