TexasET Network

Tawakoni's 14 Day Weather Summary Sponsored by North Texas Municipal Water District
Date	ETo (in)	Max Temp (f)	Min Temp (f)	Min RH (%)	Solar Rad. (MJ/m2)	Rainfall (in)	Wind 4am (mph)	Wind 4pm (mph)	Battery (v)
Apr 19, 2024 - Fri	0.10	67	57	63	9.69	0.00	7.24	4.53
Apr 18, 2024 - Thu	0.11	80	67	63	7.99	0.00	3.12	4.36
Apr 17, 2024 - Wed	0.13	81	67	56	11.94	0.00	3.00	7.42
Apr 16, 2024 - Tue	0.10	76	67	74	7.46	0.03	9.04	8.37
Apr 15, 2024 - Mon	0.10	75	65	68	7.09	0.00	7.00	7.00
Apr 14, 2024 - Sun	0.16	74	61	52	14.90	0.00	9.73	10.71
Apr 13, 2024 - Sat	0.24	78	57	26	19.49	0.00	6.93	12.74
Apr 12, 2024 - Fri	0.15	76	45	24	19.80	0.00	0.00	5.04
Apr 11, 2024 - Thu	0.20	70	49	24	19.85	0.00	5.06	10.67
Apr 10, 2024 - Wed	0.06	61	55	71	2.67	0.44	0.52	13.99
Apr 09, 2024 - Tue	0.05	66	59	84	4.08	0.49	10.37	5.36
Apr 08, 2024 - Mon	0.14	79	58	41	11.33	0.22	3.65	4.22
Apr 07, 2024 - Sun	0.21	74	58	13	18.70	0.00	8.00	7.39
Apr 06, 2024 - Sat	0.20	77	59	39	13.52	0.00	5.06	11.58
Data Summary	1.95	81	45	49	168.52	1.18	5.62	8.10

Detailed Crop Weather Data
Date	Average Temp (f)	Dew Point (f)	Max Dewpoint (f)	Min Dewpoint (f)	Wind Run (miles/day)	Soil Temp (f)
Apr 19, 2024 - Fri	62	51	59	46	129	0
Apr 18, 2024 - Thu	72	66	70	59	101	0
Apr 17, 2024 - Wed	74	66	68	64	97	0
Apr 16, 2024 - Tue	71	64	68	60	187	0
Apr 15, 2024 - Mon	70	62	65	60	148	0
Apr 14, 2024 - Sun	67	57	62	47	201	0
Apr 13, 2024 - Sat	68	50	55	46	191	0
Apr 12, 2024 - Fri	61	44	50	37	54	0
Apr 11, 2024 - Thu	59	41	44	34	146	0
Apr 10, 2024 - Wed	58	55	59	45	124	0
Apr 09, 2024 - Tue	62	58	63	55	139	0
Apr 08, 2024 - Mon	67	54	63	38	111	0
Apr 07, 2024 - Sun	67	45	60	35	126	0
Apr 06, 2024 - Sat	68	52	56	45	213	0
Data Summary	66	54	70	34	1,968	0

Heat Units
Date	Corn (heat units)	Cotton (heat units)	Sorghum (heat units)	50 Degree (heat units)	55 Degree (heat units)	60 Degree (heat units)
Apr 19, 2024 - Fri	12	2	7	12	7	2
Apr 18, 2024 - Thu	23	13	18	23	18	13
Apr 17, 2024 - Wed	24	14	19	24	19	14
Apr 16, 2024 - Tue	22	12	17	22	17	12
Apr 15, 2024 - Mon	20	10	15	20	15	10
Apr 14, 2024 - Sun	18	8	13	18	13	8
Apr 13, 2024 - Sat	17	7	12	17	12	7
Apr 12, 2024 - Fri	11	1	6	11	6	1
Apr 11, 2024 - Thu	9	0	4	9	4	0
Apr 10, 2024 - Wed	8	0	3	8	3	0
Apr 09, 2024 - Tue	12	2	7	12	7	2
Apr 08, 2024 - Mon	19	9	14	19	14	9
Apr 07, 2024 - Sun	16	6	11	16	11	6
Apr 06, 2024 - Sat	18	8	13	18	13	8
Data Summary	229	92	159	229	159	92

