TexasET Network

Farmersville's 7 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)
Aug 27, 2026 - Thu	0.20	99	75	29	18.70	0.00	1.46	4.50
Aug 26, 2026 - Wed	0.19	102	81	26	14.38	0.00	1.17	2.03
Aug 25, 2026 - Tue	0.25	100	80	21	15.33	0.00	4.18	7.33
Aug 24, 2026 - Mon	0.27	105	81	20	17.88	0.00	3.12	7.05
Aug 23, 2026 - Sun	0.24	106	80	24	18.84	0.00	1.36	4.66
Aug 22, 2026 - Sat	0.17	103	79	28	15.04	0.00	2.02	1.14
Aug 21, 2026 - Fri	0.18	101	79	29	16.69	0.00	2.73	2.66
Data Summary	1.49	106	75	25	116.86	0.00	2.29	4.19

Detailed Crop Weather Data
Date	Average Temp (f)	Dew Point (f)	Max Dewpoint (f)	Min Dewpoint (f)	Wind Run (miles/day)	Soil Temp (f)
Aug 27, 2026 - Thu	87	69	74	64	63	0
Aug 26, 2026 - Wed	89	69	75	65	61	0
Aug 25, 2026 - Tue	89	67	71	64	118	0
Aug 24, 2026 - Mon	92	69	77	64	109	0
Aug 23, 2026 - Sun	92	70	78	64	80	0
Aug 22, 2026 - Sat	91	70	77	65	45	0
Aug 21, 2026 - Fri	89	70	75	66	48	0
Data Summary	89	69	78	64	523	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)
Aug 27, 2026 - Thu	30	27	32	37	32	27
Aug 26, 2026 - Wed	33	30	35	40	35	30
Aug 25, 2026 - Tue	33	30	35	40	35	30
Aug 24, 2026 - Mon	33	30	35	40	35	30
Aug 23, 2026 - Sun	33	30	35	40	35	30
Aug 22, 2026 - Sat	32	29	34	39	34	29
Aug 21, 2026 - Fri	32	29	34	39	34	29
Data Summary	226	205	240	275	240	205

Chilling Hours
Date	Method 1 (Hours >= 32 & <= 45 F)	Method 2 (Hours <= 45 F)
Aug 27, 2026 - Thu	0	0
Aug 26, 2026 - Wed	0	0
Aug 25, 2026 - Tue	0	0
Aug 24, 2026 - Mon	0	0
Aug 23, 2026 - Sun	0	0
Aug 22, 2026 - Sat	0	0
Aug 21, 2026 - Fri	0	0
Data Summary	0	0

Chilling Hours for the 2025-2026 Season October 1, 2025 thru March 31, 2026
Month	Method 1 (Hours >= 32 & <= 45 F)	Method 2 (Hours <= 45 F)
Oct	14	14
Nov	69	78
Dec	241	290
Jan	199	352
Feb	91	105
Mar	42	46
Total	656	885

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

METHOD 1: Select your weather station. Choose the number of days to be included: 3-day summary, 5-day summary, 7-day summary or 2-week summary (by default, the 7-day summary is shown). Choose the calculator: Landscape or Crop. The calculator is automatically populated with the ETo total for the number of days listed and with rainfall. If your rainfall is different, enter it. Fill in the remaining variables and click "Compute" to determine the water needs for your landscape or crop.

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

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

NOTE:
If you utilize this site often, you may want to create an account where you can set up sites, quickly adjust crop coefficients during the growing season, and view or receive calculated water requirements by email.

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

Evapotranspiration (ETo) is an estimate of the total water requirements of a 4-inch tall, cool season grass in a deep soil growing under well-watered conditions. This reference grass is modeled after fescue and is assumed to be healthy and free of pest and disease problems. The water requirements of crops and plants are calculated from ETo by using crop coefficients (Kc). The following equation is used:

ETo x Kc = crop water requirement

Crop coefficients are different for different crops and varieties. In addition, they change based on the growing stage of the crop. In Texas, crop coefficients have been developed by USDA and Texas A&M AgriLife scientists in the Texas High Plains.

