Water Budget Irrigation Scheduling
 

In This Tutorial

 
- The Effective RootZone as a System
- Water Budget Irrigation Scheduling Explained
- Summary of Basic Steps of Water Budget Scheduling
- Data Requirements for Water Budget Irrigation Scheduling
- Water Budget Irrigation Scheduling with Micro-Irrigation Systems
- The Finer Points of a Water Budget Model
- Example of Water Budget Scheduling for a Low Frequency Irrigation System
- Example of Water Budget Scheduling for a High Frequency Irrigation System
  Purpose - The purpose of this Tutorial is to...
  - Explain one of the two main families of irrigation scheduling, "water budget"
- Summarize data requirements for performing water budget irrigation scheduling
- Explain some of the finer points of water budget scheduling
- Provide examples of water budget scheduling for both low frequency (furrow)
  and high frequency (drip) irrigation systems
   
INTRODUCTION
 

A modern irrigation system, whether furrow, border strip, sprinkle, or trickle, is an important and large investment. Whether you spend the money on precision land grading for surface irrigation, or for pump, pipes, and sprinklers/tricklers for sprinkle/trickle, you should want to make the best use of that investment. The best hardware in the world is no good if it is not used correctly.

Again, the keys to effective, efficient irrigations are knowing when, how much, and how to irrigate. Modern irrigation scheduling techniques help you to know when and how much to irrigate.   

"Irrigation Scheduling" is a generic term applied to any technique/practice that is intended to aid the farmer in determining when and how much to irrigate.  There are a number of ways in which the different techniques could be categorized.  A common way is to group them as either "water budget" and "graphical/sensor-based" methods.  This tutorial covers graphical/sensor-based (also called "bottom-line") scheduling.  

Water budget irrigation scheduling can be complex, time-consuming, and ultimately expensive.  Water budget irrigation scheduling can be complex, time-consuming, and ultimately expensive.  In many situations the graphical approach is better.  Visit the Tutorial on graphical scheduling techniques .

Again, the keys to effective, efficient irrigations are knowing when, how much, and how to irrigate. Modern irrigation scheduling techniques help you to know when and how much to irrigate.

 
  THE EFFECTIVE ROOT ZONE AS A SYSTEM
 


Review


Soil moisture levels can be measured in two ways, 1) as volumetric moisture, and 2) as soil moisture tension.  Volumetric soil moisture is a measure of how much physical water is in the soil.  Soil moisture tension is a measure of how much effort is needed to extract moisture from soil.

The most common method of describing volumetric soil moisture is a depth of water per depth of soil.   With English units this is most commonly "inches of water held per foot of soil".   (Some will use a percentage of water by volume measure.)  The measure for soil moisture tension is pressure.  The most common unit of measurement is "centibars of pressure".

 


Water budget irrigation scheduling generally works with volumetric soil moisture.   Graphical methods will use either volumetric or soil moisture tension measurements.

Soil will hold water against the pull of gravity, keeping it available for plants to extract through their root zones.  There are limits to the amount of available water. The upper limit is the Field Capacity (FC) of the soil.  The lower limit is the Permanent Wilting Point (PWP).

Essentially, field capacity is the most water the soil will hold against the pull of gravity.  You can add more water to soil that is at field capacity but the excess water will just drain down through the soil, below the effective root zone and away from the plant. 

Note however, that it may take some substantial amount of time for this excess water to drain down through the soil profile.  The amount of time depends on the type of soil and the depth of the root zone.  While this water is draining through the root zone, it is available for the crop to use.  This is a key aspect in refining Water Budget irrigation scheduling models as is explained in the Finer Points.. . section.

Permanent wilting point is the level of moisture in the soil when the plant can no longer pull water from the soil.  Thus the plant permanently wilts.  The PWP will change with the plant and soil as some plants can extract more water from a soil, or any given soil,  than others.

4. Between field capacity and permanent wilting point is "available" water.  This is water that the plant can pull from the soil through its root system.

 
  The Effective Root Zone as a System
  The Effective Root Zone is that depth of soil, determined by the farmer, where the farmer wants to control soil moisture.
 


