Surface Irrigation 522AE

Evapotranspiration and drainage requirements ET, is dependent upon: ET, is dependent upon: 1) climatic conditions 2) crop variety 3) stage of growth 4) soil moisture depletion 5) various physical and chemical properties of the soil. of the soil.

1) The seasonal distribution of reference crop "potential evapotranspiration", Etp, which can be computed with standard formulae. 1) The seasonal distribution of reference crop "potential evapotranspiration", Etp, which can be computed with standard formulae. 2) The Etp is adjusted for crop variety and stage of growth. Other factors like moisture stress can be ignored for the purposes of design computations. Other factors like moisture stress can be ignored for the purposes of design computations. Estimating ET

There are twenty commonly used methods for calculating evapotranspiration, ranging in complexity from the: There are twenty commonly used methods for calculating evapotranspiration, ranging in complexity from the: Blaney-Criddle Method using primarily mean monthly temperature Blaney-Criddle Method using primarily mean monthly temperature Methods for calculating ET

To more complete equations such as the Penman Method requiring: To more complete equations such as the Penman Method requiring: 1) Radiation 2) Temperature 3) Wind velocity 4) Humidity 5) Other factors comprising the net energy balance at the crop canopy. balance at the crop canopy.

The actual crop water demand depends on its stage of development and variety. The actual crop water demand depends on its stage of development and variety. It is estimated by multiplying ETr by a crop growth stage coefficient, Kc. It is estimated by multiplying ETr by a crop growth stage coefficient, Kc. Some irrigation water should be applied in excess to leach salts from the rooting region. Some irrigation water should be applied in excess to leach salts from the rooting region. Crop water demand

Soil moisture principles Soil moisture principles Important soil characteristics in irrigated agriculture include: Important soil characteristics in irrigated agriculture include: 1) the water-holding or storage capacity of the soil; 2) the permeability of the soil to the flow of water and air; 3) the physical features of the soil like the organic matter content, depth, texture and structure; and 4) the soil's chemical properties such as the concentration of soluble salts, nutrients and trace elements.

The total available water, TAW, for plant use in the root zone is commonly defined as: The total available water, TAW, for plant use in the root zone is commonly defined as: the range of soil moisture held at a negative apparent pressure of 0.1 to 0.33 bar (a soil moisture level called 'field capacity') and 15 bars (called the 'permanent wilting point'). the range of soil moisture held at a negative apparent pressure of 0.1 to 0.33 bar (a soil moisture level called 'field capacity') and 15 bars (called the 'permanent wilting point'). The TAW will vary from: 1.25 cm/m for silty loams to 2.6 cm/m for sandy soils.

Relationships between soil types and total available soil moisture holding capacity, field capacity and wilting point

1) Porosity, f 2) Volumetric moisture content, q 3) Saturation, S 4) Dry weight moisture fraction, W 5) Bulk density, g b 6) Specific weight, g s Other Soil Parameters

The relationships among these parameters are as follows: The relationships among these parameters are as follows: The porosity, f, of the soil is: The porosity, f, of the soil is:  the ratio of the total volume of void or pore space, Vp, to the total soil volume V: f = Vp/V Relationships

The volumetric water content, q, is the ratio of water volume in the soil, VW, to the total volume, V: The volumetric water content, q, is the ratio of water volume in the soil, VW, to the total volume, V: The saturation, S, is the portion of the pore space filled with water: The saturation, S, is the portion of the pore space filled with water: These terms are further related as follows: These terms are further related as follows: q = Vb/V S = VW/Vp q = S * f S = VW/Vp q = S * f

When a sample of field soil is collected and oven-dried, the soil moisture is reported as a dry weight fraction, W: When a sample of field soil is collected and oven-dried, the soil moisture is reported as a dry weight fraction, W:

Convert a dry weight soil moisture fraction into volumetric moisture content: Convert a dry weight soil moisture fraction into volumetric moisture content:  The dry weight fraction is multiplied by the bulk density, g b; and divided by specific weight of water, g w which can be assumed to have a value of unity. Thus: q = g b W/g w