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 following equation:

ETo x Kc / Eff = crop water requirements

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
© 2026 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
© 2026 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
© 2026 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
© 2026 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
© 2026 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
© 2026 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
© 2026 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
© 2026 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
© 2026 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
© 2026 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
© 2026 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
© 2026 Texas A&M AgriLife Extension

Station	Status	Data Available	Last Error Message	Date Error Occurred
Abilene	Updated	01/05/2024 - 08/28/2026	Soil temperature data out of range:-14366.2	02/26/2025
Allen	Not Updated	03/06/2015 - 08/27/2026	No Error Logged
Alvarado	Updated	07/08/2020 - 08/28/2026	Solar radiation data out of range:-7.1991	09/06/2023
Argyle	Updated	05/26/2018 - 08/28/2026	Solar radiation data out of range:-7999	07/17/2026
Aubrey	Updated	04/20/2022 - 08/28/2026	Solar radiation data out of range:-7.1991	06/22/2025
Austin (LCRA Redbud)	Updated	09/11/2014 - 08/28/2026	Solar radiation data out of range:0	06/26/2022
Ballinger	Not Updated	12/17/2020 - 06/07/2026	Solar radiation data out of range:-7.1991	12/24/2025
Ben Wheeler	Updated	04/20/2022 - 08/28/2026	Solar radiation data out of range:-7.1991	09/12/2025
Bryan	Not Updated	08/05/2014 - 08/27/2026	Solar radiation data out of range:-7999	07/12/2026
Buda	Updated	08/08/2018 - 08/28/2026	Relative humidity data out of range:0	08/20/2026
Burnet	Updated	07/26/2017 - 08/28/2026	Relative humidity data out of range:0	05/13/2026
Canyon	Not Updated	12/09/2022 - 08/27/2026	Solar radiation data out of range:-7999	06/19/2026
Carthage	Updated	12/10/2020 - 08/28/2026	Temperature data out of range:-304.24	10/09/2024
Cash	Not Updated	04/01/2016 - 08/27/2026	Solar radiation data out of range:-7999	01/29/2024
Cedar Creek	Updated	12/13/2020 - 08/28/2026	Solar radiation data out of range:0	06/28/2022
Cedar Park	Updated	02/11/2015 - 08/28/2026	Solar radiation data out of range:0	06/26/2022
Celina	Not Updated	06/24/2025 - 08/27/2026	No Error Logged
Conroe	Not Updated	09/05/2013 - 08/27/2026	Relative humidity data out of range:7999	05/14/2025
Cooper	Updated	09/17/2025 - 08/28/2026	No Error Logged
Corpus Christi AgriLife	Not Updated	06/08/2022 - 05/19/2026	Wind speed data out of range:873.040939162996	01/21/2025
Corpus Christi North	Not Updated	08/15/2023 - 08/25/2026	Solar radiation data out of range:-7999	07/10/2026
Corpus Christi South	Not Updated	08/15/2023 - 08/25/2026	Solar radiation data out of range:-7999	06/17/2026
Crystal City	Updated	03/29/2024 - 08/28/2026	No Error Logged
Dallas AgriLife Center	Updated	04/07/2008 - 08/28/2026	Solar radiation data out of range:-7999	06/26/2026
Delta Lake	Not Updated	03/13/2019 - 08/27/2026	Solar radiation data out of range:-7999	06/16/2026
Detroit	Updated	01/05/2024 - 08/28/2026	Relative humidity data out of range:0	06/23/2026
Dickinson	Not Updated	07/27/2016 - 08/27/2026	Solar radiation data out of range:-7999	05/09/2025
Dime Box	Updated	12/03/2024 - 08/28/2026	No Error Logged
Dripping Springs	Updated	02/11/2015 - 08/28/2026	Solar radiation data out of range:0	06/28/2022
Driscoll	Not Updated	06/07/2022 - 11/26/2025	No Error Logged
Eagle Pass	Updated	04/03/2025 - 08/28/2026	No Error Logged
El Dorado	Updated	02/06/2025 - 08/28/2026	Solar radiation data out of range:-7.1991	07/06/2025
Fabens	Updated	04/20/2022 - 08/28/2026	Solar radiation data out of range:0	06/28/2022
Farmersville	Not Updated	11/15/2012 - 08/27/2026	Solar radiation data out of range:0	06/29/2022
Forestburg	Updated	12/13/2020 - 08/28/2026	Solar radiation data out of range:0	06/28/2022
Forney	Not Updated	12/03/2012 - 08/27/2026	No Error Logged
Fort Stockton East	Updated	07/08/2020 - 08/28/2026	Solar radiation data out of range:-7.1991	08/06/2025
Fort Stockton West	Updated	12/10/2020 - 08/28/2026	Relative humidity data out of range:0	05/18/2026
Fort Worth - NOAA	Updated	08/25/2011 - 08/28/2026	Solar radiation data out of range:-0.0002808	07/11/2022
Freer	Not Updated	04/20/2022 - 08/10/2026	Solar radiation data out of range:0	06/28/2022
Garland	Not Updated	03/07/2015 - 08/27/2026	Solar radiation data out of range:0	06/28/2022
Garwood	Not Updated	12/10/2020 - 08/02/2026	Relative humidity data out of range:0	08/01/2026
Goliad	Not Updated	12/13/2020 - 04/24/2025	Solar radiation data out of range:0	06/28/2022
Halfway	Not Updated	06/28/2012 - 08/27/2026	Solar radiation data out of range:-28.7964	04/08/2024
Hebbronville	Updated	04/20/2022 - 08/28/2026	Solar radiation data out of range:-7.1991	08/17/2022
Highland-University Park	Updated	04/19/2016 - 08/28/2026	Relative humidity data out of range:0	05/25/2026
Houston	Not Updated	07/27/2016 - 08/27/2026	Solar radiation data out of range:-7999	08/07/2026
Houston North	Not Updated	07/11/2023 - 08/27/2026	Solar radiation data out of range:-7999	06/21/2026
