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How to use this site

HOW TO USE THIS WEB SITE FOR DETERMINING THE IRRIGATION WATER NEEDS OF PLANTS AND CROPS

METHOD 1: After extracting a PET summary, a calculator appears at the bottom. Fill in any needed variables, and click "Compute" to determine the water needs for your landscape or crop.

METHOD 2: Track the daily and weekly PET values and use this information subjectively to estimate water needs.

METHOD 3: Use the average historic PET values to determine seasonal irrigation water needs and establish monthly irrigation schedules.


ACCESSING PET DATA


There are three different ways to extract PET data from the database:

1. The "Weather Station" drop down list at the top right of every page will access a 14 day weather and PET summary for the selected station.

2. In the "Weather Stations" section of the Browse menu on the left, there is a link to "Search historic data." From this page you may easily view 3, 5, or 7 day PET summaries. Additionally, you may search a given range of dates by selecting "Specify Dates" and then entering a start and stop date.

3. If you utilize this site often, you may want to create a login id. By doing so, you can default the site to your desired weather station, crop/turf type, and the number of days you prefer to view. For more information, see the "Users" section of the Browse menu on the left.

Click the link below to obtain a copy of the Website User's Manual:

Texas ET Network and Website User's Manual



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What is ET?

Evapotranspiration (ET) is a measurement of the total amount of water needed to grow plants and crops. This term comes from the words evaporation (i.e., evaporation of water from the soil) and transpiration (i.e., transpiration of water by plants). Different plants have different water requirements, so they have different ET rates.

Since there are thousands of cultivated plants, we have tried to simplify matters by establishing a standard ET rate for general reference and use. The standard is referred to as the potential evapotranspiration ETo (pet) . This is the potential ET since we are assuming the crop is in a deep soil and under well watered conditions. The standard crop we are using is a cool season grass which is 4-inches tall. The technical term for this is the "Potential Evapotranspiration of a Grass Reference Crop" or "ETo" for short.

ETo depends on the climate and varies from location to location. Special weather stations are used to collect the climatic data for calculating ETo,including temperature, dew point temperature (relative humidity), wind speed, and solar radiation.

The water requirements of specific crops and turf grasses can be calculated as a fraction of the ETo. This "fraction" is the called the crop coefficient (Kc) or turf coefficient (Tc). Crop coefficients vary depending on the type of plant and its stage of growth. Detailed information on crop and turf coefficients and how to use them is presented at other locations on this Web Site.

We are using the standardized Penman-Monteith method to calculate ETo from the weather station data. This is one of a number of methods that can be used to determine ETo and ET. Several organizations, such as the International Committee on Irrigation and Drainage and the Water Requirements Committee of the American Society of Civil Engineers, have proposed establishing the Penman-Monteith method as a world-wide standard. Such a standard would help facilitate the sharing of ETo data and development of crop coefficients.

For more information on the Penman-Monteith equation see the FAO website and other methods for determining ETo, see the book:

Evapotranspiration and Irrigation Water Requirements, edited by M.E. Jensen, R.D. Burman, and R.G. Allen. Published by the American Society of Civil Engineers, New York, NY. 1990. 332pp.
Close

System Efficiencies

Typical Overall On-Farm Efficiencies
For Various Types Of Irrigation Systems.

System
Overall Efficiency

Surface
-average
-land leveling and delivery pipeline meeting desing standards
-tailwater recovery
-surge

0.5-0.8
0.5
0.7
0.8
0.6-0.91
Sprinkler
0.55-0.753
Center Pivot
0.55-0.903
LEPA
0.90-0.95
Drip
0.80-0.902

footnotes:
1. Surge has been found to increase efficiencies 8 to 28 percent over non-surge furrow systems.
2. Trickle systems are typically designed at 90 percent efficiency; short laterals (<100ft) or systems with pressure compensationg emitters may have higher efficiencies.
3. Under low wind conditions.
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Texas A&M Weather Data Format


Data Format of the TAMU weather station:

TAMU Weather Station Format
Column
Data
Units
1
Year
2
Station ID
3
Julian Date
4
Time
Hour
5
Temperature
Celcius
6
Relative Humidity
%
7
Solar Radiation
MJ/m^2
8
Wind Speed
Meters/Second
9
Rainfall
Milimeters
Close

Texas A&M Wiring Diagram

HMP45 (temp/RH)
Blue
1L
Yellow
1H
Purple
AG
Orange
C1
Red
12V
Black
AG
Clear
G
   
Anemometer
   
Black
P1
White
G
Clear
G
Wind Vane
   
Black
E2
Red
3H
White
AG
Clear
G
L1200X (pyrometer)
Red
2H
Black
2L
White
AG
Clear
G
107
(soil temp)
Clear
G
Purple
AG
Red
3L
Black
E1
TE525
(rain gauge)
Clear
G
White
AG
Black
P2
Close

Crop Coefficents

Potential Evapotranspiration (PET) is an estimate of the water requirements of a 4-inch grass in a deep soil growing under well watered conditions. The water requirements of specific crops are determined from pET through crop coeffecients (Kc). The following equation is used:

PET x Kc = crop water requirement

Crop coefficients vary for different crops, as well as for the region crops are grown in. In addition, they change based on the growing stage of the crop. Unfortunately, for Texas, we only have verified crop coefficients for the North High Plains. These coefficients were developed by the North Plains pET Network Project Team.

FAO (Food and Agriculture Organization of the United Nations) has published generalized crop coefficients which are used throughout the world where local values are not available.

Irrigation Efficiency - To adjust for irrigation system efficiency, we use the follwoing equation

PET x Kc / Eff = crop water requriement

North Plains Crop Coefficients for Sorghum

North Plains Crop Coefficients for Cotton

North Plains Crop Coefficient for Corn

North Plains Crop Coefficient for Winter Wheat

Close

Turf Coefficents

Potential Evapotranspiration, ETo (also abreviated as PET), is an estimate of the water requirements of a 4-inch grass growing in a deep soil under well-watered conditions. A turf coefficent (Tc) is needed to relate pET to the water requirements of a specific turf. For established lawns, the Tc remains constant throughout the active growing season of the grass. For warm season grasses, such as St. Augustine, the Tc is 0.6 throughout much of the year, while the Tc for cool season grasses, such as rye, is 0.8.

However, we seldomly apply this amount of water to lawns. Why? Because we do not want maximum production of grass clippings. Instead, we want to maintain a healthy, attractive turf with as little water as possible. Thus, we modify the coefficent by specifying a Quality Factor.

