Joseph Masabni, Stephen King, and Nathanael Proctor Assistant Professor and Extension Horticulturist, and former Associate Professor, and Graduate Student; all of the Texas A&M Department of Horticultural Sciences; The Texas A&M University System
A critical part of growing vegetables is determining the right amount of water to give them. If you irrigate properly, you can minimize water runoff, decrease the amount of labor needed, and produce a more bountiful, high-quality crop. Reducing runoff will also help cut down on soil erosion and fertilizer needs.
To water vegetables efficiently, you need to calculate:
- The amount of water that your plants need (irrigation requirement) under the current weather conditions of the growing season
- The amount of time to run your irrigation system to apply the water needed
Step 1: Assess your garden’s water needs
To calculate the irrigation requirement, you’ll need four measurements: effective rainfall, crop evapotranspiration, irrigation efficiency, and the crop coefficient.
Effective rainfall is the total amount of rainfall that a site receives during a specific period (usually the previous week) minus the amount lost to runoff or deep percolation from the site in that period. You can find the total rainfall for your area on the Texas ET Network website (texaset.tamu.edu). The network bases its calculations on data from weather stations across Texas (Fig. 1). Use the data from the station nearest your garden along with local information such as the amount of rain that has fallen in your specific area.
The amount of water lost to runoff or deep percolation depends on the amount of rainfall received. When little rain falls, most of the water is lost through evaporation. When much rain falls, most of the water is lost through surface runoff. Effective rainfall is calculated using the correction factors listed in Table 1. Multiply the correction factor by the amount of rainfall for that period.
For example, a gardener determines that the total rainfall received in the past week amounts to 0.5 inch. Using the correction factor in Table 1, the effective rainfall is 0.5 × 0.4 = 0.2 inch instead of 0.5 inch.
Crop evapotranspiration (ETo ) is the amount of water lost from the soil to evaporation and transpiration, which is the water that travels from the soil through the plant and out of its pores, or stomata. The current ETo values from weather stations across the state are also listed on the Texas ET Network.
Irrigation efficiency (IE) is the percentage of the water applied that can actually be used by the plants. Some of the water applied by an irrigation system evaporates before it can reach the plant roots (Fig. 2); some water runs off the site; and some will fall on soil away from the plants. The IE calculation takes into account these losses as well as the type of irrigation system you are using.
The crop coefficient (Kc) is based on the type of vegetables being grown and the current point of their growing cycle. Table 2 lists the coefficients for several types of vegetables at three stages of development— early, midseason, and harvest.
Use those four measurements and the following equation to determine the irrigation requirement for a specific crop and date (Example 1). IR (inches) = (ETo (inches) × Kc) – ER (inches)/ IE
IR = Irrigation requirement ETo = Reference evapotranspiration Kc = Crop coefficient ER = Effective rainfall = Total rainfall x correction factor IE = Irrigation efficiency
Note: If the garden received more effective rainfall than its total water requirement from the previous week, IR would equal 0 and no irrigation would be needed that week.
Step 2: How long to run your irrigation system
To determine the amount of time it will take to water your garden, you’ll first need to:
- Convert the garden dimensions from feet to square feet (length × width), and square feet to acres.
- Convert the water needs determined previously from inches to gallons.
- Determine the run time needed to apply the number of gallons of water.
Convert the garden area from feet to acres
Most people measure their gardens in square feet. Use this equation to convert the garden area into acres:
Length (feet) × Width (feet)/ 43,560
Note: You can also use an online unit converter (such as the one at unitconverters.net) to convert the garden dimensions in feet to square feet and from square feet to acres.
Convert water needs from inches to gallons
Rainfall is measured in inches, but irrigation system output is measured in gallons per minute. One inch of rain falling on 1 acre of land is equal to 27,154 gallons of water.
Use this equation to convert the amount of water that your garden needs from inches to gallons:
Gallons = 27,154 (gallons per acre-inch) × Garden area (acres) × Irrigation requirement (inches)
Determine how long to run your irrigation system
An irrigation system delivers a specific number of gallons per minute (gpm), which is called its output rating. Each irrigation system has an output rating that is specified by the manufacturer; it is an important factor to consider when choosing an irrigation system.
To find your system’s output rating, check the manufacturer’s website or product information. You can also determine it by measuring the volume of water collected in a bucket of water in 30 minutes. If you are using a hose with built-in emitters, you will also need to know how many emitters are used to collect the water volume in 30 minutes. Then you can calculate the output per emitter.
Use this equation to calculate the number of gallons of water needed:
Run time = (minutes) Irrigation requirement (gallons)/ Irrigation system output rating (gallons per minute)
Using the above equations can help you determine the water needs for a given crop.
Having the right amount of water available to the crop will increase plant health and vegetable yield and quality. This information can also help you determine the best crop to grow based on water availability and the best type of irrigation system.
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