Chilling Hours
Date	Method 1 (Hours >= 32 & <= 45 F)	Method 2 (Hours <= 45 F)
Apr 19, 2024 - Fri	0	0
Apr 18, 2024 - Thu	0	0
Apr 17, 2024 - Wed	0	0
Apr 16, 2024 - Tue	0	0
Apr 15, 2024 - Mon	0	0
Apr 14, 2024 - Sun	0	0
Apr 13, 2024 - Sat	0	0
Apr 12, 2024 - Fri	0	0
Apr 11, 2024 - Thu	0	0
Apr 10, 2024 - Wed	0	0
Apr 09, 2024 - Tue	0	0
Apr 08, 2024 - Mon	0	0
Apr 07, 2024 - Sun	0	0
Apr 06, 2024 - Sat	0	0
Data Summary	0	0

Chilling Hours for the 2023-2024 Season October 1, 2023 thru February 28, 2024
Month	Method 1 (Hours >= 32 & <= 45 F)	Method 2 (Hours <= 45 F)
Oct	36	36
Nov	82	91
Dec	137	154
Jan	196	361
Feb	75	86
Total	526	728

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

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.

In 1948, a British scientist name Howard Penman presented a theory (i.e., mathematical model) for calculating ET using local climate data. The resulting Penman equation has been modified since, but forms of this equation are still used today for this purpose.

Since there are thousands of cultivated plants, scientists simplified 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. A cool-season grass is widely used for this standard crop. 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, and solar radiation. The American Society of Biological and Agricultural Engineers has established a standard for weather stations used for calculation of ET which can be found on this website.

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 website.

We are using the standardized Penman-Monteith method to calculate ETo from the weather station data. This method is also referred to as the ASCE standard method (for more info, see: The ASCE Standardized Reference Evapotranspiration Equation | Books (ascelibrary.org) . This is one of several methods that can be used to determine ETo and ET. 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 . For information on the standardized equation and other methods for determining ET, 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. 2016.
https://doi.org/10.1061/9780784414057

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.

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

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

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 Coefficient for Corn

North Plains Crop Coefficients for Cotton

North Plains Crop Coefficients for Peanuts

North Plains Crop Coefficients for Sorghum

North Plains Crop Coefficients for Soybean

North Plains Crop Coefficient for Winter Wheat

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

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
© 2024 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
© 2024 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)

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
© 2024 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
© 2024 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
© 2024 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
© 2024 Texas A&M AgriLife Extension

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
© 2024 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
© 2024 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
© 2024 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
© 2024 Texas A&M AgriLife Extension

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
© 2024 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
© 2024 Texas A&M AgriLife Extension