The complete equation is:

ETo x Tc x Qf = turf water requirement

Turf Coefficient Values (Tc)

Warm Season 0.6
Cool Season 0.8


Quality Factor (Qf)

No Stress 1.0
Low Stress 0.8
Normal Stress 0.6
High Stress 0.5
Very High Stress 0.4
Close

Historic ETo Reference

City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Abilene 2.08 2.57 4.14 5.48 6.47 7.65 8.36 7.46 5.48 4.21 2.67 2.08
Amarillo 1.84 2.27 3.73 5.06 5.89 7.51 8.08 7.29 5.61 4.05 2.40 1.78
Austin 2.27 2.72 4.34 5.27 6.39 7.15 7.22 7.25 5.57 4.38 2.74 2.21
Brownsville 2.65 3.03 4.48 5.17 6.03 6.32 6.68 6.65 5.21 4.34 3.01 2.59
College Station 2.20 2.71 4.22 5.20 6.25 6.89 7.10 6.85 5.60 4.30 2.80 2.20
Corpus Christi 2.42 2.95 4.28 5.17 5.95 6.43 6.68 6.65 5.21 4.34 3.01 2.59
Dallas / Ft Worth 2.00 2.46 3.96 5.14 6.21 7.06 7.40 7.25 5.49 4.19 2.59 2.10
Del Rio 2.47 3.01 4.76 6.01 6.98 7.41 7.57 7.41 5.77 4.35 2.91 2.36
El Paso 2.74 3.53 6.07 8.19 9.83 11.12 9.19 8.94 7.69 5.89 3.58 2.49
Galveston 2.20 2.60 4.10 5.00 6.11 6.60 6.20 6.00 5.50 4.20 2.80 2.30
Houston 2.36 2.83 4.32 5.01 6.11 6.57 6.52 6.08 5.57 4.28 2.90 2.35
Lubbock 2.35 2.63 4.41 5.53 6.93 7.73 7.63 7.20 5.54 4.19 2.61 2.33
Midland 2.20 2.78 4.46 5.91 7.21 8.20 9.23 8.62 6.96 4.31 2.78 2.16
Port Arthur 2.25 2.63 3.95 5.09 6.12 6.60 5.81 5.61 5.46 4.18 2.76 2.23
San Angelo 2.88 3.13 5.31 7.01 8.48 9.16 9.29 8.49 6.60 5.08 3.37 2.54
San Antonio 2.42 2.90 4.42 5.47 6.47 6.97 7.31 6.99 5.64 4.44 2.85 2.36
Victoria 2.35 2.87 4.29 5.77 6.39 6.70 6.92 6.70 5.36 4.41 2.93 2.33
Waco 2.13 2.62 4.03 5.31 6.45 7.15 7.40 7.50 5.70 4.41 2.70 2.17
Wichita Falls 1.94 2.46 4.07 5.50 6.70 7.54 7.97 7.72 5.79 4.30 2.62 1.95
© 2019 Texas A&M AgriLife Extension
City Years of Data
Abilene
52
Amarillo
52
Austin
70
Brownsville
79
College Station
47
Corpus Christi
52
Dallas/Ft. Worth
26
Del Rio
44
El Paso
52
Galveston
59
Houston
31
Lubbock
89
Midland
52
Port Arthur
53
San Angelo
54
San Antonio
54
Victoria
39
Waco
68
Wichita Falls
99
© 2019 Texas A&M AgriLife Extension

Averages were computed using climatic data over the entire period of record available from the National Weather Service and compared to ETo rates based on the standardized Penman-Monteith equation where available. (August 2005)

Close

Historic Relative Humidity Reference

Average Relative Humidity
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average
Abilene
55.4
60.2
68.4
77.5
84.2
91.3
94.9

94.3

87.0
77.8
65.6
57.8
76.2
Amarillo

49.2

53.3
61.2
70.9
79.0
88.0
91.2
89.2
82.2
72.5
59.3
51.0
70.6
Austin
60.1
64.4
71.7
79.1
85.2
91.6
95.2
95.7
89.8
81.6
70.2
62.7
78.9
Brownsville
70.3
73.1
78.0
83.2
87.4
91.3
93.0

93.6

90.4
85.1
77.9
71.6
82.9
College Station
59.3
63.6
71.0
78.4
84.8
91.2
94.7
95.3
89.4
80.9
69.9
62.2
78.4
Corpus Christi
66.2
69.7
75.3
81.2
86.0
90.8
93.5
93.6
89.8
83.8
75.2
69.0
81.2
Dallas/Ft. Worth
54.2
59.9
67.8
75.9
83.2
91.6
96.2
95.7
88.3
78.6
66.0
57.3
76.2
Del Rio
63.4
68.2
76.1
83.6
88.8
94.4
97.0
96.7
91.0
82.1
71.6
64.3
81.4
El Paso
57.5
63.3
69.9
78.6
87.2
95.8
95.2
93.0
87.8
78.7
66.3
58.1
77.6
Galveston
59.4
61.5
67.0
73.5
80.2
85.5
87.7
88.1
85.0
78.0
69.1
62.9
74.8
Houston
61.8
66.0
72.7
79.0
85.3
90.8
93.7
93.5
89.0
81.4
71.7
64.9
79.2
Lubbock
53.5
58.7
66.3
75.3
82.7
90.6
92.6
91.1
84.2
75.1
63.2
54.7
74.0
Midland
57.1
62.2
69.8
78.6
86.4
92.9
94.3
93.2
86.4
77.9
66.2
59.0
77.0
Port Arthur
61.6
65.1
71.2
77.8
84.4
89.7
91.8
91.8
88.0
80.7
70.9
64.2
78.1
San Angelo
58.4
63.2
70.8
79.7
86.3
92.2
95.5
94.6
87.6
79.1
67.6
60.5
78.0
San Antonio
61.8
66.2
73.4
80.3
86.1
91.9
95.1
95.2
89.9
82.1
71.3
64.5
79.8
Victoria
63.3
67.1
73.8
80.4
85.7
90.8
93.7
94.0
89.6
83.0
73.7
66.3
80.1
Waco
57.1
61.9
69.4
78.0
84.6
92.1
96.4
96.8
89.9
80.6
68.1
59.9
77.9
Wichita Falls
53.0
58.4
67.0
76.6
83.9
92.6
97.6
97.2
88.6
78.4
64.7
55.6
76.1
© 2019 Texas A&M AgriLife Extension
Average Minimum Temperature (TMin)
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average
Abilene
31.6
36.1
42.9
52.3
61.0
68.8
72.5

71.5

64.7
54.3
42.3
34.2
52.7
Amarillo

21.9

26.3
32.2
41.6
51.8
61.2
65.7
64.1
56.7
44.9
32.1
24.5
43.6
Austin
39.5
43.0
49.3
57.7
65.1
71.4
73.8
73.6
68.8
59.2
48.5
41.7
57.6
Brownsville
51.0
53.7
59.1
65.6
71.1
74.6
75.5