The g b is defined as the specific weight of the soil particles, g s, multiplied by the particle volume or one-minus the porosity: The g b is defined as the specific weight of the soil particles, g s, multiplied by the particle volume or one-minus the porosity: g b = g b * (1 - f ) Specific Weight

The volumetric moisture contents at field capacity, q fc, and permanent wilting point, q wp, then are defined as follows: The volumetric moisture contents at field capacity, q fc, and permanent wilting point, q wp, then are defined as follows: where Wfc and Wwp are the dry weight moisture fractions at each point. where Wfc and Wwp are the dry weight moisture fractions at each point. q fc = g b Wfc/g w q wp = g b Wwp/g w q wp = g b Wwp/g w volumetric moisture contents

The total available water, TAW is the difference between field capacity and wilting point moisture contents multiplied by the depth of the root zone, RD: The total available water, TAW is the difference between field capacity and wilting point moisture contents multiplied by the depth of the root zone, RD: TAW = (q fc - q wp) RD Total Available Water

Root Depth (metres) Crop 1.5Alfalfa 1.8Almonds 1.8Apricots 1.4Artichokes 1.5Asparagus 0.9Bananas 0.9Beans 0.8Beets 0.5Broccoli 0.5Cabbage

The Soil Moisture Deficit, SMD, is a measure of soil moisture between field capacity and existing moisture content, q i, multiplied by the root depth: The Soil Moisture Deficit, SMD, is a measure of soil moisture between field capacity and existing moisture content, q i, multiplied by the root depth: SMD = (q fc - q i) * RD Soil Moisture Deficit

A similar term expressing the moisture that is allotted for depletion between irrigations is the 'Management Allowed Deficit', MAD. A similar term expressing the moisture that is allotted for depletion between irrigations is the 'Management Allowed Deficit', MAD. This is the value of SMD where irrigation should be scheduled and represents the depth of water the irrigation system should apply. This is the value of SMD where irrigation should be scheduled and represents the depth of water the irrigation system should apply. Management Allowed Deficit

Soil Moisture Measurements The soil moisture status requires periodic measurements in the field, from which one can project when the next irrigation should occur and what depth of water should be applied. The soil moisture status requires periodic measurements in the field, from which one can project when the next irrigation should occur and what depth of water should be applied. Such data can indicate how much has been applied and its uniformity over the field. Such data can indicate how much has been applied and its uniformity over the field.

Techniques for Evaluating Soil Moisture i. Gravimetric sampling Collecting a soil sample from each cm of the soil profile to a depth at least that of the root penetration. Collecting a soil sample from each cm of the soil profile to a depth at least that of the root penetration. The soil sample of gm is placed in an air tight container of known weight and then weighed. The soil sample of gm is placed in an air tight container of known weight and then weighed. The sample is then placed in an oven heated to 105° C for 24 hours with the container cover removed. The sample is then placed in an oven heated to 105° C for 24 hours with the container cover removed.

After drying, the soil and container are again weighed and the weight of water determined as the before and after readings. After drying, the soil and container are again weighed and the weight of water determined as the before and after readings. The dry weight fraction of each sample can be calculated using Eq. 5. The dry weight fraction of each sample can be calculated using Eq. 5. Knowing the bulk density, one can determine moisture contents from Eq. 6 and the soil moisture depletion from Eq. 11 Knowing the bulk density, one can determine moisture contents from Eq. 6 and the soil moisture depletion from Eq. 11

Sampling auger

Sampling tube

ii. The neutron Probe The neutron probe is inserted at various depths into an access tube and the count rate is read from the scaler. The neutron probe is inserted at various depths into an access tube and the count rate is read from the scaler. The manufacturers of neutron probe equipment furnish a calibration relating the count rate to volumetric soil moisture content. The manufacturers of neutron probe equipment furnish a calibration relating the count rate to volumetric soil moisture content.