Definition - Effective Root Zone (ERZ) - the depth of soil where you want to control soil moisture.   It may or may not be the full depth of the plant's roots.  (For example, in many vegetable crops, the effective root zone is much shallower than the full root system because of quality concerns.)  The ERZ is chosen by you.

   
  The Effective Root Zone can hold up to some maximum amount of water that is available for the crop to use.  This is called the Available Water Holding Capacity
   
 

Definition - Available Water Holding Capacity (AWHC) - the total of available water in the effective root zone, when the ERZ is at field capacity.  Thus:

AWHC = field capacity - permanent wilting point

 
 

AWHC's are a volumetric soil measurement.  For irrigation scheduling purposes, we usually talk about AWHCs as the total inches of water available in the effective root zone.  Thus we might say, "the AWHC for that field is 5.5 inches total in the 4 foot root zone", or AWHC = 5.5 inches.

  Normally you do not let the plant use up all the water available in the Effective Root Zone.  By definition this would put the plant at Permanent Wilting Point.
  A common name used to describe the amount of water allowed to be used by the crop between irrigations is the Management Allowed Depletion, MAD.
   
 

Definition - MANAGEMENT ALLOWED DEPLETION (MAD) - the amount of water that is allowed to be used by the plant between irrigations.   MADs can be described as a percentage of the AWHC or as "inches of water".  Thus we might say that "the MAD for the field is 45% of AWHC".  We are saying that we will allow the crop to use 45% of  the available water in the effective root zone between irrigations.  Or, we might say that "the MAD for the field is 3.5 inches".  We are now saying that we will allow the crop to use 3.5 inches between irrigations.

The MAD in inches is equal to the MAD percentage times the AWHC. For example...

AWHC = 5.5 inches
MAD = 45%
MAD in inches is .45 x 5.5 = about 2.5 inches

  Letting the plant use up the majority of  available water in the effective root zone between irrigations would put excessive stress on the crop, affecting yields and quality.  Also, if the plant was to use water all the way down to the permanent wilting point, it would be dead (the definition of PWP is just that, permanently wilted). 
  As will be explained later in this tutorial however, farmers desire to place more or less stress on a crop at any one time according to how they wish the crop to develop.   It is very common for winegrape growers to put stress on their vineyards purposely in order to increase quality.  A key objective of irrigation scheduling is to control the amount of stress on a crop.
  Modern farmers look at the Effective Root Zone as a system.   Among other things they try to control the amount of water in this system.

They attempt to measure or predict the amount of water in the ERZ at any one time.   As well, they try to measure or predict the amounts of water going into and out of the ERZ.

Water going into the ERZ is normally rain and irrigation.  Upflux from a high water table may be a significant factor in some areas.

Water going out of the ERZ is primarily deep percolation (from excess irrigation or infiltrated rain) and crop evapotranspiration (crop water use).

   
 

Modern water management looks at the Effective Root Zone as a system.  Managers try to:

1. Identify water going into and out of the system,
2. Measure these amounts, and
3. Control these amounts.

A schematic of the effective root zone is seen below.  The primary sources of water going into the ERZ are rainfall and irrigation.  In areas with a high water table there may be a constant upflux of water into the root zone.  Also in these areas there may be significant amounts of water moving sideways into and out of the rootzone.  The primary losses of water are crop evapotranspiration  and deep percolation from excess irrigation or rainfall.

 
  Figure 1 - Schematic of the Effective Root Zone showing the "types" of water that come into and go out of it
 
  WATER BUDGET IRRIGATION SCHEDULING EXPLAINED
  All water going into and out of the Effective Root Zone can be measured in terms of a depth of water. 

Thus, if the amounts of the different types of water going into and out of the root zone can be identified, simple addition and subtraction can indicate the net change in root zone water content.

This is the basis for one family of irrigation scheduling techniques, the "Water Budget".

   
 

All of the water going into and out of the Effective Root Zone can be measured in terms of a depth of water, usually inches...

- Rainfall is measured in inches.

- Irrigations are commonly measured as acre-inches/acre, or just inches.

- Deep percolation is water soaked into the effective root zone in excess of the soil's field capacity. Since it is either excess irrigation or excess rainfall, it too is measured in inches.