75.3

72.8
65.9
58.6
52.3
64.6
College Station
39.5
42.7
49.4
57.8
65.2
71.2
73.5
73.2
68.4
58.4
48.6
41.8
57.5
Corpus Christi
46.5
49.7
55.9
63.2
69.9
73.8
75.2
75.2
72.3
64.4
55.4
49.0
62.5
Dallas/Ft. Worth
34.0
38.7
46.3
54.0
63.2
70.7
74.8
74.2
67.1
56.0
45.3
36.9
55.1
Del Rio
39.9
43.9
51.3
59.3
66.6
72.5
74.7
74.2
69.3
59.8
48.7
40.9
58.4
El Paso
31.1
35.2
41.2
49.2
57.8
66.6
69.8
68.3
62.0
50.1
38.0
31.7
50.1
Galveston
48.1
50.6
56.8
64.6
71.9
77.3
79.4
79.1
75.6
68.2
58.1
51.6
65.1
Houston
41.3
44.3
51.1
57.8
65.5
71.3
73.2
72.8
68.4
58.7
49.7
43.5
58.1
Lubbock
25.3
29.1
35.4
45.0
54.7
63.5
66.8
65.3
58.4
47.4
35.0
27.3
46.1
Midland
29.7
33.9
40.4
49.3
58.7
66.7
69.2
68.3
61.9
51.6
38.9
31.8
50.0
Port Arthur
42.9
45.6
51.5
59.2
66.6
72.2
74.0
73.5
69.7
59.7
50.5
44.9
59.2
San Angelo
32.6
36.4
43.5
52.4
61.2
68.7
71.5
70.5
64.1
53.8
42.1
34.5
52.6
San Antonio
39.4
43.0
49.9
58.1
66.0
72.1
74.4
73.8
69.2
59.5
48.5
41.6
58.0
Victoria
43.2
46.1
53.1
60.9
68.0
73.2
74.9
74.6
70.6
61.5
52.5
45.5
60.3
Waco
36.2
40.2
46.8
55.8
64.0
71.4
74.7
74.3
67.9
57.3
46.3
38.7
56.1
Wichita Falls
29.7
34.1
41.2
50.9
59.9
68.7
72.9
72.1
64.5
53.4
40.6
32.8
51.7
© 2019 Texas A&M AgriLife Extension
Average Mean Temperature (TMean)
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average
Abilene
43.5
48.2
55.6
64.9
72.6
80.1
83.7

82.9

75.8
66.0
53.9
46.0
64.4
Amarillo

35.6

39.8
46.7
56.3
65.4
74.6
78.5
76.7
69.4
58.7
45.7
37.8
57.1
Austin
49.8
53.7
60.5
68.4
75.2
81.5
84.5
84.6
79.3
70.4
59.4
52.2
68.3
Brownsville
60.6
63.4
68.5
74.4
79.2
82.9
84.2

84.4

81.6
75.5
68.2
62.0
73.8
College Station
49.4
53.2
60.2
68.1
75.0
81.2
84.1
84.3
78.9
69.7
59.3
52.0
67.9
Corpus Christi
56.4
59.7
65.6
72.2
77.8
82.3
84.3
84.4
81.1
74.1
65.3
59.0
71.8
Dallas/Ft. Worth
44.1
49.3
57.1
65.0
73.2
81.1
85.5
84.9
77.7
67.3
55.7
47.1
65.7
Del Rio
51.7
56.0
63.7
71.4
77.7
83.4
85.8
85.5
80.2
70.9
60.2
52.6
69.9
El Paso
44.3
49.3
55.6
63.9
72.5
81.2
82.5
80.7
74.9
64.4
52.2
44.9
63.9
Galveston
53.8
56.1
61.9
69.1
76.1
81.4
83.5
83.6
80.3
73.1
63.6
57.2
70.0
Houston
51.6
55.2
61.9
68.4
75.4
81.0
83.5
83.1
78.7
70.0
60.7
54.2
68.6
Lubbock
39.4
43.9
50.9
60.2
68.7
77.0
79.7
78.2
71.3
61.2
49.1
41.0
60.1
Midland
43.4
48.0
55.1
63.9
72.5
79.8
81.8
80.7
74.2
64.7
52.6
45.4
63.5
Port Arthur
52.2
55.4
61.4
68.5
75.5
80.9
82.9
82.6
78.8
70.2
60.7
54.6
68.6
San Angelo
45.5
49.8
57.1
66.0
73.8
80.5
83.5
82.5
75.9
66.4
54.8
47.5
65.3
San Antonio
50.6
54.6
61.6
69.2
76.0
82.0
84.7
84.5
79.5
70.8
59.9
53.1
68.9
Victoria
53.3
56.6
63.4
70.7
76.8
82.0
84.3
84.3
80.1
72.2
63.1
55.9
70.2
Waco
46.7
51.1
58.1
66.9
74.3
81.8
85.6
85.5
78.9
68.9
57.2
49.3
67.0
Wichita Falls
41.4
46.3
54.1
63.7
71.9
80.7
85.2
84.6
76.6
65.9
52.6
44.2
63.9
© 2019 Texas A&M AgriLife Extension
City Years of Data
Abilene
52
Amarillo
52
Austin
70
Brownsville
79
College Station
47
Corpus Christi
52
Dallas/Ft. Worth
26
Del Rio
44
El Paso
52
Galveston
59
Houston
31
Lubbock
89
Midland
52
Port Arthur
53
San Angelo
54
San Antonio
54
Victoria
39
Waco
68
Wichita Falls
99
© 2019 Texas A&M AgriLife Extension
Close

Historic Temperature Reference

Average Maximum Temperature (TMax)
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average
Abilene
55.4
60.2
68.4
77.5
84.2
91.3
94.9

94.3

87.0
77.8
65.6
57.8
76.2
Amarillo

49.2

53.3
61.2
70.9
79.0
88.0
91.2
89.2
82.2
72.5
59.3
51.0
70.6
Austin
60.1
64.4
71.7
79.1
85.2
91.6
95.2
95.7
89.8
81.6
70.2
62.7
78.9
Brownsville
70.3
73.1
78.0
83.2
87.4
91.3
93.0

