OK! You’ve made the necessary repairs and adjustments to your irrigation system and have determined the system’s precipitation rate and uniformity (Tip Sheet 1). The next step in efficient irrigation management is to determine how often and how long to irrigate (or the irrigation frequency and irrigation duration, respectively). Ideally, your goal should be to provide the crop’s water needs for optimum growth and production, while minimizing water loss through deep drainage of water below the crop’s root zone and/or runoff of water into non-cropped areas. To accomplish this goal, you must be equipped with estimates of potential crop water-use (or evapotranspiration [ET]) and the soil’s water characteristics. This tip sheet will discuss crop ET.
Crop water-use or ET includes water that directly evaporates from soil and plant surfaces when wet (evaporation or E) and water that is transported through plant tissues from the soil via the roots, stems and leaves (transpiration or T). Crop ET varies with crop species and variety, plant size or growth stage, plant health, and weather (air temperature, humidity, solar radiation, and wind). With adequate soil water, ET increases as plant green leaf area, air temperature, wind speed, and solar radiation increases and as relative humidity decreases (i.e. the air becomes drier). Early in the growing season, plants are small and ET is low (0.05 inch per day or less). During mid-summer, as plants grow to their maximum size, ET increases to a peak value (i.e. 1/3 inch per day or more). Later in the season, as plants mature and begin to senesce (dry down), ET begins to decline, even though plant size may be large. Figure 1 shows the average daily water-use or ET of alfalfa in the Four Corners area based on measurements taken during a multiple-year study at New Mexico State University’s Agricultural Science Center at Farmington.
From about mid-April to early-August, daily water-use of a good stand of alfalfa in the Farmington area averages between 0.25 and 0.30 inch (Figure 1) and totals more than 30 inches during this time period. Over the entire alfalfa growing season (from dormancy break in February or March to the first frost in mid-October), however, ET totals 45 to 50 inches and should produce 8 tons of dry hay (or more) per acre.
The water-use curves shown in Figure 1 represent average daily values and do not show seasonal variations created by harvests (cuts). After a cut, alfalfa water-use drops significantly below the daily average (due to the drastic reduction in leaf area and transpiration potential). In comparison, just prior to beginning of bloom, alfalfa water-use will be greater than the average (i.e. 0.4 inch per day). As bloom increases and the plants begin to reach maturity, leaf area decreases and transpiration begins to decline. For maximum production and quality, alfalfa should be harvested at about 10% bloom.
Due to the time it takes for dry-down, baling, and bale pick-up, the irrigation interval will probably be longer than normal when it includes a cut. Therefore, it’s important to adjust irrigations before and after each cut so the total crop water-use (based on the daily averages) during the interval is still being satisfied. This can be accomplished by applying a heavier than normal irrigation just prior to a cut (with consideration being given to the time it takes for the field to dry down before entering with harvesting equipment). This supplies more water to satisfy the increased crop ET and also contributes to the soil-water bank that the crop can draw from prior to the next irrigation. The depth of the first irrigation after harvest should be sufficient to replace ET since the last, regularly-scheduled irrigation (based on the daily average ET).
· The normal irrigation interval is 10 days and the average daily water-use is 0.3 inches. The total water-use then for the interval is 3.00 inches.
· The last, regularly scheduled irrigation was May 18th.
· A cut is planned for June 1.
· It takes 6 days for the field to dry down before swathing.
· It takes an additional 8 days to swath, hay dry down, bale and buck.
· Therefore, the irrigation interval will be extended to about 15 days.
· Average daily ET remains at 0.30 inch. The total water-use for interval then is 4.5 inches.
In this scenario, the next regularly-scheduled irrigation would have been on May 28th (10 days after the previous irrigation). However, this would only be 4 days before harvest. To satisfy the 6-day requirement, you would apply an irrigation depth sufficient to replace crop ET since the previous irrigation (May 18th) on May 26th (6 days before the anticipated cutting and 8 days after the last irrigation). Assuming the May 18th brought the soil profile up to field capacity, and assuming crop water-use was 20% greater than average (since there was a full canopy and leaf area was at maximum) or 0.36 inch/day, you would apply enough water on May 26th to replace an ET of 2.9 inches (0.36” x 8 days). The total estimated water-use (ET) for the entire 23-day period between the normal (unadjusted) irrigation (May 18th) and the first irrigation after harvest (June 10th) is 6.9 inches (0.3” x 23 days). Theoretically then, water application depth should be sufficient to replace about 4.0 inches (6.9 inches total minus the 2.9 inches applied on May 26th).
Daily peak and total season water-use estimates for various other crops are shown in Table 1. Alfalfa, grass pasture, and mixed alfalfa /grass pasture are the three most common irrigated crops on the Hammond Conservancy District. Generally, daily ET of cool season pasture grass (i.e. tall fescue, orchard grass, meadow brome) averages about 80% that of alfalfa or about 0.20 to 0.25 inch per day in mid-summer. A good stand of mixed pasture in summer will use between 0.25 and 0.3 inch per day. Water-use of warm season grass pastures, consisting of blue grama, buffalograss, and/or bermudagrass will theoretically be about 60% that of alfalfa.
Table 1. Approximate peak daily water-use (at full canopy and maximum leaf area), full-season water-use, and expected yields at the this water-use of various crops in the Farmington area.*
*These values are based on results from research studies conducted at New Mexico State University’s Agricultural Science Center under sprinkler irrigation. Numerous factors (soil fertility, pest damage, disease, crop variety, etc.) can cause significant variation in these values, particularly yields. Because ET includes soil surface evaporation, factors that can reduce this water loss (i.e. mulching, drip irrigation, etc.) will also reduce total ET.
**cwt = hundredweight (multiply value shown by 100 to get lbs/acre)
***Based on a crop coefficient of 0.3 and 70% canopy cover.
The discussion so far has focused on actual crop water-use or ET. The irrigation requirement (IR) to satisfy this ET, however, is usually greater than ET. This is because irrigation efficiencies (IE) are less than 100% due to non-uniformity of water application (Tip Sheet 1) and sprinkler evaporation losses. A side-roll sprinkler system, if well-maintained and operated within specifications, might achieve an IE of about 80%. With this in mind, IR would be equal to ET divided by IE (expressed as a decimal). In alfalfa, for example, average daily IR would be about 0.38 inch (0.30/0.8) and total seasonal IR for maximum production would be about 60 inches. This will be discussed in more detail in an upcoming tip sheet.