Huntsville	Not Updated	01/22/2010 - 08/27/2026	Relative humidity data out of range:110	07/24/2025
Irving North	Not Updated	10/08/2013 - 08/27/2026	Solar radiation data out of range:-7999	05/12/2025
Irving South	Not Updated	10/08/2013 - 08/27/2026	Solar radiation data out of range:-7999	10/03/2023
Kennedale	Not Updated	11/12/2014 - 08/24/2026	Relative humidity data out of range:0	04/30/2026
Kingsville	Updated	12/20/2020 - 08/28/2026	Temperature data out of range:-262.84	11/27/2023
La Gloria	Updated	07/08/2020 - 08/28/2026	Temperature data out of range:-38.542	09/02/2024
La Villa	Not Updated	12/10/2020 - 05/26/2026	Relative humidity data out of range:0	08/01/2026
Ladonia 2	Updated	06/17/2025 - 08/28/2026	Relative humidity data out of range:0	07/21/2026
Lakeway	Updated	02/11/2015 - 08/28/2026	Solar radiation data out of range:0	06/26/2022
Lewisville	Not Updated	06/26/2015 - 08/27/2026	Solar radiation data out of range:-7999	02/15/2024
Lubbock	Not Updated	06/02/2020 - 08/27/2026	Solar radiation data out of range:-7999	06/04/2026
Marble Falls	Updated	09/11/2014 - 08/28/2026	Solar radiation data out of range:0	06/28/2022
Marfa	Updated	04/20/2022 - 08/28/2026	Solar radiation data out of range:-7.1991	10/19/2024
McAllen	Not Updated	06/11/2026 - 08/27/2026	Solar radiation data out of range:-7999	07/22/2026
McKinney	Not Updated	11/15/2012 - 08/27/2026	Temperature data out of range:-46.09	04/20/2023
Medina	Updated	05/25/2021 - 08/28/2026	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	Not Updated	04/01/2016 - 08/27/2026	Solar radiation data out of range:0	06/25/2022
Memorial Village	Not Updated	09/14/2017 - 08/27/2026	Solar radiation data out of range:0	06/27/2022
Menard	Updated	07/08/2020 - 08/28/2026	Solar radiation data out of range:-7.1991	02/14/2024
Mesquite	Not Updated	12/03/2012 - 08/27/2026	Solar radiation data out of range:0	06/27/2022
Mountain Home	Updated	12/03/2024 - 08/28/2026	No Error Logged
Murphy	Not Updated	03/16/2016 - 08/19/2024	Solar radiation data out of range:-7999	06/05/2025
Naples	Not Updated	04/20/2022 - 06/07/2026	Solar radiation data out of range:-7.1991	11/18/2025
Nasho	Not Updated	07/05/2018 - 08/09/2026	Solar radiation data out of range:-7999	11/15/2024
Nocona	Updated	12/10/2020 - 08/28/2026	Solar radiation data out of range:0	06/28/2022
Overton	Updated	06/07/2004 - 08/28/2026	Solar radiation data out of range:-7999	04/28/2025
Ozona-Sonora	Updated	07/08/2020 - 08/28/2026	Solar radiation data out of range:-7.1991	08/26/2025
Pflugerville	Updated	02/11/2015 - 08/28/2026	Solar radiation data out of range:0	06/27/2022
Plano	Not Updated	11/15/2012 - 08/27/2026	Solar radiation data out of range:0	06/29/2022
Princeton	Not Updated	03/07/2015 - 08/27/2026	Solar radiation data out of range:0	06/24/2022
Prosper	Not Updated	07/21/2021 - 08/27/2026	Solar radiation data out of range:-7999	06/21/2026
Providence Village	Not Updated	06/26/2015 - 08/27/2026	Solar radiation data out of range:-7999	04/18/2024
Refugio	Updated	12/22/2020 - 08/28/2026	Solar radiation data out of range:-7.1991	02/18/2025
Richardson	Not Updated	03/06/2015 - 08/27/2026	Solar radiation data out of range:0	06/27/2022
Richmond North	Not Updated	01/01/2018 - 08/27/2026	Solar radiation data out of range:-7999	04/24/2024
Richmond South	Not Updated	07/27/2016 - 08/27/2026	Temperature data out of range:-7999	07/15/2026
Rockwall	Not Updated	12/03/2012 - 08/27/2026	Solar radiation data out of range:0	06/29/2022
Round Rock	Updated	12/17/2020 - 08/28/2026	Solar radiation data out of range:0	06/28/2022
Royse City	Not Updated	03/07/2015 - 08/27/2026	Solar radiation data out of range:0	06/25/2022
Sachse	Not Updated	03/23/2016 - 12/03/2025	No Error Logged
San Angelo	Not Updated	11/03/2016 - 08/27/2026	Solar radiation data out of range:0	06/29/2022
San Antonio North	Not Updated	09/02/2005 - 08/27/2026	Solar radiation data out of range:-7999	05/01/2026
San Benito	Not Updated	04/20/2022 - 06/07/2026	Solar radiation data out of range:-7.1991	11/27/2023
San Saba	Not Updated	12/13/2020 - 05/09/2026	Relative humidity data out of range:0	08/01/2026
Schleicher County North	Not Updated	04/20/2022 - 08/16/2023	Relative humidity data out of range:0	11/27/2026
Seagoville	Not Updated	01/21/2016 - 08/27/2026	Solar radiation data out of range:0	06/27/2022
Seguin	Updated	12/10/2020 - 08/28/2026	Solar radiation data out of range:0	06/28/2022
Shafter	Updated	04/20/2022 - 08/28/2026	Solar radiation data out of range:-7.1991	08/09/2022
Sinton	Updated	07/08/2020 - 08/28/2026	Temperature data out of range:-27.886	02/23/2023
Southlake	Updated	10/16/2015 - 08/28/2026	Relative humidity data out of range:0	04/18/2026
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Uvalde Research	Not Updated	11/06/2010 - 08/27/2026	Solar radiation data out of range:0	06/23/2022
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Van Horn South	Updated	07/08/2020 - 08/28/2026	Temperature data out of range:-14366.2	10/24/2023
Victoria County West	Updated	04/20/2022 - 08/28/2026	Solar radiation data out of range:-7.1991	07/11/2025
Waco	Not Updated	02/14/2012 - 08/27/2026	Relative humidity data out of range:0	06/15/2026
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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

