Lawn Water Needs - Texas

One of the first things a professional sprinkler installer learns about irrigation and sprinkler system installations is to calculate the water requirements for lawn (grass or turf) and other plants. Below, we will explain how to determine the amount of water required for a particular grass (lawn/turf). Please note that plant requirements are different and vary depending on plant type and climate. Our focus is on the Houston, TX area and neighboring communities, including Katy, Sugar Land, Richmond, etc. This is the first step we must take to determine how long the sprinkler system needs to run.

The formula for calculating the lawn water requirement is listed below. We will define each part of the formula and explain what each variable means.


Lawn Water Requirement = PET x Tc x Qf Please refer to the charts below and the formula definitions at the bottom of the page.  

Average Monthly PET in Inches / Month
City
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Total
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
58.65
Amarillo
1.84
2.27
3.73
5.06
5.89
7.51
8.08
7.29
5.61
4.05
2.4
1.78
55.51
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
57.51
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
56.16
College Station
2.20
2.71
4.22
5.2
6.25
6.89
7.10
6.85
5.60
4.30
2.80
2.20
56.32
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
55.68
Dallas/Ft. Worth
2.0
2.46
3.96
5.14
6.21
7.06
7.40
7.25
5.49
4.19
2.59
2.10
55.85
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
61.01
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
79.26
Galveston
2.2
2.6
4.1
5.0
6.11
6.6
6.2
6.0
5.5
4.2
2.8
2.3
53.61
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
54.90
Lubbock
2.35
2.63
4.41
5.53
6.93
7.73
7.63
7.2
5.54
4.19
2.61
2.33
59.08
Midland
2.2
2.78
4.46
5.91
7.21
8.2
9.23
8.62
6.95
4.31
2.78
2.16
64.81
Port Arthur
2.25
2.63
3.95
5.09
6.12
6.6
5.81
5.61
5.46
4.18
2.76
2.23
52.69
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
71.34
San Antonio
2.42
2.9
4.42
5.47
6.47
6.97
7.31
6.99
5.64
4.44
2.85
2.36
58.24
Uvalde
2.44
2.95
4.62
5.85
6.7
7.21
7.5
7.31
5.7
4.4
2.89
2.36
59.93
Victoria
2.35
2.87
4.29
5.77
6.39
6.7
6.92
6.7
5.36
4.41
2.93
2.33
57.02
Waco
2.13
2.62
4.03
5.31
6.45
7.15
7.40
7.5
5.7
4..41
2.7
2.17
53.16
Weslaco
2.5
2.57
3.96
4.9
6.12
6.53
7.0
6.58
4.79
3.96
2.85
2.29
54.05
Wichita Falls
1.94
2.46
4.07
5.50
6.7
7.54
7.97
7.72
5.79
4.3
2.62
1.95
58.56
 
 Turf Coefficient Values (Tc)
 Warm Season Turf (ie. St. Augustine)  0.6
 Cool Season Turf (ie. Rye Grass)  0.8
 
 Quality Factor (Qf)
 No Stress (100% Growth)  1.0
 Low Stress  0.8
 Normal Stress  0.6
 High Stress  0.5
 Very High Stress  0.4


Definitions for Formula Coefficients:

PET (aka ETo) is short for “Potential Evapotranspiration”. This is the measurement of the total amount of water that a plant needs to grow.

Tc is the turf coefficient for a specific grass. In other words, it is the water requirement for the specific type of grass calculated as a fraction of the PET.

Qf is the quality factor. This is a fraction of the PET used to maintain a certain grass quality. In residential applications, you are looking for the lawn to show a good appearance. That can be achieved without reaching full growth potential.

The figures we provide apply to Texas. Please visit TexasET network to learn more of how these figures are calculated, what assumptions are made, and what methodology is used to come up with these values.

http://texaset.tamu.edu/whatis.php
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