IRRIGATION WATER MANAGEMENT Rick Schlegel Irrigation Engineer USDA - NRCS

How is irrigation water managed?  By efficiently applying the correct amount of water into the soil to meet the needs of the crop being grown.

How do we know how much water should be applied?  By knowing how much water is needed in the soil for the crop within the root zone of that crop.

How do we know how much water is needed in the root zone?  The method we are going to discuss in this presentation is a water accounting method known as “Checkbook Scheduling”.

CHECKBOOK IRRIGATION SCHEDULING  is the concept of managing your irrigation water in the same manner as you would manage your checkbook  There are deposits – rainfall and irrigation  There are withdrawals – crop usage and evaporation (Evapotranspiration) & deep percolation  There is a balance – the water balance remaining in the soil available to the crop

CHECKBOOK IRRIGATION SCHEDULING  There are overdrafts – if your crop water usage exceeds the water balance that you are managing

TYPICAL CROP ROOT ZONE THE ROOT ZONE IS OUR BANK ACCOUNT

RAINFALLRAINFALL IRRIGATIONIRRIGATION DEPOSITS

EVAPORATIONEVAPORATION TRANSPIRATIONTRANSPIRATION ET EVAPOTRANSPIRATION ET EVAPOTRANSPIRATION WITHDRAWALS DEEP PERCOLATION

“BANK ACCOUNT SIZE” IS DETERMINED BY AVAILABLE WATER CAPACITY (AWC)  THE ROOT ZONE CAN ONLY HOLD SO MUCH WATER ( FIELD CAPACITY ). THE REST IS LOST TO DEEP PERCOLATION.  “AWC” IS THE AMOUNT OF WATER HELD IN THE ROOT ZONE OF THE CROP THAT IS AVAILABLE TO THE CROP.

AWCAWC AVAILABLE WATER CAPACITY

SANDY CLAY LOAM, LOAM, & SILT LOAM 1.8 IN/FOOT SANDY CLAY LOAM, LOAM, & SILT LOAM 1.8 IN/FOOT SANDY LOAM & FINE SANDY LOAM 1.5 IN/FOOT SANDY LOAM & FINE SANDY LOAM 1.5 IN/FOOT FINE SAND AND LOAMY FINE SAND 0.9 IN/FOOT FINE SAND AND LOAMY FINE SAND 0.9 IN/FOOT CLAY, CLAY LOAM & SILTY CLAY LOAM 2.0 IN/FOOT CLAY, CLAY LOAM & SILTY CLAY LOAM 2.0 IN/FOOT ACCOUNT BALANCE: (AVAILABLE WATER) LIMITED BY SOIL TYPE AND ROOT DEPTH ACCOUNT BALANCE: (AVAILABLE WATER) LIMITED BY SOIL TYPE AND ROOT DEPTH

ALWAYS START WITH A FULL BANK ACCOUNT (PRE-WATERING IS ESSENTIAL) ALWAYS START WITH A FULL BANK ACCOUNT (PRE-WATERING IS ESSENTIAL) START FULL

EXAMPLE:  GROWING COTTON ON CLAY LOAM SOIL  MANAGE 4.0 FOOT ROOT DEPTH  CLAY LOAM HOLDS 2.0 IN/FOOT  AWC = 4.0 X 2.0 = 8.0 INCHES

How should we manage this 8.0” account ?  Most crops are able to obtain water freely from the soil until about one half of the available water is left in the soil (in our example: 4.0 inches). At that point the soil starts to become stingy and the crop has to do more and more work to retrieve the available water. As the available water goes to zero the crop will start wilting and the crop production can be permanently damaged.

 For most crops the account should be managed to replace moisture that has been used by the crop when the account balance reaches one half of the AWC.  This concept of managing the account is known as “Management Allowable Depletion” or “MAD Water Balance”.  In our example the MAD Water Balance is 4.0 inches (one half of the AWC).  The account will be managed by scheduling irrigations and recording rainfall to keep the MAD Water Balance above zero.

