The agronomic design calculations start here! GIWR – Gross Irrigation Water Requirements
We’re about to determine the final value which will tell us how big our irrigation system (capacity) shall be, in terms of total water requirements… As we have already seen, depending on our next irrigation project location, the targeted crop, along with the rest of the required parameters gathered, the crop water requirements will differ from each other accordingly.
The crop water requirements are already there! Just multiply the ETo by the kc, according to each period of the year and crop growth stage. That is as:
Crop Water Requirements = ETc = ETo x kc
Depending on which ETo value we are using (mm per day, per week, or per month, for example), we will get the related crop water requirements accordingly. Thereafter, do I mean that those Crop Water Requirements (ETc) values would be the number to be used in our irrigation design?
No, not yet. There are some points that need to be reviewed in order to get the final number: average monthly or yearly precipitation, distribution efficiency, and water quality. Have a look at those:
1.- ☔ AVERAGE RAINFALL PER YEAR
It will depend on the intensity and duration of the precipitation event, but quite often there will be some amount of rainfall water to be stored on the available to the crop (effective precipitation). Whenever the rainfall amount and spatial distribution is not enough to satisfy the crop water needs, and/or the quantity of the water supply and uniformity, need to be controlled and ensured during all the crop cycle, an irrigation system is required.
Generally, average rainfall per month or year is used for these calculations, information based on at least the previous 10-15 years data.
2.- 💠 DISTRIBUTION EFFICIENCY & APPLICATION UNIFORMITY
Here is included the capacity of the system to supply more water than the strictly required by the crops. Let’s call it a “safety factor” for the design, as there would be some water losses along all the way from the irrigation pump until the emitter (efficiency), and also knowing that not every plant will get the same amount of water (uniformity) as required.
At the design stage, this number is difficult to determine. The material, quality and characteristics of the selected emitters and pipes, along with the proper irrigation system layout for an even and uniform water application, will condition this value.
Drip irrigation systems are the most efficient in terms of water consumption. Generally talking, we can consider 90% to 95% for drip irrigation systems, and higher than 95% when referring to SDI (Sub-Surface Drip Irrigation) systems. The bigger this number, the more economical solution will get. But it also could lead to a lack of irrigation capacity within the system (if numbers and calculations are not accurately obtained).
We will cover how to ensure maximum water efficiency usage in agriculture in upcoming articles.
3.- 🔬 WATER QUALITY
If the salt concentration in the irrigation is elevated, additional water volume shall be considered in the irrigation design to wash the salt out of the root system area where the water is being applied. This will avoid further plant affection and yield to lose.
GROSS WATER REQUIREMENTS: THE DESIGN NUMBER
According to the above, and considering that there won’t be any affection by salts concentration during the irrigation season, the final water requirement value to be used on the irrigation design has to be deducting the water supplied by rainfall (if any), and considering the efficiency of the system. We get the Gross Irrigation Water Requirements:
That means, the CWR is the amount of water (per day or per week or month) that crop needs, while the GIWR is the amount of water the irrigation system shall be able to provide.
THE MAXIMUM GIWR
The CWR varies through all its cycles, so if we are working on the irrigation design, we’ll need to know when that value would be the highest one. If our irrigation system is able to supply water during the most demanding period, it will be also feasible to do it along the rest of the irrigation period.
The point is that the drip irrigation system shall be able to replace the amount of water that both, crop and solar radiation, have consumed the previous day.
When the crop water requirement will be the highest then? The answer to this question will depend on the selected crop and plantation location: that’s it, is required to combine both crop growing pattern (Kc) with the local weather (ETo) along all the life cycle. Then, identifying the maximum number is clear:
Depending on the targeted crop, its growing cycle could be referred to as an extensive or perennial crop. Thereafter, the calculations and given parameters might differ from each other. The ETo values keep constant within the selected plot, but the kc values vary according to the selected crop, and its variety/cultivar.
A.- IRRIGATION WATER REQUIREMENTS IN HORTICULTURAL/FIELD CROPS
The most important thing here is to determine the crop cycle according to the selected plant variety. From the sowing/transplanting time, to final harvest, this period and duration will depend on the crop itself and the local conditions: is not the same to calculate it for short cycle maize than a long cycle one, as well as those growing stages will differ, depending on the local climate conditions where those crops will be planted.
Also, for some field crops bearing several harvesting cycles per season, as alfalfa does, the GIWR needs to be calculated according to that growing period of higher water stress for the plants.
B.- IRRIGATION WATER REQUIREMENTS IN MULTI-SEASONAL CROPS
Either we are considering a perennial or deciduous tree, the crop water requirements differ from the early growing stages, reaching the maximum demand of nutrients and water at the mature stage, when all the vegetative organs have finally developed.