- If water going out of the root zone (deep percolation) is measured in inches, then upflux from a high water table ought to be measured in inches also.

- Finally, crop evapotranspiration is calculated in terms of acre-inches/acre per day and if we are looking at any one day, just inches.

The fact that all water going into and out of the effective root zone can be measured in terms of a depth of water is the basis for one major category of irrigation scheduling, the "Water Budget" (also known as "Checkbook" irrigation scheduling).

  Water Budget irrigation scheduling is the day-by-day accounting of the amounts of water coming into and going out of the effective root zone. 

A confusing aspect of water budget scheduling is that the process can be set up so that the accounting is in terms of the Soil Moisture Depletion, or in terms of the Available Water, or in terms of the Total Water in the effective root zone.

For the rest of this discussion the accounting will be in terms of the total soil moisture.  Although many scheduling systems track soil moisture depletions, the water budget equation is easier to understand when working with total water.

   
 

The water budget equation keeps track of water coming into and out of the effective root zone.  As will be seen shortly, the water budget equation is very simple.  However, you must decide whether to solve the equation in terms of the soil moisture depletion, in terms of the available water, or in terms of total water in the effective root zone.  It is most common solve in terms of either total water or soil moisture depletion.  This discussion will solve in terms of total water since it makes it easier to understand the equation.

   
  Assume a starting point as the total water in the effective root zone at the start of the day- call this Water start.

The total water in the root zone at the end of the day (24 hours later) will be called Water end.

Then, for the day:

  • Crop evapotranspiration will be called ETc.
  • Rainfall that infiltrates the soil will be called RAIN.
  • Any irrigation that infiltrates the soil will be called IRR.
  • Any deep percolation from excess infiltrated irrigation or rainfall will be called DEEP.
  • Any change in total water in the effective root zone from underground water movement (possibly a high water table or water moving laterally in the ground) will be called FLUX net.  

Thus, the water budget equation is:

Water end = Water start + IRR + RAIN - ETc - DEEP + FLUX net
Identifying the values and solving this equation is the basis for water budget irrigation scheduling.

 
   
 

Water budget irrigation scheduling chooses a starting point soil moisture in the effective root zone.   Then, the equation is solved on a daily basis, considering the amounts of water moving into and out of the root zone for that day. 

Example 1:

Assume the following:

- Effective root zone = 2 feet deep
- Field capacity = 4.5 inches in the 2 feet
- Current total moisture in the 2 feet = 3.2 inches = Water start
- There was no irrigation or rainfall for the day, thus IRR = 0 and RAIN = 0
- If there is no irrigation or rainfall then deep percolation must be 0 also, DEEP = 0
- There is no high water table or significant ground water movement in the root zone, thus
   FLUX net = 0
- Crop water use for the day = .25 inches = ETc

 then, the water budget equation is solved as:

Water end = Water start + IRR + RAIN - ETc - DEEP + FLUX net
Water end = 3.2 in + 0 + 0 - .25 in - 0 + 0
Water end = 2.95 inches

Example 2:

Assume the following:

 - Effective root zone = 2 feet deep
 - Field capacity = 4.5 inches in the 2 feet
 - Current total moisture in the 2 feet = 3.2 inches = Water start
 - There was 2 inches of irrigation water infiltrated for the day, thus IRR = 2
 - There was no rainfall for the day, thus RAIN = 0
 - There was 2 inches of irrigation added to the 3.2 inches already in the root zone which
   would equal 5.2 inches.  However, field capacity is only 4.5 inches, thus DEEP = 5.2 - 4.5
   = .7 inches
- There is no high water table or significant ground water movement in the root zone, thus
   FLUX net = 0
- Crop water use for the day = .25 inches = ETc

then, the water budget equation is solved as:

Water end = Water start + IRR + RAIN - ETc - DEEP + FLUX net
Water end = 3.2 in + 2 in + 0 - .25 in - .7 in + 0
Water end = 4.25 inches

   
  Note that in example 2, the irrigation water was assumed to be added to the effective root zone at the start of the day.  Thus, DEEP is calculated to be .7.  (If it was added at the end of the day then DEEP would have been calculated as .45 inches and Water end would have been 4.5 inches.)  Irrigations and rainfall can be "added" at the start or the end of a day when solving the equation.  The more normal convention is to add them at the start of the day.   Either way can cause errors depending on the actual timing of the irrigation or rain.  Thus, as will be seen later, it is normal for actual soil moisture levels to be measured periodically so that the value for Water start is reconciled with actual conditions.
   