93.6

90.4
85.1
77.9
71.6
82.9
College Station
59.3
63.6
71.0
78.4
84.8
91.2
94.7
95.3
89.4
80.9
69.9
62.2
78.4
Corpus Christi
66.2
69.7
75.3
81.2
86.0
90.8
93.5
93.6
89.8
83.8
75.2
69.0
81.2
Dallas/Ft. Worth
54.2
59.9
67.8
75.9
83.2
91.6
96.2
95.7
88.3
78.6
66.0
57.3
76.2
Del Rio
63.4
68.2
76.1
83.6
88.8
94.4
97.0
96.7
91.0
82.1
71.6
64.3
81.4
El Paso
57.5
63.3
69.9
78.6
87.2
95.8
95.2
93.0
87.8
78.7
66.3
58.1
77.6
Galveston
59.4
61.5
67.0
73.5
80.2
85.5
87.7
88.1
85.0
78.0
69.1
62.9
74.8
Houston
61.8
66.0
72.7
79.0
85.3
90.8
93.7
93.5
89.0
81.4
71.7
64.9
79.2
Lubbock
53.5
58.7
66.3
75.3
82.7
90.6
92.6
91.1
84.2
75.1
63.2
54.7
74.0
Midland
57.1
62.2
69.8
78.6
86.4
92.9
94.3
93.2
86.4
77.9
66.2
59.0
77.0
Port Arthur
61.6
65.1
71.2
77.8
84.4
89.7
91.8
91.8
88.0
80.7
70.9
64.2
78.1
San Angelo
58.4
63.2
70.8
79.7
86.3
92.2
95.5
94.6
87.6
79.1
67.6
60.5
78.0
San Antonio
61.8
66.2
73.4
80.3
86.1
91.9
95.1
95.2
89.9
82.1
71.3
64.5
79.8
Victoria
63.3
67.1
73.8
80.4
85.7
90.8
93.7
94.0
89.6
83.0
73.7
66.3
80.1
Waco
57.1
61.9
69.4
78.0
84.6
92.1
96.4
96.8
89.9
80.6
68.1
59.9
77.9
Wichita Falls
53.0
58.4
67.0
76.6
83.9
92.6
97.6
97.2
88.6
78.4
64.7
55.6
76.1
© 2019 Texas A&M AgriLife Extension
Average Minimum Temperature (TMin)
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average
Abilene
31.6
36.1
42.9
52.3
61.0
68.8
72.5

71.5

64.7
54.3
42.3
34.2
52.7
Amarillo

21.9

26.3
32.2
41.6
51.8
61.2
65.7
64.1
56.7
44.9
32.1
24.5
43.6
Austin
39.5
43.0
49.3
57.7
65.1
71.4
73.8
73.6
68.8
59.2
48.5
41.7
57.6
Brownsville
51.0
53.7
59.1
65.6
71.1
74.6
75.5

75.3

72.8
65.9
58.6
52.3
64.6
College Station
39.5
42.7
49.4
57.8
65.2
71.2
73.5
73.2
68.4
58.4
48.6
41.8
57.5
Corpus Christi
46.5
49.7
55.9
63.2
69.9
73.8
75.2
75.2
72.3
64.4
55.4
49.0
62.5
Dallas/Ft. Worth
34.0
38.7
46.3
54.0
63.2
70.7
74.8
74.2
67.1
56.0
45.3
36.9
55.1
Del Rio
39.9
43.9
51.3
59.3
66.6
72.5
74.7
74.2
69.3
59.8
48.7
40.9
58.4
El Paso
31.1
35.2
41.2
49.2
57.8
66.6
69.8
68.3
62.0
50.1
38.0
31.7
50.1
Galveston
48.1
50.6
56.8
64.6
71.9
77.3
79.4
79.1
75.6
68.2
58.1
51.6
65.1
Houston
41.3
44.3
51.1
57.8
65.5
71.3
73.2
72.8
68.4
58.7
49.7
43.5
58.1
Lubbock
25.3
29.1
35.4
45.0
54.7
63.5
66.8
65.3
58.4
47.4
35.0
27.3
46.1
Midland
29.7
33.9
40.4
49.3
58.7
66.7
69.2
68.3
61.9
51.6
38.9
31.8
50.0
Port Arthur
42.9
45.6
51.5
59.2
66.6
72.2
74.0
73.5
69.7
59.7
50.5
44.9
59.2
San Angelo
32.6
36.4
43.5
52.4
61.2
68.7
71.5
70.5
64.1
53.8
42.1
34.5
52.6
San Antonio
39.4
43.0
49.9
58.1
66.0
72.1
74.4
73.8
69.2
59.5
48.5
41.6
58.0
Victoria
43.2
46.1
53.1
60.9
68.0
73.2
74.9
74.6
70.6
61.5
52.5
45.5
60.3
Waco
36.2
40.2
46.8
55.8
64.0
71.4
74.7
74.3
67.9
57.3
46.3
38.7
56.1
Wichita Falls
29.7
34.1
41.2
50.9
59.9
68.7
72.9
72.1
64.5
53.4
40.6
32.8
51.7
© 2019 Texas A&M AgriLife Extension
Average Mean Temperature (TMean)
City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Average
Abilene
43.5
48.2
55.6
64.9
72.6
80.1
83.7

82.9

75.8
66.0
53.9
46.0
64.4
Amarillo

35.6

39.8
46.7
56.3
65.4
74.6
78.5
76.7
69.4
58.7
45.7
37.8
57.1
Austin
49.8
53.7
60.5
68.4
75.2
81.5
84.5
84.6
79.3
70.4
59.4
52.2
68.3
Brownsville
60.6
63.4
68.5
74.4
79.2
82.9
84.2

84.4

81.6
75.5
68.2
62.0
73.8
College Station
49.4
53.2
60.2
68.1
75.0
81.2
84.1
84.3
78.9
69.7
59.3
52.0
67.9
Corpus Christi
56.4
59.7
65.6
72.2
77.8
82.3
84.3
84.4
81.1
74.1
65.3
59.0
71.8
Dallas/Ft. Worth
44.1
49.3
57.1
65.0
73.2
81.1
85.5
84.9
77.7
67.3
55.7
47.1
65.7
Del Rio
51.7
56.0
63.7
71.4
77.7
83.4
85.8
85.5
80.2
70.9
60.2
52.6
69.9
El Paso
44.3
49.3
55.6
63.9
72.5
81.2
82.5
80.7
74.9
64.4
52.2
44.9
63.9
Galveston
53.8
56.1
61.9
69.1
76.1
81.4
83.5
83.6
80.3
73.1
63.6
57.2
70.0
Houston
51.6
55.2
61.9
68.4
75.4
81.0
83.5
83.1
78.7
70.0
60.7
54.2
68.6
Lubbock
39.4
43.9
50.9
60.2
68.7
77.0
79.7
78.2
71.3
61.2
49.1
41.0
60.1
Midland
43.4
48.0
55.1
63.9
72.5
79.8
81.8
80.7
74.2
64.7
52.6
45.4
63.5
Port Arthur
52.2
55.4
61.4
68.5
75.5
80.9
82.9
82.6
78.8
70.2
60.7
54.6
68.6
San Angelo
45.5
49.8
57.1
66.0
73.8
80.5
83.5
82.5
75.9
66.4
54.8
47.5
65.3
San Antonio
50.6
54.6
61.6
69.2
76.0
82.0
84.7
84.5
79.5
70.8
59.9
53.1
68.9
Victoria
53.3
56.6
63.4
70.7
76.8
82.0
84.3
84.3
80.1
72.2
63.1
55.9
70.2
Waco
46.7
51.1
58.1
66.9
74.3
81.8
85.6
85.5
78.9
68.9
57.2
49.3
67.0
Wichita Falls
41.4
46.3
54.1
63.7
71.9
80.7
85.2
84.6
76.6
65.9
52.6
44.2
63.9
© 2019 Texas A&M AgriLife Extension
City Years of Data
Abilene
52
Amarillo
52
Austin
70
Brownsville
79
College Station
47
Corpus Christi
52
Dallas/Ft. Worth
26
Del Rio
44
El Paso
52
Galveston
59
Houston
31
Lubbock
89
Midland
52
Port Arthur
53
San Angelo
54
San Antonio
54
Victoria
39
Waco
68
Wichita Falls
99
© 2019 Texas A&M AgriLife Extension
Close