Field experience suggests that these calibrations are not always accurate under a broad range of conditions so it is advisable for the investigator to develop an individual calibration for each field or soil type. Field experience suggests that these calibrations are not always accurate under a broad range of conditions so it is advisable for the investigator to develop an individual calibration for each field or soil type. Most calibration curves are linear, best fit lines of gravimetric data and scaler readings but may in some cases be slightly curvilinear Most calibration curves are linear, best fit lines of gravimetric data and scaler readings but may in some cases be slightly curvilinear

The volume of soil actually monitored in readings by the neutron probe depends on the moisture content of the soil, increasing as the soil moisture decreases. The volume of soil actually monitored in readings by the neutron probe depends on the moisture content of the soil, increasing as the soil moisture decreases. The accuracy of soil moisture determinations near the ground surface is affected by a loss of neutrons into the atmosphere thereby influencing measurements prior to an irrigation more than afterwards. The accuracy of soil moisture determinations near the ground surface is affected by a loss of neutrons into the atmosphere thereby influencing measurements prior to an irrigation more than afterwards.

As a consequence, soil moisture measurements with a neutron probe are usually unreliable within cm of the ground surface. As a consequence, soil moisture measurements with a neutron probe are usually unreliable within cm of the ground surface.

iii. Touch-and-feel As a means of developing a rough estimate of soil moisture, the Touch-and-feel method can be used. As a means of developing a rough estimate of soil moisture, the Touch-and-feel method can be used. A handful of soil is squeezed into a ball. A handful of soil is squeezed into a ball. Then the appearance of the squeezed soil can be compared subjectively to the descriptions listed in Table 2 to arrive at the estimated depletion level. Then the appearance of the squeezed soil can be compared subjectively to the descriptions listed in Table 2 to arrive at the estimated depletion level.

Merriam (1960) has developed a similar table which gives the moisture deficiency in depth of water per unit depth of soil. Merriam (1960) has developed a similar table which gives the moisture deficiency in depth of water per unit depth of soil. Over the years various investigators have compared actual gravimetric sample results to the Touch-and-Feel estimates, finding a great deal of error depending on the experience of the sampler. Over the years various investigators have compared actual gravimetric sample results to the Touch-and-Feel estimates, finding a great deal of error depending on the experience of the sampler.

iv. Bulk density Measurements of bulk density are commonly made by carefully collecting a soil sample of known volume and then drying the sample in an oven to determine the dry weight fraction. Measurements of bulk density are commonly made by carefully collecting a soil sample of known volume and then drying the sample in an oven to determine the dry weight fraction. Then the dry weight of the soil, Wb is divided by the known sample volume, V, to determine bulk density, g b: Then the dry weight of the soil, Wb is divided by the known sample volume, V, to determine bulk density, g b: g b = Wb V (12) g b = Wb V (12)

Most methods developed for determining bulk density use a metal cylinder sampler that is driven into the soil at a desired depth in the profile. Most methods developed for determining bulk density use a metal cylinder sampler that is driven into the soil at a desired depth in the profile. Bulk density varies considerably with depth and over an irrigated field. Bulk density varies considerably with depth and over an irrigated field. Thus, it is generally necessary to repeat the measurements in different places to develop reliable estimates Thus, it is generally necessary to repeat the measurements in different places to develop reliable estimates

Feel or Appearance of Soil Percent Depl etion Silt loams to clay loam Fine sandy loams to silt loams Loamy sands to fine sandy loams same no free water on ball* but wet outline on hand 0 (field capa city) easily ribbons slick feeling makes tight ball, ribbons easily, slightly sticky and slick makes ball but breaks easily and does not feel slick 0-25 pliable ball, ribbons easily slightly slick pliable ball, not sticky or slick, ribbons and feels damp balls with pressure but easily breaks slightly balls still pliable balls under pressure but is powdery and easily breaks will not ball, feels dry50-75 hard, baked, cracked, crust powdery, dry, crumbles dry, loose, flows through fingers

v. Field capacity The most common method of determining field capacity in the laboratory uses a pressure plate to apply a suction of -1/3 atmosphere to a saturated soil sample. The most common method of determining field capacity in the laboratory uses a pressure plate to apply a suction of -1/3 atmosphere to a saturated soil sample. When water is no longer leaving the soil sample, the soil moisture in the sample is determined gravimetrically and equated to field capacity. When water is no longer leaving the soil sample, the soil moisture in the sample is determined gravimetrically and equated to field capacity.

