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Impact of Water Requirements and Irrigation Scheduling
Depending on weather conditions and ground cover, citrus requires from 35 to 48 inches of water per year; grapefruit requires more water than do oranges, lemons or limes.
Water is removed from a crop by evapotranspiration , which is the removal of water that evaporates or transpires from the plants and from the underlying soil.
In the Valley, more water is lost through this process than is gained through annual rainfall.
This means that supplemental irrigation is needed for citrus crops in the Valley.
A formula has been devised to estimate the amount of water needed by a particular crop under specific local conditions.
The formula uses the rate of evapotranspiration from a standard "reference" crop, such as grass that is actively growing.
This is called the reference evapotranspiration.
To calculate the evapotranspiration from a specific crop such as citrus, multiply the reference evapotranspiration by the crop coefficient.
Crop coefficients for citrus are shown in Table 2.
The crop coefficient varies according to the crop's growth stage.
The reference evapo-
Table 2.
Citrus crop coefficients.
Jan Feb Mar Apr May June July Aug Sep Oct Nov Dec
70% canopy 0.65 0.65 0.65 0.65 0.60 0.60 0.60 0.60 0.65 0.65 0.65 0.65
50% canopy 0.60 0.60 0.60 0.60 0.55 0.55 0.55 0.55 0.60 0.60 0.60 0.60
20% canopy 0.45 0.45 0.45 0.45 0.40 0.40 0.40 0.40 0.50 0.50 0.50 0.50
Ground cover or weeds
70% canopy 0.75 0.75 0.75 0.75 0.70 0.70 0.70 0.70 0.75 0.75 0.75 0.75
50% canopy 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75
20% canopy 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.85 0.85 0.85 0.85
Locally developed crop coefficients
70% canopy 0.6 0.6 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6
Table 3.
Crop water requirements considering an average of 9 years of data and using local crop coefficients in the Lower Rio Grande Valley.
Month ET ref citrus Kc citrus Rain Rain
Jan 3.4 0.6 2.1 0.2 1.9
Feb 3.7 0.6 2.2 0.4 1.8
Mar 5.0 0.7 3.5 1.5 2.0
Apr 5.9 0.7 4.1 1.3 2.8
May 7.1 0.7 5.0 1.3 3.7
June 7.2 0.7 5.0 2.4 2.6
July 7.8 0.7 5.5 1.9 3.6
Aug 7.5 0.7 5.2 2.5 2.7
Sep 5.8 0.7 4.1 5.0 0.0
Oct 4.9 0.7 3.4 3.4 0.0
Nov 3.8 0.6 2.3 1.8 0.5
Dec 3.1 0.6 1.9 0.4 1.5
TOTAL 65.3 43.8 22.1 23.1
transpiration varies throughout the year.
Figure 1 shows the rainfall and evaporation during an average year in the Lower Rio Grande Valley.
If the soil has a ground cover such as grass or weeds, more water will be lost through evapotranspiration than that lost from bare soil, and the crop coefficient will rise.
Citrus in orchards with full grass cover can use 45 percent to 105 percent more water than can citrus in bare soil.
The crop coefficients are slightly lower at midseason than at the beginning and end of the season because the plants' stomata, or pores, close during periods of peak evapotranspiration (Table
Table 3 lists irrigation guidelines for citrus that are based on average conditions for 9 years in the Lower Rio Grande Valley.
In an average year in the Valley, citrus crops with 70 percent canopy and ground cover require about 44 inches of water; about half this amount is supplied by rainfall.
To schedule effective irrigation, producers must know the properties of the soil and the amount of water stored in it.
A balance sheet approach similar to a check register can be used to keep track of the amounts added through rainfall and irrigation and removed through crop water use or evapotranspiration.
Depletion percentages can be measured directly or estimated.
Both methods require information about a crop's rooting depth and the soil's moisture holding capacity.
Citrus roots can extend to 6 feet and, in some cases, as much as 30 feet.
Roots extract most of the water in the first 2 feet; they grow better in sandy soils that have less clay.
Studies conducted in Spain found that citrus takes from 60 percent to 80 percent of its water from the upper 20 inches of the soil.
Table 4 shows the water-holding sandy loam capacities for the top 4 feet of differHidalgo sandy ent soils in the Lower Rio Grande clay loam Valley.
Water availability varies with Rio Grande soil depth.
For example, the Hidalgo silty loam sandy clay loam soil can hold up to 0.17 inches of water per inch of soil to a depth of 28 inches; it can hold up to 0.20 inches of water per inch of soil between depths of 28 and 80 inches.
The same soil can hold between 3.8 and 8.2 inches of water in 4 feet of soil.
Producers in the Lower Rio Grande Valley use various sensors to measure soil-moisture depletion levels.
The most commonly used are granular matrix sensors, such as Watermark soil moisture sensors from Spectrum Technologies, Inc., of Plainfield, Ill.; capacitance probes such as ECH2O probes from Decagon Devices, Inc., of Pullman, Wash., and EnviroSCAN soil moisture sensors from Sentek Sensor Technologies, Australia.
During 2004, two Valley farmers installed EnviroSCAN sensors, which relayed soil moisture information through a modem to the Internet.
After the sensors scanned the soil to a depth of 4 feet, the growers could monitor the soil water levels, enabling them to manage their drip and micro-irrigation systems more precisely.
These technologies are being evaluated and offer good potential for practical use.
The cost of these devices varies dramatically, with Watermark sensors at the low end and EnviroSCAN at the high end.
Table 4.
Properties of soils in the Lower Rio Grande Valley.
Other new technologies are less useful for growers.
Neutron probes and time domain reflectometry instruments are used to measure the volume of water in the soil.
These instruments have been used only for irrigation research in the Lower Rio Grande Valley.
They are impractical for most growers because they usually require calibration and are expensive and complicated to operate.
Also, neutron probes require radiation licensing and radiation monitoring for safety.
Soil Available Water available
Soil series horizons water capacity in the top 4 ft

