Seasonal Water Needs for Colorado Crops
Understanding net crop requirement is helpful in selecting crops under limited water.
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| CoAgMet weather station |
Crop water use, consumptive use and evapotranspiration (ET), are terms that are used interchangeably to describe the water that is consumed by a crop.
Water requirements of crops depend mainly on environmental conditions. Plants use water for cooling purposes and the driving force of this process is prevailing weather conditions. Different crops have different water use requirements, under the same weather conditions. Crops will transpire water at the maximum rate when the soil water is at field capacity. When soil moisture decreases crops have to exert higher forces (energy) to extract water from the soil. Usually, the transpiration rate doesn’t decrease significantly until the soil moisture falls below 50% of field capacity.
Knowing seasonal crop water requirements is crucial for planning your crop planting mixture especially during drought years. For example, in the Greeley area, the seasonal water use of sugar beets is 30 in. while corn uses only 22 in. of water. That means, to fully irrigate sugar beets you need to apply 36% more water as compared to corn. These water requirements are net crop water use, the amount that the crop will use (not counting water losses such as deep percolation and runoff) in an average year, given soil moisture levels didn’t fall below critical levels. Under ideal conditions this net water requirement is reduced by the effective rain, which for the Greeley area is 7 in. for the growing season. The rest of the crop water requirement must be supplied by irrigation. No irrigation system is 100% efficient, so to apply the net water requirement to the entire field the amount of water applied should be increased or multiplied by the efficiency (or inefficiency) of the irrigation system. Therefore, the difference in the gross irrigation water requirement between the two crops is also increased by the irrigation system efficiency. The net water requirements in the above example, after subtracting effective rain, are 23 in. for sugar beets and 15 in. for corn. If the irrigation system is 85% efficient, 27 in. (gross irrigation amount) must be applied to the sugar beets crop and 17.6 in. to the corn crop in order to store the net water requirement in the crops’ root zone. Now the difference between the seasonal gross water requirements of sugar beets and corn is 53%. The difference in the gross irrigation requirement amounts increases as the irrigation system efficiency decreases.
Net Crop water Requirement
Net crop water requirement is estimated using models, which are based
on weather variables. Seasonal crop water requirement can be estimated
using these models by averaging weather conditions for many years, creating
an average weather year. Tables 1 and 2 are a summary of net water requirements
of different crops and effective precipitation for different locations
in eastern Colorado and western Colorado respectively. To determine the
net irrigation requirement subtract the effective rain (Av. Effective
Precipitation from Tables 1 and 2)
from the net crop water requirement. The gross irrigation water requirement
is the net irrigation requirement divided by the irrigation system efficiency
(fraction of one.) For example, corn for grain in Burlington requires
26 in. of water. Effective precipitation is 11.3 in. for the season, therefore
the net irrigation requirement is 14.7 in. The gross irrigation requirement
for a center pivot with 80% irrigation efficiency is 18.4 in. while for
a furrow irrigation system with 55% irrigation efficiency the gross irrigation
requirement is 26.8 in.
Table 1. Estimated seasonal water requirement (Consumptive Use) in Eastern Colorado*
| Burlington | Greeley | Lamar | Longmont | Rocky Ford | Springfield | Sterling | Wray | |
|
--------------------------inches/season ---------------------------------------
|
||||||||
| Alfalfa |
35.6
|
31.6
|
39.1
|
30.9
|
37.7
|
37.4
|
35.2
|
35.2
|
| Grass hay/pasture |
31.1
|
26.6
|
34.2
|
26.2
|
32.9
|
32.6
|
28.0
|
30.9
|
| Dry beans |
19.2
|
18.4
|
|
15.8
|
|
|
18.7
|
18.7
|
| Corn, grain |
26.0
|
|
26.8
|
21.7
|
27.7
|
26.7
|
|
25.4
|
| Corn, silage |
22.8
|
21.7
|
|
19.7
|
24.3
|
|
20.3
|
|
| Corn, sweet |
|
22.7
|
|
|
20.4
|
|
|
|
| Melons |
|
|
15.8
|
|
15.1
|
|
|
|
| Potatoes |
|
28.1
|
|
|
|
|
|
|
| Small vegetables |
|
17.7
|
18.8
|
|
22.2
|
|
|
|
| Sorghum, grain |
21.5
|
19.5
|
22.6
|
|
|
22.6
|
|
21.0
|
| Spring grains |
|
|
11.8
|
11.4
|
14.1
|
10.4
|
14.3
|
15.2
|
| Sugarbeets |
30.0
|
29.3
|
34.3
|
25.5
|
32.7
|
32.3
|
30.0
|
30.0
|
| Wheat, winter |
19.0
|
16.4
|
19.3
|
18.5
|
|
18.6
|
18.5
|
|
| Av. Precipitation |
16.3
|
12.2
|
15.3
|
12.7
|
12.5
|
15.4
|
14.9
|
18.5
|
| Av. Effective Precipitation |
11.3
|
7.3
|
11.0
|
7.0
|
8.9
|
10.9
|
6.7
|
12.6
|
Table 2. Estimated seasonal water requirement (Consumptive Use) in Western Colorado*
| Cortez | Durango | Gunnison | Fruita | Meeker | Monte Vista | Norwood | Walden | |
|
--------------------------inches/season ---------------------------------------
|
||||||||
| Alfalfa |
29.4
|
27.5
|
18.0
|
36.2
|
23.5
|
23.6
|
23.6
|
12.9
|
| Grass hay/pasture |
24.7
|
23.2
|
17.1
|
31.4
|
21.4
|
19.8
|
20.4
|
13.6
|
| Dry beans |
|
|
|
19.9
|
|
|
|
|
| Corn, grain |
|
|
|
25.1
|
|
|
|
|
| Corn, silage |
18.0
|
16.1
|
|
22.7
|
17.3
|
|
|
|
| Orchard w/o cover crop |
|
|
|
|
|
|
|
|
| Orchard w cover crop |
|
|
|
25.7
|
|
|
|
|
| Potatoes |
|
|
|
|
|
16.5
|
|
|
| Small vegetables |
|
|
|
18.1
|
|
6.8
|
|
|
| Spring grains (barley, wheat) |
14.8
|
16.7
|
|
19.6
|
15.5
|
12.7
|
11.4
|
|
| Sugarbeets |
|
|
|
31.6
|
|
|
|
|
| Wheat, winter |
20.1
|
18.8
|
|
18.9
|
|
|
|
|
| Av. Precipitation |
12.9
|
18.6
|
11.0
|
8.3
|
17.1
|
7.2
|
15.7
|
9.6
|
| Av. Effective Precipitation |
5.1
|
8.3
|
3.8
|
4.0
|
6.2
|
3.9
|
6.1
|
3.0
|
