TexasET Network

Royse City's 5 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 03, 2021 - Tue	0.16	89	70	37	21.55	0.00	0.00	1.86
Aug 02, 2021 - Mon	0.12	84	74	61	12.92	0.13	0.00	4.23
Aug 01, 2021 - Sun	0.15	93	76	47	16.35	0.00	0.00	4.21
Jul 31, 2021 - Sat	0.20	95	77	40	22.62	0.00	0.00	3.20
Jul 30, 2021 - Fri	0.21	96	76	38	24.88	0.00	0.00	2.66
Data Summary	0.84	96	70	44	98.33	0.13	0.00	3.23

Detailed Crop Weather Data
Date	Average Temp (f)	Dew Point (f)	Max Dewpoint (f)	Min Dewpoint (f)	Corn (heat units)	Cotton (heat units)	Sorghum (heat units)	50 Degree (heat units)	55 Degree (heat units)	60 Degree (heat units)	Wind Run (miles/day)	Soil Temp (f)
Aug 03, 2021 - Tue	79	67	71	64	28	19	24	29	24	19	20	0
Aug 02, 2021 - Mon	78	71	73	68	29	19	24	29	24	19	25	0
Aug 01, 2021 - Sun	82	71	74	66	31	24	29	34	29	24	31	0
Jul 31, 2021 - Sat	86	72	74	70	31	26	31	36	31	26	37	0
Jul 30, 2021 - Fri	86	72	74	67	31	26	31	36	31	26	26	0
Data Summary	82	70	74	64	150	114	139	164	139	114	139	0

3-Day Summary

5-Day Summary

7-Day Summary

2-Week Summary

Calculators

Landscape

Crop

Landscape Watering Calculator

1.)	ETo (pet)
2.)	Plant Coefficient
3.)	Adjustment Factor
4.)	Effective Rainfall
5.)

		Precipitation Rate
		Total Run Time
		Irrigations/Week
		Run Time/Irrigation

Crop Watering Calculator

1.)	ETo value from weather data
2.)	Select a crop coefficient
	FAO Coefficients
Or
	Texas High Plains Coefficients
3.)	Select a crop growth stage
4.)	Crop coefficient from growth stage
5.)	Enter your system efficiency
6.)	Effective Rainfall
7.)

		Precipitation Rate
		Total Run Time
		Irrigations/Week
		Run Time/Irrigation

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.

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

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

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

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

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

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

Overall Efficiency

0.5-0.8
0.5
0.7
0.8
0.6-0.9¹

TAMU Weather Station Format

L1200X (pyrometer)

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
© 2021 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
© 2021 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
© 2021 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
© 2021 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
© 2021 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
© 2021 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
© 2021 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
© 2021 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
© 2021 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
© 2021 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
© 2021 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
© 2021 Texas A&M AgriLife Extension