Historic Rainfall Reference

City Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Amarillo 0.59 0.58 0.93 1.24 2.74 3.40 2.88 2.99 1.89 1.41 0.62 0.57
Austin 2.11 2.41 2.05 3.01 4.38 3.46 2.05 2.23 3.38 3.35 2.28 2.46
Brownsville 1.33 1.31 0.90 1.63 2.31 2.85 1.69 2.46 4.95 3.36 1.61 1.18
College Station 2.87 2.88 2.50 3.77 4.73 3.79 2.24 2.43 4.30 3.64 3.07 3.15
Corpus Christi 1.57 1.88 1.33 2.06 3.09 3.19 1.84 3.33 5.30 3.54 1.56 1.60
Dallas / Ft Worth 1.94 2.44 3.12 3.15 5.43 3.18 2.09 2.10 2.42 4.01 2.43 2.50
Del Rio 0.53 0.91 0.86 1.89 2.39 1.90 1.54 1.72 2.59 1.94 0.85 0.65
El Paso 0.42 0.41 0.30 0.21 0.33 0.72 1.56 1.48 1.42 0.72 0.35 0.62
Galveston 3.33 2.58 2.43 2.55 3.46 4.14 3.77 4.23 5.36 3.17 3.33 3.59
Houston 3.70 2.99 3.48 3.49 5.22 5.13 3.25 3.79 4.45 4.65 3.89 3.64
Lubbock 0.52 0.61 0.82 1.26 2.62 2.67 2.12 2.07 2.53 1.99 0.62 0.64
Midland 0.54 0.61 0.47 0.77 2.02 1.59 1.83 1.65 2.04 1.56 0.58 0.53
Port Arthur 4.86 3.96 3.30 3.86 5.02 5.68 5.31 5.04 5.77 4.20 4.22 5.13
San Angelo 0.83 1.05 0.93 1.68 2.86 2.20 1.16 1.77 2.78 2.21 0.96 0.78
San Antonio 1.61 1.90 1.68 2.53 3.99 3.57 1.83 2.58 3.29 3.29 2.11 1.72
Victoria 2.28 2.12 2.08 2.93 4.95 4.77 3.03 3.08 5.37 3.72 2.51 2.33
Waco 2.07 2.39 2.51 3.43 4.59 2.80 1.88 1.66 3.07 2.91 2.48 2.49
Wichita Falls 1.08 1.31 1.91 2.72 4.59 3.36 2.05 2.16 2.94 2.69 1.55 1.56
Abilene 1.01 1.10 1.19 2.09 3.31 2.90 2.09 2.45 2.75 2.48 1.28 1.04
© 2019 Texas A&M AgriLife Extension
City Years of Data
Abilene
52
Amarillo
52
Austin
70
Brownsville
79
College Station
47
Corpus Christi
52
Dallas/Ft. Worth
26
Del Rio
44
El Paso
52
Galveston
59
Houston
31
Lubbock
89
Midland
52
Port Arthur
53
San Angelo
54
San Antonio
54
Victoria
39
Waco
68
Wichita Falls
99
© 2019 Texas A&M AgriLife Extension
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Current Update Status

Station Status Data Available
Allen Updated 03/06/2015 - 02/20/2019
Allen Rain Not Updated 01/20/2016 - 07/17/2018
Argyle Updated 05/26/2018 - 02/20/2019
Austin (LCRA Redbud) Updated 09/11/2014 - 02/20/2019
Bryan Updated 08/05/2014 - 02/20/2019
Buda Updated 08/08/2018 - 02/20/2019
Burnet Updated 07/26/2017 - 02/20/2019
Cash Updated 04/01/2016 - 02/20/2019
Cedar Park Updated 02/11/2015 - 02/20/2019
Conroe Not Updated 09/05/2013 - 02/19/2019
Dallas AgriLife Center Updated 04/07/2008 - 02/20/2019
Dickinson Updated 07/27/2016 - 02/20/2019
Dripping Springs Updated 02/11/2015 - 02/20/2019
Farmersville Not Updated 11/15/2012 - 02/14/2019
Farmersville Rain Not Updated 01/20/2016 - 04/26/2016
Forney Not Updated 12/03/2012 - 01/31/2019
Forney Rain Not Updated 01/20/2016 - 04/26/2016
Fort Worth - NOAA Updated 08/25/2011 - 02/20/2019
Garland Updated 03/07/2015 - 02/20/2019
Garland Rain Not Updated 01/20/2016 - 05/01/2016
Georgetown II Not Updated 10/20/1998 - 12/07/2016
Halfway Updated 06/28/2012 - 02/20/2019
Highland-University Park Not Updated 04/19/2016 - 02/19/2019
Houston Updated 07/27/2016 - 02/20/2019
Huntsville Updated 01/22/2010 - 02/20/2019
Irving North Updated 10/08/2013 - 02/20/2019
Irving South Updated 10/08/2013 - 02/20/2019
Kennedale Updated 11/12/2014 - 02/20/2019
Lakeway Updated 02/11/2015 - 02/20/2019
Lewisville Updated 06/26/2015 - 02/20/2019
Marble Falls Updated 09/11/2014 - 02/20/2019
McKinney Updated 11/15/2012 - 02/20/2019
McKinney Rain Not Updated 01/20/2016 - 05/04/2016
Melissa Not Updated 03/16/2016 - 02/14/2019
Melissa RD121 Updated 04/01/2016 - 02/20/2019
Memorial Village Updated 09/14/2017 - 02/20/2019
Mercedes Not Updated 01/01/0001 - 01/01/0001
Mesquite Not Updated 12/03/2012 - 01/31/2019
Mesquite Rain Not Updated 01/20/2016 - 04/26/2016
Murphy Not Updated 03/16/2016 - 02/08/2019
Nasho Not Updated 07/05/2018 - 02/08/2019
Overton Updated 06/07/2004 - 02/20/2019
Pflugerville Updated 02/11/2015 - 02/20/2019
Plano Updated 11/15/2012 - 02/20/2019
Plano Rain Not Updated 01/20/2016 - 05/01/2016
Princeton Updated 03/07/2015 - 02/20/2019
Princeton Rain Not Updated 01/20/2016 - 04/26/2016
Providence Village Updated 06/26/2015 - 02/20/2019
Richardson Updated 03/06/2015 - 02/20/2019
Richardson Rain Not Updated 01/20/2016 - 04/26/2016
Richmond North Updated 01/01/2018 - 02/20/2019
Richmond South Updated 07/27/2016 - 02/20/2019
Rio Grande City Not Updated 01/01/2016 - 02/12/2017
Rockwall Not Updated 12/03/2012 - 01/31/2019
Rockwall Rain Not Updated 01/20/2016 - 04/26/2016
Royse City Not Updated 03/07/2015 - 01/31/2019
Royse City Rain Not Updated 01/20/2016 - 04/26/2016
Sachse Not Updated 03/23/2016 - 02/18/2019
San Angelo Updated 11/03/2016 - 02/20/2019
San Antonio North Updated 09/02/2005 - 02/20/2019
Seagoville Updated 01/21/2016 - 02/20/2019
Southlake Updated 10/16/2015 - 02/20/2019
Spicewood Updated 07/26/2017 - 02/20/2019
Spring Updated 09/01/2016 - 02/20/2019
TAMU Turf Lab Updated 11/08/2012 - 02/20/2019
Tarrant West Updated 11/12/2014 - 02/20/2019
Tawakoni Not Updated 01/21/2016 - 02/16/2019
Uvalde Research Updated 11/06/2010 - 02/20/2019
Waco Not Updated 02/14/2012 - 02/17/2019
Weslaco Not Updated 11/20/2015 - 01/01/2017
Weslaco Annex Farm Not Updated 04/20/2016 - 02/16/2019
Winedale Updated 05/11/2017 - 02/20/2019
Wylie Updated 11/13/2012 - 02/20/2019
Wylie Rain Not Updated 01/20/2016 - 05/01/2016
Close

