Subtropical Fruit Crops Research & Education
University of California
Subtropical Fruit Crops Research & Education

Posts Tagged: rainfall

Which Way Weather?

In 2018 the Ojai Valley Land Conservancy (OVLC) accepted a grant from the Resources Legacy Fund on behalf of Watershed Coalition of Ventura County (WCVC) for a study of projected climate changes in Ventura County.  OVLC contracted with Drs. Nina Oakley and Ben Hatchett, climatologists with the Desert Research Institute (DRI), to evaluate historic climate variability and projected changes in Ventura County.  This information is needed to “paint a picture” of future climate in the watersheds of Ventura County (Ventura River, Santa Clara River, and Calleguas Creek) to support and inform climate change-related decision-making. This study provides important information for the amendment to WCVC's Integrated Regional Water Management (IRWM) Plan

You can find a copy of the report on the DRI website at:   

To view presentations and other information from the two WCVC Climate workshops conducted with Drs. Oakley and Hatchett in October of 2018, and April of this year please visit:

Some of those most interesting findings for me, are the historical data. For example, data for the years 1896 – 2018, show a tendency toward increasing maximum temperatures over the period, especially the last 10 years (Fig 1.2). But most interesting, is the increasing minimum temperatures (Fig 1.3) as compared to the maximum temperatures. Winter where is thy sting? The 2018-19 winter was the coldest in my memory, with the heater on full time at night, but there was no general frost damage this year. I can remember 1990 and 2007.

Precipitation in the South Coast region exhibits high interannual variability over the period examined. No notable long-term trends are observed (Fig. 1.4). Since approximately 2000, the 11-year running mean decreases, associated in part with the 2012–2019 drought. It is unclear whether this trend will continue in subsequent years.

There's a lot more information in the report.  READ On.

But something to keep in mind, is that we had a terrible heat wave last July, and it could easily happen again.  Growers who had their trees well hydrated before the heat arrived, sustain less or no damage to the trees and much less fruit drop.  Trees that were irrigated on the day it started to get hot, never had a chance to catch up with the heat.  Once the atmosphere starts sucking the tree dry, water movement through the soil, roots and trunk cant keep up with the demand.  Weather forecasting is pretty accurate 3 days out, and if heat is forecast, get those trees in shape.  You can run water to reduce the temperature and raise the humidity in the orchard to reduce transpirational demand which helps some.

Something we  learned last year.  What we saw and what to expect:


Map of elevational changes in Ventura County and how

elevation ventura
elevation ventura

Posted on Friday, June 14, 2019 at 6:27 AM
Tags: climate change (6), cold (5), frost (19), heat (6), rainfall (4), temperature (1)

A Little Rain Can Cause Salt Problems

Thanks for the rains that leach the soils of accumulated salts and bring on new fresh growth. Or maybe not. When we apply irrigation water with salts which with few exceptions we do in irrigated agriculture, salts accumulate in the soil. They accumulate in a certain pattern depending on the type of irrigation and soil type. There's a strong tendency for drip and microsprinklers to form a pattern of salt accumulation near the margins of the wetted patterns. This pattern is stronger with drip because the source point is always pushing a front outward from the emission point. This pattern occurs with microsprinklers, as well, although not as strongly. These patterns continue to form and accumulate as long as there is no rainfall to evenly push the salt down below the root zone. The longer the period of no rain, the larger the salt concentration at the margin.

So the way water moves is generally down. It moves in a wetting front drawn by gravity. It moves laterally too, because of the attraction water has for the soil particles. It will move laterally more in a clay soil than in a sandy soil because there are more particles in a clay soil than a sand (actually more surfaces that hold water). It also carries salt with it. Wherever the water moves, the salt moves. The more rain, the more salt is moved down. The more rain, the deeper the salt is pushed.

The problem with rain, is that if there is not enough, the salt tends to move laterally. In this wet soil solution, the salt is moving from where it is concentrated, to where there is a lower one. And if there isn't enough rain to move that salt down, it just moves back along the salt gradient, back to where the water first came from…….towards the roots. And that salt may be at such a high concentration that it can cause plant damage.

We talk about effective rainfall. This is usually about a quarter of an inch of rain. This is the amount of water to do more than just wet the dust, it's the amount to move water into the root zone. It is also moving salts into the root zone which can be a real problem. A good rain will do more than wet the dust, it will also move the salts out of harm's way in the root zone. The amount of rain necessary to do this going to depend on the salt accumulated and the soil texture. The more salt, the more rain needed. The finer the texture, the more rain. So there is no good cookbook, other than you need enough. And the first rains of the year, watch out. This is often when the highest salt accumulation and the most irregular the rains. Small amounts that can move salt into the root zone.

If there is not enough rain……………The solution !!!!!!!! Run the water to make sure there is enough to move that salt down. Crazy, but a few months ago we had just this situation. It was one of the last rains in the winter and it was not enough to move salts down, and within a week many avocados showed leaf damage. It was sad since we had all been wanting rain, and we wanted a good drenching.

So why am I bringing this up now? Well, the other night I woke up to rain, glorious rain. I enjoyed listening to it and then it stopped. I thought O NO, it's not enough. There are going to be problems. Well luckily most places didn't get and where it did, it was a dust settler. But it made me aware that with the first rains we might see this fall, growers should be on their guard.