AWCAWC MAD WATER BALANCE MADMAD STINGYSTINGY

“MAD” EXAMPLES  CROP WITH FOUR FOOT ROOT ZONE:  CLAY, CLAY LOAM, OR SILTY CLAY LOAM AWC = 8.0” MAD WB= 4.0”  SANDY CLAY LOAM, LOAM, OR SILT LOAM AWC = 7.2” MAD WB= 3.6”  SANDY LOAM OR FINE SANDY LOAM AWC = 6.0” MAD WB= 3.0”  FINE SAND OR LOAMY FINE SAND AWC = 3.6” MAD WB= 1.8”

AVAILABLE WATER CAPACITY CAN ALSO BE LIMITED BY:

PLOW PAN AWC

What will I need to do to be able to manage my water using this checkbook method?  Work with your local NRCS office to develop an Irrigation Water Management plan for each crop and field.  Basic items needed to develop the plan are: Crops to be grown, types of soils, type of irrigation system, and amount of water available to the field

What will NRCS provide?  They will develop an Irrigation Water Management plan for each field that will tell you:  1. What your MAD Water Balance will be  2. How long you will need to operate your system in order to apply the correct amount of water to the field to refill the root zone  3. How you will need to operate your system to save water, make maximum use of rainfall, prevent erosion or runoff, use of limited water at critical crop growth stages, and estimate annual irrigation water requirements.

What will I need to do after the IWM plan is provided to me ?  Manage my system in accordance with the IWM plan  Do “Checkbook Scheduling” of my irrigations by tracking evapotranspiration, rainfall, and irrigation applications  Manage my water balance in the soil to save water or make the best use of water and still meet my crop needs

I know how to determine the amount of rainfall and the amount of my irrigation application, but how do I know how much water my crop uses daily ?  Crop water usage is basically estimated from Evapotranspiration (ET) data  You can get daily ET data from two sources: an Atmometer (ETgage) in your field or from the Oklahoma Mesonet weather station in your county

What is an Atmometer or ETgage?  It is a gauge that measures ETa or ETg, that is relatively inexpensive, can be located in your field, and can be read as easily as a rain guage. It does not have to be read everyday.  Since it can be located directly in your field, I feel it gives a more accurate representation of your crop water usage than a remote weather station.  NRCS can work with you to set up the gauge in your field and teach you how to use it.

EVAPOTRANSPIRATION FROM ATMOMETER READINGS  Evapotranspiration (ET) data using an Atmometer is generally reported either as:  ETa (ET of fully mature alfalfa) or ETg (ET of fully mature grass) based on the cloth used with the Atmometer. The Pocket Scheduler you will use for your records utilizes the ETa readings.  These values are adjusted by crop coefficients based on your type crop and stage of growth  You will be working with ETa values and multiplying them by your crop coefficients to find your daily ET values for your crop. NRCS will provide you with the crop coefficients you need.

EXAMPLE:  Your crop: Cotton at Boll Formation  7/7/04 ETa = 0.25 inches  Crop Coefficient (Kc) =.80 (from NRCS)  ET for your crop =.80 X.25 =.20 inches  Your crop used.20 inches of water out of your water balance on July 7th

EVAPOTRANSPIRATION USING THE OKLAHOMA MESONET

Now what forms do I use to record all this information ? (or where is my checkbook?)  NRCS has devised two methods to allow you to keep your records:  1. Irrigation Pocket Scheduler – a booklet that you can carry in your pocket and keep all your records, your calculations, and your water balance on.  2. Automated Scheduler – using an Excel spreadsheet. This spreadsheet will do your calculations and tell you when it is time to irrigate.  NRCS can assist you with either method

USING MESONET “ET” DATA

USING EXCEL SPREADSHEET

What if my water balance says I am okay using the scheduler, but my crop appears to be suffering ?  START IRRIGATION IMMEDIATELY!!  Contact NRCS immediately to check your soil moisture in the field.  Even though the Evapotranspiration method of scheduling has proven to be quite successful, it is not fool proof. NRCS will ask you to check your soil moisture at least once a month by digging a hole into your soil profile to compare soil moisture levels against your water balance.  You will do this using the “Feel and Appearance” method following NRCS Program Aid # We have developed a form for you to use for this

SUMMARY  1. Get an Irrigation Water Management Plan from your local NRCS office for your crop and soil.  2. Get daily “ET” data from the Atmometer (ETgage) or the Oklahoma Mesonet.  3. Use the pocket or automated “checkbook scheduler” to manage your irrigations by recording daily ET, daily rainfall, irrigation applications, and water balance.  4. Request assistance from NRCS if you run into any problems.

EQIP Incentive Payment ($10/acre)  Eight basic requirements:  1. Irrigation system meets standards  2. IWM plan is followed  3. Report flow twice during season  4. Install two rain guages and record rain  5. Determine daily ET to use in scheduler  6. Spot check soil moisture monthly  7. Record irrigation amounts and durations  8. Keep scheduler & records & submit to NRCS