 

An irrigation is called for when the water level at the end of the day is equivalent to the Management Allowed Depletion.  The amount of irrigation called for is usually that amount required to take the soil moisture level back to field capacity, plus efficiency losses and required leaching.

Thus, an advantage of water budget irrigation scheduling is that it directly provides both an irrigation date and an irrigation amount.  That is, it helps to determine both...

WHEN to irrigate
and
HOW MUCH to irrigate

   
 

The water budget method provides an estimate of the soil moisture at the end of a day.  The question is whether that soil moisture is dry enough to trigger an irrigation.  That is, does the field capacity of the effective root zone minus the current soil moisture equal the management allowed depletion?  Thus, if

Field Capacity - Water end   => MAD

then call for an irrigation the next day.  The amount of irrigation is equal to (Field Capacity - Water end ) + efficiency losses and any required leaching.

 
 

Using the Water Budget Equation to Schedule (Predict) Irrigations

   
 

The water budget equation can be solved on a real-time basis.  That is, using actual weather, actual irrigations, etc.

However, the real usefulness of the water budget equation is as a predictive model.  That is, it can be used to predict irrigations if sufficient long-term, average weather data and a crop coefficient curve are available.  This data allows a prediction of daily crop water use and thus, how fast the soil moisture will be depleted.

   
 

The examples above were a calculation of the water budget on a real-time basis.  Doing so will indicate when an irrigation is needed, but doesn't predict when an irrigation will be needed.  One of the advantages of using an irrigation scheduling system is to be able to predict when irrigations are needed so as to plan for them.

Let's look at the water budget equation again.

Water end = Water start + IRR + RAIN - ETc - DEEP + FLUX net

If it is being used to predict the next irrigation, then IRR must equal 0.  Assuming we are in California in the main irrigation season it is safe to say that RAIN will be 0 (although there are ways to account for expected rainfall).  If IRR = 0 and RAIN = 0, then DEEP = 0.  And finally, just to simplify things, let us assume there is no water table to worry about.  Now the equation looks like this:

Water end = Water start - ETc

Thus, to predict when the next irrigation should occur, all that is needed is to predict the crop water use, ETc .   The Basics of Soil-Water-Plant Relationships tutorial showed that individual crop water use could be estimated if a reference evapotranspiration and a crop coefficient were available.  That is,

ETc = ETo * Kc  

where:

ETc = evapotranspiration of the crop

ETo = reference evapotranspiration (in this case, ETo signifies the ETc of a well-watered, lush grass pasture)

Kc = the crop coefficient that relates the crop's water use to the reference ETo

ETos can be estimated using a weather-based equation (which is very complex and will  not be explained here) and long-term, average weather data.  Base crop coefficients are similar year-to-year (taking into account the condition of the crop).  Thus, crop water use can be predicted.  And if crop water use can be predicted then the timing and amount of the next irrigation can be predicted using the water budget equation.

 
 

SUMMARY OF WATER BUDGET IRRIGATION SCHEDULING PROCESS

 
The basic steps for water budget irrigation scheduling are:
 
1 Determine the depth of the effective root zone.
2 Determine the starting point for soil moisture in the effective root zone.  This may be the soil moisture calculated at the end of the previous day.  Or, it may be the reading from a neutron probe or some other volumetric measurement.
3 Determine the different amounts of water going into and out of the effective root zone.  That is, calculate crop water use for that day, estimate rainfall that infiltrates (if it rains that day), estimate infiltrated irrigation water (if there was an irrigation that day), etc. etc.
4 Solve the water budget equation:

Water end = Water start + IRR + RAIN - ETc - DEEP + FLUX net
5 Determine the management allowed depletion, MAD.  Does the field capacity of the effective root zone minus the current soil moisture calculated in step 4 ( Water end ) equal the MAD?  That is, if