A field technique for finding field capacity involves irrigating a test plot until the soil profile is saturated to a depth of about one metre. A field technique for finding field capacity involves irrigating a test plot until the soil profile is saturated to a depth of about one metre. Then the plot is covered to prevent evaporation. Then the plot is covered to prevent evaporation. The soil moisture is measured each 24 hours until the changes are very small, at which point the soil moisture content is the estimate of field capacity. The soil moisture is measured each 24 hours until the changes are very small, at which point the soil moisture content is the estimate of field capacity.

vi. Permanent wilting point Generally, at the permanent wilting point the soil moisture coefficient is defined as the moisture content corresponding to a pressure of -15 atmospheres from a pressure plate test. Generally, at the permanent wilting point the soil moisture coefficient is defined as the moisture content corresponding to a pressure of -15 atmospheres from a pressure plate test. Although actual wilting points can be somewhere between -10 and -20 atm, the soil moisture content varies little in this range. Although actual wilting points can be somewhere between -10 and -20 atm, the soil moisture content varies little in this range. Thus, the -15 atm moisture content provides a reasonable estimate of the wilting point. Thus, the -15 atm moisture content provides a reasonable estimate of the wilting point.

Sandy Soils

Loamy/Silty Soils

Organic soil

Clay Soil

Objectives of evaluation The principal objective of evaluating surface irrigation systems is to identify management practices and system configurations that can be feasibly and effectively implemented to improve the irrigation efficiency. The principal objective of evaluating surface irrigation systems is to identify management practices and system configurations that can be feasibly and effectively implemented to improve the irrigation efficiency. An evaluation may show that higher efficiencies are possible by reducing the duration of the inflow to an interval required to apply the depth that would refill the root zone soil moisture deficit. An evaluation may show that higher efficiencies are possible by reducing the duration of the inflow to an interval required to apply the depth that would refill the root zone soil moisture deficit.

The evaluation may also show opportunities for improving performance through changes in the field size and topography. The evaluation may also show opportunities for improving performance through changes in the field size and topography. Evaluations are useful in a number of analyses and operations, particularly those that are essential to improve management and control. Evaluations are useful in a number of analyses and operations, particularly those that are essential to improve management and control.

The evaluation may also show opportunities for improving performance through changes in the field size and topography. The evaluation may also show opportunities for improving performance through changes in the field size and topography. Evaluations are useful in a number of analyses and operations, particularly those that are essential to improve management and control. Evaluations are useful in a number of analyses and operations, particularly those that are essential to improve management and control.

Evaluation data can be collected periodically from the system to refine management practices and identify the changes in the field that occur over the irrigation season or from year to year. Evaluation data can be collected periodically from the system to refine management practices and identify the changes in the field that occur over the irrigation season or from year to year. Evaluation the surface irrigation system is an important to optimize the use of water resources in this system. Evaluation the surface irrigation system is an important to optimize the use of water resources in this system.

A summary of the data arising from a field evaluation is enumerated below. A summary of the data arising from a field evaluation is enumerated below. There are several publications describing the equipment and procedures for evaluating surface irrigation systems, but not all give a very correct methodology for interpreting the data once collected. There are several publications describing the equipment and procedures for evaluating surface irrigation systems, but not all give a very correct methodology for interpreting the data once collected.

The data analysis depends somewhat on the data collected and the information to be derived. The data analysis depends somewhat on the data collected and the information to be derived. This section will deal with two aspects of an evaluation. This section will deal with two aspects of an evaluation. The first is the definition of the typical field infiltration relationship using the evaluation data describing the surface flow. The first is the definition of the typical field infiltration relationship using the evaluation data describing the surface flow.

The second is the evaluation of the efficiency of the irrigation event studied. The second is the evaluation of the efficiency of the irrigation event studied. Although many performance measures have been suggested, only four will be noted herein: Although many performance measures have been suggested, only four will be noted herein: 1) application efficiency 1) application efficiency 2) storage efficiency 2) storage efficiency 3) deep percolation ratio 3) deep percolation ratio 4) runoff ratio. 4) runoff ratio. These will be defined here before detailing the analyses of infiltration and performance. These will be defined here before detailing the analyses of infiltration and performance.