Lyford sandy 0-11 0.18-0.24
clay loam 11-48 0.16-0.21
clay loam 15-65 0.10-0.18
However, growers throughout the Valley have used sensors to measure soil moisture tension.
As soil moisture tension rises, plants have more difficulty extracting water.
Tools such as tensiometers and Watermark sensors are relatively inexpensive.
Watermark sensors can measure a wider tension range than can tensiometers, which read only to 60 centibars.
Centibars measure the tension in which the water is held by the soil.
The higher the tension reading, the drier the soil.
Inexpensive sensors such as Watermark can be installed at different depths and in different locations to test soil variability.
Because moisture availability includes the effects of soil texture, the readings need not be adjusted for soil type; however, the readings can be affected by soil salinity.
Tension measurements tend to remain low for extended periods as plants absorb water from the soil, then rise rapidly as available moisture levels drop.
Irrigation becomes necessary when soil moisture tension in the root zone reaches between 30 and 60 centibars.
The Watermark sensor has been observed to be slow, sometimes taking about 12 hours to show from dry to wet.
Another potential problem can be caused by the placement of the sensor in relation to the trunk of the tree and the irrigation emitter.
Start irrigation when it is not yet completely dry to allow some time for the sensor to catch up and avoid tree stress.
To reliably measure conditions in the orchard, install the soil water sensors in several locations and at different depths, and record the sensor measurements regularly.
The responsiveness of the Watermark sensors can vary, depending on the irrigation method used.
These sensors respond faster to flood irrigation than to drip or microjet spray irrigation practices.
The management allowable depletion is the deficit point at which irrigation should be triggered.
In citrus, irrigation can be triggered when the crop depletes about 55 percent to 60 percent of the soil water stored in the root zone.
For example, for a Hidalgo sandy clay loam soil with waterholding capacity of 8.2 inches and a management allowable depletion of 60 percent, irrigation is needed at the point when 4.9 inches has been used.
Table 5 shows the corresponding number of irrigations needed for a sandy clay loam in Hidalgo County with
holding capacities of 8.2 inches and 60 percent allowable depletion.
Citrus growers in the Lower Rio Grande Valley commonly flood irrigate from five to seven times per year.
However, the number of irrigations will be affected by the weather, soil type and water availability.