Get ready to irrigate with the first rains if they are insufficient for adequate leaching.


Posted on Tuesday, October 25, 2016 at 4:44 PM
Tags: rainfall (4), saliniity (1), salt (8), salt burn (4), tip burn (8)

Predicting Water Demand Along the Coast Versus in the Valley

Recently I was asked why an irrigation schedule could be projected for almond and citrus in the Central Valley (Almonds:

_Flood_Weekly_ET/Citrus: ) and why the same couldn't be done for the main avocado growing areas. Here was my response:


Generating a generic irrigation schedule for avocados along the coast is very difficult and if done would be terribly misleading. Scheduling gets really hairy along the coast where avocados are grown. As you get further from the coast the water demand (ETo) increases in many months, typically increasing in the summer. This can be most pronounced in the late winter/spring when the fog along the coast really causes a contrast between coastal and inland conditions. May in Ventura, the sun comes out for about 2 hours and in Fillmore 20 miles inland it may be 90 F at 4 PM. The fog is a major determinant for irrigation demand and it varies daily, monthly and year to year from Monterey to San Diego. So fog can throw off an irrigation schedule.

The next variable to area-wide scheduling is the topography where avocados are grown, usually slopes to improve air and water drainage. Depending on the aspect and slope position, the ETo can vary tremendously depending on the sky conditions and what those conditions are depending on the time of day (such as foggy in the morning and clear in the afternoon). So west and south facing will always be higher than north and east. The top of the slope that intercepts more wind than the bottom and will have higher ETo than the bottom of the slope. And if the trees intercept more evaporative conditions midday when the sun comes out, it will be much higher than the east side in the morning when fog is dripping off the trees (zero evaporative demand). Then as you go south from Monterey to San Diego the ETo goes up, just because of latitude and sun interception. These conditions are very different from Fresno where ETo in July is 0.6 inches per day and is the same until Sept, the sky is clear most days and trees are grown on fairly flat ground.

Now throw in rainfall. Almonds are deciduous and only count on the value of rainfall as that which is stored in the rooting zone going into spring when leaves are come out. Avocados rely on winter rain for transpiration and salt leaching. In a good year a significant portion of the total yearly ETcrop can be subtracted from the irrigation demand. In a low/no rainfall year that all needs to be made up by supplemental irrigation.

An almond grower in the Valley might be able to go onto a calendar, set the clock if they have water on demand and walk away. That's never going to happen in a coastal avocado orchard. Depending on where the avocado is grown and the ETo at that site, applied water might vary from 1.5 ac-ft per acre to 3.5. This will depend on rainfall (when and how much), water quality (which determines leaching requirement) and the system delivery (system efficiency). This system issue can be further complicated by whether the delivery is on-demand or whether a certain amount will be delivered at a certain date for a certain length of time - 24 hours or 48. This makes it difficult for the grower to put on exactly what ETo and other issues the trees would demand. In this case, the delivery system determines the schedule.

So this is why there's no chart showing ET demand for coastal avocados where the bulk are grown in California.


A CIMIS (CA Irrigation Management Information System) DWR weather station for calculating crop water requirement.

CIMIS station
CIMIS station

Posted on Monday, July 27, 2015 at 6:50 AM
Tags: avocado (289), avocado (289), citrus (339), climate (3), ETcrop (2), ETo (3), evapotranspiration (6), rainfall (4)


Standing water can lead to asphyxiation


Asphyxiation is a physiological problem that may affect certain branches, whole limbs or the entire tree.  Leaves wilt and may fall, the fruit withers and drops and the branches die back to a greater or lesser extent.  The condition develops so rapidly that it may be regarded as a form of collapse.  Usually, the larger stems and branches remain alive, and after a time, vigorous new growth is put out so that the tree tends to recover.

Asphyxiation is related to the air and water conditions of the soil.  The trouble appears mainly in fine-textured or shallow soils with impervious sub-soils.  In 1997-98, this even occurred on slopes with normally good drainage because the rains were so frequent. When such soils are over-irrigated or wetted by rains, the water displaces the soil oxygen.  The smaller roots die when deprived of oxygen.  When the stress of water shortage develops, the impaired roots are unable to supply water to the leaves rapidly enough and the tree collapses.  The condition is accentuated when rainy weather is followed by winds or warm conditions.

Canopy treatment in less severe instances consists of cutting back the dead branches to live wood.  If leaf drop has been excessive, the tree should be whitewashed to prevent sunburn.  Fruit, if mature should be harvested as soon as possible to prevent loss.  In the case of young trees, less than two years of age, recovery sometimes does not occur, and replanting should be considered if vigorous regrowth does not occur by July.

 Asphyxiation can be reduced by proper planting and grading.  If an impervious layer is identified, it should be ripped prior to planting.  The field should be graded so that water has somewhere to run off the field during high rainfall years.  Heavier soils might require planting on berms or mounds so that the crown roots have a better chance of being aerated.

Post-plant, if an impervious layer can be identified and is shallow enough to break through, ripping along side the tree of drilling 4-6 inch post holes at the corners of the tree can improve drainage.  It is important that the ripper blade or auger gets below the impervious layer for this technique to be effective.

Posted on Monday, July 9, 2012 at 9:25 AM
Tags: asphyxiation (3), irrigation (80), rainfall (4)
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