Station	Status	Data Available	Last Error Message	Date Error Occurred
Abilene	Updated	01/05/2024 - 04/19/2024	No Error Logged
Allen	Not Updated	03/06/2015 - 04/17/2024	No Error Logged
Alvarado	Updated	07/08/2020 - 04/19/2024	Solar radiation data out of range:-7.1991	09/06/2023
Argyle	Updated	05/26/2018 - 04/19/2024	Solar radiation data out of range:-7999	04/02/2024
Aubrey	Updated	04/20/2022 - 04/19/2024	Solar radiation data out of range:-7.1991	12/12/2023
Austin (LCRA Redbud)	Updated	09/11/2014 - 04/19/2024	Solar radiation data out of range:0	06/26/2022
Ballinger	Updated	12/17/2020 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Ben Wheeler	Updated	04/20/2022 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Blum	Not Updated	07/08/2020 - 04/18/2024	Solar radiation data out of range:-7.1991	07/04/2023
Bryan	Updated	08/05/2014 - 04/19/2024	Solar radiation data out of range:-7999	04/10/2024
Buda	Updated	08/08/2018 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Burnet	Updated	07/26/2017 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Canyon	Updated	12/09/2022 - 04/19/2024	Solar radiation data out of range:-7999	08/29/2023
Carthage	Updated	12/10/2020 - 04/19/2024	Solar radiation data out of range:-8.55E-06	07/01/2022
Cash	Not Updated	04/01/2016 - 04/18/2024	Solar radiation data out of range:-7999	01/29/2024
Cedar Creek	Not Updated	12/13/2020 - 12/17/2023	Solar radiation data out of range:0	06/28/2022
Cedar Park	Updated	02/11/2015 - 04/19/2024	Solar radiation data out of range:0	06/26/2022
Conroe	Not Updated	09/05/2013 - 01/13/2024	Solar radiation data out of range:0	06/29/2022
Corpus Christi AgriLife	Updated	06/08/2022 - 04/19/2024	Solar radiation data out of range:500	07/07/2022
Corpus Christi North	Not Updated	08/15/2023 - 04/16/2024	Solar radiation data out of range:-7999	03/21/2024
Corpus Christi South	Not Updated	08/15/2023 - 04/17/2024	Solar radiation data out of range:-7999	04/14/2024
Crystal City	Updated	03/29/2024 - 04/19/2024	No Error Logged
Dallas AgriLife Center	Not Updated	04/07/2008 - 04/17/2024	Solar radiation data out of range:-7999	04/19/2024
Delta Lake	Not Updated	03/13/2019 - 02/18/2024	No Error Logged
Detroit	Updated	01/05/2024 - 04/19/2024	No Error Logged
Dickinson	Updated	07/27/2016 - 04/19/2024	Temperature data out of range:-7999	11/14/2023
Dripping Springs	Updated	02/11/2015 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Driscoll	Updated	06/07/2022 - 04/19/2024	No Error Logged
Fabens	Updated	04/20/2022 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Farmersville	Updated	11/15/2012 - 04/19/2024	Solar radiation data out of range:0	06/29/2022
Forestburg	Updated	12/13/2020 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Forney	Updated	12/03/2012 - 04/19/2024	No Error Logged
Fort Stockton East	Updated	07/08/2020 - 04/19/2024	Solar radiation data out of range:-7.1991	03/27/2024
Fort Stockton West	Updated	12/10/2020 - 04/19/2024	Solar radiation data out of range:-7.1991	02/07/2023
Fort Worth - NOAA	Updated	08/25/2011 - 04/19/2024	Solar radiation data out of range:-0.0002808	07/11/2022
Freer	Updated	04/20/2022 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Garland	Updated	03/07/2015 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Garwood	Not Updated	12/10/2020 - 04/18/2024	Solar radiation data out of range:-7.1991	11/01/2023
Goliad	Updated	12/13/2020 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Halfway	Updated	06/28/2012 - 04/19/2024	Solar radiation data out of range:-28.7964	04/08/2024
Highland-University Park	Not Updated	04/19/2016 - 04/18/2024	Solar radiation data out of range:0	06/27/2022
Houston	Updated	07/27/2016 - 04/19/2024	Solar radiation data out of range:-7999	04/29/2023
Houston North	Updated	07/11/2023 - 04/19/2024	Solar radiation data out of range:-7999	04/10/2024
Huntsville	Not Updated	01/22/2010 - 08/07/2023	Relative humidity data out of range:108	04/21/2023
Irving North	Updated	10/08/2013 - 04/19/2024	Solar radiation data out of range:0	06/29/2022
Irving South	Updated	10/08/2013 - 04/19/2024	Solar radiation data out of range:-7999	10/03/2023
Jersey Village	Updated	03/06/2020 - 04/19/2024	Solar radiation data out of range:-7999	04/13/2024