Grower's Guide

Using PET for Determining Crop Water Requirements and Irrigation Scheduling


Click to download a hard copy of the grower's guide

Contents

I. How to Use

      Adjusting for Irrigation System Efficiency

        Adjusting for Rainfall

      II. Where to Find PET Information

      III. What is Potential Evapotranspiration (PET)?

      IV. Crop Coefficients

          Choosing and Using Crop Coefficients

          Soil Moisture Monitoring

          Table 1. Sorghum Crop Coefficients

          Table 2. Cotton Crop Coefficients

          Table 3. Corn Crop Coefficients

        V. Irrigation System Efficiencies

            Table 4. Typical overall On-Farm Efficiencies

          VI. Understanding FAO Crop Coefficients

              Table 5. FAO Mean Crop Coefficients, Kc.

              Table 6. FAO General Crop Growth Stages.

            VII. Acknowledgments





            I. HOW TO USE PET

            To calculate the water requirements of a crop, we multiply the PET times the crop coefficient using the following equation:

            PET x Kc = crop water requirements (equation 1)

            where:

                PET is the sum of daily PET over the time period of interest, such as the 3-day total, the weekly total, etc.

                Kc is the crop coefficient corresponding to the current stage of crop growth.

              Example 1: the 5-day PET total is 1.32 inches. My sorghum is in the "heading" growth stage. What are the water requirements? (Note: from Table 1, the "heading" crop coefficient is 1.10)

                  1.32 inches x 1.10 = 1.45 inches

                Thus, I need to apply 1.45 inches to replace the water used by the sorghum in the last 5 days.

                Adjusting for Irrigation System Efficiency

                It may be necessary to increase the amount of irrigation water in order to compensate for poor irrigation system efficiency. Irrigation system efficiency is defined in Section V of this guide. Table 4 gives the typical ranges of on-farm irrigation systems.

                To adjust for irrigation system efficiency, use the following equation:

                PET x Kc ÷ Eff = irrigation water requirements (equation 2)

                where:

                    Eff is the overall efficiency of the irrigation system.

                  Example 2. I am irrigating with a low-pressure center pivot. I estimate that my overall system efficiency is 85%. What are my irrigation water requirements for the sorghum in example 1?

                      1.32 inches x 1.10 ÷ 0.85 = 1.71 inches.

                    Adjusting for Rainfall

                    Rainfall reduces the amount of water we must supply by irrigation to meet plant water requirements. However, not all rainfall becomes available for use by plants and crops. Depending on such factors as soil type, duration and intensity of rainfall, soil moisture levels, etc., a portion of the rainfall will be lost to runoff and deep percolation (water moving below the root zone).

                    In irrigation scheduling, the term " effective rainfall " refers to that portion of rainfall which infiltrates and is stored in the root zone. Effective rainfall must be estimated for each field and rainfall event. The irrigation requirement determined with equations (1) or (2) should be reduced by the amount of effective rainfall.

                    Alternatively, soil moisture monitoring devices can be use to determine soil moisture levels and to determine when irrigations should be re-started following rains.



                    II. WHERE TO FIND PET INFORMATION

                    For persons with Internet access, PET and weather information is provided for about 10 locations in Central and South Texas on the Texas PET Web Site. The address is:

                        http://texaset.tamu.edu

                      Persons without Internet access should contact their water district or County Extension Agent to see if this information is being provided locally in another way.

                      Persons on the Texas High Plains should contact the Texas A&M Research and Extension Center in Amarillo at (806) 359-5401 about subscription to the High Plains PET Network where PET data is sent out every three days by FAX.



                      III. WHAT IS POTENTIAL EVAPOTRANSPIRATION (PET)?

                      Evapotranspiration (ET) is a measurement of the total amount of water needed to grow plants and crops. This term comes from the words evaporation (i.e., evaporation of water from the soil) and transpiration (i.e., transpiration of water by plants). Different plants have different water requirements, so they have different ET rates.

                      Since there are thousands of cultivated plants, we have tried to simplified matters by establishing a standard ET rate for general reference and use. The standard is referred to as the potential evapotranspiration (PET). This is the potential ET since we are assuming the crop is in a deep soil and under well watered conditions. The standard crop we are using is a cool season grass which is 4-inches tall. The technical term for this is the " Potential Evapotranspiration of a Grass Reference Crop " or "PET" for short.

                      PET depends on the climate and varies from location to location. Special weather stations are used to collect the climatic data for calculating PET, including temperature, dew point temperature (relative humidity), wind speed, and solar radiation.

                      The water requirements of specific crops are calculated as a percentage of the PET. This "percentage" is the called the crop coefficient (Kc). Crop coefficients depend on the type of crop and its stage of growth. Detailed information on crop coefficients are given later.