Station	Status	Data Available	Last Error Message	Date Error Occurred
Allen	Updated	03/06/2015 - 08/03/2021	Incomplete daily data	08/04/2021
Allen Rain	Updated	01/20/2016 - 08/03/2021	No Error Logged
Alvarado	Updated	07/08/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Argyle	Updated	05/26/2018 - 08/03/2021	Incomplete daily data	08/04/2021
Austin (LCRA Redbud)	Updated	09/11/2014 - 08/03/2021	Incomplete daily data	06/29/2021
Ballinger	Updated	12/17/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Baytown Rain (77520)	Not Updated	01/01/0001 - 01/01/0001	No Error Logged
Baytown Rain (77521)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Blum	Updated	07/08/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Brookshire Rain (77423)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Bryan	Updated	08/05/2014 - 08/03/2021	Incomplete daily data	08/04/2021
Buda	Updated	08/08/2018 - 08/03/2021	No Error Logged
Burnet	Updated	07/26/2017 - 08/03/2021	Incomplete daily data	07/17/2021
Carthage	Updated	12/10/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Cash	Not Updated	04/01/2016 - 06/10/2021	Incomplete daily data	06/11/2021
Cedar Creek	Updated	12/13/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Cedar Park	Updated	02/11/2015 - 08/03/2021	No Error Logged
Channelview Rain (77530)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Conroe Rain (77306)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Crosby Rain (77532)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Cypress Rain (77429)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Cypress Rain (77433)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Dallas AgriLife Center	Updated	04/07/2008 - 08/03/2021	Incomplete daily data	08/04/2021
Delta Lake	Updated	03/13/2019 - 08/03/2021	Incomplete daily data	08/04/2021
Dickinson	Updated	07/27/2016 - 08/03/2021	Incomplete daily data	08/04/2021
Dripping Springs	Updated	02/11/2015 - 08/03/2021	Incomplete daily data	07/04/2021
Farmersville	Updated	11/15/2012 - 08/03/2021	Incomplete daily data	08/04/2021
Farmersville Rain	Not Updated	01/20/2016 - 06/14/2021	No Error Logged
Forestburg	Updated	12/13/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Forney	Updated	12/03/2012 - 08/03/2021	Incomplete daily data	08/04/2021
Forney Rain	Not Updated	01/20/2016 - 05/29/2021	No Error Logged
Fort Stockton East	Updated	07/08/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Fort Stockton West	Updated	12/10/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Fort Worth - NOAA	Not Updated	08/25/2011 - 03/22/2021	Relative humidity data out of range:343	03/23/2021
Friendswood Rain (77546)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Garland	Updated	03/07/2015 - 08/03/2021	Incomplete daily data	08/04/2021
Garland Rain	Not Updated	01/20/2016 - 08/02/2021	No Error Logged
Garwood	Updated	12/10/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Georgetown II	Not Updated	10/20/1998 - 12/07/2016	No Error Logged
Goliad	Not Updated	12/13/2020 - 08/01/2021	Incomplete daily data	08/02/2021
Halfway	Updated	06/28/2012 - 08/03/2021	Incomplete daily data	08/04/2021
Highland-University Park	Updated	04/19/2016 - 08/03/2021	Incomplete daily data	08/04/2021
Hockley Rain (77447)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Horizon City	Not Updated	07/08/2020 - 11/16/2020	No Error Logged
Houston	Updated	07/27/2016 - 08/03/2021	Incomplete daily data	08/04/2021
Houston Rain (77007)	Not Updated	05/14/2021 - 06/15/2021	Incomplete daily data	05/15/2021
Houston Rain (77008)	Not Updated	01/01/0001 - 01/01/0001	No Error Logged
Houston Rain (77013)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77014)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77015)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77017)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77018)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77019)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77023)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77025)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77029)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77031)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77032)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77035)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77036)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77037)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77039)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77040)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77042)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77044)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77045)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77047)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77048)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77049)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77051)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77054)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77055)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77058)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77059)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77060)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77063)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77064)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77066)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77068)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77070)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77071)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77072)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77073)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77075)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77077)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77079)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77080)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77082)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77083)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77084)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77087)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77088)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77089)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77090)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77091)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77092)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77093)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77094)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77095)	