- Author: Mary Bianchi
Capturing Precipitation - How much rainfall do I need to capture?
Managing precipitation to your advantage is really a three step process (Lal and Stewart, 2012).
ü Step 1 - maximize preciptitation captured in the soil
ü Step 2 - minimize the evaporation of the stored soil moisture
ü Step 3 - maximize plant water use efficiency
The first step of the process is often thought of as “effective rain”. Effective rainfall refers to the percentage of rainfall which becomes available to plants and crops. It considers “losses” due to runoff, evaporation and deep percolation (Klein, 2011). In the past we might have considered deep percolation as a loss. We now know that percolation “losses” may be a vital resource in sustaining our groundwater basins. As we move into the fall of 2015, we have the opportunity to plan for effective rainfall by managing the orchard floor for maximum capture of precipitation. This will help provide stored soil moisture for plant growth as well as deep percolation of water to groundwater
The following figure illustrates some of the important points about effective rainfall and reminds us of what we can do to maximize capture of precipitation (1). We want to maximize 2 (infiltration during a rain event), 3 (surface capture), 6 (infiltration from surface capture), 7 (percolation to ground water), and 8 (rootzone storage for use by the crop). We want to minimize 4 (runoff) and 5 (evaporation).
When rain water ((1) falls on the soil surface, some of it infiltrates into the soil (2), some stagnates on the surface (3), while some flows over the surface as runoff (4). When the rainfall stops, some of the water stagnating on the surface (3) evaporates to the atmosphere (5), while the rest slowly infiltrates into the soil (6). From all the water that infiltrates into the soil ((2) and (6)), some percolates below the rootzone (7), while the rest remains stored in the rootzone (8). From FAO Irrigation Water Management 1985 http://www.fao.org/docrep/r4082e/r4082e05.htm#4.1.4 effective rainfall
Larry Stein from Texas A&M wrote a very good basic explanation “So What Constitutes an Effective Rain Event ?” (Stein, 2011) We can use his approach to look at managing precipitation in the Central Coast. Understanding these concepts can help you manage precipitation in your operation.
For example, the majority of olive roots are in the top 18 inches of soil. So how much rainfall do we need to capture to refill the rootzone of an olive grove in Paso Robles? We need to know:
ü The amount and intensity of rainfall
ü The infiltration rate of the soil (how fast the soil takes in water). Sandy soils take water in more quickly.
ü How much water the soil will hold in the rootzone of the grove
Average rainfall for Paso Robles in January is about 2.75 inches. Table 1 shows that olives on a sandy loam soil might be able to infiltrate 1 to 1.5 inches per hour. If all that rain comes in one storm then as much as 1.25 inches may either run off (4) or pond (3) in the low spots until it can infiltrate.
Average rainfall in Paso Robles in January would be adequate to refill the rootzone of olives (8) on a sandy loam soil, IF all of the rainfall infiltrates (2), and none is lost to evaporation (5) or runoff (4).
Table 1. General soil water storage and depletion characteristics for three different soil types (Klein, 2011)
|
Soil Texture |
||
|
Sands |
Loams |
Clays |
Water infiltration rate (inches / hour) |
2.0 – 6.0 |
0.6 – 2.0 |
0.2 – 0.6 |
Available water (inches / foot) |
1.0 – 1.5 |
1.5 – 2.5 |
2.5 – 4.0 |
Days to depletion when ET – 0.2 inches / day |
5 – 7.5 |
7.5 – 12.5 |
12.5 – 20.0 |
Amount of water to wet to 18 inches in a dry soil (inches) |
1.5 |
2.25 – 3.0 |
3.75 |
Cover crops help keep the soil surface from crusting as well as protecting the soil surface from erosion. Their roots provide channels for water to infiltrate into the soil. Remember that cover crops may also be using water stored in the rootzone (8). When facing drought conditions, it may be advantageous to manage with low residue cover crops to reduce the amount of water extracted from the rootzone. Here's a link to a video on low residue cover crops and their impact on runoff from work by UC Cooperative Extension Advisors in Monterey County https://www.youtube.com/watch?v=k0oVVJ_BA7s
Klein, L. 2011. So What Constitutes an Effective Rain Event? http://aggie-horticulture.tamu.edu/earthkind/drought/drought-management-for-commercial-horticulture/so-what-constitutes-an-effective-rain-event/ .
Lal. R.and, B.A. Stewart. 2012. Soil Water and Agronomic Productivity
/h3>- Author: Ben Faber
Soil likes to be covered at all times. It doesn't “want” to be exposed to the elements, so you either cover it (plants, asphalt, paper etc.) or it will cover it for you with plants (weeds). If it can't be covered fast enough, it disappears – erodes. This can be from wind or rain or just natural movement down slope. Plants that are managed for other than their agricultural return are called cover crops, although they can also have a crop that is saleable. Often weeds can be managed to be a cover crop, as well.
We are looking at a possibly wet winter and many tree crops grown on hillsides and sloping ground are prone to soil erosion. Covers can be grown year round, but that usually means they require water all year round. That means they need an irrigation system dedicated to their needs. It also means having extra water which may be limiting.
A winter cover crop that grows out in the winter, does its thing (although that “thing” can include lots of other things, e.g. insectary, nutrients, water retention, etc.), and then dies or goes dormant, can be ideal. It also requires less water than a permanent cover.
