Vasudha Sharma, Extension irrigation specialist
What is Evapotranspiration (ET)?
Evapotranspiration (ET) includes two terms: evaporation (E) from the soil surface and transpiration (T) from the leaves. It is very difficult to estimate or measure these two components independently, so they are often calculated and measured together. Over a growing season, 70% to 80% of all ET is used up as transpiration and remaining 20% to 30% of ET is direct evaporation from the soil.For transpiration, water moves from the soil through the crop’s root system and comes out from the leaves. The process of transpiration is essential for cooling the leaves and it also facilitates the movement of nutrients from the soil into the plant, thus directly influencing the grain production. As the crop grows, the canopy covers or shades the soil surface, which reduces the evaporation from the soil as we move into the growing season. By the mid-season, leaf area is sufficient to cover the soil surface to the maximum. At that time, transpiration becomes the dominant component.
What are different factors that affect ET?
Crop type and growth stage
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Figure 1. Average (2016-2020) crop water use rates for corn and soybean at Becker, MN. |
Weather
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Figure 2. Comparison of average (2016-2020) growing season ET with 2021 growing season ET for corn and soybean. Assumes full cover at 60 and 54 days after planting for corn and soybeans, respectively. |
Water holding capacity of the soil
Water holding capacity is the maximum amount of water that soil can store to be extracted by the plants. It is the water held between field capacity and permanent wilting point. The total available water in the soil root zone for a specific crop is equal to the crop’s rooting depth multiplied by the available water-holding capacity per unit depth of the soil.Residue cover
The residue cover has a significant impact on the evaporation of water from the soil surface. Tillage increases the exposed surface area of the soil, which increases evaporation and runoff, and may reduce water infiltration because of compaction in the tillage process. Through research, it has been found that the plots with residue on the soil surface could save 3-4 inches of irrigation as compared to bare soil (University of Nebraska).How do I calculate ET?
Crop Evapotranspiration (ETc) can be estimated using a two-step approach to determine the crop-specific water use (ETc) as per the following equation:ETc = ETref x Kc (Equation 3)
Days after planting | Corn | Soybean |
---|---|---|
10 | 0.19 | 0.23 |
20 | 0.22 | 0.33 |
30 | 0.33 | 0.53 |
40 | 0.53 | 0.77 |
50 | 0.79 | 0.94 |
60 | 1.00 | 0.97 |
70 | 0.99 | 0.89 |
80 | 0.97 | 0.59 |
90 | 0.94 | 0.29 |
100 | 0.89 | 0.14 |
110 | 0.82 | 0.09 |
120 | 0.72 | 0.06 |
130 | 0.58 | |
140 | 0.40 | |
150 | 0.17 |
Kc numbers are adapted from ASCE manual 70 and were adjusted based on the following assumption: Assumes full cover at 60 and 54 days after planting for corn, and beans respectively.
Alfalfa-reference and grass-reference evapotranspiration (ETref) are calculated using the standardized Penman-Monteith equation. Climate variables measured by a weather station are used as input data to calculate ETref. Climatic data included are air temperatures, solar radiation, humidity and wind.Tools for reference ET:
When to stop irrigating?
Determining the amount and timing of the last few irrigations of the season is one of the most critical water management decisions. Discontinuing too early in the season to save water or reduce pumping costs could mean a much greater reduction in yield returns than the cost of pumping. On the other hand, irrigating right up to crop maturity may mean using 1 to 3 inches more irrigation water than necessary.The two basic irrigation water management strategies that an operator should keep in mind when predicting the last irrigation are:
- To provide adequate soil moisture in the root zone to carry the crop to maturity without reducing yields.
- To deplete the soil moisture farther than normal (i.e., 60-70% of available water can be depleted at maturity) when nearing maturity. This will minimize irrigation water supply needs, fuel and labor for the season and allow the off-season precipitation to recharge the soil profile.
Table 3. Estimated normal water requirements for corn (95 RM) and soybeans between various growth stages and maturity in Minnesota.
Growth stage | Water use (ET) to maturity (inches) |
---|---|
Corn | |
Blister (R2) | 7.0-7.5 |
Milk (R3) | 4.8-5.3 |
Dough (R4) | 3.2-3.6 |
Beginning dent (R4.7) | 2.1-2.4 |
Full dent (R5) | 1.6-1.8 |
1/2 Milk line (R5.5) | 0.9-1.2 |
Soybean | |
Full flowering (R2) | 6.8-7.6 |
Full pod (R4) | 4.0-4.8 |
Beginning seed (R5) | 2.7-3.3 |
Full seed (R6) | 1.0-1.4 |
Beginning maturity (R7) | 0.4-0.7 |
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