Field Capacity - Water end   => MAD

then call for an irrigation the next day.  The amount of irrigation is equal to (Field Capacity - Water end ) + efficiency losses and any required leaching.
6 Periodically update the calculated soil moisture with an actual reading from the field.  This is required since it is impossible to correctly evaluate all variables in the water budget equation and there are inherent timing problems with RAIN and IRR.
7 Predict irrigation dates and amounts by using long-term average reference evapotranspiration data, crop coefficient curves, and knowledge of how the effective root zone develops to predict the variables used in the water budget equation.
 

  BASIC DATA REQUIREMENTS FOR WATER BUDGET IRRIGATION SCHEDULING
 


The actual water budget equation is simple, a starting soil moisture level and (up to) five numbers to add or subtract to determine the ending soil moisture. The problems, and the costs, of water budget scheduling come with identifying those numbers. Accurate water budget irrigation scheduling requires knowledge of the following...

  • The field capacity and available water holding capacity of your soils

  • The effective root zone of your crops throughout the season

  • Agronomic factors that determine how much stress you want the crop under between irrigations (allows a choice of MAD)

  • Daily reference evapotranspiration

  • A crop coefficient curve that relates the actual crop evapotranspiration, ETc, to the reference ET

  • Effective rainfall, that is, rain that actually soaks into the soil and is not runoff

  • Infiltrated irrigation depths, how much water delivered to a field infiltrates the soil

  • Knowledge of high water tables or significant sub-surface water movement

Most people doing water budget scheduling will also take actual measurements of soil moisture periodically to make sure the process is accurate.   (Refer to the Tutorial on Graphical Irrigation Scheduling for a discussion of the different methods of measuring soil moisture.)

  There is a lot of data needed to perform water budget irrigation scheduling accurately.  However, there are sources for all required data and professional consultants providing irrigation scheduling services gather this data as part of the service
   
 

Each of the above requirements will be discussed briefly.

Soil Moisture Holding Capacities - The NRCS can usually provide estimates of the holding capacities of soils in your area. The UC Extension specialists also will have information, including publication #21463, "Holding Capacities of California Soils".

Effective root zones - It can be very difficult to set effective root zones and may take a couple of years of experience, especially with annual crops. Your UC Cooperative Extension specialist or consulting agronomist will have information for you. Make sure you consider any restrictions on the root zones due to hard pans or high water tables.

Obviously, the ERZ of an annual is going to change constantly up to plant maturity. But the ERZ does not have to be the full depth of the rooting system. For example, in cotton it is common to cut off the ERZ at a maximum 4 foot root zone, even though the full system might go to 6 foot or more.  ERZs for vegetables are also generally shallower than the actual full rooting depth.

Management Allowed Depletions - Management Allowed Depletions (MAD) are a measure of how much stress is to be applied to a crop. MAD's may change with the season.  Be aware of the different growth stages of your crops and how they should be manipulated during these stages. Also, if you have fields with high salinity, the MAD is likely to be lower than normal.

Many times, MAD's are "backed in to".  For example, the farmer is checking a field and finally decides to irrigate. The irrigation scheduling system is checked for the soil moisture level at the time of the irrigation. This is then converted to a MAD for future use.

   
  In many cases, the advantage of using irrigation scheduling is to alert you that a field is getting close to the MAD so that you can begin looking at it closer. It is never recommended that irrigation scheduling be the sole ruler of when to irrigate. However, irrigation scheduling will always provide an estimate of how much water to put back into the soil.
   
 

Daily reference evapotranspirations - there are several kinds of reference ET that are in use.  Most common are ETp, ETo, and ETpan.   ETp is the water use of a lush, well-watered alfalfa.  ETpan is the evaporation from a standard U.S. Weather Bureau Class A evaporation pan.  ETo is the water use of a lush, well-watered grass pasture.  You can use any reference that you wish, probably set by the availability of one or the other.  However, you MUST use a crop coefficient curve that is keyed to that particular reference.  You cannot use a crop coefficient curve developed for use with ETo when ETp is the reference used in the water budget.