Kennedale	Updated	11/12/2014 - 04/19/2024	Solar radiation data out of range:-7.2E-05	07/11/2022
Kingsville	Updated	12/20/2020 - 04/19/2024	Temperature data out of range:-262.84	11/27/2023
La Gloria	Updated	07/08/2020 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
La Villa	Updated	12/10/2020 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Ladonia	Updated	05/18/2019 - 04/19/2024	Solar radiation data out of range:-7999	12/04/2023
Lakeway	Updated	02/11/2015 - 04/19/2024	Solar radiation data out of range:0	06/26/2022
Lewisville	Updated	06/26/2015 - 04/19/2024	Solar radiation data out of range:-7999	02/15/2024
Lubbock	Updated	06/02/2020 - 04/19/2024	Solar radiation data out of range:-7999	03/15/2024
Marble Falls	Updated	09/11/2014 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Marfa	Not Updated	04/20/2022 - 04/18/2024	Solar radiation data out of range:-7.1991	04/05/2024
McKinney	Updated	11/15/2012 - 04/19/2024	Temperature data out of range:-46.09	04/20/2023
Medina	Updated	05/25/2021 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Melissa	Not Updated	03/16/2016 - 02/12/2024	Solar radiation data out of range:0	06/12/2022
Melissa RD121	Updated	04/01/2016 - 04/19/2024	Solar radiation data out of range:0	06/25/2022
Memorial Village	Updated	09/14/2017 - 04/19/2024	Solar radiation data out of range:0	06/27/2022
Menard	Updated	07/08/2020 - 04/19/2024	Solar radiation data out of range:-7.1991	02/14/2024
Mesquite	Updated	12/03/2012 - 04/19/2024	Solar radiation data out of range:0	06/27/2022
Murphy	Updated	03/16/2016 - 04/19/2024	Solar radiation data out of range:-7999	04/02/2024
Naples	Updated	04/20/2022 - 04/19/2024	Solar radiation data out of range:-7.1991	11/30/2022
Nasho	Not Updated	07/05/2018 - 02/18/2024	Solar radiation data out of range:-7999	02/09/2024
Nocona	Updated	12/10/2020 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Overton	Updated	06/07/2004 - 04/19/2024	Solar radiation data out of range:-7999	05/08/2023
Ozona-Sonora	Updated	07/08/2020 - 04/19/2024	Solar radiation data out of range:-7.1991	05/18/2023
Pflugerville	Updated	02/11/2015 - 04/19/2024	Solar radiation data out of range:0	06/27/2022
Plano	Updated	11/15/2012 - 04/19/2024	Solar radiation data out of range:0	06/29/2022
Princeton	Not Updated	03/07/2015 - 04/16/2024	Solar radiation data out of range:0	06/24/2022
Prosper	Updated	07/21/2021 - 04/19/2024	Solar radiation data out of range:-7999	04/02/2024
Providence Village	Updated	06/26/2015 - 04/19/2024	Solar radiation data out of range:-7999	04/18/2024
Refugio	Updated	12/22/2020 - 04/19/2024	Solar radiation data out of range:-7.1991	11/29/2023
Richardson	Updated	03/06/2015 - 04/19/2024	Solar radiation data out of range:0	06/27/2022
Richmond North	Updated	01/01/2018 - 04/19/2024	Solar radiation data out of range:0	06/28/2022
Richmond South	Updated	07/27/2016 - 04/19/2024	Solar radiation data out of range:-7999	05/14/2023
Rockwall	Updated	12/03/2012 - 04/19/2024	Solar radiation data out of range:0	06/29/2022
Round Rock	Not Updated	12/17/2020 - 04/10/2024	Solar radiation data out of range:0	06/28/2022
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Shafter	Updated	04/20/2022 - 04/19/2024	Solar radiation data out of range:-7.1991	08/09/2022
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Tarrant West	Updated	11/12/2014 - 04/19/2024	Solar radiation data out of range:-3.6E-05	07/11/2022
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Van Horn North	Updated	12/10/2020 - 04/19/2024	Solar radiation data out of range:-7.1991	03/23/2024
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Victoria County West	Updated	04/20/2022 - 04/19/2024	Solar radiation data out of range:-7.1991	04/17/2023
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Water Valley	Updated	04/02/2024 - 04/19/2024	No Error Logged
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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	K_C	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	K_C	DAYS AFTER PLANTING
Seeding	.07	0 - 10
1^st Sqr	.22	32 - 40
1^st Blom	.44	55 - 60
Max Blom	1.10	70 - 90
1^st 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	K_C	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, K_c, for Arid Climates¹

