In a recent meeting the topic of where to go for irrigation information came up. Well there's no substitute for attending a class in irrigation, such as offered at Cal Poly SLO (http://www.itrc.org/classes/iseclass.htm ,
but here's some written sources to get you started thinking.
So after five years of drought a grower told me he finally gets it. Farming avocados in Goleta with limited well water and poor quality and expensive, rationed delivery, he has finally cut out trees that were not performing well. These were wind sept trees, areas with root and crown rot. Tree were either stumped or removed altogether. The idea is to focus on those trees that are productive, and are they ever. They are getting the water they need and their schedule has been changed. Previously irrigation had been on a fixed schedule of 2 weeks and got 24 hours per set. The schedule was dictated by the time it took to get around to all 300 acres. Now trees are irrigated one to two times per week with shorter sets, from 5-8 hours depending on the time it takes to get 18 inches of penetration in sandy ground. Yields per acre have significantly increased, largely because non-productive areas have been eliminated and the remaining trees are getting what they need.
He is also anticipating irrigation needs – projecting a schedule.
The driving forces for water loss in avocados in decreasing effect are sunlight (day length, cloud cover), wind, humidity, and temperature. More light, more transpiration. More wind, more water loss, lower humidity, more water loss. And least of all if temperature which is what we normally respond to, but which the tree responds the least. Usually, though the most desiccating conditions occur during periods of high, dry winds that blow out of the Great Basin – the dread Santa Anas or in the case of Santa Barbara the Sundowners. When they start blowing, it's hard to play catchup. It takes a while for water to infiltrate and for the roots to start taking it up and filling all the drained leaves and stems. Now the grower more carefully watches the forecasts and makes sure to get water on before the high demand conditions arrive. With multiple blocks he wants to get all of them wetted to a normal irrigation depth and then it's time to start the cycle over again. He keep an eye on the nearest CIMIS station to see how much the water demand is increasing and adjusts the irrigation frequency. Importantly, h keep your eye on how fast the soil in the root zone is drying out and then makes even smaller and more frequent applications.
When it comes to making the water meet the needs of the trees, he is really customizing each irrigation.
The Irrigation Training & Research Center (ITRC) of Cal Poly San Luis Obispo tested 28 different pressure-compensating models of microirrigation emitting devices from a total of nine manufacturers in order to compare independent laboratory testing with manufacturer specifications.
The test results indicate that:
The majority of ~0.5 gallon-per-hour emitters (drippers), regardless of manufacturer exhibited:
Good uniformity of manufacturer
Had excellent response to pressure variation
Had consistent flow rates within the nominal operating pressure range
But that the percentage of well-performing products decreased as the designed flow rate increased. Many of the emitters designated as microsprinklers or sprayers, although pressure compensating did not compensate at the normal operating pressures. Often the pressure compensating feature did not start performing until much higher pressures were achieved. Often this occurred when clogging occurred and this clogging often occurred where the pressure diaphragm was located and was not performing. Sediment would get in back of the diaphragm. Effectively the emitters were not pressure compensating. The testing procedure of numerous medium and high flow models also found individual pieces were found to be defective. These faulty emitters had a measurable effect on the evaluation for those models.
Read more at: http://www.itrc.org/reports/pdf/emitters.pdf
Sand media filters are commonly used in agricultural microirrigation systems. They have the advantages of simplicity and large capacities and are favored by many farmers and designers over other filtration hardware when there is a lot of organic matter in the water. The Irrigation Training and Research Center (ITRC) at Cal Poly San Luis looked at sand filters to see if it were possible to use lower-than-accepted backflush pressure and thereby reduce the total pressure required for irrigation systems. By lowering backflush pressure it would be possible to design a system that could run at an overall lower pressure and hence cost. The various components of microirrigation systems run at lower pressures than the backflush pressures recommended for most sand media filters.