Not Updated	05/28/2021 - 06/15/2021	No Error Logged
Houston Rain (77099)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Houston Rain (77502)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Huffman Rain (77336)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Humble Rain (77338)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Humble Rain (77396)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Huntsville	Updated	01/22/2010 - 08/03/2021	Incomplete daily data	08/04/2021
Irving North	Updated	10/08/2013 - 08/03/2021	Incomplete daily data	08/04/2021
Irving South	Updated	10/08/2013 - 08/03/2021	Incomplete daily data	08/04/2021
Jersey Village	Not Updated	03/06/2020 - 07/27/2021	Incomplete daily data	07/28/2021
Katy Rain (77449)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Katy Rain (77450)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Katy Rain (77493)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Katy Rain (77494)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Kingsville	Updated	12/20/2020 - 08/03/2021	Incomplete daily data	07/21/2021
La Gloria	Updated	07/08/2020 - 08/03/2021	Incomplete daily data	07/21/2021
La Porte Rain (77571)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
La Villa	Updated	12/10/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Ladonia	Updated	05/18/2019 - 08/03/2021	Incomplete daily data	08/04/2021
Lakeway	Updated	02/11/2015 - 08/03/2021	Incomplete daily data	06/29/2021
League City Rain (77573)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Lewisville	Updated	06/26/2015 - 08/03/2021	Incomplete daily data	08/04/2021
Lubbock	Updated	06/02/2020 - 08/03/2021	Incomplete daily data	08/04/2021
Magnolia Rain (77354)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Magnolia Rain (77355)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Marble Falls	Updated	09/11/2014 - 08/03/2021	Incomplete daily data	07/01/2021
McKinney	Updated	11/15/2012 - 08/03/2021	Incomplete daily data	08/04/2021
McKinney Rain	Not Updated	01/20/2016 - 07/27/2021	No Error Logged
Medina	Updated	05/25/2021 - 08/03/2021	Incomplete daily data	07/21/2021
Melissa	Not Updated	03/16/2016 - 06/03/2021	Incomplete daily data	08/03/2021
Melissa RD121	Updated	04/01/2016 - 08/03/2021	Incomplete daily data	08/04/2021
Memorial Village	Not Updated	09/14/2017 - 07/30/2021	Incomplete daily data	07/31/2021
Menard	Updated	07/08/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Mercedes	Not Updated	01/01/0001 - 01/01/0001	Rainfall data out of range:83.9	08/04/2021
Mesquite	Updated	12/03/2012 - 08/03/2021	Incomplete daily data	08/04/2021
Mesquite Rain	Updated	01/20/2016 - 08/03/2021	No Error Logged
Murphy	Not Updated	03/16/2016 - 08/18/2020	Incomplete daily data	08/03/2021
Nasho	Updated	07/05/2018 - 08/03/2021	Incomplete daily data	08/04/2021
New Caney Rain (77357)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Nocona	Updated	12/10/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Overton	Updated	06/07/2004 - 08/03/2021	Incomplete daily data	08/04/2021
Ozona-Sonora	Not Updated	07/08/2020 - 07/30/2021	Incomplete daily data	07/31/2021
Pasadena Rain (77506)	Not Updated	06/01/2021 - 06/15/2021	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 - 06/15/2021	No Error Logged
Pflugerville	Updated	02/11/2015 - 08/03/2021	Incomplete daily data	07/30/2021
Pinehurst Rain (77362)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Plano	Updated	11/15/2012 - 08/03/2021	Incomplete daily data	08/04/2021
Plano Rain	Not Updated	01/20/2016 - 08/02/2021	No Error Logged
Plantersville Rain (77363)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Porter Rain (77365)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Princeton	Updated	03/07/2015 - 08/03/2021	Incomplete daily data	08/04/2021
Princeton Rain	Not Updated	01/20/2016 - 08/02/2021	No Error Logged
Prosper	Updated	07/21/2021 - 08/03/2021	Incomplete daily data	08/04/2021
Providence Village	Updated	06/26/2015 - 08/03/2021	Incomplete daily data	08/04/2021
Refugio	Not Updated	12/22/2020 - 07/24/2021	Incomplete daily data	07/25/2021
Richardson	Updated	03/06/2015 - 08/03/2021	Incomplete daily data	08/04/2021
Richardson Rain	Updated	01/20/2016 - 08/03/2021	No Error Logged
Richmond North	Updated	01/01/2018 - 08/03/2021	Incomplete daily data	08/04/2021
Richmond Rain (77407)	Not Updated	06/01/2021 - 06/15/2021	No Error Logged
Richmond South	Updated	07/27/2016 - 08/03/2021	Incomplete daily data	08/04/2021
Rockwall	Updated	12/03/2012 - 08/03/2021	Incomplete daily data	08/04/2021
Rockwall Rain	Updated	01/20/2016 - 08/03/2021	No Error Logged
Round Rock	Updated	12/17/2020 - 08/03/2021	Incomplete daily data	07/21/2021
Royse City	Updated	03/07/2015 - 08/03/2021	Incomplete daily data	08/04/2021
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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..

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