CROP	K_c1²	K_c2	K_c3
Alfalfa Hay³	0.30	1.25	1.10
Artichokes	0.95	1.00	0.95
Asparagus	0.30	0.95	0.30
Banana, 1^st year	0.50	1.15	1.10
Banana, 2^nd 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 K_c2 and K_c3 represent those for an arid climate (RH_min~ 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 K_c3 can be modified as follows:

K_c2=K_{c2 table} - 0.0015 RH_min + 0.01 U₂

K_c3=K_{c3 table} - 0.0015 RH_min + 0.01 U₂ when K_c3 > or = 0.4

K_c3 = K_{c3 table} + 0.001 (RH_min-20) when K_c3 < 0.4

²General values for K_c1 under infrequent soil wetting (~each 10 days). K_c1 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. K_c3 for alfalfa seed, which is never cut for forage or hay is about 0.5.
⁴ The first K_c3 value is for harvest at high grain moisture. The second K_c3 value is for harvest after complete field drying of grain (to about 18%).
⁵ Crucifers include cabbage, cauliflower, broccoli, and Brussel sprouts.
⁶ The K_c3 value represents the K_c 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 K_c3 value for potatoes is about 0.40 for long season potatoes with vine kill.
⁹The K_c3 value for sugar beets is 0.60 if no irrigation occurs during the last month.

Table 6 . FAO Generalized Crop Growth Stages

K_cValues	Growth Stage	Description
K_c1	Initial	The average K_C value from planting to about 10% ground cover.
K_c1-K_c2	Rapid Growth	From 10% ground cover to 75% cover or to peak water use, which ever comes first.
K_c2	Mid season	The average value from the end of the rapid growth stage until water use begins to decline due to crop aging.
K_c2-K_c3	Late season	From when K_C begins to decline until harvest or when water use ceases or becomes minimal.
K_c3	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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Katy Rain (77493)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Katy Rain (77494)	Updated	06/01/2021 - 04/19/2024	No Error Logged
La Porte Rain (77571)	Updated	06/01/2021 - 04/19/2024	No Error Logged
League City Rain (77573)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Magnolia Rain (77354)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Magnolia Rain (77355)	Updated	06/01/2021 - 04/19/2024	No Error Logged
McKinney Rain	Updated	01/20/2016 - 04/19/2024	No Error Logged
Mesquite Rain	Not Updated	01/20/2016 - 04/16/2024	No Error Logged
New Caney Rain (77357)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Pasadena Rain (77506)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Pasadena Rain (77507)	Not Updated	01/01/0001 - 01/01/0001	No Error Logged
Pearland Rain (77581)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Pinehurst Rain (77362)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Plano Rain	Updated	01/20/2016 - 04/19/2024	No Error Logged
Plantersville Rain (77363)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Porter Rain (77365)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Princeton Rain	Not Updated	01/20/2016 - 04/16/2024	No Error Logged
Richardson Rain	Not Updated	01/20/2016 - 04/17/2024	No Error Logged
Richmond Rain (77407)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Rockwall Rain	Not Updated	01/20/2016 - 04/17/2024	No Error Logged
Royse City Rain	Updated	01/20/2016 - 04/19/2024	No Error Logged
Seabrook Rain (77586)	Updated	06/01/2021 - 04/19/2024	No Error Logged
South Houston Rain (77587)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Splendora Rain (77372)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Spring Rain (77373)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Spring Rain (77382)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Spring Rain (77389)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Tomball Rain (77375)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Tomball Rain (77377)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Waller Rain (77484)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Webster Rain (77598)	Updated	06/01/2021 - 04/19/2024	No Error Logged
Wylie Rain	Updated	01/20/2016 - 04/19/2024	No Error Logged

Calculators