CIMIS, a state-wide network of weather stations and computers, is administered by the Department of Water Resources, CIMIS program. The information developed by CIMIS is free to the public.  It can be accessed by telephone or from the CIMIS web site.  Many newspapers and radio shows will report CIMIS data. 

There are also numerous suppliers of individual, on-farm weather stations.  These can provide much more accuracy than a CIMIS station, especially when rainfall is a significant factor.

Crop coefficients - The actual evapotranspiration of the crop (ETc) depends on the following...

  • Type of plant (some plants use more water than others)

  • Stage of growth (a mature plant uses more than a seedling)

  • Condition of plant (stress from whatever cause, insects, fertilizer, excess salinity, or lack of soil water, decreases ETc)

  • Climate (higher temperature, lower humidity, and higher wind will increase ETc)

Of the factors just mentioned, the reference ET accounts for the climatic variability. 

The "type of plant", the "stage of growth", and general crop condition are accounted for by the crop coefficient, Kc.  Crop Coefficients define the relationship between reference ET's (ETr)  and ETc, the actual crop ET.  As noted previously ETc is calculated using ETr and the Kc as...

        ETc = ETr X Kc

For example, if the ETr for a certain day was .25 inches and the crop coefficient for a crop was .8, then...

        ETc = ETr X Kc
        ETc = .25 inches X .8
        ETc = .20 inches

Crop coefficients are different for different crops and for different growth stages of any one crop.  The crop coefficient curve is a set of relationships between the crop and reference ET throughout the crop's growing season.  For example, the Kc for cotton may be .15 at 20% of its development and 1.05 at peak growth.  The figure below is an example Kc curve for an annual crop.  You can see that the Kc is very small as the plant is just coming out of the ground.  It goes into a rapid growth stage and then levels off at peak water use, declining as the crop matures and then is cut off. 

Note in the Curve below that the Kc value is keyed to the percent of cover before full cover, but then is keyed to the number of days after full cover.  Some crop coefficient curves, notably those published by the University of California, are keyed to specific dates.  Others may be keyed to specific growth stages.

 
 
Figure 2 - Example crop coefficient curve for an annual crop
   
  Deciduous trees may immediately go to a Kc of .6 when leafing in the Spring but peak at a Kc of .75 for the rest of the season.  The figure below is a common crop curve for deciduous trees.
 
 


Figure 3 - Example crop coefficient curve for a permanent crop

 
Important! If you are using reference ET information and crop coefficient curves to keep track of your crop's water use, make sure that the Kc curve you are using matches the reference ET.  For example, if you are growing cotton and using the ETo information (a reference of pasture) from the CIMIS system you would use a Kc curve keyed to ETo.   But if you had your own weather station that calculated ETp (a reference of alfalfa) you would use a Kc curve that was keyed to ETp.
   
 

There are many sources for crop coefficient curves.  Some curves and explanations as to their use can be found in the UC Extension publications 21427 and 21428, "Using Reference ET and Kc's to Estimate ETc". Publication 21427 is for field crops and Publication 21428 is for trees and vines.   UC also publishes 21454 "Irrigation Scheduling, A Guide for Efficient On-Farm Water Management.   It contains tables of crop coefficients and long-term, average ETo for many locations in the state.  Again, be aware of what ET reference was used in developing the Kc curve that you use.

Effective Rainfall - Rainfalls are reported by radio, TV, and newspapers.  Many times they get their information from weather stations at airports.  Actual rainfall in your field can vary widely from reported.  Rain gauges are cheap.  Place one near your fields to get a more accurate measure of the actual rainfall.  The CIMIS system also reports rainfall.

Note also that the total rainfall may not have soaked into the ground.  Depending on the storm's intensity and duration, along with pre-existing moisture conditions, there might be significant runoff.  It is your experience that judges how much rainfall is EFFECTIVE rainfall, that is, how much actually infiltrates.

Infiltrated Irrigation - There are many different methods used to estimate the depth of water applied with an irrigation.  Some schedulers will take an actual soil moisture measurement a day or two after an irrigation to see what went on.   Some schedulers will assume that the irrigation is excessive (especially with surface irrigations) and that soil moisture goes to field capacity (zero depletion) during an irrigation.

