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

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

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

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

ACCESSING PET DATA

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

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

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

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

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

Texas ET Network and Website User's Manual

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

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

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

ETo depends on the climate and varies from location to location. Special weather stations are used to collect the climatic data for calculating ETo, including temperature, dew point temperature (relative humidity), wind, and solar radiation. The American Society of Biological and Agricultural Engineers has established a standard for weather stations used for calculation of ET which can be found on this website.

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

We are using the standardized Penman-Monteith method to calculate ETo from the weather station data. This method is also referred to as the ASCE standard method (for more info, see: The ASCE Standardized Reference Evapotranspiration Equation | Books (ascelibrary.org) . This is one of several methods that can be used to determine ETo and ET. Organizations, such as the International Committee on Irrigation and Drainage and the Water Requirements Committee of the American Society of Civil Engineers, have proposed establishing the Penman-Monteith method as a world-wide standard. Such a standard would help facilitate the sharing of ETo data and development of crop coefficients.

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

Evapotranspiration and Irrigation Water Requirements, edited by M.E. Jensen, R.D. Burman, and R.G. Allen. Published by the American Society of Civil Engineers, New York, NY. 2016.
https://doi.org/10.1061/9780784414057

Using PET for Determining Crop Water Requirements and Irrigation Scheduling Click to download a hard copy of the grower's guide Contents I. How to Use Adjusting for Irrigation System Efficiency Adjusting for Rainfall II. Where to Find PET Information III. What is Potential Evapotranspiration (PET)? IV. Crop Coefficients Choosing and Using Crop Coefficients Soil Moisture Monitoring Table 1. Sorghum Crop Coefficients Table 2. Cotton Crop Coefficients Table 3. Corn Crop Coefficients V. Irrigation System Efficiencies Table 4. Typical overall On-Farm Efficiencies VI. Understanding FAO Crop Coefficients Table 5. FAO Mean Crop Coefficients, Kc. Table 6. FAO General Crop Growth Stages. VII. Acknowledgments I. HOW TO USE PET To calculate the water requirements of a crop, we multiply the PET times the crop coefficient using the following equation: PET x Kc = crop water requirements (equation 1) where: PET is the sum of daily PET over the time period of interest, such as the 3-day total, the weekly total, etc. Kc is the crop coefficient corresponding to the current stage of crop growth. Example 1: the 5-day PET total is 1.32 inches. My sorghum is in the "heading" growth stage. What are the water requirements? (Note: from Table 1, the "heading" crop coefficient is 1.10) 1.32 inches x 1.10 = 1.45 inches Thus, I need to apply 1.45 inches to replace the water used by the sorghum in the last 5 days. Adjusting for Irrigation System Efficiency It may be necessary to increase the amount of irrigation water in order to compensate for poor irrigation system efficiency. Irrigation system efficiency is defined in Section V of this guide. Table 4 gives the typical ranges of on-farm irrigation systems. To adjust for irrigation system efficiency, use the following equation: PET x Kc ÷ Eff = irrigation water requirements (equation 2) where: Eff is the overall efficiency of the irrigation system. Example 2. I am irrigating with a low-pressure center pivot. I estimate that my overall system efficiency is 85%. What are my irrigation water requirements for the sorghum in example 1? 