But there is a big problem here. Establishment of an introduced cover still requires water. Rainfall in Southern California is erratic and there may be early rains to germinate seed, but it may not be consistent enough to get the plants established. In fact, they may die for lack of further rain or be delayed.
Delayed germination means that soil is cooler and there is less growth. The real growth may occur after there has been sufficient rainfall by January and February when the soils are cooler and there is even less chance for growth
So when rainfall is doing its worst, there's no effective cover. Or what cover there is, is what has germinated from “native” seed. It may not have the characteristics you want for management: low stature, low entanglement with the trees, low water use, holds the soil without holding up harvest, etc.
So what do you do? There are several approaches. You can move the sprinklers out into the middles and irrigate up the seed. If you are in a limited water situation, you can do alternate middles, not cover cropping the whole area, or every third middle. Whatever it takes to break the surface flow of water. Or you can turn to mulching. Put down sufficient mulch in a middle or every other middle to break overland water flow.
Cover cropping is easier than mulching, but it takes water and timing.
Below are two websites with descriptions of cover crops and how to distinguish them from “weeds”. Often a good cover can be the residential weeds. A low–lying cover allows pickers in to get lemons without mess and fear of snakes. It also means that it can be more easily treated (mowed, weed whipped) at the end of the rainy season to reduce fire hazard.
1) Characteristics of different cover crops
http://asi.ucdavis.edu/programs/sarep/research-initiatives/are/nutrient-mgmt/cover-crops-database1
2) Weed identification from the UC IPM
- Author: Ben Faber
Oh, oh is there going to be a wet fall and winter? If it comes and washes the accumulated salts of the last four years out of the root zones of citrus and avocado, that's a good thing. But what happens if there is a little too much of the good stuff? In the winter of 2005, Venture got over 40” of rain, which is 200% of what is normal. The last time big rains occurred prior to that was in the winter of 1997-98. That year the rains were evenly spaced on almost a weekly basis through the winter and into the late spring and over 50" fell. That year we had major problems with both citrus and avocados collapsing from asphyxiation. The same occurred in 2005, but not so pronounced. This winter could also see some collapse.
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 alongside the tree or 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.
We don't know what the future holds. Hopefully rain that does some good without too much harm.
Avocado asphyxiation 2005
- Author: Ben Faber
California is suffering historic drought conditions. The information on this webpage offers farmers and ranchers links to valuable resources, carried out by researchers and specialists, on a vast array of issues they are facing during this extremely dry and difficult year. For general drought information please click on the links below.
For drought issues with a particular crop, please see the left navigation buttons where you will find resources for your specific agricultural needs.
UC ANR California Institute for Water Resources Drought Information
http://ciwr.ucanr.edu/California_Drought_Expertise/
UC Davis Rangeland Watershed Laboratory Managing Drought http://rangelandwatersheds.ucdavis.edu/main/drought.html#RancherPerspectives
Coping with Declining Groundwater Levels
CIMIS Drought Tips
View film excerpts from University of California Researchers and Academics on a large variety of expert water and drought topics
Insights: Water and Drought Online Seminar Series
Irrigation Scheduling Tools
UC Drought Management-Evapotranspiration Scheduling
Soil Moisture Monitoring
UC Drought Management-Soil Moisture Monitoring
Extension- Soil Moisture Sensors
Drought Strategies
Irrigation Scheduling during a Drought
- Author: Ben Faber
How to Manage PestsUC Pest Management Guidelines
I have had a number of requests to identify fruit spotting on lemons. It turns out to be Septoria fungus which can show up on leaves, stems and fruit. The key to this is to make sure there is a fungicide on the tree in the fall before the wet weather kicks in
CitrusSeptoria SpotPathogen: Septoria citri(Reviewed 9/08, updated 9/08) In this Guideline: SymptomsEarly symptoms of Septoria spot appear as small, light tan to reddish brown pits on fruit, 0.04 to 0.08 inch (1 to 2 mm) in diameter, which usually do not extend beyond the oil-bearing tissue. Advanced lesions are blackish, sunken, extend into the albedo (white spongy inner part of rind), and are up to 0.8 to 1.2 inch (20 to 30 mm) in diameter. Dark brown to black fruiting bodies often develop in these lesions, which usually do not extend beyond the oil-bearing tissue. The spots are much more conspicuous after the fruit has changed from green to yellow or orange. Small spots may develop into large, brown blotches during storage or long-distance transportation. Septoria citri may also cause similar spotting on leaves or twigs that are weakened by frost or pests. Comments on the DiseaseThe Septoria fungus causes spotting of Valencia oranges, late-season navel oranges, and occasionally of lemons and grapefruit. It occurs in the San Joaquin Valley and interior districts of southern California during cool, moist weather. Infections begin when Septoria conidia are transported throughout the tree by rainfall. The spores germinate with additional moisture from rain or dews and commonly infect cold-injured fruit tissue and mechanical injuries. The damage to the rind lowers the grade of the fruit and results in culling. Septoria spot may be confused with copper injury and other abiotic and biotic agents. ManagementApply a preventive copper spray in late fall or early winter, just before or after the first rain. In years with heavy rainfall, additional applications may be necessary. For California oranges (Navels and Valencias) shipped to Korea:
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