The conclusions are:
There are substantial pressure differences amongst different models and designs during backflush and filtration
The main pressure loss is at the backflush valves
If designed right large backflush flow rates can be accomplished at low backflush pressures (this is critical for proper detritus removal).
There are substantial differences among underdrains of various models which affects pressure requirements
No large intimal high pressure was necessary to break up the media bed, a common practice.
Different underdrain designs create different patterns of cleaning the media.
There were substantial differences among models in the amount of sand discharged from the system at backflow rate of 190 GPM. Sand discharge should actually be avoided since it's an indication of preferential flow and poor cleaning.
These are some new ideas, and even though they are meant to reduce pressure and energy use, they are also good management suggestions.
If this strikes your fancy, read more at http://www.itrc.org/reports/mediafilters.htm
A crucial aspect to microsprinkler and dripper performance is maintaining the size of the orifice as it was delivered from the factory. Even small changes in the size of the orifice can have significant effects on the volume of water distributed in the orchard. One of the most common causes of a decrease in orifice size or even clogging is a result of the high lime content of our waters. Calcium carbonate precipitation can readily be observed by the whitish deposits that form on emitters and microsprinklers. With the drought it's important to make that water go farther.
Reasons for carbonate precipitation include the following:
1. Change in the pH of the groundwater. This can occur when groundwater is pumped. Pumping reduces the pressure of the groundwater as it flows into the well. This reduction releases dissolved carbon dioxide gas causing the pH of the groundwater to increase. The pH increase can result in carbonate precipitation.
2. Evaporation of water in the dripper or microsprinkler. Evaporation increases the concentration of chemicals dissolved in the water remaining in the emitter. Because of its low solubility in water, calcium carbonate readily precipitates during evaporation.
3. Increase in water temperature. The solubility of calcium carbonate is reduced as water temperature increases.
4, Injection of certain chemicals, such as bleach or some fertilizers that interact with the water.
The problem of lime precipitation depends primarily on the pH of the water. At pH values less than 6, mostly dissolved carbon dioxide and a small amount of carbonic acid exist in the water. At pH values between about 6.5 and 10, bicarbonate is the dominant species. When water evaporates from the irrigation system the bicarbonate precipitates as lime if there is adequate calcium in the water. The potential for carbonate clogging is highest when bicarbonate concentration in the water exceeds 2 milliequivalents per liter (meq/L) and the pH exceeds 7.5.
This relationship of bicarbonate to water pH indicates that lowering the pH will prevent or reduce carbonate clogging of the system. Lowering the pH, dissolves any existing carbonate precipitation and prevents the formation of lime deposits.
A water's pH is lowered by injecting acids. The common acids, such as sulfuric and hydrochloric (muriatic) have been used, as well as the more expensive citric and nitric acids. An acid/fertilizer compound of a combination of urea/sulfuric acid (N-pHURIC ®) has proven to be useful and much safer than straight acids. This acidifying material is most commonly used for water treatment rather than as a source of nitrogen. If the material is used in any significant amount, its nitrogen contribution to the fertilizer program needs to be considered.
Determining frequency and amounts of acid to prevent clogging can be fairly matter of fact. Depending on the rate at which carbonate precipitation occurs, acid injection may only need to occur intermittently during the irrigation cycle to. It might only require 30-60 minutes to maintain a pH of 4. With more problematic waters, continuous acid injection to maintain a pH between 5 and 7 may be necessary.
The amount of acid needed to lower the pH to a desired level depends on the bicarbonate/carbonate concentration in the water and the target pH. The water can be sent to a lab for determining the acid amount or a trial and error approach can be used. Acid can be added to water in increments while measuring the pH until the desired pH is reached. Water pH can be measured with pH test strips or a hand held pH meter. Test kits are also readily available at swimming pool supply stores.
Other than acid for correcting lime clogging, there are several other amendments being sold on the market. Sodium hexametaphosphate has also been used and works against iron and manganese clogging. The small amount of sodium is not a problem. It is safer than an acid, but costs a bit more.