1.32 inches x 1.10 ÷ 0.85 = 1.71 inches. Adjusting for Rainfall Rainfall reduces the amount of water we must supply by irrigation to meet plant water requirements. However, not all rainfall becomes available for use by plants and crops. Depending on such factors as soil type, duration and intensity of rainfall, soil moisture levels, etc., a portion of the rainfall will be lost to runoff and deep percolation (water moving below the root zone). In irrigation scheduling, the term " effective rainfall " refers to that portion of rainfall which infiltrates and is stored in the root zone. Effective rainfall must be estimated for each field and rainfall event. The irrigation requirement determined with equations (1) or (2) should be reduced by the amount of effective rainfall. Alternatively, soil moisture monitoring devices can be use to determine soil moisture levels and to determine when irrigations should be re-started following rains. II. WHERE TO FIND PET INFORMATION For persons with Internet access, PET and weather information is provided for about 10 locations in Central and South Texas on the Texas PET Web Site. The address is: http://texaset.tamu.edu Persons without Internet access should contact their water district or County Extension Agent to see if this information is being provided locally in another way. Persons on the Texas High Plains should contact the Texas A&M Research and Extension Center in Amarillo at (806) 359-5401 about subscription to the High Plains PET Network where PET data is sent out every three days by FAX. III. WHAT IS POTENTIAL EVAPOTRANSPIRATION (PET)? Evapotranspiration (ET) is a measurement of the total amount of water needed to grow plants and crops. This term comes from the words evaporation (i.e., evaporation of water from the soil) and transpiration (i.e., transpiration of water by plants). Different plants have different water requirements, so they have different ET rates. Since there are thousands of cultivated plants, we have tried to simplified matters by establishing a standard ET rate for general reference and use. The standard is referred to as the potential evapotranspiration (PET). This is the potential ET since we are assuming the crop is in a deep soil and under well watered conditions. The standard crop we are using is a cool season grass which is 4-inches tall. The technical term for this is the " Potential Evapotranspiration of a Grass Reference Crop " or "PET" for short. PET depends on the climate and varies from location to location. Special weather stations are used to collect the climatic data for calculating PET, including temperature, dew point temperature (relative humidity), wind speed, and solar radiation. The water requirements of specific crops are calculated as a percentage of the PET. This "percentage" is the called the crop coefficient (Kc). Crop coefficients depend on the type of crop and its stage of growth. Detailed information on crop coefficients are given later. We are using the Penman-Monteith method to calculate PET from the weather station data. This is one of a number of methods that can be used to determine PET and ET. Several organizations, such as the International Committee on Irrigation and Drainage and the Water Requirements Committee of the American Society of Civil Engineers have proposed establishing the Penman-Monteith method as a world-wide standard. Such a standard would help facilitate the sharing of PET data and development of crop coefficients. IV. CROP COEFFICIENTS Potential Evapotranspiration (PET) is an estimate of the water requirements of a 4-inch grass in a deep soil growing under well watered conditions. The water requirements of other crops are determined from PET through crop coefficients (Kc). Crop coefficients vary for different crops. They also change depending on the growing stage of the crop. Unfortunately, we only have verified crop coefficients for the Texas North High Plains for cotton, sorghum, corn and wheat (see Tables 1-3). These coefficients were developed by the North Plains PET Network Project Team. Another source is the FAO (Food and Agriculture Organization of the United Nations) who has published a long list of generalized crop coefficients which are used throughout the world where local values are not available (see Tables 5&6). Choosing and Using Crop Coefficients For cotton, sorghum and corn, I recommend using the North High Plains crop coefficients. These have been verified in research and on-farm irrigation studies, and