If using a sprinkle or trickle system, you should know what the application rate of the system is (refer to the Advisory page "Planning a Sprinkler Irrigation").  Then, it is a simple matter of multiplying the application rate times the set time to estimate the gross irrigation.

In most irrigation scheduling systems, a GROSS irrigation is reported so that the total amount of water delivered to the field can be tracked.  The net IRR, used in the water budget equation, is determined by applying an efficiency factor.   That is...

    IRR = GROSS x EFF

The efficiency factor used may not be what would commonly be called irrigation efficiency.  Rather, it should be more like an infiltration efficiency.   Remember that irrigation efficiency is a measure of how much applied water is beneficially used.  However, we are interested in how much irrigation water that was applied to the field infiltrated the soil.   This can be a difficult concept.   However, many irrigation specialists can develop estimates of distribution uniformity.  The DU is then used to determine IRR from a gross irrigation minus surface runoff and immediate evaporation.  That is,

    IRR = GROSS * DU * (1 - RO) * (1 - EVAP)

    where

    IRR = amount of infiltrated irrigation water to be used in water budget equation
    GROSS = gross amount of irrigation water applied to field
    DU = distribution uniformity
    RO = percent of applied water that becomes surface runoff
    EVAP = percent of applied water that becomes immediate evaporation

The calculation for IRR is another reason why periodic checks of actual soil moisture are needed.  DUs are a measure of how evenly water infiltrates across a field.  It is actually a measure of the lowest (or some average of the lowest areas) amount of infiltration.  Depending on where you are measuring soil moisture in the field, you may want to use a number for DU that is higher than the actual DU to better represent conditions at that site.

Upflux from high Water Tables and Lateral Groundwater Movement - This is a very difficult number to estimate.   However, areas that are affected by these conditions are generally under study.   Check with your local NRCS or UC Extension Agent if you suspect that a high water table or local groundwater movement is contributing water to the effective root zone.

 
 

WATER BUDGET SCHEDULING WITH MICRO-IRRIGATION SYSTEMS

 

It is not actually recommended that "classic" water budget irrigation scheduling be done for fields using micro-irrigation systems.  This situation usually implies a high-frequency irrigation schedule.  This means that the ability to predict an irrigation date is not that important.  The system is being run on a fairly frequent basis since one of the advantages of a micro system is the ability to maintain a constant, relatively high soil moisture level.  In most situations, water budget scheduling systems have different report formats for micro systems.  With these reports it is more common to see a recommended time and/or amount of irrigation for the next 7 to 14 days.

Basically the water budget system as applied to micro-irrigation:

  1. Develops an estimate of crop water use for the next 7 - 14 days
  2. Utilizes a known system water application rate and irrigation efficiency to estimate hours of operation
  3. Periodic measurements of actual soil or plant moisture will guide the scheduler in modifying the crop coefficient curve.  Modifications to the reference ET estimate based on current weather (see Finer Points... below) may also be made.

The Figure below is an irrigation scheduling report from one commercial, computerized  irrigation scheduling system.  Note the different report formats for the different types of irrigation systems.

 
 

Figure 4 - Example commercial irrigation scheduling report (source: CropData system from Peter Canessa)

 
 

However, if you do want to perform water budget scheduling be aware that the irrigation system does not wet the entire field.  Thus, the crop water use, which is calculated using the entire field, must be extracted from the wetted area only.   This will dry down the effective root zone quicker and the scheduler may have to account for dry soil effects (see Finer Points.. . below).

