Posts Tagged: ET

What Happens to Trees on a Hill?

Many orchards in California are planted on slopes, the most extreme examples are usually avocado orchards with some slopes exceeding 50%. They pose difficulties in harvesting because of the steepness, but also in their irrigation. These slopes can be north/south/east/west facing or all of the quadrants in the same orchard. The plantings can be of varying steepness and at different positions (toe, top, mid-slope). These positions affect solar radiation which is the main driver of evapotranspiration, but also wind interception. South and west facing slopes intercept the most sunlight, while north and east intercept the least. The top of the slope usually intercepts the most sunlight during the day and also the most wind. There can be 100% difference in the amount of ET depending on the position on the slope. That is, some trees require twice as much water as others because they are getting more energy that drives water loss.

When looking at an older avocado grove, the trees are usually larger at the bottom of the slope where there is the least wind and most irrigation water interception. This is where the soil is the deepest and has the greatest moisture reserve. The soils at the top of the slope are the shallowest and get the greatest amount of energy driving water loss. Trees on the north side are often tall from greater soil depth and moisture reserve and less ET demand. As the solar angle changes during the year (lower in the sky during the winter), the proportion of ET in these different positions changes.

Right. OK. We know this. The problem is that many smaller orchards are laid out so that there is one valve controlling the amount of water going to all the different positions. The trees at the bottom of the slopes get the same as those at the top. Those on the north side get the same as those on the south side. This basically sets up an orchard for stress. Stress that leads to disease and impacts on yields and ultimately the longevity of the orchard.

Add to this, irrigation performance varies with pressure and many orchards have very little pressure compensation. Often trees at the top of the slope have the lowest pressure and output. The distribution uniformity is often terrible. Not only are the normal problems of broken and clogged emitters an issue, but also pressure loss from elevation differences.

So where you plant on a hillside should be part of the irrigation design. In different positions on the hillside there are different water requirements and unless they are irrigated differently, there can be major differences in tree response. These different irrigation requirements should be incorporated into the irrigation design by creating as many different irrigation blocks as possible. A valve for the top of the slope, another for the north and south slopes, etc. These can be incorporated easily in the initial design and not so easily customized after the trees have been planted.

At some point, for optimum tree performance, tree health and water use efficiency, growers should recognize the need for irrigating to trees' needs according to slope position. Avocado growers have it harder than most growers.

Read more about an ET study done on a hill:

http://www.avocadosource.com/CAS_Yearbooks/CAS_86_2002/cas_2002_pg_099-104.pdf

hill trees

Fog Contributions to Avocado and Citrus Water Use - Adjusting Irrigation

Drought may not be the right time to be thinking about this, or maybe it is.  It concerns managing water and any time a grower uses water more effectively the crop performs better.  But fog can be a significant factor in water management.

As fog passes through a tree canopy, it is absorbed by leaves and coats them. Before the tree will transpire water, the water coating must first be evaporated before the tree loses internal water. This water use is not accounted for in a water budget schedule using evapotranspiration based inputs, such as from CIMIS. For deciduous trees, this is often not of concern, because in the winter they don't have leaves and therefore are not transpiring anyway. For evergreen subtropicals like citrus and avocado, this could be an important source of water.

In many situations in the Central Valley and along the coast there can be periods where fog can represent a significant proportion of the water requirement for an orchard. These periods would be for winter tule fog in the Valley and along the coast in the spring and early summer. A recent publication by Rick Snyder at UC Davis has just been released that shows how this fog water can be incorporated into an irrigation schedule. You can see it at the UC's California Institute for Water Resources website: http://anrcatalog.ucanr.edu/pdf/8532.pdf, http://ciwr.ucanr.edu/california_drought_expertise/droughttips/

fog

Capturing Rain

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)

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

rain

ATMOMETERS FOR IRRIGATION MANAGEMENT

Efficient and precise irrigation management is becoming increasingly important inCaliforniaagriculture, both for maximizing crop quality and for conserving water. The most advanced irrigation scheduling strategy is based on local measurements of reference evapotranspiration (ETo), which is converted to crop evapotranspiration (ETc) with an appropriate crop coefficient (kc).

To be able to use this method, an irrigation manager needs to have locally accurate ETo values throughout the growing season. However, the highly variable microclimates that characterize many farming areas often make it difficult to use data from distant weather stations; therefore an accurate local measurement may often be preferable to relying on a regional value.

One inexpensive option for measuring ETo locally is to use a simple atmometer (Fig. 1). Atmometers are water-filled devices, in which the actual evaporation of water is measured over time. In their simplest form, the atmometer is outfitted with a graduated sight glass on the water supply tank which allows the user to easily measure the evaporation that occurred over a given period. In practice, this type of atmometer is most suited for making readings at multiple day intervals, for example once per week, or on days when irrigation is applied.

The performance of atmometers versus more expensive weather stations was evaluated on theCentralCoastin 2003. In this study, atmometers were placed adjacent to seven weather stations throughout the area, and weekly values for both methods were compared (Fig. 2). The results indicate that the atmometers and weather stations have very comparable ETo readings, with the atmometers indicating somewhat lower ETo values under conditions of lower evapotranspiration.

Like any technique, using atmometers has advantages and disadvantages. Advantages include their very low cost and ease of operation, with no computer or power required. Disadvantages include the potential for damage by freezing weather, the need to refill the water supply (every three to six weeks), and the need to read the gauge manually. Also, if they are installed in a large open area, birds may tend to perch on the evaporating surface and foul it with their droppings; for this reason several wires are installed on top of the device to

discourage birds from perching there. In general, atmometers function quite reliably with few problems.

Converting atmometer ETo readings to the amount of irrigation run time required to replenish the soil moisture lost to evapotranspiration is fairly straightforward. A relatively simple example for a sprinkler-irrigated field is presented below in Table 1.

(Note: To convert Gallons to Inches: Gallons ÷ Area (square feet) ÷ ?0.6234 = Inches

To convert Inches to Gallons: Inches * Area (square feet) ÷ 1.604 = Gallons)

 Atmometer installed on a fence post

Using Evapotranspiration (ETo) for Scheduling Irrigations: An Improvement on Guessing?

Irrigation Calculator developed by Reuben Hofshi, Shanti Hofshi and Ben Faber.

citrus irrigation

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