Impact of Water Requirements and Irrigation Scheduling

Depending on weather conditions and ground cover, citrus requires from 35 to 48 inches of water per year; grapefruit requires more water than do oranges, lemons or limes.

Water is removed from a crop by evapotranspiration , which is the removal of water that evaporates or transpires from the plants and from the underlying soil.

In the Valley, more water is lost through this process than is gained through annual rainfall.

This means that supplemental irrigation is needed for citrus crops in the Valley.

A formula has been devised to estimate the amount of water needed by a particular crop under specific local conditions.

The formula uses the rate of evapotranspiration from a standard "reference" crop, such as grass that is actively growing.

This is called the reference evapotranspiration.

To calculate the evapotranspiration from a specific crop such as citrus, multiply the reference evapotranspiration by the crop coefficient.

Crop coefficients for citrus are shown in Table 2.

The crop coefficient varies according to the crop's growth stage.

The reference evapo-

Table 2.

Citrus crop coefficients.

Jan Feb Mar Apr May June July Aug Sep Oct Nov Dec

70% canopy 0.65 0.65 0.65 0.65 0.60 0.60 0.60 0.60 0.65 0.65 0.65 0.65

50% canopy 0.60 0.60 0.60 0.60 0.55 0.55 0.55 0.55 0.60 0.60 0.60 0.60

20% canopy 0.45 0.45 0.45 0.45 0.40 0.40 0.40 0.40 0.50 0.50 0.50 0.50

Ground cover or weeds

70% canopy 0.75 0.75 0.75 0.75 0.70 0.70 0.70 0.70 0.75 0.75 0.75 0.75

50% canopy 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75 0.75

20% canopy 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.80 0.85 0.85 0.85 0.85

Locally developed crop coefficients

70% canopy 0.6 0.6 0.7 0.7 0.7 0.7 0.7 0.7 0.7 0.6 0.6 0.6

Table 3.

Crop water requirements considering an average of 9 years of data and using local crop coefficients in the Lower Rio Grande Valley.

Month ET ref citrus Kc citrus Rain Rain

Jan 3.4 0.6 2.1 0.2 1.9

Feb 3.7 0.6 2.2 0.4 1.8

Mar 5.0 0.7 3.5 1.5 2.0

Apr 5.9 0.7 4.1 1.3 2.8

May 7.1 0.7 5.0 1.3 3.7

June 7.2 0.7 5.0 2.4 2.6

July 7.8 0.7 5.5 1.9 3.6

Aug 7.5 0.7 5.2 2.5 2.7

Sep 5.8 0.7 4.1 5.0 0.0

Oct 4.9 0.7 3.4 3.4 0.0

Nov 3.8 0.6 2.3 1.8 0.5

Dec 3.1 0.6 1.9 0.4 1.5

TOTAL 65.3 43.8 22.1 23.1

transpiration varies throughout the year.

Figure 1 shows the rainfall and evaporation during an average year in the Lower Rio Grande Valley.

If the soil has a ground cover such as grass or weeds, more water will be lost through evapotranspiration than that lost from bare soil, and the crop coefficient will rise.

Citrus in orchards with full grass cover can use 45 percent to 105 percent more water than can citrus in bare soil.

The crop coefficients are slightly lower at midseason than at the beginning and end of the season because the plants' stomata, or pores, close during periods of peak evapotranspiration (Table

Table 3 lists irrigation guidelines for citrus that are based on average conditions for 9 years in the Lower Rio Grande Valley.

In an average year in the Valley, citrus crops with 70 percent canopy and ground cover require about 44 inches of water; about half this amount is supplied by rainfall.

To schedule effective irrigation, producers must know the properties of the soil and the amount of water stored in it.

A balance sheet approach similar to a check register can be used to keep track of the amounts added through rainfall and irrigation and removed through crop water use or evapotranspiration.

Depletion percentages can be measured directly or estimated.

Both methods require information about a crop's rooting depth and the soil's moisture holding capacity.

Citrus roots can extend to 6 feet and, in some cases, as much as 30 feet.

Roots extract most of the water in the first 2 feet; they grow better in sandy soils that have less clay.

Studies conducted in Spain found that citrus takes from 60 percent to 80 percent of its water from the upper 20 inches of the soil.