Example - Assume the following:

    - drip irrigated almond orchard planted on 24 x 24 spacing
    - the irrigation system wets an area of 200 square feet around each tree
    - crop ETc is estimated at .25 inches/day

The crop ETc of .25 inches/day implies that there is a depth of .25 inches of water being extracted from the entire field, or the 24 x 24 spacing for each tree.  (This is because crop coefficient curves are developed based on the entire field.)  However, water is only be applied to a 200 square foot area.  Thus, the volume of water that the tree is using must be extracted from this area only.  Thus,

    ETceff = ETc * Planted / Wetted

where:

    ETceff is the depth of water extracted from the wetted area and thus, used in the water budget equation

    ETc is the estimated crop water use

    Planted is the planted area for each plant, in this case 24 x 24 = 576 square feet

    Wetted is the wetted area due to the irrigation system, in this case 200 square feet

Thus,

    ETceff = ETc * Planted / Wetted

    ETceff = .25 * 576 / 200 = .72 inches

 
 

THE FINER POINTS OF WATER BUDGET IRRIGATION SCHEDULING

   
 

Water budget irrigation scheduling basically tries to model the physical process of water movement into the soil, through the soil, and through the plant.  This requires a lot of data and experience if it is to be accurate. 

There are many other factors to consider beyond the simple water budget equation seen above.

   
 

Modeling physical processes can be very complex and involved.   It can result in numerous calibration points.  If any of these calibrations are wrong, then the results of the model will be wrong.  The points discussed below concern those calibrations that are felt essential for a scheduling system to be reasonably accurate.  

Modifying the base crop coefficient - There are two basic factors that need to be considered:

  1. Crop condition - note that crop coefficient curves are usually keyed to a planting date and a full cover date.  However, large variations in weather (a cold spell or excessive temperatures) can accelerate or delay crop development significantly.   The scheduler must be alert to whether the calculated normal crop coefficient is representative of the actual field conditions.
  2. Dry soil - with low frequency irrigation systems in lighter soils, the effect of dry soil on crop ET must be considered.  That is, as the soil dries out, it starts to reduce normal crop ET at a certain point.  The effect will be more or less depending on the growth stage, soil type, and ET rate.

Deep percolation- that isn't - As noted, the soil of the effective root zone has a field capacity.  If irrigation or rainfall infiltrates beyond that level it will become deep percolation- or will it?  Normally it takes a significant amount of time for excess water to move through the root zone.  It will take longer in finer soils or with deeper root zones.  As this excess water is moving through the root zone it is available for the plant to use.  Thus, in effect, normal depletion of soil water will be delayed as excess water works its way through the root zone.

Upward flux from a high water table - This is obviously going to be a difficult number to identify in any case.  However, the scheduler must be aware of when this number will become significant.  That is, at a shallower effective root zone there may be no upflux.  If scheduling an annual crop the scheduler must be aware of when the effective root zone becomes deep enough to be affected by a high water table.  And, it may be that the effect will be greater as the root zone expands.

Inactive versus active rootzones in micro-irrigated fields - water budget irrigation scheduling with micro-irrigated fields is made more complex by the fact that not all of the field is wetted.  Thus, crop ET must be accounted for in the wetted part of the field only, where roots are active (see ... Scheduling with Micro-Irrigation above)..  It has been seen that the area of active roots in a micro-irrigated field may be much greater than usual in the spring, following winter rains.  The winter rains wet the entire field.  This activates dormant root systems and/or provides a reservoir of water, beyond irrigation water,  that can move into the normal area of active roots.  In either case, the effect is to delay the start of spring irrigations.

Long-term, average reference ET versus expected reference ET - water budget systems commonly use long-term, average weather data to generate predicted reference ET.  However, hot and cold spells can last for significant periods of time.   The scheduler might want to increase or decrease the predicted reference ET based on the current weather pattern or predicted weather.

Wet-soil ET - evaporation from a wet soil surface must be accounted for after an irrigation or rainfall.  This accounts for excess water at the soil surface that will immediately evaporate rather than become deep percolation or stored in the root zone and available for crop ETc.  The effect is usually accounted for on a decreasing basis for about three days.  It can be a fairly complicated calculation if accounting for the effects of crop shading.

Rainfall - rainfall in the main irrigation season is not a consideration in some growing areas.  However, even in drier climates rainfall may be important with annual crops in the spring.  If rainfall is significant, and long-term data indicates some consistency, rainfall may be added on a per day basis, much like UPFLUX.   Or, near-term weather forecasts will be examined and rain added manually in the predictive phase of modeling.  Make sure that only rainfall that infiltrates the soil is used.

 
   
 
 
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