should vary no more than about 10% for other parts of the state. The North High Plains crop coefficients are listed by stage of growth in Tables 1-3. Please note that these dates are provided as a general guide only, as crop growth rate is affected by many factors including variety, current weather, soil moisture conditions, etc. For other crops, refer to the FAO Crop Coefficients until researchers are able to verify coefficients for specific regions in Texas. For many crops, we would expect these general coefficients to be within about 10%. Soil Moisture Monitoring I highly recommend soil moisture monitoring using gypsum blocks, watermark sensors, tensiometers, the "feel" method, or other devices for measuring the current water status in the root zone. This provides an excellent check to ensure that irrigations are keeping up with crop water demand. V. IRRIGATION SYSTEM EFFICIENCIES No irrigation system is 100% efficient. For sprinkler irrigation systems, we can lose anywhere from 10% to 40% of the water in the air before the water reaches the ground depending on wind and other weather conditions. The amount of water lost to spray drift is referred to as the application efficiency. For drip and surface irrigation system, our biggest concern is how evenly distributed the water is over the field or along laterals and rows. This is referred to as the distribution efficiency. The term overall efficiency is a combination of both the application and distribution efficiencies. The normal ranges in on-farm overall efficiencies are listed in Table 4. Under some situations, we will need to increase the amount of irrigation to compensate for water lost due to the inefficiencies of the system. Table 1. Sorghum Crop Coefficients. GROWTH STAGE1 KC DAYS AFTER PLANTING2 Seeding 0.40 3 - 4 Emerg 0.40 5 - 8 3-leaf 0.55 19 - 24 4-leaf 0.60 28 - 33 5-leaf 0.70 32 - 37 GPD 0.80 35 - 40 Flag 0.95 52 - 58 Boot 1.10 57 - 61 Heading 1.10 60 - 65 Flower 1.00 68 - 75 S Dough 0.95 85 - 95 H Dough 0.90 95 - 100 Blk lyr 0.85 110 - 120 Harvest 0.00 125 - 140 1Sorghum will bloom at different times depending on locating , planting date, and maturity of the variety. 2The Days After Planting are for a medium-early to medium late variety. Table 2. Cotton Crop Coefficients. GROWTH STAGE KC DAYS AFTER PLANTING Seeding .07 0 - 10 1st Sqr .22 32 - 40 1st Blom .44 55 - 60 Max Blom 1.10 70 - 90 1st open 1.10 105 - 115 25% open .83 115 - 125 50% open .44 135 - 145 95% open .44 140 - 150 Pick .10 140 - 150 Table 3. Corn Crop Coefficients. GROWTH STAGE1 KC DAYS AFTER PLANTING Seed 0.25 0 - 5 Emerg 0.35 5 - 8 4-leaf 0.45 24 - 28 5-leaf 0.70 29 - 34 6-leaf 0.85 35 - 40 8-leaf 1.00 43 - 46 10 leaf 1.15 51 - 58 12-leaf 1.20 57 - 65 14-leaf 1.25 65 - 75 Tassel 1.25 72 - 82 Silk 1.30 75 - 85 Blister 1.30 85 - 95 Milk 1.30 95 - 105 Dough 1.20 100 - 110 Dent 1.00 110 - 120 ½ mat 0.90 115 - 125 Blk lyr 0.70 125 - 135 Harvest 0.00 140 + 1 Note: 50% silking occurs in 85 days in Corpus Christi/Coastal bend area and in 75 days in the Uvalde area. Table 4. Typical overall On-Farm Efficiencies System Overall Effeciency Surface 0.50 - 0.80 a. average 0.50 b. land leveling and delivery pipeline meeting design standards 0.70 c. tailwater recovery with (b) 0.80 d. surge 0.60 - 0.901 Sprinkler 0.55 - 0.753 Center Pivot 0.55 - 0.903 LEPA 0.90 - 0.95 Drip 0.80 - 0.902 1. Surge has been found to increase efficiencies 8 to 28 percent over non-surge furrow systems. 2. Trickle systems are typically designed at 90 percent efficiency; short laterals (<100ft) or systems with pressure compensationg emitters may have higher efficiencies. 3. Under low wind conditions. VI. FAO CROP COEFFICIENTS With the FAO method, crop coefficients (Table 5) are represented by straight lines connecting four general growth stages, as indicated in the following figure. Table 6 gives a more detailed explanation of the crop coefficient values and their respective growth stages. Table 5. FAO Mean Crop Coefficients, Kc, for Arid Climates1 CROP Kc12 Kc2 Kc3 Alfalfa Hay3 0.30 1.25 1.10 Artichokes 0.95 1.00 0.95 Asparagus 0.30 0.95 0.30 Banana, 1st year 0.50 1.15 1.10 Banana, 2nd year 0.70 1.20 1.10 Barley, Wheat, Oats 0.30 1.15 0.20 Beans, Green 0.40 1.00 0.90 Beans, Dry and Pulses 0.40 1.15 0.35 Beets, Table 0.40 1.05 0.95 Berries 0.30 1.05 0.40 Carrots 0.50 1.10 0.80 Castor Beans 0.40 1.15 0.50 Celery 