Evapotranspiration (ETo) 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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New Caney Rain (77357)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Pasadena Rain (77506)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Pasadena Rain (77507)	Not Updated	01/01/0001 - 01/01/0001	No Error Logged
Pearland Rain (77581)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Pinehurst Rain (77362)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Plano Rain	Not Updated	01/20/2016 - 01/06/2025	No Error Logged
Plantersville Rain (77363)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Porter Rain (77365)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Princeton Rain	Not Updated	01/20/2016 - 12/23/2025	No Error Logged
Richardson Rain	Not Updated	01/20/2016 - 12/23/2025	No Error Logged
Richmond Rain (77407)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Rockwall Rain	Not Updated	01/20/2016 - 12/22/2025	No Error Logged
Royse City Rain	Not Updated	01/20/2016 - 12/23/2025	No Error Logged
Seabrook Rain (77586)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
South Houston Rain (77587)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Splendora Rain (77372)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Spring Rain (77373)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Spring Rain (77382)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Spring Rain (77389)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Tomball Rain (77375)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Tomball Rain (77377)	Not Updated	06/01/2021 - 12/06/2025	No Error Logged
Waller Rain (77484)	Not Updated	06/01/2021 - 12/06/2025	No Error Logged
Webster Rain (77598)	Not Updated	06/01/2021 - 12/23/2025	No Error Logged
Wylie Rain	Not Updated	01/20/2016 - 12/23/2025	No Error Logged

Calculators