                      We are using the Penman-Monteith method to calculate PET from the weather station data. This is one of a number of methods that can be used to determine PET and ET. Several organizations, such as the International Committee on Irrigation and Drainage and the Water Requirements Committee of the American Society of Civil Engineers have proposed establishing the Penman-Monteith method as a world-wide standard. Such a standard would help facilitate the sharing of PET data and development of crop coefficients.

                      IV. CROP COEFFICIENTS

                      Potential Evapotranspiration (PET) is an estimate of the water requirements of a 4-inch grass in a deep soil growing under well watered conditions. The water requirements of other crops are determined from PET through crop coefficients (Kc). Crop coefficients vary for different crops. They also change depending on the growing stage of the crop.

                      Unfortunately, we only have verified crop coefficients for the Texas North High Plains for cotton, sorghum, corn and wheat (see Tables 1-3). These coefficients were developed by the North Plains PET Network Project Team. Another source is the FAO (Food and Agriculture Organization of the United Nations) who has published a long list of generalized crop coefficients which are used throughout the world where local values are not available (see Tables 5&6).

                      Choosing and Using Crop Coefficients

                      For cotton, sorghum and corn, I recommend using the North High Plains crop coefficients. These have been verified in research and on-farm irrigation studies, and should vary no more than about 10% for other parts of the state.

                      The North High Plains crop coefficients are listed by stage of growth in Tables 1-3. Please note that these dates are provided as a general guide only, as crop growth rate is affected by many factors including variety, current weather, soil moisture conditions, etc.

                      For other crops, refer to the FAO Crop Coefficients until researchers are able to verify coefficients for specific regions in Texas. For many crops, we would expect these general coefficients to be within about 10%.

                      Soil Moisture Monitoring

                      I highly recommend soil moisture monitoring using gypsum blocks, watermark sensors, tensiometers, the "feel" method, or other devices for measuring the current water status in the root zone. This provides an excellent check to ensure that irrigations are keeping up with crop water demand.


                      V. IRRIGATION SYSTEM EFFICIENCIES

                      No irrigation system is 100% efficient. For sprinkler irrigation systems, we can lose anywhere from 10% to 40% of the water in the air before the water reaches the ground depending on wind and other weather conditions. The amount of water lost to spray drift is referred to as the application efficiency.

                      For drip and surface irrigation system, our biggest concern is how evenly distributed the water is over the field or along laterals and rows. This is referred to as the distribution efficiency. The term overall efficiency is a combination of both the application and distribution efficiencies.

                      The normal ranges in on-farm overall efficiencies are listed in Table 4. Under some situations, we will need to increase the amount of irrigation to compensate for water lost due to the inefficiencies of the system.

                      Table 1. Sorghum Crop Coefficients.

                      GROWTH STAGE1

                      KC

                      DAYS AFTER PLANTING2

                      Seeding

                      0.40

                      3 - 4
                      Emerg

                      0.40

                      5 - 8
                      3-leaf 0.55 19 - 24
                      4-leaf 0.60 28 - 33
                      5-leaf 0.70 32 - 37
                      GPD 0.80 35 - 40
                      Flag 0.95 52 - 58
                      Boot 1.10 57 - 61
                      Heading 1.10 60 - 65
                      Flower 1.00 68 - 75
                      S Dough 0.95 85 - 95
                      H Dough 0.90 95 - 100
                      Blk lyr 0.85 110 - 120
                      Harvest 0.00 125 - 140

                      1Sorghum will bloom at different times depending on locating , planting date, and maturity of the variety.

                      2The Days After Planting are for a medium-early to medium late variety.

                      Table 2. Cotton Crop Coefficients.

                      GROWTH STAGE

                      KC

                      DAYS AFTER PLANTING

                      Seeding

                      .07

                      0 - 10
                      1st Sqr .22 32 - 40
                      1st Blom .44 55 - 60
                      Max Blom 1.10 70 - 90
                      1st open 1.10 105 - 115
                      25% open .83 115 - 125
                      50% open .44 135 - 145
                      95% open .44 140 - 150
                      Pick .10 140 - 150



                      Table 3. Corn Crop Coefficients.

                      GROWTH STAGE1

                      KC

                      DAYS AFTER PLANTING

                      Seed

                      0.25

                      0 - 5
                      Emerg

                      0.35

                      5 - 8
                      4-leaf 0.45 24 - 28
                      5-leaf 0.70 29 - 34
                      6-leaf 0.85 35 - 40
                      8-leaf 1.00 43 - 46
                      10 leaf 1.15 51 - 58
                      12-leaf 1.20 57 - 65
                      14-leaf 1.25 65 - 75
                      Tassel 1.25 72 - 82
                      Silk 1.30 75 - 85
                      Blister 1.30 85 - 95
                      Milk 1.30 95 - 105
                      Dough 1.20 100 - 110
                      Dent 1.00 110 - 120
                      ½ mat 0.90 115 - 125
                      Blk lyr 0.70 125 - 135
                      Harvest 0.00 140 +

                      1 Note: 50% silking occurs in 85 days in Corpus Christi/Coastal bend area and in 75 days in the Uvalde area.

                      Table 4. Typical overall On-Farm Efficiencies

                      System Overall Effeciency
                      Surface 0.50 - 0.80
                      a. average 0.50
                      b. land leveling and delivery pipeline meeting

                      design standards

                      0.70
                      c. tailwater recovery with (b) 0.80
                      d. surge 0.60 - 0.901
                      Sprinkler 0.55 - 0.753
                      Center Pivot 0.55 - 0.903
                      LEPA 0.90 - 0.95
                      Drip 0.80 - 0.902

                      1. Surge has been found to increase efficiencies 8 to 28 percent over non-surge furrow systems.

                      2. Trickle systems are typically designed at 90 percent efficiency; short laterals (<100ft) or systems with pressure compensationg emitters may have higher efficiencies.

                      3. Under low wind conditions.


                      VI. FAO CROP COEFFICIENTS

                      With the FAO method, crop coefficients (Table 5) are represented by straight lines connecting four general growth stages, as indicated in the following figure. Table 6 gives a more detailed explanation of the crop coefficient values and their respective growth stages.