0.35 1.10 1.00 Citrus 0.65 0.80 0.65 Citrus w/cover or weeds 0.70 1.05 1.05 Clover Hay3 0.60 1.20 1.05 Coffee 0.90 0.90 0.90 Conifer Trees 1.20 1.20 1.20 Corn, Field 0.40 1.15 0.60, 0.354 Corn, Sweet 0.40 1.15 1.05 Cotton 0.40 1.20 0.65 Cucumber, Fresh Market 0.35 0.95 0.75 Cucumber, Machine Harvest 0.35 0.95 0.95 Crucifers5 0.40 1.05 0.90 Dates 0.95 0.95 0.95 Deciduous Orchard 0.50 1.00 0.656 Deciduous Orchard w/cover or weeds 0.80 1.25 0.856 Eggplant 0.40 1.05 0.85 Flax 0.30 1.10 0.20 Grapes 0.30 0.80 0.40 Grass Pasture 0.80 0.80 0.80 Groundnuts 0.40 1.05 0.60 Hops 0.30 1.00 0.60 Kiwi 0.30 1.05 1.05 Lentil 0.30 1.15 0.25 Lettuce 0.30 1.00 0.95 Melons 0.40 1.00 0.75 Millet 0.30 1.10 0.25 Mint 0.60 1.10 1.10 Olives 0.40 0.70 0.70 Onion, Dry 0.50 1.05 0.80 Onion, Green 0.50 1.00 1.00 Open Water 1.15 1.15 1.15 Palm Trees 0.95 0.95 0.95 Peas, Fresh 0.40 1.10 1.05 Peas, Dry/Seed 0.40 1.10 0.30 Peppers, Fresh 0.35 1.05 0.85 Pistachios 0.20 1.10 0.40 Potato 0.40 1.10 0.758 Pumpkin 0.40 1.00 0.75 Radishes 0.30 0.85 0.80 Rice 1.10 1.25 1.00 Safflower 0.35 1.15 0.20 Sorghum, Grain 0.30 1.05 0.50 Sorghum, Sweet 0.30 1.20 0.50 Soybeans 0.35 1.10 0.45 Spinach 0.30 1.00 0.95 Squash 0.30 0.95 0.75 Strawberries 0.40 0.90 0.70 Sugar Beet 0.30 1.15 1.009 Sugar Cane 0.40 1.25 0.70 Sunflower 0.30 1.15 0.35 Tea 1.00 1.00 1.00 Tomato 0.40 1.20 0.65 Walnut Orchard 0.50 1.00 0.65 Winter Wheat 0.30 1.15 0.20 Footnotes to Table 5. 1Values for Kc2 and Kc3 represent those for an arid climate (RHmin ~ 20%) with moderate wind speed (0-5 m s-1, averaging 2 m s-1) and are used for general irrigation water requirements. For humid or windier conditions, Kc2 and Kc3 can be modified as follows: Kc2=Kc2 table - 0.0015 RHmin + 0.01 U2 Kc3=Kc3 table - 0.0015 RHmin + 0.01 U2 when Kc3 > or = 0.4 Kc3 = Kc3 table + 0.001 (RHmin-20) when Kc3 < 0.4 2General values for Kc1 under infrequent soil wetting (~each 10 days). Kc1 is a function of wetting interval and potential evaporation rate during the initial and development periods. 3 The coefficients for hay crops represent immediately following cutting; full cover; and immediately before cutting, respectively. An overall mean coefficient for alfalfa hay which takes into account cutting effects is 1.05 and for clover is 1.00. Kc3 for alfalfa seed, which is never cut for forage or hay is about 0.5. 4 The first Kc3 value is for harvest at high grain moisture. The second Kc3 value is for harvest after complete field drying of grain (to about 18%). 5 Crucifers include cabbage, cauliflower, broccoli, and Brussel sprouts. 6 The Kc3 value represents the Kc prior to leaf drop. 7 Cool season grass varieties include dense strands of bluegrass, ryegrass, and fescue. Warm season varieties include bermuda grass and St. Augustine grass. The 0.90 values for cool season grass represent a 0.06 to 0.08m height under general turf conditions. 8 The Kc3 value for potatoes is about 0.40 for long season potatoes with vine kill. 9The Kc3 value for sugar beets is 0.60 if no irrigation occurs during the last month. Table 6 . FAO Generalized Crop Growth Stages Kc Values Growth Stage Description Kc1 Initial The average KC value from planting to about 10% ground cover. Kc1-Kc2 Rapid Growth From 10% ground cover to 75% cover or to peak water use, which ever comes first. Kc2 Mid season The average value from the end of the rapid growth stage until water use begins to decline due to crop aging. Kc2-Kc3 Late season From when KC begins to decline until harvest or when water use ceases or becomes minimal. Kc3 Harvest The average value at harvest or the end of the water use season. Note: For the rapid growth and the late season stage, it is assumed that KC values increase or decrease linearly with time. VII. ACKNOWLEDGEMENTS Information on crop growth periods, time periods after planting, and other advice provided by Charles, Stickler, Associate Professor and Extension Agronomist, Texas A&M Center-Uvalde. For More Information For more information on the Penman-Monteith equation and other methods for determining PET, see the book: Evapotranspiration and Irrigation Water Requirements, edited by M.E. Jensen, R.D. Burman, and R.G. Allen. Published by the American Society of Civil Engineers, New York, NY. 1990. 332pp. Written by Guy Fipps, Associate Professor and Extension Agricultural Engineer, Texas A&M University, College Station..