Table 4 shows the water-holding sandy loam capacities for the top 4 feet of differHidalgo sandy ent soils in the Lower Rio Grande clay loam Valley.

Water availability varies with Rio Grande soil depth.

For example, the Hidalgo silty loam sandy clay loam soil can hold up to 0.17 inches of water per inch of soil to a depth of 28 inches; it can hold up to 0.20 inches of water per inch of soil between depths of 28 and 80 inches.

The same soil can hold between 3.8 and 8.2 inches of water in 4 feet of soil.

Producers in the Lower Rio Grande Valley use various sensors to measure soil-moisture depletion levels.

The most commonly used are granular matrix sensors, such as Watermark soil moisture sensors from Spectrum Technologies, Inc., of Plainfield, Ill.; capacitance probes such as ECH2O probes from Decagon Devices, Inc., of Pullman, Wash., and EnviroSCAN soil moisture sensors from Sentek Sensor Technologies, Australia.

During 2004, two Valley farmers installed EnviroSCAN sensors, which relayed soil moisture information through a modem to the Internet.

After the sensors scanned the soil to a depth of 4 feet, the growers could monitor the soil water levels, enabling them to manage their drip and micro-irrigation systems more precisely.

These technologies are being evaluated and offer good potential for practical use.

The cost of these devices varies dramatically, with Watermark sensors at the low end and EnviroSCAN at the high end.

Table 4.

Properties of soils in the Lower Rio Grande Valley.

Other new technologies are less useful for growers.

Neutron probes and time domain reflectometry instruments are used to measure the volume of water in the soil.

These instruments have been used only for irrigation research in the Lower Rio Grande Valley.

They are impractical for most growers because they usually require calibration and are expensive and complicated to operate.

Also, neutron probes require radiation licensing and radiation monitoring for safety.

Soil Available Water available

Soil series horizons water capacity in the top 4 ft

Lyford sandy 0-11 0.18-0.24

clay loam 11-48 0.16-0.21

clay loam 15-65 0.10-0.18

However, growers throughout the Valley have used sensors to measure soil moisture tension.

As soil moisture tension rises, plants have more difficulty extracting water.

Tools such as tensiometers and Watermark sensors are relatively inexpensive.

Watermark sensors can measure a wider tension range than can tensiometers, which read only to 60 centibars.

Centibars measure the tension in which the water is held by the soil.

The higher the tension reading, the drier the soil.

Inexpensive sensors such as Watermark can be installed at different depths and in different locations to test soil variability.

Because moisture availability includes the effects of soil texture, the readings need not be adjusted for soil type; however, the readings can be affected by soil salinity.

Tension measurements tend to remain low for extended periods as plants absorb water from the soil, then rise rapidly as available moisture levels drop.

Irrigation becomes necessary when soil moisture tension in the root zone reaches between 30 and 60 centibars.

The Watermark sensor has been observed to be slow, sometimes taking about 12 hours to show from dry to wet.

Another potential problem can be caused by the placement of the sensor in relation to the trunk of the tree and the irrigation emitter.

Start irrigation when it is not yet completely dry to allow some time for the sensor to catch up and avoid tree stress.

To reliably measure conditions in the orchard, install the soil water sensors in several locations and at different depths, and record the sensor measurements regularly.

The responsiveness of the Watermark sensors can vary, depending on the irrigation method used.

These sensors respond faster to flood irrigation than to drip or microjet spray irrigation practices.

The management allowable depletion is the deficit point at which irrigation should be triggered.

In citrus, irrigation can be triggered when the crop depletes about 55 percent to 60 percent of the soil water stored in the root zone.

For example, for a Hidalgo sandy clay loam soil with waterholding capacity of 8.2 inches and a management allowable depletion of 60 percent, irrigation is needed at the point when 4.9 inches has been used.

Table 5 shows the corresponding number of irrigations needed for a sandy clay loam in Hidalgo County with

holding capacities of 8.2 inches and 60 percent allowable depletion.

Citrus growers in the Lower Rio Grande Valley commonly flood irrigate from five to seven times per year.

However, the number of irrigations will be affected by the weather, soil type and water availability.