                      Table 5. FAO Mean Crop Coefficients, Kc, for Arid Climates1

                      CROP

                      Kc12

                      Kc2 Kc3
                      Alfalfa Hay3 0.30 1.25 1.10
                      Artichokes 0.95 1.00 0.95
                      Asparagus 0.30 0.95 0.30
                      Banana, 1st year 0.50 1.15 1.10
                      Banana, 2nd year 0.70 1.20 1.10
                      Barley, Wheat, Oats 0.30 1.15 0.20
                      Beans, Green 0.40 1.00 0.90
                      Beans, Dry and Pulses 0.40 1.15 0.35
                      Beets, Table 0.40 1.05 0.95
                      Berries 0.30 1.05 0.40
                      Carrots 0.50 1.10 0.80
                      Castor Beans 0.40 1.15 0.50
                      Celery 0.35 1.10 1.00
                      Citrus 0.65 0.80 0.65
                      Citrus w/cover or weeds 0.70 1.05 1.05
                      Clover Hay3 0.60 1.20 1.05
                      Coffee 0.90 0.90 0.90
                      Conifer Trees 1.20 1.20 1.20
                      Corn, Field 0.40 1.15 0.60, 0.354
                      Corn, Sweet 0.40 1.15 1.05
                      Cotton 0.40 1.20 0.65
                      Cucumber, Fresh Market 0.35 0.95 0.75
                      Cucumber, Machine Harvest 0.35 0.95 0.95
                      Crucifers5 0.40 1.05 0.90
                      Dates 0.95 0.95 0.95
                      Deciduous Orchard 0.50 1.00 0.656
                      Deciduous Orchard w/cover or weeds 0.80 1.25 0.856
                      Eggplant 0.40 1.05 0.85
                      Flax 0.30 1.10 0.20
                      Grapes 0.30 0.80 0.40
                      Grass Pasture 0.80 0.80 0.80
                      Groundnuts 0.40 1.05 0.60
                      Hops 0.30 1.00 0.60
                      Kiwi 0.30 1.05 1.05
                      Lentil 0.30 1.15 0.25
                      Lettuce 0.30 1.00 0.95
                      Melons 0.40 1.00 0.75
                      Millet 0.30 1.10 0.25
                      Mint 0.60 1.10 1.10
                      Olives 0.40 0.70 0.70
                      Onion, Dry 0.50 1.05 0.80
                      Onion, Green 0.50 1.00 1.00
                      Open Water 1.15 1.15 1.15
                      Palm Trees 0.95 0.95 0.95
                      Peas, Fresh 0.40 1.10 1.05
                      Peas, Dry/Seed 0.40 1.10 0.30
                      Peppers, Fresh 0.35 1.05 0.85
                      Pistachios 0.20 1.10 0.40
                      Potato 0.40 1.10 0.758
                      Pumpkin 0.40 1.00 0.75
                      Radishes 0.30 0.85 0.80
                      Rice 1.10 1.25 1.00
                      Safflower 0.35 1.15 0.20
                      Sorghum, Grain 0.30 1.05 0.50
                      Sorghum, Sweet 0.30 1.20 0.50
                      Soybeans 0.35 1.10 0.45
                      Spinach 0.30 1.00 0.95
                      Squash 0.30 0.95 0.75
                      Strawberries 0.40 0.90 0.70
                      Sugar Beet 0.30 1.15 1.009
                      Sugar Cane 0.40 1.25 0.70
                      Sunflower 0.30 1.15 0.35
                      Tea 1.00 1.00 1.00
                      Tomato 0.40 1.20 0.65
                      Walnut Orchard 0.50 1.00 0.65
                      Winter Wheat 0.30 1.15 0.20

                      Footnotes to Table 5.

                      1Values for Kc2 and Kc3 represent those for an arid climate (RHmin ~ 20%) with moderate wind speed (0-5 m s-1, averaging 2 m s-1) and are used for general irrigation water requirements. For humid or windier conditions, Kc2 and Kc3 can be modified as follows:

                          Kc2=Kc2 table - 0.0015 RHmin + 0.01 U2

                            Kc3=Kc3 table - 0.0015 RHmin + 0.01 U2 when Kc3 > or = 0.4

                            Kc3 = Kc3 table + 0.001 (RHmin-20) when Kc3 < 0.4

                          2General values for Kc1 under infrequent soil wetting (~each 10 days). Kc1 is a function of wetting interval and potential evaporation rate during the initial and development periods.
                          3 The coefficients for hay crops represent immediately following cutting; full cover; and immediately before cutting, respectively. An overall mean coefficient for alfalfa hay which takes into account cutting effects is 1.05 and for clover is 1.00. Kc3 for alfalfa seed, which is never cut for forage or hay is about 0.5.
                          4 The first Kc3 value is for harvest at high grain moisture. The second Kc3 value is for harvest after complete field drying of grain (to about 18%).
                          5 Crucifers include cabbage, cauliflower, broccoli, and Brussel sprouts.
                          6 The Kc3 value represents the Kc prior to leaf drop.
                          7 Cool season grass varieties include dense strands of bluegrass, ryegrass, and fescue. Warm season varieties include bermuda grass and St. Augustine grass. The 0.90 values for cool season grass represent a 0.06 to 0.08m height under general turf conditions.
                          8 The Kc3 value for potatoes is about 0.40 for long season potatoes with vine kill.
                          9The Kc3 value for sugar beets is 0.60 if no irrigation occurs during the last month.


                          Table 6 . FAO Generalized Crop Growth Stages

                          Kc Values

                          Growth Stage

                          Description

                          Kc1

                          Initial

                          The average KC value from planting to about 10% ground cover.

                          Kc1-Kc2

                          Rapid Growth

                          From 10% ground cover to 75% cover or to peak water use, which ever comes first.

                          Kc2

                          Mid season

                          The average value from the end of the rapid growth stage until water use begins to decline due to crop aging.

                          Kc2-Kc3

                          Late season

                          From when KC begins to decline until harvest or when water use ceases or becomes minimal.

                          Kc3

                          Harvest

                          The average value at harvest or the end of the water use season.

                          Note: For the rapid growth and the late season stage, it is assumed that KC values increase or decrease linearly with time.



                          VII. ACKNOWLEDGEMENTS

                          Information on crop growth periods, time periods after planting, and other advice provided by Charles, Stickler, Associate Professor and Extension Agronomist, Texas A&M Center-Uvalde.

                          For More Information

                          For more information on the Penman-Monteith equation and other methods for determining PET, see the book: Evapotranspiration and Irrigation Water Requirements, edited by M.E. Jensen, R.D. Burman, and R.G. Allen. Published by the American Society of Civil Engineers, New York, NY. 1990. 332pp.

                          Written by Guy Fipps, Associate Professor and Extension Agricultural Engineer, Texas A&M University, College Station..

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                          Texas A&M AgriLife Extension
                          Charles Swanson
                          Extension Program Specialist - Landscape Irrigation
                          2117 Texas A&M University
                          College Station, TX 77843
                          P: (979) 845-5614
                          clswanson@tamu.edu
                          Texas A&M AgriLife Extension
                          Guy Fipps
                          Professor & Extension Specialist - Irrigation
                          2117 Texas A&M University
                          College Station, TX 77843
                          P: (979) 845-7454
                          g-fipps@tamu.edu
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